Scientech 2502

Optical Fiber Communication Scientech 2502

Product Tutorials Ver. 1.1

Designed & Manufactured byAn ISO 9001:2008 company Scientech Technologies Pvt. Ltd. 94, Electronic Complex, Pardesipura, Indore - 452 010 India, + 91-731 4211100,

: [email protected] , [email protected] ,

: www.ScientechWorld.com


Optical Fiber Communication Scientech 2502 Table of Contents

Safety Instructions

4

Introduction

5

Features

6

Technical Specifications

7

Introduction to Fiber Optic

9

Theory of Fiber Optic

8

Advantages of Fiber Optic System

16

Characteristics of Optical Fiber

18

Experiments •

•

•

•

•

•

•

•

•

•

•

•

Experiment 1 Setting up Fiber Optic analog link

44

Experiment 2 Setting up Fiber Optic digital link

46

Experiment 3 Intensity Modulation System using analog input signal

48

Experiment 4 Intensity Modulation System using digital input signal

50

Experiment 5 Frequency Modulation System

53

Experiment 6 Pulse Width Modulation System

55

Experiment 7 Study of Propagation loss in Optical Fiber

57

Experiment 8 Study of Bending Loss

59

Experiment 9 Measurement of Optical Power using Optical power meter

61

Experiment 10 Measurement of Propagation Loss using OPM

63

Experiment 11 Measurement of Numerical Aperture

65

Experiment 12 2


•

•

•

•

•

•

•

•

•

•

•

•

•

•

Characteristics of E-O converter using OPM

67

Experiment 13 V-I Characteristics of Photo LED (E - O converter)

69

Experiment 13A V-I Characteristics of Photo Detector (E - O converter)

71

Experiment 14 Characteristics of Fiber Optic Communication Link

73

Experiment 15 Setting up of FO voice link using Intensity Modulation

75

Experiment 16 Setting up of FO voice link using FM

77

Experiment 17 Setting up of FO voice link using PWM

79

Experiment 18 Study of switched fault in Intensity Modulation System

82

Experiment 19 Study of switched fault in FM

84

Experiment 20 Study of switched fault in PWM

86

Experiment 21 Computer Communication using RS232 interface

78

Experiment 22 BIT rate measurement

90

Experiment 23 Sensitivity

91

Experiment 24 Determination of Power Margin (Power Budget)

92

Experiment 25 To Measure Bit Error Rate

93

Experiment 26 To Study and Observation of Eye Pattern Frequently Asked Questions

96 101

Glossary

109

Warranty

112

List of Accessories

112

•

3


Safety Instructions Read the following safety instructions carefully before operating the product. To avoid any personal injury, or damage to the product, or any products connected to it; Do not operate the instrument if you suspect any damage within.

The instrument should be serviced by qualified personnel only. For your Safety: Use proper Mains cord

: Use only the mains cord designed for this product. Ensure that the mains cord is suitable for your country.

Ground the Instrument

: This product is grounded through the protective earth conductor of the mains cord. To avoid electric shock the grounding conductor must be connected to the earth ground. Before making connections to the input terminals, ensure that the instrument is properly grounded.

Observe Terminal Ratings : To avoid fire or shock hazards, observe all ratings and marks on the instrument.

Use only the proper Fuse

: Use the fuse type and rating specified for this product.

Use in proper Atmosphere : Please refer to operating conditions given in the manual.

•

Do not operate in wet / damp conditions.

•

Do not operate in an explosive atmosphere.

•

Keep the product dust free, clean and dry.

4


Introduction

Scientech TechBooks are compact and user friendly learning platforms to provide a modern, portable, comprehensive and practical way to learn Technology. Each TechBook is provided with detailed Multimedia learning material which covers basic theory, step by step procedure to conduct the experiment and other use ful information. Scientech 2502 Fiber Optic Communication TechBook demonstrate full Duplex method of transmitting information from one place to another by sending pulses of light through an Optical fiber the light forms electromagnetic wave that is modulated to carry information. Fiber Optics Communication Scientech 2502 is Advanced Fiber Optic TechBook is basically designed to learn the communication techniques in Fiber Optics. The TechBook demonstrates properties of Fiber Optics Transmitter & Receiver, characteristics of Fiber Optics Cable, different types of Modulation / Demodulation techniques and PC to PC communication via fiber link using RS232 interface. It can also be used to demonstrate various Digital Communication Techniques via Fiber Optic link using Scientech Digital Communication TechBooks.

5


Features

•

Full Duplex Analog & Digital Trans-receiver

•

Single Module covering large number experiments with Optical Power Meter

•

660 nm & 950 nm (Optional Laser Source) channel with Transmitter & Receiver

•

AM-FM-PWM modulation / demodulation

•

PC-PC comm. with RS232 ports & software

•

On board Function Generator

•

Crystal Controlled Clock

•

Functional Blocks indicated on-board mimic

•

Input-output & test points provided on board

•

On board voice link

•

Built in DC Power Supply

•

Numerical Aperture measurement jig and mandrel for bending loss measurement

•

Switched faults on Transmitter & Receiver

of

experiments

including

6


Technical Specifications Transmitter

:

2 in number, fiber optic LED having peak wave length of emission 660 nm & 950 nm (Optical Laser Source)

Receiver

:

2 in number, fiber optic photo (-) detector

Modulation Techniques

:

1. direct amplitude modulation & demodulation 2. Frequency modulation / demodulation 3. Pulse width modulation / demodulation

Drivers

:

Analog & separately

digital

for

both

channels

Comparator

:

2 Nos.

AC Amplifiers

:

2 Nos.

Clock

:

Crystal controlled - 4.096 MHz

PLL Detector

:

1 No

Filters

:

2 Nos, 4th order Butter worth Filter (3.4 KHz cut- off frequency)

Analog Band Width

:

350 KHz

Digital Band Width

:

2.5 MHz

Function Generator

:

1. 1 KHz Sine wave (Amplitude adjustable) 2. 1 KHz Square wave (TTL)

Voice Link

:

Fiber Optics voice link using microphone & speaker (built in)

PC-PC Communication

:

Using two interface.

RS 232 Port

:

9 Pin D- type connector

Baud Rate

:

19200 baud

Switched Faults

:

Four in transmitter & four in receiver

Fiber Optic Cable

:

Connector type standard SMA (sub miniature assembly) duly polished fiber at both end for maximum transmission & perfect round spot for numerical aperture measurement

Cable type

:

Step indexed multimode PMMA plastic cable

Core Refractive Index

:

1.492

Clad Refractive Index

:

1.406

channels

through

RS232

7


Numerical Aperture

:

Better than 0.5

Acceptance angle

:

Better than 60 degrees

Fiber Diameter

:

1000 microns

Outer Diameter

:

2.2 mm

Fiber Length

:

0.5 m & 1 m

Inter connections

:

2 mm sockets

Test Points

:

50 nos.

Power Supply

:

110-220V, ± 10%, 50 / 60 Hz

Power Consumption

:

Interconnections

:

4 mm Banana sockets

Test points

:

50 Nos.

Dimensions (mm)

:

W 326 x H 52 x D 525

Weight

:

2.5 Kg. (approximately)

Product Tutorials

:

Online (Theory, procedure, reference results, etc).

4.5 VA (approximately)

8


Theory Introduction to Optical Fiber:

Communication can be broadly defined as the transfer of information from one point to another. Before Fiber optics came along, the primary means of real time data communication was electrical in nature. It was accomplished by using copper wire or by modulating information on to an electromagnetic wave which acts as a carrier for the information signal. All these methods have one problem in common the communication had to be over a straight line path. Fiber optics provides an alternative means of sending information over significant distances using light energy. Light is utilized for communication has major advantages because it can be modulated at significant higher frequencies than electrical signals. That is till 1870, when an Irish physicist John Tyndall carried out a simple experiment. He filled a container with water and shone light into it. In the darkened room he pulled the bung from the opposite end of the container. The light shone out, of course but in which direction? The light followed the curved path of water. The light was guided and a new science was born. This was due to a property of light ca lled refraction. Understanding how Fiber optics are made and function for uses in everyday life is an intriguing work of art combined with science. Fiber optics has been fabricated from materials that transmit light and are made from a bundle of very thin glass or plastic Fibers enclosed in a tube. One end is at a source of light and the other end is a camera lens, used to channel light and images around the bends and corners. Fiber optics has a highly transparent core of glass, or plastic encircled by a covering called "cladding". Light is stimulated through a source on one end of the Fiber optic and as the light travels through the tube, the cladding is there to keep it all inside. A bundle of Fiber optics may be bent or twisted without distorting the image, as the cladding is designed to reflect these lighting images from inside the surface. This Fiber optic light source can carry light over mass distances, ranging from a few inches to over 100 miles. There are two kinds of Fiber optics. The single-mode Fiber optic is used for high speed and long distance transmissions because they have extremely tiny cores and they accept light only along the axis of the Fibers. Tiny lasers send light directly into the Fiber optic where there are low-loss connectors used to join the Fibers within the system without substantially degrading the light signal. Then there are multi-mode which have much larger cores and accept light from a variety of angles and can use more types of light sources. Multi-mode Fiber optics also uses less expensive connectors, but they cannot be used over long distances as with the single-mode Fiber optics. Fiber optics has a large variety of uses. Most common and widely used in communication systems, Fiber optic communication systems have a variety of features that make it superior to the systems that use the traditional copper cables. The uses of fiber optics with these systems use a larger information-carrying capacity where they are not hassled with electrical interference and require fewer amplifiers then the copper cable systems. Fiber optic communication systems are installed in large networks of Fiber optic bundles all around the world and even under the oceans. Many Fiber optic testers are available to provide you with the best Fiber optic 9


equipment. In Fiber optic communication systems, lasers are used to transmit messages in numeric code by flashing on and off at high speeds. This code can constitute a voice or an electronic file containing, text, numbers, or illustrations, all by using Fiber optics. The light from many lasers are added together onto a single Fiber optic enabling thousands of currents of data to pass through a single Fiber optic cable at one time. This data will travel through the Fiber optics and into interpreting devices to convert the messages back into the form of its original signals. Industries also use Fiber optics to measure temperatures, pressure, acceleration and voltage, among an assortment of other uses.

Optical Fiber Communication System

Optical Fiber Communication System

10


Here, the information source provides an amplified electrical signal to a transmitter comprising an electrical stage, which drives an optical source to give conversion, may be either a semiconductor, LASER (Light Amplification by Stimulated Emission of Radiation) or LED. The transmission medium consists of optical source, which provides an electrical to optical conversion, an optical Fiber cable used for transmission of signal and the receiver, consists of an optical detector, which drives a further electrical stage and hence provides demodulation of optical carrier. This electrical signal is amplified and applied to the destination example speaker. Photo diodes (P-I-N or Avalanche) and in some instances photo transistors and photo conductors are utilized for detection of optical signal and optical to electrical conversion. The optical carrier may be modulated using an analog or digital information signal. Analog modulation involves the variation of light e mitted from the optical source in continuous manner. In digital modulation however, discrete changes in the light intensity are obtained (i.e. ‘On-Off’ pulses) although often simpler to implement, analog modulation with an optical Fiber communication system is less efficient, requiring a far higher signal to noise ratio (SNR) at the receiver than digital modulation. Also, linearity needed for analog modulation is not provided by semiconductor optical sources especially at high modulation frequencies. Principle of operation of Optical Fiber:

The principle of operation of optical Fiber lies in the behavior of light. It is a widely held view that light always travels in straight line and at constant speed. Of course, the light propagates in straight lines, but when it is reflected inside the optical Fiber million and trillion times by the clad, each movement comprising of a straight line and consequently because of such reflections, it acquires the shape of the optical Fiber. So effectively, it is said to have been traveling along the Fiber. It changes its direction only if there is a change in the dielectric medium as also illustrated by the Tyndall’s experiment. To understand the propagation of light within an optical Fiber it is necessary to take into account refractive index of the dielectric medium. Refractive index of a medium is defined as the ratio of velocity of light in vacuum to velocity of light in medium. Refractive index =

Velocity of light in vaccum Velocity of light in medium

Since, the velocity of light in any solid, transparent material is less than in vacuum the refractive index of such material is always greater than 1.0. A ray of light travels slowly in an optically dense medium than one that is less dense. Now, the direction that the light approaches the boundary between the two materials is very important. When a ray is incident on the interface between two dielectrics of differing refractive indices, refraction occurs. The light is refracted and also partly reflected internally in the same medium; which is referred as Partial Internal Reflection. It may be observed that the ray approaching the interface is propagating in a dielectric of refractive index n1 and is at an angle φ 1 to the normal at the surface of the interface. If the dielectric on the other side of interface has a refractive index n2 which is less

11


than n1 , then the refraction is such that the ray path in this lower index medium is at angle φ 2 to the normal where φ 2 is greater than φ 1 .

The angle of incidence φ 1 and refraction φ 2 are related to each other and to refractive indices of dielectrics by Snell's law of refraction which states that: n1 sinφ 1 = n2 sinφ 2 sin φ 1 sin φ 2

=

n2 n1

It is this change in refractive indices which causes the change in the path of the incident ray as evident from the Snell’s law. Larger the change in the refractive indices larger change in the direction of the incident ray. The sine of the angles will be in the ratio of their refractive indices. As the angle of incident ray increases, the angle of refraction also increases even faster and when the angle of refraction becomes 90° thereafter, if the angle of incidence is increased a condition is arrived where the incident ray is totally reflected in the same medium from where it has emerged; this is referred as the total internal reflection.

12


Total Internal Reflection:

Since, the angle of refraction is always greater than the angle of incidence, when the incident medium is denser than the refraction medium. Thus, the angle of refraction is 90° and the refracted ray emerges parallel to the interface between the dielectrics. This is the limiting case of refraction and this angle of incidence is known as critical angle φ c . The value of critical angle is given by: φ 2 = 90° φ1 = φ c Substituting this in the equation for Snell’s law g ives n1 sinφ c = n2 sin 90°

∴ sinφ c =

n2 n1

At angles of incidence greater than the critical angle the light is reflected back into the originating dielectric medium. This behavior of light is termed as Total Internal Reflection. Here,

Angle of Incidence = Angle of Reflection

This is the mechanism by which light may be considered to propagate down an optical fiber with low loss. shown in next figure below illustrates the transmission of a light ray in an optical fiber via a series of total internal reflection at the interface of the silica core and slightly lower refractive index silica cladding.

The light ray shown in next figure is known as meridian ray as it passes through the axis of the fiber core. It is generally used when illustrating the fundamental transmission properties of optical fiber. 13


Acceptance Angle:

Since, only rays with an angle greater than critical angle at the core cladding interface are transmitted by total internal reflection, it is clear that not all rays entering the fiber core will continue to propagate down the length.

Shown in next figure illustrates two incident rays I and B. It may be observed that ray ‘I’ enters the fiber core at an angle θ i less than θ a (conical half angle for the fiber explained herein under) to the fiber axis and is refracted at the air- core interface before transmission to the core- cladding interface at an angle φ more than the critical angle φ c . This ray is totally internally reflected and propagated along the fiber. While incident ray ‘B’ is incident into the fiber core at an angle θ b greater than θ a and will be transmitted to the core- cladding interface at an angle less than φ c and will not be totally internally reflected instead will be refracted into cladding and eventually lost by the radiation. Thus, for rays to be transmitted by total internal reflection within the fiber core they must be incident on the fiber core within an acceptance cone defined by conical half angle θ a . Hence, θ a is the maximum angle to the axis at which light may enter the fiber in order to be propagated and is referred to as the acceptance angle for the fiber? Here θ i < θ a < θ b

14


Numerical Aperture:

It gives the relationship between the acceptance angle and the refractive indices of the three media involved viz. the core, the c ladding and air. In the shown in next figure above φ i corresponds to θ i ; when φ i approaches φ c ; θ i approaches θ a By Snell's law of refraction:n0 sin θ a = n1 sin(90° − φ c )

{where φ c is the critical angle}

= n1 cos φ c = n1(1 − sin c 2φ )1/ 2 = n1 (1 −

n22 n 21

)1/ 2

{as sin φ c =

n2 n1

}

= n12 − n2 2 This term is referred as numerical aperture of the Wave Guide – Optical Fiber.

Numerical Aperture

= n0 sinθ a = (n12 − n22 )1/ 2 = (n12 − n22 )

= (n1 − n2 )(n1 + n2 )

Where, n0 = Refractive index of air n1 = Refractive index of core n2 = Refractive index of cladding The Numerical Aperture is a very useful measure of light collecting ability of a fiber. It directly relates to the refractive indices of the core and cladding. As we observe from the above equation, greater the absolute value of the indices of core and cladding, greater the numerical aperture; similarly, greater the difference between the refractive indices greater the numerical aperture. In accordance to the requirement of the numerical aperture, the material for the core and cladding is chosen, keeping in view the other parameters and requirements for transmission.

15


Advantages of FIBER Optic System 

Enormous Potential Band Width (BW):

The information carrying capacity of a transmission system is directly proportional to the carrier frequency of the transmitted signals. The optical carrier frequency in the range 1013 to 1016 Hz. (generally near infrared around 14 15 10 or 10 Hz) yields a far greater potential transmission B.W. than metallic cable system. (i.e. coaxial cable Bandwidth up to 500 MHz) or even milli meter wave radio system, (i.e. system currently operating with modulation Bandwidth of 700 MHz). Thus the optical Fibers have enormous transmission bandwidths and high data rate. Using wavelength division multiplexing operation, the data rate or information carrying capacity of optical fibers is enhanced to many orders of magnitude. At present the Bandwidth available to fiber system is not fully utilized by modulation at several GHz over hundred km. and hundreds of MHz over 300 Km with intervening electronics (repeaters) is possible. Therefore, the information carrying capacity of optical fiber system has proved far superior to best copper cable available, by comparison losses in coaxial cable systems restrict. A much-enhanced Bandwidth utilization for an optical fiber can be achieved by transmitting several optical signals each at different centre wavelengths in parallel on the same fiber. This wavelength division multiplexed operation particularly with dense packing of the optical wavelength (or fine frequency spacing) offers potential information carrying capacity. 

Small size and weight :

Optical Fibers have very small diameter in the ranges from 10 micrometers to 50 micrometers. The space occupied by the fiber cable is negligibly small compared to conventional electrical cables. Hence, when they are covered with protective coatings they are far smaller & lighter. This is a tremendous boon towards the alleviation of duct congestion in cities and allowing expansion of signal transmission in mobiles e.g. aircrafts, ships etc. 

Electrical Isolation :

Optical Fibers are fabricated from glass or plastic polymers, they are electrical insulators therefore they do not exhibit earth loop, interference problems, electromagnetic wave or any high current lightening. This property makes them suitable for communication in electrically hazardous environment as fiber create no arcing or spark hazard at abrasions or short circuit & usually fiber do not contain sufficient energy to ignite vapors or gases. It is also suitable in explosive environment.

16

Optical Fiber Communication Scientech 2502 

Immunity to Interference and Cross talk :

Optical fibers form a dielectric wave-guide and therefore are free from Electro Magnetic Interference (E.M.I), Radio Frequency Interference (R.F.I) or switching transients. It is not susceptible to lightening striker if used overhead rather than underground. Moreover it is easy to ensure that there is no optical interference between fibers. Since optical interference among different Fibers is not possible, cross talk is negligible even many Fibers are cabled together. 

Signal Security :

The light from optical fibers does not radiate significantly and therefore they provide a high degree of signal security. Unlike in copper cables, a transmitted signal cannot be drawn from a fiber without tampering it. Thus, the optical fiber communication provides 100% signal security. A transmitted optical signal cannot be obtained from a fiber in a non-invasive manner (i.e. without drawing optical power form the fiber). In theory, any attempt to acquire a message signal transmitted optically may be detected. This feature is obviously attractive for military & banking. 

Low transmission loss:

Due to the usage of ultra low loss Fibers and the erbium doped silica Fibers as optical amplifiers ,Optical fibers results in low attenuation or transmission loss in comparison with the best copper conductor. It facilitates the implementation of communication links with extremely wide repeater spacing thus reducing both system cost and complexity. This quality along with already proven modulation B W capability of fiber cable, it is used in long haul telecommunication applications. Hence for long distance communication Fibers of 0.002 dB/km are used. Thus the repeater spacing is more than 100 km. 

Potential Low Cost :

The glass that generally provides optical fiber transmission medium is made from sand not a scarce resource. In comparison with copper conductors, optical fiber offers low cost line communication. This is because many miles of optical cable are easier and less expensive to install than the same amount of copper wire or cable. 

Thinner:

Fiber optics is thinner than copper wire cables, so they will fit in smaller, more crowded places. This is important for underground cable systems, like in cities, where space needs to be shared with sewer pipes, power wires, and subway systems. 

Non-flammable

Since fiber optics send light instead of electricity, fiber optics are nonflammable. This means there is not a fire hazard. Fiber optics also does not cause electric shocks, because they do not carry electricity.

17


Ruggedness and flexibility

The fiber cable can be easily bend or twisted without damaging it. Further the fiber cables are superior than the copper cables in terms of handling, installation, storage, transportation, maintenance, strength and durability. 

Low cost and availability

Since the Fibers are made of silica which is available in abundance. Hence, there is no shortage of material and optical fibers offer the potential for low cost communication. 

Reliability

The optical Fibers are made from silicon glass which does not undergo any chemical reaction or corrosion. Its quality is not affected by external radiation. Further due to its negligible attenuation and dispersion, optical fiber communication has high reliability. All the above factors also tend to reduce the expenditure on its maintenance. The disadvantages of optical Fibers are: 

Price - Even though the raw material for making optical Fibers, sand, is abundant and cheap, optical Fibers are still more expensive per metre than copper. Although, one Fiber can carry many more signals than a single copper cable and the large transmission distances mean that fewer expensive repeaters are required.



Fragility - Optical Fibers are more fragile than electrical wires.



Affected by chemicals - The glass can be affected by various chemicals including hydrogen gas (a problem in underwater cables.)



Opaqueness - Despite extensive military use it is known that most Fibers become opaque when exposed to radiation.



Requires special skills - Optical Fibers cannot be joined together as a easily as copper cable and requires additional training of personnel and expensive precision splicing and measurement equipment.

18


The Optic Fiber:

The simplest Fiber optic cable consists of two concentric layers of transparent materials. The inner portion the core transports the light, the outer covering (the cladding) must have a lower refractive index than the core so the two of them are made up of different materials. To provide mechanical protection for the cladding an additional plastic layer; called Primary Buffer is added. Some constructions of optic Fiber have additional layers of buffers that are then referred to as Secondary Buffers. It is very important to note that the whole Fiber-Core, Cladding & Primary Buffer is solid and the light is confined to the core by the Total Internal Reflection due to the difference in the refractive index of the core as compared to that of cladding.

19


Single Mode versus Multi Mode:

As we have already seen that there are particular angles of propagation defined by cone of acceptance, which can be transmitted down the optic fiber. At these angles, the electromagnetic wave that the light can set up a number of completes patterns across the fiber. The number of complete patterns called Modes depends on the dimensions of the optic fiber core. There are essentially two different types of fiber optic transmission schemes in use viz. 

Single Mode



Multi Mode

Single Mode:

As the name suggests the single mode cable is able to propagate only one mode (Electromagnetic wave). This is used in long distance and/or, high-speed communication. It is beneficial over long distances since it completely eliminates a problem known as Intermodal Dispersion associated with Multimode cables. All our long distance telephone conversations are now carried by single mode optic fiber system over at least some part of the route.

20


Multi Mode:

The term multimode means that the diameter of the fiber optic core is large enough to propagate more than one mode (Electro Magnetic Wave). Because of the multiple modes the pulse that is transmitted down the fiber tends to become stretched over distance this is referred to as dispersion & has the effect of reducing the available bandwidth. These are typically used in applications such as LAN (Local Area Networks) & FDDI (Fiber Distr ibuted Area Interface)

Optical Fiber Index Profile

Index profile is the refractive index distribution across the core and the cladding of a fiber. Some optical fiber has a step index profile, in which the core has one uniformly distributed index and the cladding has a lower uniformly distributed index. Other optical fiber has a graded index profile, in which refractive index varies gradually as a function of radial distance from the fiber center. Graded-index profiles include powerlaw index profiles and parabolic index profiles. Step Index And Graded Index Fibers:

The first type of fiber optic cable put to use was called step index. In this design, the cladding has a different index of refraction than the core. The light bounces off the side and is reflected back into the fiber core. The problem with this design is that the reflected light must travel a slightly longer distance, than that which travels down the centre of the fiber, thus limiting the maximum transmission rate. This design was improved with the use of Graded index fiber. In this design, the index of refraction decreases in proportion to the distance away from the centre of the fiber core. The light moves more quickly in the outer portion thus compensating for the additional distance. The change in index has the effect of bending. The light reflects back towards the core. This change increases the transmission capacity by a reasonable factor. In the newest single mode design, the diameter of the fiber core is so small that all the light travels in a straight line. Even the latest fiber optic facility in use today uses less than 5% of the maximum theoretical capacity of a single mode fiber. 21


Some of the optical fibers in use are:

Multimode step index fibers.



Multimode graded index fibers.



Single mode step index fibers.



Plastic - clad fibers.



All plastic fibers.

Dimensions of fiber optic cables are written as a ratio e.g. a cable with cladding diameter of 125 microns and fiber core diameter of 62.5 or 50 microns will be referred to as 62.5 /125 or 50 / 125 fibers. That is if the diameter of the core is D cr and the diameter of the clad is Dcd both in microns (1 micron = 10 - 6 meters), then the dimensions of the Fiber optic cable will be denoted as D cr / Dcd.

22


Choice of Operating Frequency:

Once we had the LASER and the new optic fiber available, every thing was in place for a significant upsurge in communications. This resulted in two driving forces: one towards the ability to send more data faster and secondly to send the data to greater distances without being re-amplified. More Data Faster:

As the transmission rate of data is increased, the required bandwidth increases and this can be best accommodated by increasing the carrier frequency. This premise has stood us in good stead over many years. The speech and poor quality music transmissions on the medium frequency, AM radio, gives way to the higher frequency of FM radios which accommodate the increased bandwidth necessary for improved music quality. When television required even higher data rates, we responded by moving to even higher frequencies. These previous experience rather suggested that the light used for fiber optic communications should be of the highest frequency possible. But there was a surprise in store! Lower Frequencies Mean Lower Losses:

The first experiments used visible light of different colors (frequencies). As the losses were measured, we found that the higher frequencies caused more losses. The losses actually increased by the 4th power of the frequency. This means that a 4 tripling of the frequency would result in the losses increasing by 3 or 81 times. We therefore have two conflicting influences: High frequency = High Data Rates Low frequency = Long Ranges At the moment, long distance communication is more important than achieving the ultimate in data transmission rates. Therefore in most real installations, we tend to go for the relatively low frequencies of infrared light that is just below the visible spectrum.

23


Fiber Windows:

We now have an infrared range between 800 nm – 1700 nm (1 nano meter = 10 -9 meter) with one part of it around 1380 nm that is to be avoided due to high losses because of Hydroxyl Ion Absorption as is clear from the graph optical wavelength versus losses per kilometer (dB) . It seemed sensible to agree on standard wavelengths so that equipment from different manufacturers can be made compatible.

This has resulted in three standard wavelength slabs called windows. The windows were really the result of looking at the available light sources. Some wavelengths of LED and LASER light are easier and less expensive than others to produce. The design and manufacture of the optic fiber is then optimized for these frequencies. Note: The infrared light is very dangerous to eyes which can cause irreversible damages and since it is invisible, care should be taken to ensure that the optic fiber is not live.

24


Losses in Optic Fiber: 

Attenuation



Material Absorption Losses



Linear Scattering Losses



•

Ray Leigh Scatter

•

Mie Scattering

Non Linear Scattering



Micro Bending and Macro Bending



Dispersion



•

Inter modal Dispersion

•

Intra modal Dispersion

Attenuation :

Transmission of light is not 100 % efficient. Some photons of light are lost, causing attenuation of signal. Several mechanisms are involved, including absorption by materials within the fiber, scattering of light out of the core caused by environmental factors. The degree of attenuation depends on the wavelength of light transmitted. Attenuation measures the reduction in signal strength by comparing output power with input power. Measurements are made in decibels (dB). It is defined as: dB loss α = 10 log

Pi 10

Po

25




Material Absorption Losses :

It is a loss mechanism related to the material composition and fabrication process of the fiber that result in the dissipation of some of the transmitted optical power as heat in wave-guide. The absorption of light may be intrinsic (caused by one or more major components of glass) or extrinsic (caused by impurities within the glass).

26


Linear Scattering Losses :

Linear scattering mechanisms cause the transfer of some or all of the optical power contained within one propagating mode to be transferred linearly (proportionally) into a different mode. This process tends to result in attenuation of the transmitted light as the transfer may be to a leaky or radiation mode that doesn't continue to propagate within the fiber core, but is radiated from the fiber. It is mainly of two types.

} Ray Leigh Scattering } Mie Scattering Ray Leigh Scattering:

When the infrared light strikes a very-very small place where the materials in the glass are imperfectly mixed, this gives rise to localized changes in the refractive index resulting in the light being scattered in all directions. Some of the light escapes the optic fiber, some continues in the correct direction and some is returned towards the light source. This is called backscatter .

Mie Scattering:

These result from the non - perfect cylindrical structure of the wave-guide. It may be the caused by the imperfections such as irregularities in the core cladding interface core, cladding refractive index difference along the fiber length, diameter fluctuations, strains and bubbles. The scattering created by such in homogeneities is mainly in the forward direction. 

Non Linear Scattering :

Optical wave-guide does not behave linearly, several non-linear effects occur, which in the case of scattering cause disproportionate attenuation usually at high optical power level. This non-linear scattering causes the optical power from one mode to be transferred in either the forward or backward direction to the same, or other modes at different frequency. It depends critically upon the optical power density within the fiber and hence only becomes significant above threshold power levels.

27




Micro Bending and Macro Bending :

A problem that often occurs in cabling of the optical fiber is the twisting of the fiber core axis on a microscopic scale within the cable form. This phenomenon, known as micro bending result from small lateral forces exerted on the fiber during the cabling process and it causes losses due to radiation in both multimode and single mode fiber.



Macro bends :

The light propagates down the optic fiber solely because the incident angle exceeds the critical angle. If a sharp bend occurs, the normal and the critical angle move round with the fiber. The incident ray continues in a straight line and it finds itself approaching the core - cladding boundary at an angle less than the critical angle and much of light is able to escape.

28




Dispersion :

When an electrical pulse energizes a LASER, it launches a short flash or light along the optic fiber. It is an unfortunate fact that the light burst becomes longer as it moves along the fiber optic cable. The light spreads out. This effect is called 'Dispersion', shown in next figure the light pulse shown before and after it has traveled through the cable.

It is going to limit how fast we can send data - how many bits per second we can transmit through a fiber optic link. In fact it is the main limit to the data transmission rate for long distance communication system. If we send flashes of LASER light down a long link in which dispersion is a problem, the flashes will merge at the far end and the ON/OFF states will not be distinguished by the receiver. Over a given transmission path, there are only two remedies. Firstly, we could reduce the transmission rate so that even allowing for the spreading effect of the dispersion; the ON-OFF states are still clearly separated. This is not a very exciting solution and would clash with one of the main reasons for using optic fiber. There are two types of Dispersion:

}


}


Inter modal Dispersion:

You will recall that, to be propagated down the core of the optic fiber, the light must enter at an angle greater than the critical angle. Let us consider just two such rays of light as they travel along a section of optic fiber.

29


Ray ‘A’ will reach the far end before Ray ‘B’ since it is traveling a shorter distance. Assuming that rays A and B are part of the same pulse of light and start at the same time, we can now see how the spreading of the pulses can occur. Each and every ray being propagated at its own angle will arrive at slightly different times at the far end. This spreading effect will occur all along the fiber so it is also important to appreciate that the longer the optic fiber, the greater the dispersion. Transmission rates that are actually possible on an optic fiber therefore depend in its length. In practice, there are only particular angles of propagation that can be transmitted down the optic fiber. Intra modal Dispersion:

This form of dispersion occurs in both multi mode and single mode optic fibers. It is only really significant in single mode usage since, being very slight; it is completely swamped by the inter modal dispersion in the multimode case. The cause is simple enough - the refractive index of material is determined to some extent by the wavelength of the light source. Can you see how this causes dispersion? A change in refractive index will change the speed of that particular wavelength of light. Now if your light source produces different wavelengths at the same time, we will have components of the transmitted light pulse traveling at the same time, we will have components of the transmitted light pulse traveling at different speeds. The total package of light will spread out - hence the dispersion. 

Cure for Inter modal Dispersion :

A large core diameter means many modes and severe inter modal dispersion. The cure for this type of dispersion is quite simple. Reduce the core size; the number of modes decreases and inter modal dispersion is reduced. We can do better than just reducing inter modal dispersion, we can completely eliminate it. Simply make the core so small that only one mode is propagated. A single ray cannot possibly go at two different speeds so inter modal dispersion cannot occur. In practice the core is reduced to about 9 µ m (micron). The optic fiber that now carries only a single mode is now referred to as a 'single mode fiber'. Single mode fiber is used for all long distance and/or high-speed communications. All long distance telephone conversations are now carried by single mode fiber optic systems over some parts of the route. The larger core optic fibers for short and medium distances carry many modes and are called 'Multimode'.

30


The Cure for Intra modal Dispersion :

The cure is apparently so simple; use a light source that emits only one wavelength of light. Unfortunately, it has not yet been invented. Our light sources in current use are the LED and the LASER. Study shown in next figure and decide which of the two would cause the lesser amount of Intra modal Dispersion.

The LASER would cause less intra modal dispersion because its light is more concentrated around the central wavelength. The spread of wavelength measured between the points where the power output falls to half of the peak power is called the spectral width. Some LASERS have spectral widths as low as 0.1 nm (nanometre). The low spectral width together with its high power and fast switching makes the LASER first choice for long distance communications using single mode optic fiber. Also there are some losses due to coupling in between the fibers at LED and photo detector ends. Applications of Macro bends: A Live Fiber Detector:

Here is the problem long distance fiber optic systems employ powerful LASER operating in the infrared region of the spectrum. This infrared light has two properties that are very significant to the engineers and technicians working on the system. We have various pieces of test equipment that can be used to check the system. The' live ' fiber detector is able to find which fibers are carrying data in most day to day checks, but read the instruction manual first to ensure that the instrument is suitable for the type of optic fiber you are checking.

31


This device has a pair of spring-loaded jaws. The fiber under test is slipped in between them and when the jaws close it will cause the fiber to be bent sufficiently to cause a macro bend. The escaping light can be detected by the photocell and used to activate the LED indicator. One flaw in the system is that it relies on the buffer being transparent to the infrared light. The Optical Time Domain Reflectometer (OTDR):

The OTDR is a measuring instrument that uses backscatter. It is the most versatile piece of test equipment that we have for making measurements on fiber optic systems. It provides us with two different measurements: 

It can measure the magnitude of any losses that occur along optic fiber.



It can measure distance along the optic fiber.

32


Losses:

As the light moves along the optic fiber, the light intensity is attenuated by the losses in the optic fiber and so the reflections returned to the OTDR receiver become weaker. Measurement of the amplitude of the returned signals tells us the optic fiber loss in dB/Km. if a macro bend has occurred, it would show up as a drop in the signal level at a particular point. (If the optic fiber has been cut, a connector should be fitted in such a way that end face of the glass causes a reflection of energy. It is also usual for this to occur at the extreme end of the optic fiber. This cause a localized increase in energy returned to the Optical Time Domain Reflectometer (OTDR). This reflection called as Fresnel (the‘s’ is not pronounced) reflection shown up as a small spike on the display. There is always a Fresnel Reflection at the start of the fiber due to the connector on the front panel of the OTDR. Distance:

We obtain timing information by starting the display and the pulse generator at the same instant. This is achieved by the synchronizing pulse that switches on both the LASER and the receiver at the same instant. If we know how long it takes for the backscatter light to return to the OTDR then we only have to know how fast the infrared light is traveling along the optic fiber to be able to calculate how far the light has traveled some light returns after say, 500 ns, it follows that it has traveled to a total of 100 meters. This represents 50 meters out along the optic fiber and 50 meters back. You will remember that the actual speed of propagation is determined by the refractive index of the core of the optic fiber. Speed of propagation = speed of light in free space / refractive index of the core (The refractive index of the optic fiber being tested must be punched in to the OTDR otherwise all the distance will miscalculated. The value of the refractive index is coated by the manufacturer). The synchronizing pulse simply provides a start to the generator and to the display circuits to allow them to determine the travel-time of the laser light and the backscatter.

33


Shows in next figure Fiber Optic System together with its appearance on OTDR screen. Notice that both the macro bends and fusion splices are shown as a sudden loss of power at a particular point. Indeed, it is not possible to distinguish between macro bed and a fusion splice just by observing the OTDR display. It just shows a localized loss. The loss may appear as a vertical drop or a sloping line depending upon the speed at which the screen being scanned on the OTDR. The connector has a similar loss but it also has a Fresnel reflection. Typical value of losses: Fusion splice - 0.05 dB Connector - 0.2 dB Macro bend: 0 dB to more than 8 dB depending on the severity of the macro bend. Two Other Applications of Back scatter. Distributive Temperature Sensing (DTS):

The amount of backscatter occurring in an optic fiber is dependent upon the manufacture of the optic fiber, the optic window used, and also upon the temperature of the optic fiber. Now, when we find a characteristic of the optic fiber that depends on the temperature, it is but a small step away from using the effect to measure temperatures. This new technique is called Distributive Temperature Sensing (DTS). Basically it is an optic fiber connected to equipment operating just like an OTDR that is then passed through the areas to be measured. If it passes through a refrigerator (minimum temperature of 190°C or 310 °F). shows in next figure e.g. the trace on the OTDR will show the backscatter level falling to a level dependent upon the temperature in the refrigerator. Similarly, a heated area (maximum temperature 460°C or 860°F) would return a higher level of backscatter. 34


Security:

You will recall that one of the advantages of the fiber optic system is the high level of security offered. We know however, that a macro bend would allow the light to escape and hence the data to be copied. An OTDR monitoring the line would immediately detect the power loss of the macro bend and be able to measure its distance along the optic fiber to an accuracy of approximately 0.1 meters (4 inches) the same immediate detection would occur as with the security matting Shows in next figure.

35


Fiber optic links:

Fiber optic links can be used for transmission of digital as well as analog signals. Basically a fiber optic link contains three main elements, a transmitter, an optical fiber and a receiver. The transmitter module takes the input signal in electrical form and then transforms it into optical (light) energy containing the same information. The optical fiber is the medium, which takes the energy to the receiver. At the receiver light is converted back into electrical form with the same pattern as originally fed to the transmitter. Transmitter:

Fiber optic transmitters are typically composed of a buffer, driver and optical source. The buffer provides both an electrical connection and isolation between the transmitter & the electrical system supplying the data. The driver provides electrical power to the optical source. Finally, the optical source converts the electrical current to the light energy with the same pattern. Commonly used optical sources are light emitting diodes (LED) and LASER beams. Simple LED circuits, for digital and analog transmissions are shown below.

Shows in next figure Tran conductance drive circuits for analog transmissioncommon emitter configuration. The transmitter sect ion comprises of: 

Function Generator



Frequency Modulator



Pulse Width Modulator Block.

The Function Generator generates the input signals that are going to be used as information to transmit through the fiber optic link. The output voltage available is 1 KHz sinusoidal signal of adjustable amplitude, and fixed amplitude 1 KHz square wave signal. The modulator section accepts the information signal and converts it into suitable form for transmission through the fiber optic link. 36


The Fiber Optic Link:

Emitter and Detector circuit on board form the fiber optic link. This section provides the light source for the optic fiber and the light detector at the far end of the fiber optic links. The optic fiber plugs into the connectors provided in this part of the board. Two separate links are provided. The Receiver: The Comparator circuit, Low Pass Filter, Phase Locked Loop, AC Amplifier Circuits form receiver on the board. It is able to undo the modulation process in order to recover the original information signal. In this experiment the TechBook board is used to illustrate One-Way communication between digital transmitter and receiver circuits.

Shows in next figure a simple drive circuit for binary digital transmission consisting of a common emitter-saturating switch. Modulation:

In order to transmit information via an optical fiber communication system it is necessary to modulate a property of the light with the information signal. This property may be intensity, frequency, and phase with either digital or analog signals. The choices are indicated by the characteristics of optical fiber, the available optical sources and detectors, and considerations of the over all system.

37


Intensity Modulation:

In this system the information signal is used to control the Intensity of the source. At the far end, the variation in the amplitude of the received signal is used to recover the original information signal.

The audio input signal is used to control the current through an LED which in turn controls the light output. The light is conveyed to the detector 1 circuit by optic fiber. The detector is a phototransistor that converts the incoming light to a small current which flows through a series resistor. This gives rise to a voltage whose amplitude is controlled by the received light intensity. The voltage is now amplified within the detector circuit and if necessary, amplified further by amplifier circuit. The Analog Bias Voltage:

There are two problems using amplitude modulation with an analog signal. The first is to do with the signal itself.

If you glance at the shown in next figure you will see that analog wave form moves positive & negative of the zero line. The second problem is that it is the shape of the waveform that carries the information. Ideally the emitter characteristic would be a straight line. Even so we would loose the negative going half cycles as shown in next figure below:

38


The answer is to superimpose the sinusoidal signal on positive voltage called the bias voltage so that both halves of the incoming signal have an effect on the light intensity. The combination of linear characteristic would be ideal but the real characteristic is not completely straight. However it does have a straight section that we can use if we employ a suitable value of bias voltage. Shown in next figure ideal and practical situations.

39


Digital Modulation:

With digital modulation, discrete changes in light intensity are obtained (i.e. ‘On-Off’ pulses) shown in next figure a block schematic of a typical digital optical fiber link.

Initially, input digital signal from the information source is suitably encoded for optical transmission. The LED drive circuit directly modulates the intensity of the light with encoded digital signal. Hence, a digital optical signal is launched into the optical fiber cable. An amplifier to provide gain follows the phototransistor used as detector. Finally, the signal obtained is decoded to give the original digital information. Digital Bias Voltage:

In case of a digital signal the only information which needs to be conveyed is the ’On’ state and ‘Off’ state. The digital Input signal is entirely positive going as shown in next figure.

So, there is no negative part of the signal to be lost and further more any distortion due to non-linearity of the characteristic is of no importance; all we need to know is whether the signal is ‘On’ or ‘Off’. There is no need therefore to generate a bias voltage. When amplitude modulation is used with a digital input we employ a comparator at the receiving end of the fiber to make the waveform square again called 'cleaning it up'.

40


Frequency Modulation:

In the traditional form of FM the carrier frequency is changed or modulated by the amplitude of the analog signal. In a fiber optic system this is not feasible since both our light sources; the LED and the LASER are fixed frequency devices. In fiber optic systems FM is achieved by using the original analog input signal to vary the frequency of a train of digital pulses.

Frequency Modulation

41


A circuit called Voltage Controlled Oscillator usually abbreviated as VCO achieves this. The digital pulses are communicated through the optic fiber and squared up at the receiver by a comparator in the same way as it was in amplitude modulation system. At this point, we convert the digital train back to the original analog signal by means of the Phase Locked Loop Detector (PLL). The PLL circuit performs a very simple function. It monitors an incoming signal and produces a DC Voltage output. If the input frequency increases, the DC voltage increases. If the frequency decreases, the DC voltage decreases in this way the original analog signal is recovered. The output of the PLL contains many unwanted frequency components. A Low Pass Filter removes these and then finally the signal is amplified to the desired level. Pulse width modulation:

Pulse width modulation (PWM) is an alternative to frequency modulation. They are both digital transmission. In FM, you will remember, the incoming analog signal is used to change the frequency of the digital stream. In pulse width modulation the amplitude of the analog signal to be transmitted as the changes in the width of the pulse. It is an extremely simple system of modulation. Assume an input signal at zero volts.

The digital stream and the average voltage level would be as shown in figure next.

42


Pulse Width Modulation

If the input voltage moves to a positive value, the pulse width will increase and since the waveform is ‘On’ longer than it is ‘Off’ the average value increases. Similarly, if the input signal goes negative the width of the pulse will decrease. The average value of the digital voltage now decreases. You will now appreciate that the average voltage level is increasing and decreasing in accordance with the input voltage. At the far end of the transmission system the digital pulses are cleaned up by the comparator and then simply passed through a low pass filter. The filter removes the square waves but the average level remains to form the output signal. At this stage, the output signal is increasing and decreasing in step with the input, but you will remember that the OV input signal produced a DC level at the output. This DC level must now be removed. We do this by means of blocking capacitors at t he input to the final amplifier. Recommended Testing Instruments for Experiments:

Oscilloscope 20 MHz Dual Trace Scientech 201

43


Experiment 1 Objective: Setting up Fiber Optic Analog Link

Study of a 650nm fiber optic analog link in this experiment you will study the relationship between the input signal and received signal. Equipments Required: 

Scientech 2502 TechBook with Power Supply cord



Optical Fiber cable



Cathode ray Oscilloscope with necessary connecting probe

Connection Diagram:

44


Procedure: 

Connect the Power Supply cord to the main power plug & to TechBook Scientech 2502.



Ensure that all switched faults are ‘Off’.



Make the following connections as shown in next figure

•

Connect the 1 KHz sine wave output to emitter l's input.

•

Connect the Fiber Optics cable between emitter output and detectors input.

•

Detector l's output to AC amplifier 1 input.



On the board, switch emitter l's driver to analog mode.



Switch ON the Power Supply of TechBook and Oscilloscope.



Observe the input to emitter 1 (TP5) with the output from AC amplifier 1 (TP28) and note that the two signals are same.

Questions: 

What is meant by index profile?



What is the drawback of multimode Fibers?



What is Fiber optics?

45


Experiment 2 Objective: Setting up Fiber Optic Digital Link

Study of a 650 nm fiber optic digital link In this experiment you will study the relationship between the input signal and received signal. Equipments Required: 




Optical Fiber cable




Connection Diagram:

46


Procedure: 

Connect the Power Supply cord to the main the Power Supply to the board.






Make the following connections as shown in next figure.

•

Connect the 1 KHz square wave output to emitter l's input.

•

Connect the fiber optic cable between emitter emitt er output and detectors input.

•

Detector 1's output to comparator 1’s input.

•

Comparator l's output to AC amplifier l's input.



On the board, switch emitter 1's driver to digital mode.



Switch ON the Power Supply of TechBook and Oscilloscope.



Monitor both the inputs to comparator 1 (TP13 & 14). Slowly adjust the comparators bias preset, until DC Level on the input (TP13) lays mid way between the high and low level level of the signal on the positive input (TP14).



Observe the input to emitter 1 (TP 5) with the output from AC amplifier 1 (TP28) and note that the two signals are same.

Questions: 

Why single mode Fibers are used for long distance transmission?



What is optical Fiber?



What is step index profile?

47


Experiment 3 Objective: Study of Intensity Modulation Technique using Analog input signal.

To obtain intensity modulation of the analog signal, transmit it over a fiber optic cable and demodulate the same at the t he receiver and to get back the original or iginal signal. Equipments Required: 




Optical Fiber cable



Cathode ray Oscilloscope with necessary connecting co nnecting probe

Connection Diagram:

48


Procedure: 

Connect the Power Supply cord to the main power plug & to TechBook Scientech 2502




•

Connect the FG output marked 1 KHz sine wave to input if emitt er 1.

•

Plug in a fiber optic link from output of emitter 1 LED to the photo transistor of the detector 1.

•

Detector 1 output TP 10 to input of Amplifier TP 27.



In the emitter 1 block switch the mode select to analog.



Turn the 1 KHz preset in Function Generator block to fully clockwise (maximum amplitude) position.



Switch on the Power Supply of the TechBook and Oscilloscope.



With the help of dual trace Oscilloscope observe the input signal at emitter 1 TP 5 also; observe the output from the detector 1. It should carry a smaller version of the original 1 KHz sine wave, illustrating that the modulated light beam has been reconverted back into an electrical signal.



The output from detector 1 is further amplified by AC amplifier 1. This amplifier increases the amplitude of the received signal, and also removes the DC component, which is present at detector output. Monitor the output of amplifier 1 TP28 and adjust the gain adjust 1 preset until the monitored signal has same amplitude as that applied to emitter 1 Input TP 5 .



While monitoring the output of Amplifier 1 TP 28 change the amplitude of modulating sine wave by varying the 1 KHz preset in the Function Generator block. Note that as expected, the amplitude of the receiver output signal changes.

Questions: 

What is the function of transmitter, optical Fiber and receiver?



Where Fiber optics links can be used?



What is spectral width?

49


Experiment 4 Objective: Study of Intensity Modulation Technique using digital I nput signal

The objective of this experiment is to obtain intensity modulation of digital signal, transmit it over fiber optic cable and demodulate the same at the receiver end to get back the original signal. Equipments Required: 




Optical Fiber cable




Connection Diagram:

50


Procedure: 





•

Connect the 1 KHz square wave socket in Function Generator block to emitter 1 input.

•

Connect an optic fiber link between emitter 1 output & Detector 1 input with the help of connector provided.

•

Detector output to comparator l's non-inverting (+ve) input.



Switch the mode switch in emitter block to digital mode. This ensures that signal applied to the driver's input cause the emitter LED to switch quickly between ‘On’ & ‘Off’ states.



Examine the Input to emitter 1 TP 5 on an Oscilloscope this 1 KHz square wave is now being used to amplitude modulate emitter I emitter LED.



Examine the output of detector 1 TP 10. This should carry a smaller version of original I KHz square wave illustrating that the modulated light beam has been reconverted into an electrical signal.



Monitor both input to comparator 1, at TP 13 & 14 and slowly adjust the "Comparator bias 1 preset until the DC Level on the negative input TP 13 lies midway between the high & low level of the signal on the positive input TP. 14. This DC level is comparator's threshold level.



Examine the output of comparator 1 TP15 Note that the original digital modulating signal has been reconstructed at the receiver.



Once again carefully flex the fiber optic cable we can see that there is no change in output on bending the fiber. The output amplitude is now independent of the bend radius of the cable and that of length of cable, provided that detector output signal is large enough to cross the comparator threshold level. This illustrates one of the advantages of amplitude modulation of a light beam by digital rather than analog means. Also, non-linear ties within the emitter LED & photo transistor causing distortion of the signal at the receiver output are the disadvantages associated with amplitude modulating a light source by analog means. Linearity is not a problem if the light beam is switched ‘On’ & ‘Off’ with a digital signal, since the detector output is simply squared up by a comparator circuit. To overcome problems associated with amplitude modulation of a light beam by analog means, analog signals are often used to vary or modulate some characteristic of a digital signal (e.g. frequency or pulse width.). The digital signal being used to switch the light beam ‘On’ & ‘Off’. The next two experiments illustrate how an analog signal can be used to modulate two specific characteristics of a digital signal.

51


Questions: 

What is intensity modulation?



What is the function of LASER?



How the modulated signal is detected?

52


Experiment 5 Objective: The Frequency Modulation System Equipments Required: 




Optical Fiber cable




Connection Diagram:

Procedure: Connect the Power Supply cord to the main power plug & to TechBook  Scientech 2502. 





•

Connect Function Generator 1 KHz sine wave signal to frequency modulator input.

•

Frequency modulator output TP2 to the emitter 1 input at TP5.

•

Connect the optic fiber between the emitter 1 circuit and the detector 1 circuit.

•

Detector 1 output TP10 to comparator 1 input at TP14.

53


•

Comparator 1 output TP15 to the PLL detector input at TP23.

•

PLL detector output at TP26 to the low pass filter 1 input at TP19

•

Low Pass Filter 1 output TP20 to A C amplifier 1 input at TP27



Switch emitter l's driver to digital mode. This ensures that fast changing digital signal applied to the drivers input causes the emitter LED to switch quickly between ‘On’ & ‘Off’ states.



Turn the 1 KHz preset in the Function Generator block to fully anticlockwise (Zero amplitude) position.



Switch ON the Power Supply of the TechBook and Oscilloscope.



Monitor the output of the voltage controlled oscillators (VCO) in the frequency modulator block TP2. Note that the frequency of this digital signal is at present constant, since the modulating 1 KHz sine wave has zer o amplitude.



Examine the output of detector 1 (TP10 and check that the transmitted digital pulses are successfully detected at the receiver).



With the help of dual trace Oscilloscope monitor both inputs to comparator1. Now adjust the bias 1 preset until the bias input at TP13 is halfway between the top and bottom of the square wave on TP14. You will remember that the function of the comparator is to clean up the square wave after its transmission through the fiber optic link.



The output of comparator 1 drives the input of the PLL detector which produces a signal whose average level is proportional to the frequency of the digital stream. This average level is then extracted by low pass filter 1, and amplified by AC Amplifier1 to produce the original analog signal at the amplifiers output TP28. Examine TP28 and note that the output voltage is zero. This is expected since there is currently no modulating voltage in the transmitter.



While monitoring the input to the frequency modulator block TP1 and the output from AC amplifier 1 TP 28 turn the 1KHz preset to its fully clockwise maximum amplitude) position. Note that the modulating 1 KHz signal now appears at the amplifiers output. If necessary, adjust the amplifiers gain, adjust 1 preset until the two monitored signal are equal in amplitude.



In order to fully understand how this frequency modulation transmitter/ receiver system works, examine the inputs and outputs of all functional blocks within the system, using an Oscilloscope.

Questions: 

How the FM signal is generated?



What are the various detection techniques of FM signals?



Why FM is used for short distance communication?

54


Experiment 6 Objective: The Pulse Width Modulation System Equipments Required: 




Optical Fiber cable




Connection Diagram:

Procedure: 




Ensure that all switched faults are set to ‘Off’.




• FG' s 1KHz sine wave signal to the Pulse width modulator input TP3 • Pulse width modulator output TP4 to emitter 1 input TP5 • Connect the optic fiber between the emitter 1 circuit and detector 1 circuit. • Detector 1 output TP10 to comparator & input at TP14. • Comparator 1 output TP15 to LPF 1 at TP19. 55


• LPF 1 output TP20 to A C amplifier 1 input at TP27. 

Switch emitter 1’s driver to digital mode. This ensures that fast changing digital signals applied to the drivers input because the emitter LED to switch quickly between ‘On’ & ‘Off’ states.



Turn the 1 KHz preset of Function Generator block to fully anticlockwise (zero amplitude) position.






Monitor the output of the pulse width modulator block TP4. Note that the pulse width of this digital signal is at present constant, since the modulating 1 KHz sine wave has zero amplitude.



Examine the output Detector TP10 and check that the transmitted digital pulse is successfully detected at the receiver.



Monitor both inputs’ comparator 1 TP13 & TP14 and if necessary, slowly adjust the comparator's bias preset, until the DC Level on the negative input TP13 lies midway between the high and low level of the signal on t he positive input TP14.



The average level of comparator l's output is extracted by LPF 1 and then amplified by AC amplifier which also removes the DC offset. Since, the average level of the comparator output is proportional to the pulse width, the original analog signal appears at the amplifiers output TP28. Examine TP28 and note that the output voltage is zero. This is expected since there is currently no modulating voltage at the transmitter.



While monitoring the input to the pulse width modulator block TP3 and the output from AC amplifier 1 TP28 turn the 1 KHz preset to its fully clockwise (maximum amplitude position). Note that the modulating 1 KHz signal now appears at the amplifiers output. If necessary, adjust the amplifiers gain adjust 1 preset until the two monitored signals are equal in amplitude.



In order to fully understand how this pulse width modulation transmitter/ receiver system works, examine the inputs and outputs of all functional blocks within the system using an Oscilloscope.

Questions: 

What is PWM?



What is the advantage of using PWM in communication systems?



What is the function of comparator circuit?

56


Experiment 7 Objective: Study of Propagation Loss in Optical Fiber

To measure propagation or attenuation loss in optical fiber Equipments Required: 




Optical Fiber cable




Connection Diagram:

57


Procedure: 

Connect Power Supply cord to the main power plug & to TechBook Scientech 2502.




•

Function Generator’s 1 KHz sine wave output to Input 1 socket of emitter 1 circuit via 4 mm lead.

•

Connect 0.5 m optic fiber between emitter 1 output and detector l's input.

•

Connect detector 1 output to amplifier 1 input socket via 4mm lead.






Set the Oscilloscope channel 1 to 0.5 V / Div and adjust 4 - 6 div amplitude by using X 1 probe with the help of variable pot in Function Generator block at input 1 of Emitter 1.



Observe the output signal from detector TP10 on CRO.



Adjust the amplitude of the received signal same as that of transmitted one with the help of gain adjust potentiometer in AC amplifier block. Note this amplitude and name it V1.



Now replace the previous FG cable with 1 m cable without disturbing any previous setting.



Measure the amplitude at the receiver side again at output of amplifier 1 socket TP 28. Note this value end name it V2.

Calculate the propagation (attenuation) loss with t he help of following formula. V1 / V2 = e-  (L1 + L2) Where  is loss in nepers / meter

1 neper = 8. 686 dB L 1 = length of shorter cable (0.5 m) L 2 = Length of longer cable (1 m) Questions: 

How to measure propagation losses?



By what optical cable is made up of?



What is step index Fiber?

58


Experiment 8 Objective: Study of Bending Loss

The object of this experiment into study bending loss Equipments Required: 




Optical Fiber cable






Mandrel

Connection Diagram:

59


Procedure: 

Connect Power Supply cord to the main power plug & to TechBook Scientech 2502.



Make the connections as shown in next figure.

•

Function Generator 1 KHz sine wave output to input socket of emitter Circuit via 4 mm lead.

•

Connect 0.5 m optic fiber between emitter output and detectors input.

•

Connect Detector output to amplifier input socket via 4 mm lead.



Switch ‘On’ the Power Supply of the TechBook and Oscilloscope.



Set the Oscilloscope channel 1 to 0.5 V/ Div and adjust 4-6 div amplitude by using X 1 probe with the help of variable pot in Function Generator Block at input of Emitter.



Observe the output signal from detector (TP8) on CRO.



Adjust the amplitude of the received signal as that of transmitted one with the help of gain adjusts potentiometer in AC amplifier block. Note this amplitude and name it V 1 .



Wind the fiber optic cable on the mandrel and observe the corresponding AC amplifier output on CRO, it will be gradually reducing, showing loss due to bends.

Questions: 

What is the reason of bending losses?



What is core and cladding?



What is the function of cladding?

60


Experiment 9 Objective: Measurement of Optical Power using Optical Power Meter

To measure optical power using optical power meter. Equipments Required: 




Optical Fiber cable






Power Meter Scientech 2551 with Power Supply cord

Connection Diagram:

61


Procedure: 

Connect the Power Supply cord to mains supply and to the TechBook Scientech 2502.






Connect the fiber optic cable between emitt ers 1's output & power meter's input.



On the board, switch emitter l's driver to analog mode. Keep the power meter's wavelength selector switch in 660 nm



Switch ON the Power Supply of the TechBook and power meter.



Note the reading displayed in power meter.



Switch the wavelength selector switch to 950 nm positions. & note the reading displayed on power meter.



Perform the same experiment with emitter 2.

Questions: 

How the power is measured using power meter?



What is wavelength of light?



What do you understand by fiber bending?

62


Experiment 10 Objective: Measurement of propagation loss using Optical Power Meter

To measure propagation loss in optical fiber using optical power meter. Equipments Required: 




Optical Fiber cable






Power Meter Scientech 2551 with Power Supply cord

Connection Diagram:

63


Procedure: 




Keep the mode switch in emitter 1 circuit in analog mode



Connect the 0.5m fiber cable in between the emitter LED & I/P of power meter.



Switch on the instrument Optical Fiber Communication & power meter (Keep the wavelength switch in 660 nm, position). Note the reading in power meter.



Replace the 0.5m fiber cable with the 1m cables without disturbing any setting. Again note the reading in power. This reading will be lesser then the previous one, indicating that the propagation loss increases with increase in length.



Perform the same experiment with emitter 2.

Questions: 

How Propagation Loss in optical fiber is measured?



What are the various types of losses in fiber?



What is the formula used for measurement of losses?

64


Experiment 11 Objective: Measurement of Numerical Aperture

Measurement of the Numerical Aperture (NA) of the fiber Equipments Required: 




Optical Fiber cable



Numerical Aperture measurement Jig

Connection Diagram:

65


Procedure: 




Connect the frequency generator's 1 KHz sine wave output to input of emitter 1 circuit. Adjust its amplitude at 5Vpp.



Connect one end of fiber cable to the output socket of emitter 1 circuit and the other end to the numerical aperture measurement jig. Hold the white screen facing the fiber such that its cut face is perpendicular to the axis of the fiber.



Hold the white screen with 4 concentric circles (10, 15, 20 & 25mm diameter) vertically at a suitable distance to make the red spot from the fiber coincide with 10 mm circle.



Record the distance of screen from the fiber end L and note the diameter W of the spot.



Compute the numerical aperture from the formula given below W NA = -------------- = sinθmax

√4L2 + W2 

Vary the distance between in screen and fiber optic cable and make it coincide with one of the concentric circles. Note its distance.



Tabulate the various distances and diameter of the circles made on the white screen and computes the numerical aperture from the formula given above.

Inferences: The N.A. recorded in the manufacturer's data sheet is 0.5. The variation in the observation is due to fiber being used. The Acceptance Angle is given by 2sin!max. The deviation from the data sheet is again due to fiber being used. Questions: 

What is numerical aperture?



Write the formula for numerical aperture?



What is the significance of numerical aperture? 66


Experiment 12 Objective: Characteristics of E-O Converter using OPM

Study of characteristics of E-O converter using OPM Equipments Required: 




Optical Fiber cable






Power Meter ST 2551 with Power Supply cord

Connection Diagram:

67


Procedure: 







Put emitter 1 block in digital mode. Connect the bias 1 on comparator 1 block preset TP13 to the emitter 1 I/P at TP5.



Adjust the bias l preset to its minimum setting fully counter clockwise. Connect the Fiber Optics cable between the emitter 1 LED & power meter.






Note down the reading from power meter.



Vary the bias preset so as to vary the voltage applied to emitter 1 LED.



Record the change in power meter reading corresponding to change in forward voltage. Plot the graph between forward voltage and power meter reading.



Perform the experiment with emitter 2.

Questions: 

What is the function of optical power meter?



What is the full form of LED?



Why LED is not used for long distance transmission?

68


Experiment 13 Objective: V-I Characteristics of Photo LED ( E - O converter)

The aim of this experiment is to plot the characteristic of LED. Equipments Required: 




Optical Fiber cable




Connection Diagram:

69


Theory :

LED’s and LASER diodes are the commonly used sources in optical communication systems, whether the system transmits digital or analog signal. It is therefore, often necessary to use linear electrical to optical converter to allow its use in intensity modulation & high quality analog transmission systems. LED's have a linear optical output with relation to the forward current over a certain region of operation.

Procedure: 

Connect Power Supply to the board.



Ensure that all switched faults are in OFF condition.



Put emitter 1 block in Digital Mode



Make connections as given below.

•

Connect the bias 1 preset on comparator to the emitter 1 input.

•

Adjust the bias 1 preset to its minimum setting fully counter clockwise. Now look down the emitter 1 LED Socket and slowly advance the setting of the bias 1 preset until in subdued lighting the light from LED is just visible.



Connect the DMM between + 12V supply (Red Socket) and tp of Input of Emitter LED. The DMM will now read the forward voltage (V f)



Measure the voltage drop across the 1k (R9) current limiting resistors by connecting DMM between tp of Input of Emitter LED and tp6 (tp38 if old kit). The forward current is given by dividing the readings by 1k. This If is known as threshold current.





DVM reading If = ------------------- mA 1000 Vary the bias 1 preset so as to vary the forward voltage (as 1.0, 1.5…4.0), note the corresponding If (forward current). Record these values of Vf and If & plot the characteristic between these two.

Questions: 

What are the characteristics of fiber optics link?



What is the function of transmitter?



How light signals are converted back to t he electrical signals?

70


Experiment 13A Objective: Characteristics of Photo Detector (E - O converter)

The aim of this experiment is to plot the characteristic of Photo Detector. Equipments Required: 




Optical Fiber cable




Connection Diagram:

Theory:

Photo Transistors and Photo Diodes are the commonly used detectors in optical communication systems, whether the system receives digital or analog signal. It is therefore, often necessary to use linear optical to electrical converter to allow its use in intensity demodulation & high quality analog receiving systems. Photo Diodes have a linear electrical output with relation to the light intensity over a certain region of operation.

71


Procedure: 

Connect Power Supply to the board.



Ensure that all switched faults are in OFF condition.



Put emitter 1 block in Digital Mode



Make connections as given below.

•

Connect the bias 1 preset on comparator to the emitter 1 input.

•

Adjust the bias 1 preset to its minimum setting fully counter clockwise. Now look down the emitter 1 LED Socket and slowly advance the setting of the bias 1 preset until in subdued lighting the light from LED is just visible.

•

Connect the fiber optic cable between emitter output and detectors input.



Connect the DMM between + 12V supply (Red Socket) and tp of Input of Emitter LED. The DMM will now read the forward voltage (V f)



Measure the voltage drop across the 75E resistors by connecting DMM between tp of output of Photo Transistor and Ground. The detector current is given by dividing the readings by 75E.





DVM reading0 Id = ------------------- mA 75 Vary the bias 1 preset so as to vary the forward voltage (as 1.0, 1.5…4.0), note the corresponding If (forward current). Record these values of Vf and Id & plot the characteristic between these two.

72


Experiment 14 Objective: Characteristics of Fiber Optics communication Link

The aim of experiment is to st udy the Vin (AC) versus Vo (AC). Equipments Required: 




Optical Fiber cable




Connection Diagram:

73


Procedure: 





•

Function Generator 1 KHz sine wave output to input socket of emitter 1 circuit via 4mm lead.

•

Connect optic fiber between emitter l’s output and detector 1’s input.

•

Connect Detector 1 output to amplifier 1 input socket via 4nm lead.



Switch on the Power Supply.



Set the amplitude of the Function Generator to 2V p-p.



Observe the transmitted and received signal on CRO. V o (output voltage) should be in the same order as Vin (input voltage).



Next set Vin to suitable values and note the values of Vo.



Tabulate and plot a graph V o versus Vin & compute Vo/Vin.

Questions: 

What is the advantage of amplitude modulation in terms of bandwidth requirement?



How amplitude modulation signal is generated?



What is the detection process amplitude modulated signals?

74


Experiment 15 Objective: Setting up of Fiber Optics voice link using Intensity Modulation

Study of voice communication through Fiber Optic cable using amplitude modulation Equipments Required: 




Optical Fiber cable




Connection Diagram:

75


Procedure: 





• Connect the FG output marked 1 KHz sine wave to input if emitt er 1. • Plug in a fiber optic link from output of emitter 1 LED to the photo transistor of the detector 1.

• Detector 1 output TP 10 to input of Amplifier TP 27. • In the emitter 1 block switch the mode select to analog. • Turn the 1 KHz preset in Function Generator block to fully clockwise (maximum amplitude) position. 




With the help of dual trace Oscilloscope observe the input signal at emitter 1 TP 5 also; observe the output from the detector 1. It should carry a smaller version of the original 1 KHz sine wave, illustrating that the modulated light beam has been reconverted back into an electrical signal.



The output from detector 1 is further amplified by AC amplifier 1. This amplifier increases the amplitude of the received signal, and also removes the DC component, which is present at detector output. Monitor the output of amplifier 1 TP28 and adjust the gain adjust 1 preset until the monitored signal has same amplitude as that applied to emitter 1 Input TP 5 .



While monitoring the output of Amplifier 1 TP 28 change the amplitude of modulating sine wave by varying the 1 KHz preset in the Function Generator block. Note that as expected, the amplitude of the receiver output signal changes.



Disconnect the emitter 1’s input to 1 KHz sine wave socket.



Make the following additional connections (as shown in next figure).



Audio input block's input to microphone.



Connect the output of audio input block to emitter l’s input.



AC Amplifier's output to input of audio output block.



Observe that same audio output is available on the speaker as fed to the microphone.

Questions: 

What is the drawback of FM modulation in terms of bandwidth requirement?



How the FM signals are generated?



What is the function of AC amplifier? 76


Experiment 16 Objective: Setting up Fiber Optics voice link using Frequency Modulation

The objective of this experiment is to demonstrate voice transmission through optic fiber using F M. Equipments Required: 




Optical Fiber cable




Connection Diagram:

77


Procedure: 








• Connect Function Generator 1 KHz sine wave signal to frequency modulator input.

• Frequency modulator output TP2 to the emitter 1 input at TP5. • Connect the optic fiber between the emitter 1 circuit and the detector 1 circuit.

• Detector 1 output TP10 to comparator 1 input at TP14. • Comparator 1 output TP15 to the PLL detector input at TP23. • PLL detector output at TP26 to the low pass filter 1 input at TP19 • Low Pass Filter 1 output TP20 to A C amplifier 1 input at TP27 

Switch emitter l's driver to digital mode. This ensures that fast changing digital signal applied to the drivers input causes the emitter LED to switch quickly between ‘On’ & ‘Off’ states.



Turn the 1 KHz preset in the Function Generator block to fully anticlockwise (Zero amplitude) position.



Disconnect the 1 KHz sine wave output from in put of F M block.



Make the following additional connections as shown in next figure without disturbing previous setting.



Plug the Microphone in the input of Audio input block.



Output of Audio input block to input of FM block.



Output of AC Amp block to the Power Supply.



Speak in the Microphone and listen the same in the speaker / Headphone.

Questions: 

What is the drawback of FM modulation in terms of bandwidth requirement?



How the FM signals are generated?



What is the function of AC amplifier?

78


Experiment 17 Objective: Setting up of Fiber Optic Voice Link using PWM

Study of voice transmission through Fiber Optic cable using PWM Equipments Required: 




Optical Fiber cable




Connection Diagram:

79


Procedure: 








• FG' s 1KHz sine wave signal to t he Pulse width modulator input TP3 • Pulse width modulator output TP4 to emitter 1 input TP5 • Connect the optic fiber between the emitter 1 circuit and detector 1 circuit. • Detector 1 output TP10 to comparator & input at TP14. • Comparator 1 output TP15 to LPF 1 at TP19. • LPF 1 output TP20 to A C amplifier 1 input at TP27. 

Switch emitter 1’s driver to digital mode. This ensures that fast changing digital signals applied to the drivers input because the emitter LED to switch quickly between ‘On’ & ‘Off’ states.



Turn the 1 KHz preset of Function Generator block to fully anticlockwise (zero amplitude) position.






Monitor the output of the pulse width modulator block TP4. Note that the pulse width of this digital signal is at present constant, since the modulating 1 KHz sine wave has zero amplitude.



Examine the output Detector TP10 and check that the transmitted digital pulse is successfully detected at the receiver.



Monitor both inputs’ comparator 1 TP13 & TP14 and if necessary, slowly adjust the comparator's bias preset, until the DC Level on the negative input TP13 lies midway between the high and low level of the signal on t he positive input TP14.



The average level of comparator l's output is extracted by LPF 1 and then amplified by AC amplifier which also removes the DC offset. Since, the average level of the comparator output is proportional to the pulse width, the original analog signal appears at the amplifiers output TP28. Examine TP28 and note that the output voltage is zero. This is expected since there is currently no modulating voltage at the transmitter.



While monitoring the input to the pulse width modulator block TP3 and the output from AC amplifier 1 TP28 turn the 1 KHz preset to its fully clockwise (maximum amplitude position). Note that the modulating 1 KHz signal now appears at the amplifiers output. If necessary, adjust the amplifiers gain adjust 1 preset until the two monitored signals are equal in amplitude.

80


In order to fully understand how this pulse width modulation transmitter/ receiver system works, examine the inputs and outputs of all functional blocks within the system using an Oscilloscope.



Disconnect the PWM input from 1 KHz sine wave socket



Make the following additional connection (as shown in next figure) without disturbing any previous settings



•

Plug the microphone into input of audio input block

•

Output of audio input block to input of PWM block

•

Output of AC Amp block to input of audio output block

Observe that the same audio sound is available in the speaker as fed to microphone.

Questions: 

What is frequency band for voice signals?



By what means the voice signals are converted into electrical signals?



Why PWM method is generally preferred for communication system?

81


Experiment 18 Objective: Study of Switched Fault in Intensity Mode

Study of the effect of switched fault number 1& 5 on amplitude modulated system Equipments Required: 




Optical Fiber cable




Connection Diagram:

82


Procedure: 

Connect the Power Supply cord to the main power plug & to TechBook Scientech 2502 .




•

Connect the FG output marked 1 KHz sine wave to input if emitt er 1.

•

Plug in a fiber optic link from output of emitter 1 LED to the photo transistor of the detector 1.

•

Detector 1 output TP 10 to input of Amplifier TP 27.






See that AM System is operat ing correctly.



Adjust the gain adjust potentiometer in the AC amplifier circuit to provide a sinusoidal signal of same amplitude as input.



Switch on fault 1. All other faults are set to ‘Off’.



This fault removes the bias normally present on emitter 1's LED in analog mode so that distortion occurs when analog amplitude modulation takes place.



Turn ‘Off’ the fault 1 and check that A.M. system is operating correctly. Adjust the preset to provide a sinusoidal signal of 4 V peak to peak at the output.



Switch ‘On’ fault 5. This shorts the output and negative input of AC amplifier 2. So that amplifier gain is always +1, irrespective of the position of the gain adjust 2 preset.



Observe the output, and vary the gain adjust preset. Is there any change in output?



Switch fault 5 ‘Off’.



Switch Power Supply ‘Off’.

Questions: 

What do you mean by intensity modulation?



What is the significance of switch fault?



Which type of optical source is used for intensity modulation?

83


Experiment 19 Objective: Study of switched Faults in FM System

To study the Effect of fault number 4, 6, & 8 in F.M. Syste m Equipments Required: 




Optical Fiber cable




Connection Diagram:

Procedure: 








•

Connect Function Generator 1 KHz sine wave signal to frequency modulator input.

•

Frequency modulator output TP2 to the emitter 1 input at TP5.

84


•

Connect the optic fiber between the emitter 1 circuit and the detector 1 circuit.

•

Detector 1 output TP10 to comparator 1 input at TP14.

•

Comparator 1 output TP15 to the PLL detector input at TP23.

•

PLL detector output at TP26 to the low pass filter 1 input at TP19

•

Low Pass Filter 1 output TP20 to A C amplifier 1 input at TP27



Set the emitter 1 to digital mode.






Check, that FM System is operating correct ly.



Adjust the preset in AC amplifier block to give a sinusoidal signal of amplitude same as input.



Switch fault 4 ‘On’ all other faults set to ‘Off’. This fault affects the phase locked loop detector between the voltage controlled oscillator (VCO) and phase comparator (exclusive OR gate). The result is that the PLL no longer follows changes in the frequency of the input signal.



Observe the system output at TP 28 and the output of PLL block.






Switch fault 6 ‘On’. This changes the DC bias on frequency modulator VCO input from +2.5V to 0V, So that the VCO no longer oscillates irrespective of the signal applied to its input.



Observe the system output and the FM block's output.






Switch fault 8 ‘On’. These shorts pin 11 'R' of IC9 to junction of R90 &C32.



Observe the output of system and also PLL output.



Switch ‘Off’ fault 8 and check the operation of FM System.

Questions: 

What is the function of VCO?



What is FM?



What is the function of PLL while detect ing the transmitted signals?

85


Experiment 20 Objective: Study of switched Faults in PWM System

To study the Effect of Switched Faults 2, 3 & 7 on pulse width modulation system. Equipments Required: 




Optical Fiber cable




Connection Diagram:

86


Procedure: 

Connect Power Supply cord to the mains and to the TechBook Scientech 2502.







•

FG' s 1KHz sine wave signal to t he Pulse width modulator input TP3

•

Pulse width modulator output TP4 to emitter 1 input TP5

•

Connect the optic fiber between the emitter 1 circuit and detector 1 circuit.

•

Detector 1 output TP10 to comparator & input at TP14.

•

Comparator 1 output TP15 to LPF 1 at TP19.

•

LPF 1 output TP20 to A C amplifier 1 input at TP27.






Switch on the Power Supply.



Check the correct operation of the PWM system.



Switch on fault number 2. This open circuits the feed back loop of the first stage of detector 2's voltage amplifier, so that the final amplifier output at TP 28 saturates.



Observe the output at detector 2 it saturates at + 10V, and observe the system output, it goes to zero. Try to the explain reason behind it.






Switch on fault number 3.



This open circuits the positive. In put of first comparator and due to this the system output goes zero. Try to explain reason behind it.



Switch ‘Off’ fault 3.



Switch fault 7 ‘On’. This switches ‘Off’ constant current source to PWM so that output level of modulator is permanently high.



Observe the output of PWM, it goes permanently high and output of system to zero. Try explaining reasons for it.



Switch off fault 7 check correct operation of PWM System.

Questions: 

What is the significance of Switched Faults?



What is full form of PWM?



What is the function of PWM?

87


Experiment 21 Objective: Computer to Computer communication using RS232 interface via Fiber Optic Link

There are 2 fibers optic Links provided on Optical Fiber Communication. We shall utilize these two links to communicate from one PC to other & via-versa. That means it is a Duplex system of communication using fiber optic link. The Software developed will help to transmit & receive messages from the computers. Equipments Required: 




Optical Fiber cable



Personal computer - 2 Nos. 486 or Pentium, DOS 6.0 or onwards, CD drive



RS232 cables for connecting PC's to TechBook - 2 Nos.

Connection Diagram:

88


Procedure: 

Keep one PC towards left and another towards right of Optical Fiber Communication.



Load the software in PC 1 & PC2, with the help o f the CD supplied.



Keep one of the COM port free on each of PC to connect the RS232 cables. Keep baud rate of both PC equal, say 57600.



Switch off both the PC's.



Make the following connections on the Optical Fiber Communication.

• • • • • •

Connect Fiber Link on CHI (emitter to Detector).

•

Switch on the TechBook. Observe input to emitter 1 and output of comparator 1. Adjust bias of comparator 1 for square wave output.

•

Switch 1 KHz square wave from the input of CHI to input of CH2 (emitter) and adjust comparator 2 bias for square wave output. Switch off the TechBook.

• •

Make connections as shown in diagram 17 and Switch on the TechBook.

•

Connect PC1 & PC2 to D type connectors. (Any to anyone) switch on the PC's and the TechBook & start working. Whatever you type in PC1 will be seen on the transmit column of PC1 and will also be received in the receive column of PC2 simultaneously & vice versa.

•

Remove any of the fiber links. To transmit & receive of that link is disconnected.

•

Change baud rate of any of the PCs & you will find that data is not transmitted. Keep the baud rate same.

•

Reduce the baud rate of both the PCs & you will see that transmit rate is lower. Switch off the TechBook and the PCs.

Connect Fiber Link on CH2 (emitter to Detector). Connect output of Detector 1 to comparator 1 input. Connect output of Detector 2 to comparator 2 inputs. Connect 1 KHz square wave to input of CHI (e mitter). Keep mode switch of both channels to digital and all Switched Faults in ‘Off’ position.

Optical Fiber Communication is ready for connection to PC's. Switch off the TechBook.

Questions: 

What RS 232C stands for?



What is the pin configuration for 25 pin D connector?



What is the pin configuration for 9 pin D connector? 89


Experiment 22 Objective: Bit Rate Measurement

Determining the bit rate supported by the fiber optic link Equipments Required: 




Optical Fiber cable



Oscilloscope with Power Supply cord

Procedure: 

Set up the fiber optic digital link as explained earlier, and ensure that the link is working satisfactorily.



Remove the on board TTL output from the emitter input and connect the TTL output of square wave generator to emitter input.



Keep the frequency at 10 KHz.



Observe the received output on the Oscilloscope.



Vary the frequency of the TTL input observing the output each time (You can adjust the comparator's bias preset).



Note the frequency at which the output is distorted or reduces to zero. The bit rate supported by the link is twice the frequency reading corresponding to zero/distorted output in bits per second.

Questions: 

How to determine the bit rate?



What is optical fiber link?

90


Experiment 23 Objective: Determining the sensitivity of the Fiber Optic Link Procedure:

Sensitivity is defined as the minimum power incident on the photo detector in order to establish the link. 

Set up the fiber optic digital link as explained earlier using 0.5m cable, and ensure that the link is working satisfactorily.



Remove the on board TTL output from the emitter input and connect the output of square wave generator to emitter input.



Observe the output of the detector on the Oscilloscope.



Remove the end of the fiber connected to the detector and connects it to the optical power meter.



Note the reading on the power meter Po. This reading is the power being transmitted to the receiver from the source.



Remove the fiber end which is connected to the power meter and connect it back to receiver.



Slowly reduce the amplitude of the square wave till the output being viewed on the Oscilloscope reduces to zero.



Remove the fiber end from the receiver and connect it to the power meter. Note the reading on the power meter Ps. This gives the measure of sensitivity of the receiver.

Questions: 

Define the sensitivity?



What are the elements of fiber optics link?

91


Experiment 24 Objective: Determination of Power Margin (Power Budget)

The Power margin is defined by: Po-Pi-Ps Where, Po is the power transmitted Pi is the power lost in the fiber Ps the sensitivity of the rece iver Procedure: 

Assuming that the power lost in the 0.5m fiber Pi is negligible, Po -Ps gives power margin of the link.



Repeat the sensitivity experiment with 1 m fiber optic cable.



In this case calculate the Pi as follows:

Connect the 1m fiber to the source and power meter. Note the power meter reading. We call it p1. Assuming that the power lost in the 0.5m fiber is negligible, Pi- is P1- Po. Po-Pi-Ps is the power margin for this link. Questions: 

How to determine power Margin?



What do you understand by power budget?

92


Experiment 25 Objective: To Measure Bit Error Rate. Apparatus Required: 

Eye Pattern and BER Measurement Module. (Optional)



Scientech 2501/ Scientech 2502 Optical Fiber Communication platform.



Patch Cords



Optical Fiber Cable



+ 5V DC, 500 mA Adapter

Theory:

In telecommunication transmission, the bit error rate (BER) is a Ratio of bits that have errors relative to the total number of bits received in a transmission. The BER is an indication of how often a packet or other data unit has to be retransmitted because of an error. Too high a BER may indicate that a slower data rate would actually improve overall transmission time for a given amount of transmitted data since the BER might be reduced, lowering the number of packets that had to be resent. Connection Diagram:

93


Procedure:

This Eye Pattern and BER Measurement Module is to be used with Optical Fiber Communication Trainer Scientech 2501/ Scientech 2502. 

Connect Power Supply to Optical Fiber communication Trainer Scientech 2501/Scientech 2502.



Ensure that all switched faults are off.



Connect the fiber optic cable between emitt er output and detector input.



Connect +5V DC, 500 mA adapter to the board.



Make the connections as shown in next figure.

•

Connect clock frequency of 64 KHz from CLK Generator to CLK In of Data generator using patch cord.

•

Connect Data out of Data Generator to of Data In of optical Link.

•

Now connect a patch cord from Data In of Optical Link of Module to Input of emitter section of Optical Fiber Communication Trainer Scientech 2501/ Scientech 2502.

•

Now connect a patch cord from detector section of Optical Fiber Communication Trainer Scientech 2501/ Scientech 2502 to Data out of Optical Link of Module.

•

Connect Data Out of Optical Link Section to Signal In of Noise Generator.

•

Make the ground common of Trainer and Module using patch cord.



Keep toggle switch towards Bit Error Counter.



Switch ON the power of Trainer and Module.



Initially Adjust Level pot at middle position.

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Observe the Error Count on 7-Segment Display of Bit Error Counter by using the start/ stop switch for any time duration T.



Adjust Level pot for minimum and maximum position to observe effect of variable noise on the error count.

94


Calculation and Observations: Sr. No.

CLK Frequency

Time Duration (td)

Total No. of Transmitted Bits

Bit Error Count (E)

Bit Error Rate = E/N

N= CLK * td

Measuring Bit Error Rate

A BERT (bit error rate tester) is a procedure or device that measures the BER for a given transmission. The BER, or quality of the digital link, is calculated from the number of bits received with error divided by the number of bits transmitted. BER= Bits received with Error /Total bits transmitted Observations: 

We can observe that for a fixed clock frequency, as we increase the level of Noise, Bit Error Count increases.



We can also observe that for a fixed level of Noise, Bit Error Count are less in No. for lower clock frequencies and more for higher clock frequencies.

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Experiment 26 Objective: Study and Observation of Eye Pattern. Apparatus Required: 

Eye Pattern and BER Measurement Module. (Optional)



Scientech 2501/ Scientech 2502 Optical Fiber Communication platform.



Patch Cords



Optical Fiber Cable



+ 5V DC, 500 mA Adapter

Theory:

The eye-pattern technique is a simple but powerful measurement method for assessing the data-handling ability of a digital transmission system. This method has been used extensively for evaluating the performance of wire systems and can also be applied to optical fiber data links. The eye-pattern measurements are made in the time domain and allow the effects of waveform distortion to be shown immediately on an Oscilloscope. Connection Diagram:

96


Procedure:

This Eye Pattern and BER Measurement Module is to be used with Optical Fiber Communication Trainer Scientech 2501/ Scientech 2502. 

Connect Power Supply to Optical Fiber communication Trainer Scientech 2501/ Scientech 2502.



Ensure that all switched faults are off.



Connect the fiber optic cable between emitt er output and detector input.



Connect +5V DC, 500 mA adapter to the board.



Make the connections as shown in figure.

• Connect clock frequency of 64 KHz from CLK Generator to CLK In of Data generator using patch cord.

• Connect Data out of Data Generator to of Data In of optical Link. • Now connect a patch cord from Data In of Optical Link of Module to Input of emitter section of Optical Fiber Communication Trainer Scientech 2501/ Scientech 2502.

• Now connect a patch cord from detector section of Optical Fiber Communication Trainer Scientech 2501/ Scientech 2502 to Data out of Optical Link of Module.

• Connect Data Out of Optical Link Section to Signal In of No ise Generator. • Make the ground common of Trainer and Module using patch cord. 

Keep toggle switch towards Eye Pattern Socket.



Connect channel 1 (CH Y) of Oscilloscope to Eye Patt ern Socket.



Connect EXT. TRIG. Of Oscilloscope to CLK In of Data Generator and press the EXT. TRIG. Switch.



Adjust the time base as well as Cal pot of Oscilloscope to get Eye Pattern as shown in next figure.

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Observe the Eye pattern for different clock frequencies and different Noise Level.

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Observation:

Oscilloscope Setting

Eye Pattern: Without error & with error in data


98



99


Received Data with no error, less error and more error

Conclusion:

As clock frequency increases the EYE opening becomes smaller.

100


Frequently Asked Questions 

How fiber optics is fabricated?

Fiber optics has been fabricated from materials that transmit light and are made from a bundle of very thin glass or plastic fibers enclosed in a tube. One end is at a source of light and the other end is a camera lens, used to channel light and images around the bends and corners. 

What is fiber optics?

Fiber optics has a highly transparent core of glass, or plastic encircled by a covering called "cladding". Light is stimulated through a source on one end of the fiber optic and as the light travels through the tube, the cladding is there to keep it all inside. 

Why single mode fibers are used for long distance transmission? The single-mode fiber optic is used for high speed and long distance transmissions because they have extremely tiny cores and they accept light only along the axis of the fibers. Tiny lasers send light directly into the fiber optic where there are low-loss connectors used to join the fibers within the system without substantially degrading the light signal.



What is the drawback of multimode fibers?

Multi-mode fibers which have much larger cores and accept light from a variet y of angles and can use more types of light sources. Due to this reason they cannot be used over long distances transmissions. 

What is optical fiber?

The fiber optic cable consists of two concentric layers of transparent materials. The inner portion the core transports the light, the outer covering the cladding must have a lower refractive index than the core so the two of them are made up of different materials. 

List the advantages of optical fiber as waveguide?

Advantages of Fiber Optic System are as follows: Enormous Potential Band Width (BW) Small size and weight Electrical Isolation Immunity to Interference and Cross talk Signal Security Low transmission loss Potential Low Cost Thinner Non-flammable Ruggedness and flexibility Low cost and availability Reliability 101


List the disadvantages of optical fibers as waveguide?

Disadvantages of optical fibers systems are as follows: Price Fragility Affected by chemicals Opaqueness Requires special skills 

What is meant by index profile?

Index profile is the refract ive index distribution across the core and the c ladding of a fiber. 

What is step index profile?

Step index profile in which core has one uniformly distr ibuted index and the cladding has a lower uniformly distributed index. 

What is graded index profile?

Graded index profile, in which refractive index varies gradually as a function of radial distance from the fiber centre. 

What is the principle of operation of Optical Fiber?

The principle of operation of optical fiber lies in the behavior of light. It is a widely held view that light always travels in straight line and at constant speed. Of course, the light propagates in straight lines, but when it is reflected inside the optical fiber million and trillion times by the clad, each movement comprising of a straight line and consequently because of such reflections, it acquires the shape of the optical fiber. So effectively, it is said to have been traveling along the fiber. It changes its direction only if there is a change in the dielectric medium. 

How refractive index is defined?

Refractive index of a medium is defined as the ratio of velocity of light in vacuum to velocity of light in medium. Refractive index = 

Velocity of light in vaccum Velocity of light in medium

How refraction occurs?

When a ray is incident on the interface between two dielectrics of differing refractive indices, refraction occurs. 

What is partial internal reflection?

If the light is refracted and also partly reflected internally in the same medium then it is referred as Partial Internal Reflection. 102


What Snell's law of refraction states?

The angle of incidence φ 1 and refraction φ 2 are related to each other and to refractive indices of dielectrics by Snell's law of refraction which states that: n1 sinφ 1 = n2 sinφ 2 sin φ 1 sin φ 2

=

n2 n1

It is this change in refractive indices which causes the change in the path of the incident ray as evident from the Snell’s law. Larger the change in the refractive indices larger change in the direction of the incident ray. 

What is the relationship between incident ray and angle of refraction?

As the angle of incident ray increases, the angle of refraction also increases even faster and when the angle of refraction becomes 90° thereafter, if the angle of incidence is increased a condition is arrived where the incident ray is totally reflected in the same medium from where it has emerged; this is referred as the total internal reflection. 

How the critical angle is defined?

Since, the angle of refraction is always greater than the angle of incidence, when the incident medium is denser than the refraction medium. Thus, the angle of refraction is 90° and the refracted ray emerges parallel to the interface between the dielectrics. This is the limiting case of refraction and this angle of incidence is known as critical angle φ c . 

What is total internal reflection?

At angles of incidence greater than the critical angle the light is reflected back into the originating dielectric medium. This behavior of light is termed as Total Internal Reflection. 

What is the condition of total internal reflection?

Angle of Incidence = Angle of Reflection 

What is acceptance angle?

The maximum angle to the axis at which light may enter the fiber in order to be propagated hence it is referred to as the acceptance angle for the fiber. 

What is numerical aperture?

It gives the relationship between the acceptance angle and the refractive indices of the three media involved viz. the core, the cladding and air.

103


Write the formula for numerical aperture?

umerical Aperture

n0 sin a

(n 2 n 2 )1/ 2 1 2

(n 2 1

n 2) 2

(n n )( n n ) 1 2 1 2

Where, n0 = Refractive index of air n1 = Refractive index of core n2 = Refractive Refract ive index of cladding 

What is the significance of Numerical Aperture?

The Numerical Aperture is a very useful measure of light collecting ability of a fiber. It directly relates to the refractive indices of the core and cladding. As we observe from the above equation, greater the absolute value of the indices of core and cladding, greater the numerical aperture; similarly, greater the difference between the refractive indices greater the numerical aperture. 

Give the classification of optical fiber?



What is single mode fiber?

In single mode fiber only one mode (Electromagnetic wave) is able to propagate. 

Where the single mode fiber is used?

This is used in long distance and/or, high-speed communication. 

Why single mode fiber is used for long distance transmission?

It is beneficial over long distances since it completely eliminates a problem known as Inter as Inter modal Dispersion associated Dispersion associated with Multimode cables. 

What is the dimension of core and cladding for single mode?

Core: 8.3 um Cladding: 125 um 104


What is the meaning of multi mode fiber?

The term multimode means that the diameter of the fiber optic core is large enough to propagate more than one mode (Electro Magnetic Wave). 

What is dispersion?

Because of the multiple modes the pulse that is transmitted down the fiber tends to become stretched over distance this is referred to as dispersion. 

What is the effect of dispersion?

Available bandwidth is reduced. 

What is the application of multimode fiber? fiber?

These are typically used in applications such as LAN (Local Area Networks) & FDDI (Fiber Distributed Area Interface) 

What is the dimension of core and cladding for multi mode?

Core: 50 um Cladding: 125 um 

What are the various losses in optical fiber?

Losses in Optic Fiber are as follows:

• • •

Attenuation Material Absorption Losses Linear Scattering Losses Ray Leigh Scatter Mie Scattering

• • •

Non Linear Scattering Micro Bending and Macro Bending Dispersion Inter modal Dispersion Intra modal Dispersion



What are the reasons for attenuation of signals?

Several mechanisms are involved, including absorption by materials within the fiber, scattering of light out of the core caused by environmental factors. The degree of attenuation depends on the wavelength of light t ransmitted. 

What is the definition of attenuation?

Attenuation measures the reduction in signal strength by comparing output power with input power. Measurements are made in decibels (dB). ( dB). It I t is defined as: Pi DB loss α = 10 log 10 P o

105




What material absorption loss indicates?

It is a loss mechanism related to the material composition and fabrication process of the fiber that result in the dissipation of some of the transmitted optical power as heat in wave-guide. 

What is linear scattering means?

Linear scattering mechanisms cause the transfer of some or all of the optical power contained within one propagating mode to be transferred linearly (proportionally) into a different mode. 

Why scattering of light occurs in optical fibers?

This process tends to result in attenuation of the transmitted light as the transfer may be to a leaky or radiation mode that doesn't continue to propagate within the fiber core, but is radiated from the fiber. 



Give the types of scattering?

•

Ray Leigh Scattering

•

Mie Scattering

What is backscattering?

When the infrared light strikes a very-very small place where the materials in the glass are imperfectly mixed, this gives rise to localized changes in the refractive index resulting in the light being scattered in all directions. Some of the light escapes the optic fiber, some continues in the correct direction and some is returned towards the t he light source. This is called backscatter . 

What are the causes of Mie Scattering?

These result from the non - perfect cylindrical structure of the wave-guide. It may be the caused by the imperfections such as irregularities in the core cladding interface core, cladding refractive index difference along the fiber length, diameter fluctuations, strains and bubbles. The scattering created by such in homogeneities is mainly in the forward direction. 

What is micro bending?

A problem that often occurs in cabling of the optical fiber is the twisting of the fiber core axis on a microscopic scale within the cable form. This phenomenon, known as micro bending result from small lateral forces exerted on the fiber during the cabling process and it causes losses due to radiation in both multimode and single mode fiber. 

What happens when sharp bend occurs in fiber?

The light propagates down the optic fiber solely because the incident angle exceeds the critical angle. If a sharp bend occurs, the normal and the critical angle move round with the fiber. The incident ray continues in a straight line 106


and it finds itself approaching the core - cladding boundary at an angle less than the critical angle and much of light is able to escape. 

What is the effect of dispersion on light in optical fiber?

When an electrical pulse energizes a LASER, it launches a short flash or light along the optic fiber. It is an unfortunate fact that the light burst becomes longer as it moves along the fiber optic cable. The light spreads out. 

List the types of dispersion?

There are two types of Dispersion:



•


•


What is the condition for light to be entered into the optical fiber?

Light to be propagated down the core of the optic fiber, t he light must enter at an angle greater than the critical angle. 

How the spreading of pulses occurs in Inter model dispersion?

When each and every ray is propagated at its own angle will arrive at slightly different times at the far end. This spreading effect will occur all along the fiber so it is also important to appreciate that the longer the optic fiber, the greater the dispersion. Transmission rates that are actually possible on an optic fiber therefore depend in its length. 

What is the effect of change in refractive index on light?

A change in refractive index will change the speed of that particular wavelength of light. 

What is Intra modal Dispersion?

When the light source produces different wavelengths at the same time, the components of the transmitted light pulse traveling at the same time, and then the components of the transmitted light pulse traveling at different speeds. The total package of light will spread o ut - hence the Intra modal dispersion occurs. 

How to cure inter modal Dispersion?

A large core diameter means many modes and severe inter modal dispersion. The cure for this type of dispersion is quite simple. Reduce the core size; the number of modes decreases and inter modal dispersion is reduced. 

How inter modal Dispersion is completely eliminated?

To eliminate inter modal dispersion completely simply make the core so small that only one mode is propagated. A single ray cannot possibly go at two different speeds so inter modal dispersion cannot occur. 

Why LASER is used to reduce intra modal dispersion?

107


The LASER would cause less intra modal dispersion because its light is more concentrated around the central wavelength.



What is spectral width?

The spread of wavelength measured between the points where the power output falls to half of the peak power is called the spectra l width. 

Why LASER is used for long distance transmission?

Some LASERS have spectral widths as low as 0.1 nm (nanometer). The low spectral width together with its high power and fast switching makes the LASER first choice for long distance communications using single mode optic fiber. 

What is full form of OTDR?

Optical Time Domain Reflect meter 

What is the function of OTDR?

The OTDR is a measuring instrument that uses backscatter. It is the most versatile piece of test equipment that we have for making measurements on fiber optic systems. 

What can be measured with the help of OTDR?

It provides us with two different measurements:



1.

It can measure the magnitude of any losses that occur along optic fiber.

2.

It can measure distance along the optic fiber.

Where fiber optics links can be used?

Fiber optic links can be used for transmission of digital as well as analog signals. 

Fiber optics links consists of how many elements?

Basically a fiber optic link contains three main elements, a transmitter, an optical fiber and a receiver. 

What is the function of transmitter, optical fiber and receiver?

The transmitter module takes the input signal in electrical form and then transforms it into optical (light) energy containing the same information. The optical fiber is the medium, which takes the energy to the receiver. At the receiver light is converted back into electrical form with the same pattern as originally fed to the transmitter. In this system the information signal is used to control the Intensity of the source. At the far end, the variation in the amplitude of the received signal is used to recover the original information signal.

108


Glossary Acceptance Angle: The angle above which, the core of an optical fiber accepts incoming light, usually measured from the fiber axis. Angle of Incidence: The angle between incident ray and the normal to a reflecting or refracting surface. Attenuation: Reduction of signal magnitude or loss normally measured in decibels. Fiber attenuation is normally measured per unit length in decibels per kilometer. Avalanche Photodiode: A semiconductor photo detector with integral detection and amplification stages. Electrons generated at p-n junction are accelerated in a region where they free an avalanche of other electrons. A PD can detect faint signals but require higher voltage than other semiconductor elect ronics. Back Scattering: Scattering of light in the direction opposite to that in which it was originally traveling. Bandwidth: The highest frequency that can be transmitted in analog operation or (especially for digital system) the information carrying capacity of the signal. Baud: The number of signal level transitions per second in digital data for common coding schemes, this equals bits per second. Bit Error Rate (BER): The fraction of bits transmitted incorrectly. Cladding: The layers of glass or other transparent material surrounding the light carrying core of an optical fiber. It has lower refractive index than the core, and thus confines light in the core. Critical Angle: The smallest angle at which a meridional ray may be totally reflected within a fiber at the core - cladding interface. Core: The central part of optical fiber that carries light. Dark Current: The noise current generated by a photo diode in dark. Decibel: A logarithmic comparison of power levels, defined as ten times the base ten logarithm of the ratio of two power levels. Detector: A transducer that provides an electrical output signal in response to an input Index Matching Gel: A gel or fluid whose refractive index is close to the core index that reduces refractive index discontinuities that can cause reflective losses. Intensity: Power per unit solid angle. Infrared: Wave length longer than 700 nm and shorter than about 1 nm. We can not see infrared radiation but can feel it as heat. Transmission in optical fiber is best in infrared at wavelengths of 1100-1600 nm. ISDN: Acronym of Integrated Services Digital Network is a digital standard calling for 144K bits/sec transmission, corresponding to two 64 K bits/sec digital voice channels and one 16 K bits/data channel. 109


LASER: Acronym of Light Amplification by Stimulated Emission of Radiation. A device that produces monochromatic coherent light through stimulated emission most lasers used in fiber optic communication is solid state semiconductor devices. LED: Acronym of Light Emitting Diode, a semiconductor device which emits light from a p-n junction (when biased with an electrical current) Light may exit from the junction strip edge or from its surface (depending on device structure) Light: Strictly speaking, electromagnetic radiation with properties similar to visible light includes the invisible near-infrared radiation in most fiber optic communication system. Mode: An electromagnetic field distribution that satisfies theoretical requirement for propagation in waveguide. Micro bending: Tiny bends in a fiber that allow light to leak out and increase loss. Macro bending: In an optical fiber all macroscopic deviations of the axis from a straight line distinguished from micro bending. Meridian Ray: A light ray that passes through the axis of optical fiber. It is generally used when illustrating the fundamental transmission pro perties of optical fiber. Material Dispersion: Light impulse broadening caused by various wavelengths of light traveling at different velocities through a fiber. Material dispersion increases with increasing spectral width of the source. Numerical Aperture: A characteristic parameter of any given fiber's light gathering capability defined by the sine of half angle over which a fiber can accept light. It is multiplied by the refractive index of the medium containing the light.

N.A = n o sin !a = (n12 -n 22 ) 1 2 Noise Equivalent power: The rms value of optical power which is required to produce an rms signal to noise ratio of 1, and indication o f noise level which defines the minimum detectable signal level. Optic fiber: This is the length of clear material along which we propagate the light. Optical Time Domain Reflectometer (OTDR): A method for measuring transmission characteristics by sending a short pulse of light through fiber and the resulting back scatter and reflection is measured as a function of time. It is useful in estimating attenuation coefficient as a function of distance and identifying defects and other localized losses. Polarization: Alignment of the electric and magnetic fields that make up an electro magnetic wave normally refers to the electric field. If all light waves have the same alignment; light is said to be polarized. Rayleigh Scattering: Scattering by refractive index fluctuations ( non-homogeneities in material density or composition) that are small with respect to wavelength. Reflectance: The ratio of reflected power to incident power. Frequently referred to as optical density or as a percent in communication applications, generally expressed in dB. 110


Radiometer: An instrument distinct from photometer, to measure power (watts) of electromagnetic radiation. Responsivity : The ratio of detector output to input, usually measured in units of amperes per watt. SMA: Abbreviation of sub-miniature assembly. Signal – to - Noise Ratio: The ratio of signal to noise, measured in decibels, an indication of signal quality in analog system. Skew Ray: A ray that does not intersect the axis of a fiber is known as skew ray. Total Internal Reflection: The total internal reflection occurs when light strikes an interface at angles of incidence (with respect to normal) greater than the critical angle. Wavelength: The distance an electromagnetic wave travels in the time it takes to oscillate through a complete cycle. Wavelength of light is measured in nanometers or micro meters.

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Scientech 2502

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