OPTICAL TIME DOMAIN REFLECT REFLECTOMETER OMETER
L t C d r H a rs rs h a v a r d h a n 2011 2011 CRF 3 631
CONTENT •
Basic Introduction
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Reflection is the key
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Block diagram
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Evaluation of few traces
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Factors affecting OTDR trace
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Cost analysis
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Optical Time Domain Reflectometer (OTDR) •
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A device for characterizing a fiber optic cable An optical pulse is transmitted through the fiber and the resulting backscatter and reflections to the input are measured as a function of time
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Useful in estimating attenuation as a function of distance
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Identifying defects and other localized losses
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Check the quality of fiber optic links by measuring backscatter
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FRESNEL’S REFLECTION •
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Light moves from a medium of a refractive index (n1 )into a second medium with refractive index (n2) Both reflection and refraction of the light may occur. The Fresnel equations describe what fraction of the light is reflected and what fraction is refracted
Source-wikipedia
sin n sin
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RAYLEIGH BACKSCATTERING •
Result of the elastic collisions between the light wave and the silica molecules in the fiber.
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Accounts for about 96 percent of attenuation in optical fiber Light scattered in all directions after interaction with silica
some portion of the light propagates forward
deviates out of the propagation path and escapes from the fiber core reflected back toward the light source
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REFLECTION IS THE KEY •
Rayleigh backscattering –
used to calculate the level of attenuation in the fiber as a function of distance
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Expressed in dB/km
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shown by a straight slope in an OTDR trace
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Higher wavelengths are less attenuated than shorter ones and thus require less power to travel over the same distance in a standard fiber
Fresnel’s reflection –
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detects physical events along the link. When the light hits an abrupt change in index of refraction (e.g., from glass to air) a higher amount of light is reflected back, creating Fresnel reflection
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thousands of times bigger than the Rayleigh backscattering.
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Fresnel reflection is identifiable by the spikes in an OTDR trace
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Reflections from connectors, mechanical splices, fiber breaks
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HOW OTDR WORKS •
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An OTDR calculates the distances to Events by measuring the time elapsed between transmission of the light and reception of the reflection The rising edge of the reflection of the front panel connector and the reflection from a connector/splice/cracks The distance and the time measured depends upon the the refractive index of the fiber
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BLOCK DIAGRAM
Adapted from http://www.EXFO.com/application note 194
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EVENTS •
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Anything that causes loss or reflections other than normal scattering of the fiber material Applies to all kind of connectors as well as damages such as bendings, cracks or breaks. The vertical axis is the power axis and the horizontal one is the distance axis
Source- OTDR pocket book by Agilent Technologies
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BEGINNING OF A FIBER •
For a normal straight connector, the beginning of a fiber always shows a strong reflection at the front connector
Source- OTDR pocket book by Agilent Technologies
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FIBER END
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A strong reflection at the end of the fiber before the trace drops down to noise level
Source- OTDR pocket book by Agilent Technologies
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FIBER BREAK •
If the fiber is interrupted or broken, this is called a break
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Breaks are non-reflective Events. The trace drops down to noise level
Source- OTDR pocket book by Agilent Technologies
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CONNECTOR OR MECHANICAL SPLICE •
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Connectors within a link cause both reflection and loss A mechanical splice has a similar signature to a connector. Usually it has lower loss and reflection values
Source- OTDR pocket book by Agilent Technologies
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FUSION SPLICE •
A fusion splice is a non-reflective Event, only loss can be detected.
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Modern fusion splices are so good, they may be nearly invisible
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In the case of a bad splice, we may see some reflection Source- OTDR pocket book by Agilent Technologies
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CRACKS •
A crack refers to as a partially damaged fiber that causes reflection and loss
Source- OTDR pocket book by Agilent Technologies
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PATCHCORDS •
Patch cords are used to connect the OTDR to the fiber under test
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The initial reflection is not covering the beginning of the fiber
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This allows better examination of the first connector
Source- OTDR pocket book by Agilent Technologies
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FACTORS AFFECTING OTDR TRACE •
Dead zone
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Pulse width
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Dynamic range
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DEAD ZONE •
The time during which the detector is temporary blinded by a high amount of reflected light, until it recovers and can read light again –
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Assume yourself driving on a highway during night
More reflection causes the detector to take more time to recover, resulting in a longer dead zone
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Determines how close together two events can be measured
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Dead zone increases as the pulse width increases
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PULSE WIDTH •
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The time during which the laser is ON The shorter the pulse, the less energy it carries and the shorter the distance it travels due to the loss along the link (i.e., attenuation, connectors, splices, etc.). A long pulse carries much more energy for use in extremely long fibers
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NOISY TRACE •
Increased averaging time –
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Results in a considerable improvement in SNR, while maintaining the good resolution of the short pulse. does not improve SNR indefinitely.
Longer Pulse width –
Use the next available higher pulse (more energy).
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Dead zones extend along with the pulse width
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DYNAMIC RANGE •
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Maximum optical loss an OTDR can be analyzed from the backscattering level down to a specific noise level. It is the maximum length of fiber that the longest pulse can reach Bigger the dynamic range (in dB), the longer the distance reached. Typical values 20-50 dB Directly related to pulse width Specified for the longest pulse width at a three-minute averaging time with signal-to-noise ratio (SNR) = 1 Improved by using longer pulse width and decreasing noise through averaging
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COST ANALYSIS – M/s Fluke OTDRs
Measurement Type
Model
Cost
Multimode OTDR
OF-500-M
$9,265.00
Singlemode OTDR
OF-500-S
$10,295.00
Multimode plus Single Mode OTDR
OF-500-MS
$14,415.00
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THANK YOU
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REFERENCES •
http://www.EXFO.com/application note 194 by Jimmy Gagnon, Product Specialist, Optical Business Unit
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OTDR pocket guide – Agilent Technologies
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Wikipedia
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TYPES OF FO CABLES •
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Single-mode fibers – used to transmit one signal per fiber. They have small cores(9 microns in diameter) and transmit infra-red light from laser. Multi-mode fibers – used to transmit many signals per fiber (used in computer networks). They have larger cores(62.5 microns in diameter) and transmit infra-red light from LED.
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FO CABLE •
Core – thin glass center of the fiber where light travels.
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Cladding – outer optical material surrounding the core
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Buffer Coating – plastic
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coating that protects
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the fiber.
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TYPES OF CONNECTOR TYPE
FULL NAME
MT-RJ
Mechanical Transfer Registered Jack / Media Termination recommended jack
Snap (duplex)
SC
Subscriber Connector / Square connector / Standard Connector
Snap (push2.5 mm pull coupling)
Datacom and telcom; extremely common
ST / BFOC
Straight Tip/Bayonet Fiber Optic Connector
Bayonet
Multimode, rarely single-mode;
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COUPLING TYPE
SIZE
USE
2.45×4.4 mm
Duplex multimode connections
2.5 mm
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INDEX OF REFRACTION (IOR) •
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The Index of Refraction is a way of measuring the speed of light in a material. Light travels fastest in a vacuum. Index of Refraction is calculated by dividing the speed of light in a vacuum by the speed of light in some other medium (such as glass in the case of fiber optics!).
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OTDR SETUP – IOR •
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Each different optical glass fiber has a different refractive index profile consistent with it’s type and manufacture process. Typical G.652.B singlemode fiber from Draka has an index number of 1.467 @ 1310nm and 1.468 @ 1550nm
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The longer the wavelength, the faster the light travels through the core.
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The user must set the OTDR to the proper GIR (Group Index of Refraction).
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If the GIR is not set to the proper number, the OTDR may overestimate or underestimate linear cable footage.
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