FOUR WAVE MIXING NONLINEARITY EFFECT IN WAVELENGTH DIVISION MULTIPLEXING RADIO OVER FIBER SYSTEM
HAFIZ ABD EL LATIF AHMED HABIB
UNIVERSITI TEKNOLOGI MALAYSIA
FOUR WAVE WAVE MIXING MIXING NONLINEARITY NONLINEARITY EFFECT EFFECT IN WAVELENGTH DIVISION MULTIPLEXING RADIO OVER FIBER SYSTEM 2006/2007-III HAFIZ ABD EL LATIF AHMED HABIB
Elmitd Elmitdaad aad,, 3 Exten Extensio sion, n, Khartoum, Sudan.
ULY 2007
DR. RAZALI BIN NGAH
JULY 2007
i
“I hereby declare that I have read this project report and in my opinion this project report is sufficient in terms of scope and quality for the award of the degree of Master of Engineering Engineering (Electrical- Electronics and Telecommunications)”
Signature
: ………………………………………...
Name of Supervisor
: DR. RAZALI BIN NGAH
Date
: ………………………………………...
ii
FOUR WAVE MIXING NONLINEARITY EFFECT IN WAVELENGTH DIISION MULTIPLEXING EADIO OVER FIBER SYSTEM
HAFIZ ABD EL LATIF AHMED HABIB
A project report submitted submitted in partial fulfillment fulfillment of the requirements requirements for the award award of the degree degree of Master of Engineer Engineering ing (Electrical-Electronics and Telecommunications)
Faculty of Electrical Engineering Universiti Technologi Malaysia
JULY 2007
iii
I declare that this project report entitled “Four Wave Mixing Nonlinearity Effect in Wavelength Wavelength Division Multiplexing Multiplexing Radio over over Fiber System” is the result of my own research except as cited in references. The project has not been accepted for any degree and is not concurrently submitted in candidature of any other degree.
Signature Signature : …………………………… …………………………………… ……… Name
: Hafiz Abd El Latif Ahmed
Date
:
iv
DEDICATION
To my beloved beloved late mother, mother, may her soul rest in Paradise. Paradise.
v
ACKNOWLEDGEMENTS
Praise and thanks to Allah (SWT) who gave me the strength and courage to complete this project. I would like to express sincere thanks to my supervisor Dr. Razali bin Ngah for his invaluable invaluable guidance throughout throughout the course of this project. project. His guidance, guidance, ideas, encouragemen encouragement, t, affable nature, nature, kindness kindness and support support were greatly helpful. Even with his busy busy schedu schedule, le, he spent spent conside considerab rable le amount amount of time helpin helping g me through through the different phases of this project.
Speci Special al ackno acknowl wled edge geme ment nt to Dr. Dr. Abde Abdellwahab wahab Moha Mohamm mmed ed Ali, Ali, for for his his valu valuab able le sugg suggest estio ions ns,, and and revisi revision on of this this proj projec ect. t. Speci Special al than thanks ks to Eng. Eng. Mr. Mr. Abdelrazig Saeed Mohamed for his assistance and support and to Eng. Mr. Reza Abdolee for the many interesting interesting discussions discussions I have had with him.
I wish to thank my parents, for their daily prayers, giving me the motivation and strength, strength, and encouraging encouraging me to accomplish accomplish and achieve my goals.
A special acknowledgment must be given to my brothers and sisters for their motivation help and support during my academic period at UTM. I am indebted to them and words words will never express the gratitude gratitude I owe to them. Last but not least, sincere thanks and gratitude to my lovely wife Najat and my sons Mohamm Mohammed ed and Awab who inspired inspired me by their, courag courage, e, suppor supportt and patience throughout the period of my study.
vi
ABSTRACT
The integration of wireless and optical networks is a potential solution for the increasing capacity and mobility as well as decreasing costs in the access networks. Optical Optical networ networks ks are fast, fast, robust obust and error error free, free, however however,, there there are nonlin nonlineari earity ty obstacles obstacles preventing preventing them from being perfect media. The performance performance of wavelength wavelength divi divisi sion on mult multip iplex lexin ing g (W (WDM DM)) in radi radio o over over fiber fiber (RoF (RoF)) syst system emss is foun found d to be strongly strongly influenced by nonlinearity nonlinearity characteristics characteristics in side the fiber. The effect of four wave mixing (FWM) as one of the influential factors in the WDM for RoF has been studied here using Optisystem and Matlab. From the results obtained, it is found that the FWM effects have become significant at high optical power levels and have become even more significant when the capacity of the optical transmission line is incr increas eased ed,, which which has has been been done done by either either incre increasi asing ng the the chan channe nell bit bit rate, rate, and and decreasing the channel spacing, or by the combination of both process. It is found that that when when the chann channel el spacin spacing g is 0.1 0.1 nm, 0.2 0.2 nm and and 0.5 nm the FWM powe powerr is respectively, becomes about -59 dBm, -61 dBm and -79 dBm. This result confirms that the fiber nonlinearities play decisive role in the WDM for RoF system. The simulation results obtained here are in reasonable agreement as compared with other numerical simulation results obtained, elsewhere, using different simulation tools.
vii
ABSTRAK
Integr Integrasi asi talian talian tanpa tanpa wayar wayar dan rangka rangkaian ian optik optik menjad menjadii potens potensii kepada kepada penyelesaian untuk peningkatan kapasiti dan mobiliti dan seterusnya mengurangkan kos kos capa capaia ian n rang rangka kaia ian. n. Rang Rangka kaia ian n opti optik k adal adalah ah pant pantas as,, berk berkes esan an,, dan dan tida tidak k mempunyai masalah. Namun begitu halangan ‘nonlinearity ‘ nonlinearity’’ menghalangnya menjadi media yang sempurna. sempurna. Prestasi jarak gelombang gelombang pembahagi pemultipleksan pemultipleksan (WDM) dala dalam m radi radio o mela melalu luii fibe fiberr (RoF (RoF)) sist sistem em amat amatla lah h dipe dipeng ngar aruh uhii oleh oleh ciri ciri-c -cir irii ‘nonlinearity’ nonlinearity ’ didalam fiber. Kesan ‘ four four wave mixing ’ (FWM) yang menjadi salah satu satu fakt faktor or berp berpen enga garu ruh h dala dalam m WDM WDM untu untuk k RoF RoF tela telah h dika dikaji ji meng menggu guna naka kan n Optisystem dan Matlab. Keputusan yang diperolehi mendapati bahawa kesan FWM menjad menjadii pentin penting g pada pada optik optik kuasa kuasa aras tinggi tinggi dan sangat sangat pentin penting g apabil apabilaa kapasit kapasitii talian talian pengha penghanta ntaraan raan optik optik bertamb bertambah, ah, sama sama ada dengan dengan mening meningkat katkan kan kadar kadar bit saluran saluran,, mengur mengurang angkan kan penjara penjarakan kan saluran saluran,, ataupun ataupun kedua-d kedua-duan uanya ya sekali. sekali. Ianya Ianya didapati didapati bahawa apabila penjarakan saluran saluran adalah 0.1 nm, 0.2 nm, dan 0.5 nm kuasa FWM FW M masin masingg-ma masin sing g adal adalah ah lebih lebih kuran kurang g –59 –59 dBm dBm, -61 -61 dBm, dBm, dan dan –79 –79 dBm. dBm. Keputusan ini mengesahkan bahawa ‘ fiber ‘ fiber nonlinearities’ nonlinearities’ memainkan peranan utama dalam WDM untuk sistem RoF. Keputusan simulasi berangka yang diperolehi juga bersamaan dengan keputusan model analisis anali sis yang diperolehi melalui Matlab.
viii
TABLE OF CONTENTS
CHAPTER
PAGE
DECLARATION
iii
DEDICATION
iv
ACKNOWLEGEMENT
v
ABSTRACT
vi
ABSTRAK
vii
TABLE OF CONTENTS
viii
LIST OF TABLES
xi
LIST OF FIGURES
xii
LIST OF ABBREVIATION
xv
LIST OF SYMBOLS
xvii
LIST OF APPENDICES
xix
INTRODUCTION
1
1.1
Introduction
1
1.2
Problem Background
2
1.3
Problem Statement
3
1.4
Objective of the Project
4
1.5
Scope of the Project
5
1.6
Organization of the Thesis
5
1
2
TITLE
RADIO OVER FIBER TECHNOLOGY
7
2.1
Introduction
7
2.2
What is Radio over Fiber?
8
2.3
Benefits of RoF Technology
9
ix
2.4
2.5
2.3.1
Low Attenuation Loss
9
2.3.2
Large Bandwidth
10
2.3.3
Immunity to Radio Frequency Interference
11
2.3.4
Easy Installation and Maintenance
11
2.3.5
Reduced Power Consumption
12
2.3.6
Multi-o ti-op perato ator and and Multilti-sservi rvice Operat ration
12
2.3.7
Dynamic Resource Allocation
13
The Application of Radio over Fiber Technology
13
2.4.1
Cellular Networks
13
2.4.2
Satellite Communications
14
RoF Multiplexing Techniques
15
2.5.1
Sub-Carrier Multiplexing in RoF System
15
2.5. 2.5.2 2
Wave Wa vele leng ngth th Divi Divisi sion on Mult Multip iplex lexin ing g in RoF RoF Syst System em
17
NON-LINEAR EFFECTS
19
3.1
Introduction
19
3.2
Types of Fibers
20
3.3
Fiber Losses
20
3.4
Fiber Nonlinearities
21
3.4.1
Self Phase Modulation
22
3.4.2
Cross Phase Modulation
23
3.4.3
Four Wave Mixing
25
3.4.3
Stimulated Brillouin Scattering
29
3.4.5
Stimulated Raman Scattering
30
3
METHODOLOGY
32
4.1
Introduction
32
4.2
Simulation using Optisystem Software
32
4.3
The Simulation Model
33
4.4
Simulation of the Four Wave Mixing ef e f fe c t
35
4.5
Simulation of FWM for Higher Number of Channels
40
4.6
Effec ffectt of Diff Differ eren entt Powe Powerr Level evel of the the Signa ignall Sou Sources rces
41
4.7
Effect of Increase Dispersion of the Fiber Optic
41
4.8
Effect of Increase Effective Area f the the Fiber Optic
42
4
x 4.9
Mo Modelling the Effect of FWM
42
RESULTS AND DISCUSSIONS
47
5.1
Introduction
47
5.2
Simulation of the Four Wave Mixing Effect
47
5.3 5.3
Simu Simulat latio ion n Resu Result ltss with withou outt the the Exte Extern rnal al Modu Modulat lated ed Sign Signal al
48
5.3.1
Effect of Channel Spacing
48
5.3.2 5.3.2
Effect Effect of Different Different Power Level Level of the the Signal Signalss Sources Sources 51
5.3. 5.3.3 3
Effe Effect ct of Incr Increa ease se Disp Disper ersi sion on of the the Fibe Fiberr Opti Opticc
53
Simul imulat atio ion n Resul esults ts with with the the Exte Extern rnal al Mod Modulat ulated ed Sig Signal nal
54
5.4.1
Effect of Channel Spacing
55
5.4.2 5.4.2
Effect Effect of Differ Different ent Power Power Level Level of the the Sign Signals als Sour Sources ces 58
5.4. 5.4.3 3
Effe Effect ct of Incr Increa ease se Disp Disper ersi sion on of the the Fibe Fiberr Opti Opticc
61
5.4. 5.4.4 4
Effe Effect ct of Incr Increas easee Effe Effecti ctive ve Area Area of the the Fibe Fiberr Opti Opticc
61
5
5.4
5.5
Simu imulati ation of FWM for Hig Higher Number of Channels els
5.5.1 5.5.1
Simulation Simulation Results Results for for Four Four Signal Signal Source Source without without External External Modulated Signal
5.5.2 5.5.2
62
63
Simulation Simulation Results Results for for Four Four Signal Signal Source Source without without External External Modulated Signal
65
5.6
Discussion
67
5.7
Analytical Modelling
68
5.8
FWM reduction
70
5.8.1
Effect of Unequal Channel S pacing
70
5.8.1
Effe ffect of Increas ease Effe ffecti ctive Are Area of the the Fiber Optic
72
CONCLUSIONS AND RECOMMENDATIONS
73
6.1
Conclusion
73
6.2
Recommendations for Future Work
74
6
REFERENCE
75
Appendix
77
xi
LIST OF TABLES
TABLE NO.
TITLE
PAGE
3.1
Comparison between SBS and SRS
31
4.1
Global parameters
37
4.2
CW Laser sources parameters
38
4.3
DM 2x1 m ultiplexer parameters
38
4.4
Main tab and dispersion tab parameters for optical fiber
38
4.5
Nonlinear tab parameters for optical f iber
39
4.6
Numerical tab and PMD tab parameters for optical fiber
39
xii
LIST OF FIGURES
FIGURE NO.
TITLE
PAGE
2.1
The Radio over Fiber System Concept
8
2.2
Operating regions of optical fiber
10
2.3
These robust RAPs are connected to the central base station via the ROF
2.4
SubCa ubCarr rrie ierr Mult Multip iple lexi xin ng of Mix Mixed Digi Digita tall and and Anal Analo ogue gue Signals
2.5
14
16
WDM system using multiple wavelength channels and optical amplifiers
17
3.1
Frequency chirping effect
23
3.2
Four wave mixing products
26
3.3
The arising new frequency components due to FWM
27
3.4
FWM products versus channel count
28
3.5
FWM mixing efficiency in s ingle-mode fibers
29
4.1
Direct Modulation
33
4.2
External modulation
33
4.3
Simulation model with external modulated signal
34
4.4
Simulation model without external modulated signal
35
4.5
Simulation model with three channels
40
4.6
flowchart illustrate the modeling steps
40
4.7
The phase matching condition of two different wavelengths
5.1
Opt Optica ical spe spectr ctrum at the input of the fiber when chan chann nel spacing is set to 0.1 nm
5.2
45
48
Opt Optica ical spe spectr ctrum at the onput of the fiber when channel spacing is set to 0.1 nm
49
xiii 5.3
Opt Optica ical spe spectr ctrum at the input of the fiber when chan chann nel spacing is set to 0.2 nm
5.4
Opt Optica ical spe spectr ctrum at the input of the fiber when chan chann nel spacing is set to 0.2 nm
5.5
58
Optic ptical al spec spectr tru um at the the outp output ut of the the fibe fiberr when hen inpu inputt power is set at 20 dBm
5.18 .18
57
Optic ptical al spec spectr tru um at the the outp output ut of the the fibe fiberr when hen the the channel spacing is set at 0.5 nm
5.17 .17
57
Optical cal spectru trum at the inp input of the fib fiber when the channel spacing is set at 0.5 nm
5.16 .16
56
Optic ptical al spec spectr tru um at the the outp output ut of the the fibe fiberr when hen the the channel spacing is set at 0.2 nm
5.15
56
Optical cal spectru trum at the inp input of the fib fiber when the channel spacing is set at 0.2 nm
5.14 .14
55
Optic ptical al spec spectr tru um at the the outp utput of the the fib fiber when when the the channel spacing is set at 0.1 nm
5.13
54
Optical cal spectru trum at the inp input of the fib fiber when the channel spacing is set at 0.1 nm
5.12 .12
53
Outp Output ut optic ptical al spec spectr trum um when hen the the disp ispersi ersio on of fib fiber optic is set to 16.75 ps/nm/km
5.11
52
Opt Optical spectrum at the output of the fiber when input power is set to -10 dBm
5.10 .10
52
Opt Optica ical spe spectr ctrum at the output of the the fibe iber when input power is set to 20 dBm
5.9
51
Opt Optical spectrum at the outpu tput of the fiber when inp input power is set to 20 dBm
5.8
50
Opt Optica ical spe spectr ctrum at the onput of the fiber when channel spacing is set to 0.5 nm
5.7
50
Opt Optica ical spe spectr ctrum at the input of the fiber when chan chann nel spacing is set to 0.5 nm
5.6
49
59
Optic ptical al spec spectr tru um at the the outp output ut of the the fibe fiberr when hen inp input power is set at 10 dBm
59
xiv
5.19 .19
Optic ptical al spec spectr tru um at the the outp output ut of the the fib fiber when when inpu inputt power is set at -10 dBm
5.20 .20
60
Optic ptical al spec spectr tru um at the the outp output ut of the the fib fiber when when inpu inputt power is set at 0 dBm
5.21
61
Optica ical spe spectru ctrum m at the the outpu tput of the the fib fiber when the 2
effective area of the fiber optic is set at 76.5 μ m 5.22 .22
Four our opti optica call spec spectr tru um at the the inp input of the the fib fiber when hen the the channel spacing is set at 0.1 nm
5.23 .23
63
Four our outp utput opti optica call spec spectr tru um chan chann nels els when when the the chan chann nel spacing is set at 0.1 nm
5.24 .24
64
Four our outp utput opti optica call spec spectr tru um chan chann nels els when when the the chan channe nell spacing is set at 0.5 nm
5.25 .25
64
Four our inp input opti optica call spec spectr trum um chan chann nels els when when the the chan chann nel spacing is set at 0.1 nm
5.26 .26
65
Four our outp utput opti optica call spec spectr tru um chan chann nels els when when the the chan chann nel spacing is set at 0. 1 nm
5.27 .27
62
66
Four our outp utput opti optica call spec spectr tru um chan chann nels els when when the the chan channe nell spacing is set at 0.5 nm
66
5.28
Power per channel vs. FWM power
69
5.29
Channel spacing versus FWM power
69
5.30
Opti Opticcal spect ectrum rum at the input of the fiber when the the channel spacing is unequal
5.31 .31
71
Optic ptical al spec spectr tru um at the the outp output ut of the the fib fiber with with uneq unequa uall channel spacing
5.32 .32
71
Optic ptical al spec spectr tru um at the the outp output ut of the the fibe fiberr when hen the the 2
effective area of the fiber optic is set at 76.5 μ m
72
xv
LIST OF ABBREVIATIONS
RoF
-
Radio over Fiber
SPM
-
Self Phase Modulation
XPM
-
Cross Phase Modulation
FWM
-
Four Wave Mixing
SRS
-
Stimulated Raman Scattering
SBS
-
Stimulated Brillouin Scattering
WDM
-
Wavelength Division Multiplexing
DWDM
-
Dense Wavelength Division Multiplexing
SMF
-
Single Mode Fiber
nm
-
nanometer
E/O
-
Electrical-To -Optical Converter
O/E
-
Optical - To Electrical- Converter
RF
-
Radio Frequency
IF
-
Intermediate Frequency
CW
-
Continuous Wave
RAU
-
Radio Antenna Unit
THz
-
Teri hertz
OTDM
-
Optical Time Division Multiplexing
SCM
-
Sub-Carrier Multiplexing
EMI
-
ElectroMagnetic Interference
IM-DD
-
Intensity Modulation and Direct Detection
OFM
-
Optical Frequency Multiplication
GSM
-
Global System for Mobile communication
MVDS
-
Multipoint Video Distribution Service
MBS
-
Mobile Broadband System
xvi GHz
-
Gigahertz
RHD
-
Remote Heterodyne Detection
TDM
-
Time Division Multiplexing
OADM
-
Optical Add-Drop Multiplexer
LED
-
Light Emitting Diode
GVD
-
Group velocity dispersion
ITU
-
International Telecommunication Union
MUX
-
Multiplexer
NDSF
-
Non Dispersion Shifted Fiber
PMD
-
Polarization Mode Dispersion
NRZ
-
Non-Return to zero
xvii
LIST OF SYMBOLS
A
-
Pulse amplitude
Aeff
-
Effec fective area of optical fiber
c
-
Speed of light
co
-
Spee d of light in vacuum
D
-
Dispersion parameter
dijk
-
Degeneracy factor
E
-
Electric field, vector
E
-
Electric field, scalar
f
-
Frequency
I
-
Intensity
L
-
Length
Leff
-
Effective length
n
-
Refractive index
no
-
Waveleng ength depend endent refra fracti ctive index
n2
-
Nonlinear refractive index
n2/Aeff
-
Nonlinear coefficient
P
-
Total polarization, vector
P
-
Power
pi
-
Input power
r
-
Radius
t
-
Time
z
-
Distance
α
-
Attenuation constant [1/m]
αdB βi
-
Attenuation constant [db/km]
-
Propagation constant of the mode i
γ
-
Nonlinear parameter
xviii
εo
-
Vacuum permittivity
λ
-
Wavelength
λ o
-
Center wavelength
τ
-
Normalized time constant
ωi
-
Angular frequency
-
j order susceptibility
(j)
χ
th
xix
LIST OF APPENDICES
APPENDIX
A
TITLE
PAGE
A Matlab Program for FWM power and channel spacing
78
1
CHAPTER 1
INTRODUCTION
1. 1
Int ro rod uc uct io ion
In the the past past,, dati dating ng back back to the the begi beginn nnin ing g of the the huma human n civi civili liza zati tion on,, communication was done through signals, voice or primitive forms of writing and gradually developed to use signaling lamps, flags, and other semaphore tools.
As time passed by, the need for communication through distances, to pass info inform rmat atio ion n from from one one place place to anot anothe her, r, becam becamee neces necessar sary y and and the the inve invent ntio ion n of teleg telegrap raphy hy brou brough ghtt the the worl world d into into the the elect electric ricalal-co comm mmun unic icati ation on.. The major major revolu revolutio tion n that that affected affected the the world world howeve howeverr was the inventi invention on of the telepho telephone ne in 1876. 1876. This This event event has drastic drastically ally transfor transformed med the develo developme pment nt of commun communicat ication ion technol technology ogy.. Today’ Today’ss long long distan distance ce commun communicat ication ion has the ability ability to transmit transmit and receive a large amount of information in a short period of time.
Since the development of the first-generation of optical fiber communication syste systems ms in the the early early 80’s 80’s [4], [4], the the optic optical al fiber fiber comm commun unic icati ation on techn technol olog ogy y has has develop developed ed fast to achieve achieve larger larger transmi transmissio ssion n capacit capacity y and longer longer transmi transmissio ssion n distance, to satisfy the increased demand of computer network. Since the demand on the increasing system and network capacity is expected, more bandwidth is needed because of the high data rates application, such as video conference and real-time image transmission, and also to achieve affordable communication for everyone, at
2 anytime anytime and place place [1]. [1]. The communic communicatio ation n capabil capabilitie itiess allow allow not only human human to human human commun communicat ication ion and contact contact,, but also human to machin machinee and machine machine to machine machine interac interactio tion. n. The commun communicat ication ion will will allow allow our visual, visual, audio, audio, and touch touch sense, to be contacted as a virtual 3-D presence [3].
To keep keep up with with the the capaci capacity ty incr increas easin ing g requ require ireme ment nt,, new new devi devices ces and and technologies with high bandwidth are greatly needed by using both electronic and optical technologies together to produce a new term Radio over Fiber (RoF). The progress made so far has been impressive, where information rate at 1 terabits/s can be handled by a single fiber [5].
RoF is a technology used to distribute RF signals over analog optical links. In such RoF systems, broadband microwave data signals are modulated onto an optical carrier at a central location, and then transported to remote sites using optical fiber. The base-stations then transmit the RF signals over small areas using microwave antennas and. Such a technology is expected to play an important role in present and future wireless networks since it provides an end user with a truly broadband access to the the netw networ ork k whil whilee guara guarant nteei eeing ng the the incr increas easin ing g requ require ireme ment nt for for mobi mobilit lity. y. In addition, since it enables the generation of millimeter-wave signals with excellent properties, and makes effective use of the broad bandwidth and low l ow transmission t ransmission loss charact characteris eristics tics of optical optical fibers, fibers, it is a very very attracti attractive, ve, cost-ef cost-effect fective ive and flexibl flexiblee system configuration.
1.2 1.2
Prob Proble lem m Back Backgr grou ound nd
Normally light waves or photons transmitted through RoF have little interaction with each other, and are not changed by their passage through the fiber (except for absorption and scattering). However, there are exceptions arising from the interactions between light waves and the material transmitting them, which can affect optical signals in RoF. These processes generally are called nonlinear effects because their strength typically depends on the square (or some higher power) of intensity rather than simply on the amount of light present. This means that nonlinear
3 such as self phase modulation (SPM), cross phase modulation (XPM), four wave mixi mixing ng (FWM (FWM), ), stimu stimula lated ted raman raman scatte scatterin ring g (SRS (SRS), ), and and stim stimul ulate ated d brill brillou ouin in scattering effects (SBS) are weak at low powers, but can become much stronger when light reaches high intensities [7]. This can occur either when the power is increased, or when it is concentrated in a small area-such as the core of an optical fiber. Nonlinear optical devices have become common in RoF applications, such as to convert the output of lasers to shorter wavelengths by doubling the frequency. The nonlinearities in RoF are small, but they accumulate as light passes through many kilo kilome mete ters rs of fiber. fiber. Nonl Nonlin inear ear effect effectss are comp compara arativ tively ely small small in optic optical al fibers fibers transmitting a single optical channel. They become much larger when wavelengthdivision multiplexing (WDM) packs many channels into a single fiber [9].
WDM puts many closely spaced wavelengths into the same fiber where they can interact with one another. It also multiplies the total power in the fiber. A singlechan channe nell syste system m may may carry carry powe powers rs of 3 milli milliwa watts tts near near the the tran transmi smitt tter. er. DWDM DWDM multiplies the total power by the number of channels, so a 40-channel system carries 120 mW. That's a total of 2 mW per square micrometer-or 200,000 watts per square centimeter [11]. Several nonlinear effects are potentially important in RoF, although some have produce more troublesome than others. Some occur in systems carrying only a single optical channel, but others can occur only in multichannel systems.
1.3
Problem oblem Statemen atementt
The The rapi rapid d deve develo lopm pmen entt of the the wire wirele less ss comm commun unic icat atio ion n netw networ orks ks has has increased the need of the optical signal processing. The link lengths have grown to thousands of kilometers without need to convert optical signals back and forth to electric form, and the transmission speeds of terabits per second are feasible today [5]. This ever-growing demand for the high speed communication has forced to use higher bit rates as well as transmission powers.
Nonlinear effects on communication have become significant at high optical power levels and have become even more important since the development of
4 erbium-doped fiber amplifier (EFDA) and DWDM systems. By increasing the capacity
of the optical transmission line, which can be done by increasing channel bit rate, decreasing channel spacing or the combination of both, the fiber nonlinearities come to play even more decisive role.
The origin of the nonlinearities is the refractive index of the optical fiber, which which is varies varies with with the the intens intensity ity of the optical optical signal signal.. This This intens intensity ity-de -depen penden dentt component of the refractive index includes several nonlinear effects, such as SPM, XPM, FWM, SRS, and SBS, and becomes significant when high powers are used. Although the individual power in each channel may be below the level needed to produce nonlinearities, the total power summed over all channels can quickly become significant. The combination of high total optical power and large number of channe channels ls at closely closely spaced wavelen wavelength gthss is a source source for many many kinds kinds of nonlin nonlinear ear interactions.
Form the above-mentioned reasons, this study is aimed to gain insight into nonl nonlin inea earr effec effectt caus caused ed speci specific ficall ally y by FW FWM M in the the WDM WDM for for RoF RoF syst system em and and measure the coefficient behind these nonlinear effects. Nonlinear coefficient of the RoF may become an important parameter, when new optical long-haul transmission lines and networks are being deployed.
1.4
Objec jectiv tivee of the Project oject
The main objective of this project is to evaluate the FWM in WDM for RoF technology technology,, in order to calculate calculate the impairments impairments associated with long-distance long-distance high bit rate optical fiber communication systems. In order to achieve the objective, optis optisys yste tem m and and matl matlab ab prog program rammi ming ng softw softwar aree will will be used used respe respecti ctive vely ly in the the nume numeri rica call simu simula lati tion on and and the the anal analyt ytic ical al mode modell llin ing g will will be veri verifi fied ed thro throug ugh h comparison with optisystem simulation.
5 1.5
Scope of the Pro Projec jectt
To study the efficiency of the FWM in WDM for RoF optical network, two appr approa oach ches es were were follo followe wed d in this this proj project ect.. The The first first appr approa oach ch is the the nume numeric rical al simulati simulation on using using Optisy Optisystem stem softwar softwaree which which almost almost replica replicates tes a real system. system. The second aproach is the analytical modeling, which is simple and faster to analyze its performance. MATLAB programming is used to implement the analytical model. To verify the analytical system, a comparison is made with the Optisystem software. Since Routing and wavelength assignment algorithm (RWA) needs to set up the path immediately immediately to reduce network network delays, the analytical model developed in this project can be used to calculate the impairments fast enough so that the routing decisions can be made efficiently, to achieve a chieve optimal systems.
1.6
Organiza ganizatio tion n of the Pr Project oject
Chapter 1 provides the introduction to this project where brief background of the study problem and to the statement of the problem. Followed by the objective, and and the the scop scopee of the the stud study. y. Chap Chapte terr 2 revi review ewss the the lite litera ratu ture re,, whic which h incl includ udes es intr introd oduc uctio tion n to the the RoF, RoF, the the bene benefit fits, s, and and appl applica icatio tions ns of the the Radi Radio o over over Fibe Fiber r Technology in both satellite and mobile radio communications. In addition various type typess of RoF RoF Mult Multip iplex lexin ing g Tech Techni niqu ques, es, such such as Sub Sub carrie carrierr multi multipl plex exin ing g and and wave wavelen lengt gth h divi divisio sion n multi multipl plex exin ing, g, have have also also bee bee cove covered red.. Chap Chapter ter 3 prov provid ides es information about the fiber characteristics, and the non linear effects such as SPM, FWM, SBS, SRS, and XPM. Chapter 4 describes the methodological processes by showing detailed diagram of the methods implemented as well as highlighting briefly the steps those have been foll follow owed ed to achiev achievee the the objec objectiv tivee of this this proj project ect.. Chap Chapte terr 5 prese present ntss the the resul results ts derived derived from the methods methods explained explained where some analyses and simulations simulations were done based on the FWM effects. Finally the conclusions of the study, as well as some suggestions suggestions for future work were summed up in Chapter 6.
6
CHAPTE CHAPTER R2
RADIO-OVER-FIBER TECHNOLOGY
2.1 2.1
Intro ntrodu duct ctio ion n
The integration of wireless and optical networks is a potential solution for increasing capacity and mobility as well as decreasing costs in the access network, by RoF. The concept of RoF means to transport information over optical fiber by modulating the light with the radio signal. This modulation can be done directly with the radio signal or at an intermediate intermediate frequency. frequency. RoF technique technique has the potentiality potentiality to the backbo backbone ne of the wireles wirelesss access access networ network. k. Such architec architectur turee can give give several several advantages, such as reduced complexity at the antenna site, radio carriers can be allocated allocated dynamically to the different antenna sites, and Transparency Transparency and scalability scalability [10].
RoF technology is now ubiquitous in the telecommunications infrastructure. Fiber Fiber optics optics and WDM technolog technology y have have increas increased ed signif significan icantly tly the transmis transmissio sion n capaci capacity ty of toda today' y'ss tran transp spor ortt netw networ orks ks,, and and they they are are play playin ing g impo import rtan antt roles roles in supporting the rapidly increasing data traffic.
7 2.2 2.2
What Wh at is Radi Radio o over over Fibe Fiber? r?
RoF technology entails the use of optical fiber links to distribute RF signals from a central location (headend) to Remote Antenna Units (RAUs). In narrowband communicatio communication n systems and Wireless Local Area Network (WLANs), (WLANs), most of signal processing (including coding, multiplexing, RF generation, modulation, etc) are made in central stations (CS-s) rather than in the base station (BS-s) [1]. The signal between CS and BS is transmitted in the optical band via a RoF network. This architecture makes design of BS-s quite simple. In the simplest case, the BS consists mainly from optical-to-electrical (O/E) and electrical-to-optical (E/O) converters, an antenna and some microwave circuitry (two amplifiers and a diplexer).
The central centralizat ization ion of Radio Radio Freque Frequency ncy (RF) (RF) signal signal process processing ing functio functions ns enables equipment sharing, dynamic allocation of resources, and simplifies system operation and maintenance. These advantages could be translated into major system installation installation and operational operational savings, especially especially in wide-covera wide-coverage ge broadband broadband wireless communication systems, where a high density is necessary. Figure 2.1 shows that the concept of RoF system.
Figure 2.1
The Radio over Fiber System Concept Concept [5]
8 2.3 2.3
Bene Be nefi fits ts of RoF RoF Te Tech chno nolo logy gy
The RoF technol technology ogy holds holds many many advant advantage agess compar compared ed to the electro electronic nic signal distribution. Some of these advantages will be given in the following sections.
2.3.1 2.3.1
Low Attenu Attenuati ation on Loss Loss
Electrical Electrical distribution distribution of high-freque high-frequency ncy microwave microwave signals signals through through either free space or transmission lines always causes problems besides its high cost. In free space, losses due to absorption and reflection increase with frequency, where as in transmission lines, the rise of impedance with frequency leads to very high losses. Therefore, distributing high frequency radio signals electrically over long distances requires expensive regenerating equipment, as for mm-waves, their distribution via the use of transmission transmission lines is not feasible even for short distances. distances.
The alternative solution for this problem is to distribute baseband signals or signals at low intermediate frequencies (IF) from the switching centre (headend) to the BS [1]. The baseband or IF signals are up-converted to the required microwave, or mm-wave frequency at each base station and amplified amplified before being radiated. radiated. This system configuration is the same as the one used in the distribution of narrowband mobi mobile le comm commun unica icatio tion n syste systems ms.. Sinc Sincee optic optical al fibe fiberr offer offerss very very low low loss, loss, RoF RoF technology can be used to achieve both low-loss distribution of mm-waves, as well as simplification of RAUs at the same time.
Single Mode Fibers (SMFs) made from glass (silica), have attenuation losses below 0.2 dB/km and 0.5 dB/km in the 1550 and 1300 nm windows, respectively as shown in Figure 2.2 [6].
9
Figure 2.2.
2.3.2 2.3.2
Operating regions of optical fiber [2]
Large Large Bandwi Bandwidth dth
Optical fibers offer enormous bandwidth. There are three main transmission windows, which offer low attenuation in the wavelength region of 850, 1310, and 1550 nm respectively respectively [6] as shown in Figure 2.2.
For a single SMF optical fiber, the combined bandwidth of the three windows is in the excess of 50 THz. commercial systems utilize only a fraction of this capacity (1.6 THz) [5].
The high optical bandwidth enables high speed signal processing that may be more more diff difficu icult lt or impo impossi ssibl blee to do in elect electro roni nicc syste systems. ms. Furth Furtherm ermor ore, e, signa signall processing in the optical domain makes it possible to use cheaper low bandwidth optic optical al comp compon onen ents ts such such as laser laser diod diodes es and and modu modulat lator ors; s; in addi additi tion on,, it is still still capable to handle high bandwidth signals.
The utilization of enormous bandwidth, which is primary source of receiver and transmission transmission data, offered by optical fibers is however, however, severely hampered by the limitat limitation ion of bandwi bandwidth dth in electro electronic nic systems. systems. This This probl problem em is referred referred to as the “electronic bottleneck” [3]. The solution of the electronic bottleneck lies in effective
10 multiplexing Optical Time Division Multiplexing (OTDM) and DWDM techniques. In analogue optical systems, including RoF technology, the Sub-Carrier Multiplexing (SCM (SCM)) is used used to incre increase ase optic optical al fibe fiberr band bandwi widt dth h utili utilizat zatio ion. n. In SCM, SCM, sever several al micro microwa wave ve subc subcar arrie riers, rs, which which are modu modula lated ted with with digi digita tall or anal analog ogue ue data, data, are combined and used to modulate the optical signal, to be carried on a single fiber. This makes RoF systems cost-effective.
2.3.3 2.3.3
Immunity Immunity to Radio Radio Frequ Frequency ency Interfer Interference ence
Immunity Immunity to ElectroMagne ElectroMagnetic tic Interference Interference (EMI) is a very attractive property of RoF technology technology,, especially especially for microwave microwave transmission. transmission. This is so because signals are transmitted in the form of light through the fiber. Due to this immunity, fiber cables are preferred even for short connections at mm-waves. EMI immunity is the immuni immunity ty to eavesdr eavesdropp opping ing,, which which is an import important ant charact characteris eristic tic of optical optical fiber fiber communications as it provides privacy and security.
2.3.4 2.3.4
Easy Installat Installation ion and Maintena Maintenance nce
In RoF systems, complex and expensive equipments are kept at the headend, thereby making the Remote Antenna Unit (RAUs) simpler. For instance, most RoF techniques eliminate the need for a local oscillator and related equipments at the RAU. In such cases a photodetector, an RF amplifier and an antenna make up the RAU. Modulation and switching equipment is kept in the headend and is shared by several RAUs. This arrangement leads to smaller and lighter RAUs by effectively reduc reducin ing g syst system em insta installa llatio tion n and and main mainten tenan ance ce cost costs. s. Easy Easy insta installa llatio tion n and and low low maintenance costs of RAUs are very important requirements for mm-wave systems, because of the large number of the required RAUs. In applications where RAUs are not easily accessible, the reduction in maintenance requirements leads to a major operational cost savings [10]. The usage of smaller number of RAUs also leads to a reduced environmental impact.
11 2.3.5 2.3.5
Reduced Reduced Power Power Consumpt Consumption ion
Reduced power consumption is a consequence of having simple RAUs with reduce reduced d equipm equipments ents.. Most Most of the comple complex x equipm equipment entss are kept kept at the central centralized ized headend. In some applications, the RAUs are operated in passive mode. For instance, some 5 GHz Fiber-Radio systems employing pico-cells can have the RAUs operate in a passi passive ve mode mode [10] [10].. Redu Reduced ced powe powerr cons consum umpt ptio ion n at the the RAU RAU is signi signific fican antt considering that the RAUs are sometimes placed in remote locations and have not been fed by the power grid.
2.3.6 2.3.6
Multi-Oper Multi-Operator ator and MultiMulti-Ser Service vice Oper Operation ation
RoF offers offers system system operati operationa onall flexibi flexibility lity.. Depend Depending ing on the microw microwave ave gene genera ratio tion n techn techniq ique ue,, the the RoF RoF dist distrib ribut utio ion n syste system m can be made made signa signal-f l-for orma matt transparent. The Intensity Modulation and Direct Detection (IM-DD) technique can be made to operate as a linear system and, therefore, as a transparent system. This can be achiev achieved ed by usin using g low low disp disper ersio sion n fibe fiberr (SMF (SMF)) in comb combin inati ation on with with prepremodulated RF subcarriers (SCM). In that case, the same RoF network can be used to distribute distribute multi-operator multi-operator and multi-service multi-service traffic resulting in huge economic savings [11]. The principle of Optical Frequency Multiplication (OFM) can also be used to achieve multi-service operation in combination with either WDM or SCM, because its tolerance to chromatic dispersion.
2.3.7 2.3.7
Dynamic Dynamic Resource Resource Allocation Allocation
Since the switching, modulation, and other RF functions are performed at a centralized headend, it is possible to allocate the capacity dynamically. In a RoF distribution system for Global System for Mobile communications (GSM) traffic, more capacity can be allocated to a certain area during the peak times and then reallocated to other areas when off-peak. This can be achieved by allocating optical
12 wavelengths, through WDM [1]. Allocating the capacity dynamically as the need for it arises, obviates the requirement for allocating permanent capacity, which would be a waste waste of resou resource rcess in the the cases cases where where traffi trafficc load loadss vary vary frequ frequen ently tly by larg largee margins. margins. Furthermore, Furthermore, having the centralized centralized headend headend facilitates facilitates the consolidation consolidation of other other signal signal process processing ing functio functions ns such such as mobilit mobility y functio functions ns and macro macro diversi diversity ty transmission [1].
2.4
The Applicati Applications ons of Radio-ove Radio-over-Fibe r-Fiberr Tech Technolog nology y
Some
of
the
applications
of
RoF
technology
include
satellite
communications, mobile radio communications, broadband access radio, Multipoint Video Distribution Distribution Services Services (MVDS), (MVDS), Mobile Mobile Broadband Broadband System (MBS), vehicle communications and control, and wireless LANs over optical networks. Two of the main application areas of RoF technology are briefly discussed below.
2.4.1 2.4.1
Cellul Cellular ar Networ Networks ks
The The fiel field d of mobi mobile le netw networ orks ks is an impo import rtan antt appl applic icat atio ion n area area of RoF RoF techn technol olog ogy. y. The The ever ever-ri -risin sing g numb number er of mobi mobile le subs subscr crib ibers ers coup couple led d with with the the increasing demand for broadband services have kept sustained pressure on mobile networks to offer increased capacity. Therefore, mobile traffic (GSM) can be relayed cost effectively between the SCs and the BSs by exploiting the benefits of SMF technol technology ogy.. Other Other RoF functio functionali nalities ties such such as dynami dynamicc capacit capacity y allocati allocation on offer offer significant significant operational operational benefits to cellular networks. networks.
13
BS Figure 2.3
These robust robust RAPs are are connected connected to the central central base base station via the RoF links [10]
2.4.2 2.4.2
Satellite Satellite Communica Communication tionss
Satellite communication was one of the first practical applications of RoF technology. One of the applications involves the remoting of antennas to suitable locations at satellite earth stations. In this case, small optical fiber links of less than 1km and operating at frequencies between 1 GHz and 15 GHz are used [10]. By doing so, high frequency equipment can be centralized.
The second application involves the remoting of earth stations themselves. With the use of RoF technology, the antenna needs not to be within the control area (e.g. Switching Centre). They can be sited many kilometers away for the purpose of impr improv oved ed satel satellit litee visi visibi bilit lity y or redu reducti ction on of inter interfer feren ence ce from from othe otherr terres terrestri trial al systems systems.. The Switch Switching ing equipm equipment ent may also be approp appropriat riately ely sited, sited, taking taking in to consideration the environmental or accessibility reasons or reasons relating to the cost of premises, without requiring requiring to be in the vicinity of the earth station antennas. antennas.
14 2.5 RoF Multiplex Multiplexing ing Techn Technique iquess
RoF multiplexing techniques is the process of multiplexing wavelength of different frequency onto a single fiber. This operation cerates many virtual fibers, each capable of carrying carrying different different signal. signal. RoF multiplexing uses wavelengths to transmit data parallel by bit or serial by character, which increases the capacity of the fiber by assigning incoming optical signals to specific frequency (wavelengths) within designated frequency band and then multiplexing multiplexing the resulting signal out on to one fiber.
2.5.1 2.5.1
Sub-Carr SubCarrier ier Multiplex Multiplexing ing in in RoF RoF Syste Systems ms
Subcarrier Multiplexing (SCM) is a simple and cost effective approach for exploiting optical fiber bandwidth in analogue optical communication systems in general and in RoF systems in particular. In SCM, the RF signal is used to modulate an opti optical cal carrie carrierr at the the tran transmi smitt tter’ er’ss side. side. This This resul results ts in an optic optical al spect spectru rum m consisting of the original optical carrier f 0 , plus two side-tones located at f 0 ± f SC, , where f SC is the subcarrier frequency. If the subcarrier itself is modulated with data (analogue (analogue or digital), digital), then sidebands sidebands centered at f 0 ± f ± f SC are produced as illustrated in Figure 2.4.
2.4 GHz
Figure 2.4
SubCarrier multiplexing multiplexing of mixed digital and analogue signals [11]
15 In order to multiplex multiple channels of mixed digital and analogue signals to one optical carrier, the multiple sub-carriers are first combined and then used to modulate the optical carrier as shown in Figure 2.3. At the receiver’s side the subcarr carrie iers rs are are reco recove vere red d thro throug ugh h dire direct ct dete detect ctio ion n and and then then radi radiat ated ed.. Diff Differ eren entt modulation modulation schemes may be used on separate sub-carriers. One sub-carrier sub-carrier may carry digital data, while the other may be modulated with an analogue signal such as video or telephone traffic. therefore, SCM is found to support the multiplexing of various kinds kinds of mixed mixed mode mode broadb broadband and data. Modulatio Modulation n of the optical optical carrier carrier may be achieved by either directly modulating the laser, or by using external modulators.
SCM may be used in both IM-DD and Remote Heterodyne Detection (RHD) RoF techniques. SCM in combination with IM-DD has been used in RoF systems fed by multimode fiber. However, these systems have been used mainly for transmitting WLAN signals signals at frequencies frequencies below 6 GHz [11].
2.5.2 2.5.2
Wavelengt Wavelength h Division Division Multip Multiplexin lexing g in RoF RoF System Systemss
WDM WDM are are pass passiv ivee devi device cess that that comb combin inee ligh lightt sign signal alss with with diff differ eren entt wavelengths, coming from different fibers, onto a single fiber. They include dense wave wavelen lengt gth h divi divisio sion n multi multipl plex exers ers (DWD (DWDM) M),, devi device cess that that use use optic optical al (anal (analog og)) multiplexing techniques to increase the carrying capacity of fiber networks beyond levels that can be accomplished via time division multiplexing (TDM)
The use of WDM for the distribution of RoF signals as illustrated in figure, has gained importance recently. WDM enables the efficient exploitation of the fiber network’s bandwidth. These systems can achieve capacities over 1 Tb/s over a single fiber. At the same time, bit rates on a single channel have risen to 10 Gb/s and systems operating at 40 Gb/s channel rates are becoming commercially available. The channel spacing in WDM can be decreased to 50 GHz or even to 25 GHz and thus, it is possible to use hundreds of channels. However, if the channel spacing is dropped to 50 GHz instead of 100 GHz, it will become much harder to upgrade the systems to operate at 40 Gb/s due to the nonlinear effects.
16
Figure 2.5
WDM system using multiple wavelength channels and optical amplifiers [10]
However, the transmission of RF signals is seen as inefficient in terms of spectrum spectrum utilization, since the modulation modulation bandwidth bandwidth is always a small fraction of the carrier signal frequency. Therefore, methods to improve the spectrum efficiency have been proposed. RoF on WDM systems have been reported. Carriers modulated with mm-wave mm-wavess are dropped dropped from from and added to a fiber fiber ring ring using using Optical Optical Add-Dr Add-Drop op Multiplexers (OADM). The OADM are placed at base stations and tuned to select the desired optical carriers to drop [10] [11].
17
CHAPTER 3
NON-LINEAR EFFECTS
3.1 3.1
Intro ntrodu duct ctio ion n
The fundamental component that makes the optical communication possible is the optical fiber. The phenomenon which guides the light along the optical fiber is the total total intern internal al reflecti reflection. on. It is an optical optical phenomen phenomenon on which which occurs occurs when when the incident light is completely reflected. In case of materials with different refractive indices, light will be reflected and refracted at the boundary surface. This will occur only from higher refractive index to a lower refractive index such as light passing from from glass glass to air. air. This This phen phenom omen enon on form formss the the basis basis of optic optical al comm commun unica icatio tion n through fibers.
An opti optical cal fibe fiberr is a diel dielect ectric ric wave wavegu guid ide, e, it is cylin cylindr drica ical, l, and and guid guides es the the ligh lightt parallel to the axis. The cylindrical structure is dielectric with a radius “a” and refrac refractiv tivee inde index x of “n1” is the the call called ed the the core core of the the fibe fiberr and and the the lay layer that that encompasses encompasses this structure structure is called the cladding. The Cladding has a refractive refractive index “n2” which is lesser than “n1 ”. This This helps helps in provid providing ing mechani mechanical cal strengt strength h and reduci reducing ng scatteri scattering ng losses. losses. It also preven prevents ts the core core from from surface surface contam contamina inatio tion. n. cladding doesn’t take part in light propagation.
18 3.2 3.2
Type Typess of Fibe Fibers rs
Fibers Fibers can be classi classifie fied d accord according ing to the core’s core’s materi material al compos compositi ition. on. If the refractive index of the core is uniform and changes abruptly at the cladding boundary, then it is called as Step-index fiber. If the refractive index changes at each radial distance, then it is called as Graded-index fiber. These fibers can be divided into single mode and multi mode fibers. The single mode fibers operate in only one mode of propagation. Multimode fibers can support hundreds of modes.
Both laser diodes and light emitting diodes (LED) can be used as light wave sources in fiber-optical communication systems. When compared to Laser diodes, LEDs are less less expensi expensive, ve, less less comple complex x and have a longer longer lifeti lifetime; me; however however,, their their optica opticall powers are typically small and spectral linewidths are much wider than that of laser diod diodes es.. In multi ultimo mode de fiber fiberss diff differ eren entt mode modess trav travel el with with diff differ erent ent spee speed, d, whic which h is commonly commonly referred to as intermodal intermodal dispersion, dispersion, giving room to pulse spreading. spreading. In single single mode fibers, different signal frequency components travel in different speed within the fundamental mode and this result in chromatic dispersion. Since the effect of chromatic dispersion is proportional the spectral linewidth of the source, laser diodes are often used in high-speed optical systems because of their narrow spectral linewidth.
3.3
Fibe iber Losses sses
For For effi effici cien entt reco recover very y of the the recei received ved sign signal al,, the the sign signal al to nois noisee rati ratio o at the the receiver must be considerably high. Fiber losses will affect the received power eventually reducing the signal power at the receiver. Hence optical fibers suffer heavy loss and degrad degradati ation on over long long distan distances ces.. To overcom overcomee these these losses losses,, optica opticall amplif amplifier ierss were were invented, which significantly boosted the power in the spans in between the source and receiver. However, optical amplifiers introduce amplified spontaneous emission (ASE) noises which are proportional to the amount of optical amplifications they provide; low
19 loss in optical fibers is still a critical requirement in long distance optical systems to efficiently recover the signal at the receiver.
Attenuation Coefficient is a fiber-loss parameter, which expressed in the units of dB/Km. For short wavelengths; the loss may exceed 5 dB/Km and makes it unsuitable for long distance transmission [2]. These losses are mainly due to material absorption and Rayleigh scattering. Material absorption is the phenomenon exhibited by silica fibers. The intrin intrinsic sic absorp absorptio tion n is caused caused by the fused fused silica silica and the extri extrinsi nsicc absorp absorptio tion n is caused by the impurities in silica. The other contributing factor is the Rayleigh scattering which is caused by the density fluctuations in the fiber. These fluctuations change the refractive index on a smaller scale. Light scattering in such medium is called Rayleigh scattering [7].
In multi-mode fibers, intermodal dispersion is the dominant contributor of signal waveform distortion. Although intermodal dispersion is eliminated in single mode fibers, different frequency component of optical signal carried by the fundamental mode still travel in slightly different speed giving rise to a wavelength-dependent group delay. As the the group group dela delay y depen depends ds on wave wavele leng ngth th,, diff differ eren entt amoun amountt of time time is take taken n for for the the different spectral components to reach a certain distance. Due to this effect, the optical signal with a certain spectral width spreads with time when it travels through the fiber. This pulse spreading is important and needs to be determined.
3.4 3.4
Fibe Fiberr Nonl Nonlin inea eari riti ties es
Even though though optica opticall networ networks ks are fast, fast, robu robust, and error error free, free, still still nonlin nonlinear earity ity obstacles exist, which prevent it from being a perfect medium.
The nonlinear nonlinear effects of the fibers fibers play a detrimental detrimental role in the light propagation. Nonlinear Kerr effect is the dependence of the refractive index of the fiber on the power
20 that propagating through it. This effect is responsible for self phase modulation (SPM), cross phase modulation (XPM) and four wave mixing (FWM). The other two important effects are stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS).
3.4.1 3.4.1
Self Phase Phase Modulati Modulation on
In fibers, the refractive refractive index always has some dependence dependence on the optical intensity intensity whic which h is the the optic ptical al powe powerr per per effe effect ctiv ivee area area.. This This rela relati tion on can can be give given n as [6]: [6]:
P n n0 n2 I n0 n2 .........................................................................(3.1 A eff where no is the ordinary refractive index , n2 is the nonlinear nonlinear refractive refractive index co -efficient, -efficient, Aeff is is the effective core area, and P is the power of the optical signal.
This This nonlin nonlinear earity ity is known known as Kerr Kerr nonlin nonlinear earity ity.. This This produc produces es Kerr Kerr effect effect in whic which h the the prop propag agat atin ing g sign signal al is phas phasee modu modula late ted d by the the carr carrie ier. r. This This lead leadss to a phenomenon called Self-phase Self-phase modulation that converts power fluctuations into phase fluctuation fluctuationss in the same wave [8].
In a material where the refractive index depends on a varying signal intensity propagating along the fiber, it will produce time varying refractive index. Higher refractive index at the peak of the pulse is produced, when compared to the edges of the pulse. These results a time varying phase change d change d θ/dt. θ/dt. Due to this change, change, the frequency of the optica opticall signal signal underg undergoes oes a frequen frequency cy shift shift from from its initial initial value. value. This This effect effect is known known as frequen frequency cy chirpi chirping, ng, in which which differ different ent parts parts of the pulse pulse undergo undergo differ different ent phase change as shown in Figure 3.1 [8]. The rising edge experiences a shift towards the higher frequency and the trailing edge experiences a shift towards the lower frequency. Since this effect depends heavily on the signal intensity, SPM has more effect on high intensity signal pulses.
21
Figure 3.1
Frequency chirping effect
In case of fibers having the group velocity dispersion (GVD) effects, the pulse normally broadens which leads to difficulty in the receiver side to decode the signal. When the chromatic dispersion is negative, the edges of frequencies which experienced high higher er shif shifts ts tend tend to move move away away from from the the cent centre re of the the puls pulse. e. The The edge edgess of the the frequencies which experienced lower shifts tend to move away from the centre in the opposite direction. Thus the GVD affected pulse will be broadened at the end of the fiber, and and the the chir chirpi ping ng wors worsen enss due due to this this effe effect ct.. Ther Theref efor oree the the SPM SPM can can wors worsen en the the performance of the optical system in the case of long haul transmission.
3.4.2 3.4.2
Cross Cross Phase Phase Modulati Modulation on
As with with Equat Equatio ion n 3.2, 3.2, the the refr refract activ ivee index index of the the fibe fiberr depen depends ds on the the time time varying signal intensity, which results time varying refractive index. This phenomenon
22 leads to an effect called XPM. XPM has more pronounced effect in the case of WDM systems in which more optical channels are transmitted simultaneously. In the case of XPM, the phase shift depends on the power of other channel. The total phase shift can be represented as [6].
j NL Leff Pj 2
wher wheree
...... ... ..... .. .... ...... .... ...... .... ..... . ... ..... .. .... ...... .... ...... ... ..... .. .... ...... .... ...... .................... ( ................... ( . 3.2) P .... m j
m
3
2
is the the nonnon-li line near ar phase phase shif shiftt for for the the jth channel, M=½(N -N ) varies from 1 to
-1
5 W /Km, Leff is the effective length of the fiber, and P j and Pm are the power for the channels channels i and j respectivel respectively. y.
On the right-hand-side of Equation 3.2, the first term represents effect of SPM and the second term represents that of XPM. The factor of 2 in this equation implies that XPM is twice as effective as SPM for the same amount of power [7]. The phase shift which is directly created by XPM at the end of the fiber depends on the bit patterns and powers of the neighboring channels. The effect of XPM also depends on the wavelength separation between the signal channel and the neighboring channel. If the channels are separated widely, then the XPM effects are relatively weak because the two bit streams walk walk-o -off ff from from each each othe otherr quic quickl kly. y. In case case of the the DWDM DWDM syst system ems, s, the the chan channe nell wavelength separation is very narrow, which leads to strong XPM effect. Since XPM results in an inter channel crosstalk, its effect, to some extent, also depends on the bit pattern of the two channels. channels.
To analyze the effect of XPM and SPM, the nonlinear Schrödinger equation can be used which represented as [2]:
A i 2 2 A 2 A i A A...............................................................(3.3) ...............................................................(3.3) 2 2 z
23 By increasing the effective area, nonlinearities can be reduced. Aeff is about 80 µm2 for standard fibers and 50 µm2 for dispersion shifted fibers [6].
3.4.3 3.4.3
Four Four Wave Wave Mixing Mixing
FWM is a phenomenon that occurs in the case of DWDM systems in which the wavelength channel spacing are very close to each other. This effect is generated by the third order distortion that creates third order harmonics. As shown in Figure 3.2, these cross products interfere with the original wavelength and cause the mixing. In fact, these spurio spurious us signal signalss fall fall right right on the origin original al wavele wavelengt ngth h which which result resultss in diffic difficult ulty y in filtering them out. In case of 3 channel system, there will be 9 cross products, where 3 of them will be on the original wavelength. This is caused by the channel spacing and fiber dispersion. If the channel spacing is too close, then FWM occurs. If the dispersion is less lesser er,, then then FW FWM M is highe higherr sinc sincee disp disper ersi sion on is inve invers rsel ely y propor roporti tiona onall to mixi mixing ng efficiency. efficiency. As can be seen in Figure 3.2, the cross product lies right on the original signal signal which poses problem when filtering.
In gener general al,, for for N wavel waveleng ength thss input input channe channell ther theree will will be M cros crosss mixi mixing ng products and an d are a re given by [22] [22]
M
N 2
N 1 ..........................................................................................(3.4)
24
Figure 3.2
Four wave mixing products
If the WDM system system is considered considered as a sum of N N monochromatic monochromatic plane waves, it is possible to solve the arising channels angular frequencies. Considering a simple threewavelength wavelength ( 1, 2, and 3) system system that is experie experienci ncing ng FWM distor distortio tion, n, nine nine cross cross
1, 2 , and 3 see Figure 3.3 that involve two or more of the products are generated near original wavelengths. There are additional products generated, however they fall well away from the original input wavelengths.
25
Figure 3.3
(a) two input signals ω1 and ω 2 (b) three three input input signals signals ω1, ω 2 and ω 3 and the arising arising new frequency frequency components due to FWM
Assuming that the input wavelengths are 1 = 1551.72 nm , 2 = 1552.52 nm, and 1553.3 .32 2 nm. The interf interferi ering ng wavele wavelengt ngths hs generat generated ed around around the origin original al three three 3 = 1553 wavelength system are:
1 + 2- 3 = 1550.92 nm 1- 2 + 3 = 1552.52 nm 2+ 31 = 1554.12 nm 1-2+ 3 =1552.52 nm 21- 3 = 1550.12 nm 23- 1 = 1554.92 nm
2+ 3- 1 = 1554.12 nm 22- 1 = 1553.32 nm 23 -2 = 1554.12 nm It can be seen that three of the interfering products fall right on top of the original three signals and the remaining six products fall outside of the original three signals. These six wavelengths wavelengths can be optically filtered filtered out. The three interfering interfering products products that fall on top of the original signals are mixed together; and cannot be removed by any means. Figure 3.2 shows the results graphically. The three tall solid bars are the three original signals. The shorte shorterr cross cross-ha -hatch tched ed bars bars repres represent ent the nine nine interf interferi ering ng product products. s. The number number of the 3
2
interfering products increases as ½ • (N -N ) where N is the number of signals.
26 Figure 3.4 show that the number of the interfering products rapidly becomes a very large. Since there is no way to eliminate the products that falling on top of the original signals, the priority is to prevent them from forming in the first place.
Figure 3.4
FWM products products versus versus channel count [22]
Therefore Therefore two factors factors strongly strongly influence the magnitude magnitude of the FWM products, products, referred referred to as the the FW FWM M effi effici cienc ency. y. The firs firstt fact factor or is the the chann channel el spac spacin ing; g; where where the the mixi mixing ng efficiency increases dramatically as the channel spacing becomes closer. Fiber dispersion is the second factor, factor, and the mixing efficien efficiency cy is invers inversely ely proporti proportiona onall to the fiber dispersion, being strongest at the zero-dispersion point. In all cases, the FWM mixing efficiency is expressed in dB, and more negative values are better since they indicate a lower mixing efficiency.
Figur iguree 3.5 3.5 show showss the the magn magnit itud udee of FW FWM M mixi mixing ng effi effici cien ency cy vers versus us fibe fiber r dispersion and channel spacing. If a system design uses NDSF with dispersion of 17 ps/nm/km and the minimum recommended International Telecommunication Union (ITU) DWDM spacing of 0.8 nm, then the mixing efficiency is about -48 dB and will have little impact. On the other hand, if a system design uses DSF with a dispersion of 1 ps/nm/km and a non-standard spacing of 0.4 nm, then the mixing efficiency becomes -12 dB and and will will have have a sever severee impa impact ct on the the syst system em perfo perform rmanc ance, e, perhap perhaps, s, maki making ng the the recovery of the transmitted signal impossible. The magnitude of the mixing efficiency
27 will vary widely as these parameters vary. The data presented is intended to illustrate the principles only.
Figure 3.5
FWM mixing mixing efficienc efficiency y in single-mode single-mode fibers fibers [22] [22]
FWM is independent of the used bit rate; however, it is critically dependent on channel spacing and chromatic dispersion. Therefore, the effects of FWM must be considered even even at modera moderatete-bit bit-ra -rate te system systems, s, if the channel channel spacin spacing g is small small or the chromat chromatic ic dispersion of the fiber is low. Thus, it is possible to minimize the effects of FWM by increasing the channel spacing and the chromatic dispersion of the fiber.
3.4.4 3.4.4
Stimulat Stimulated ed Brill Brillouin ouin Scatt Scatterin ering g ((SBS) SBS)
SBS falls under the category of inelastic scattering in which the frequency of the scattered light is shifted downward. This results in the loss of the transmitted power along the fiber. At low power levels, this effect will become negligible. SBS sets a threshold on the transmitted transmitted power, above which considerable considerable amount of power is reflected. reflected. This back reflection will make the light to reverse direction and travel towards the source. This usuall usually y happens happens at the connect connector or interf interfaces aces where there there is a change change in the refrac refractiv tivee
28 index. As the power level increases, more light is backscattered since the level would have crossed the SBS threshold. threshold. The parameters which decide the threshold are the wavelength and the line width of the transmitter.
Lower line width experiences lesser SBS and the increase in the spectral width of the source will reduce SBS. In the case of bit streams with shorter pulse width, no SBS will occur. The value of the threshold depends on the RZ and NRZ waveforms, which are used to modulate the source. It is typically 5 mW and can be increased to 10 mW by increasing the bandwidth of the carrier greater than 200 MHz by phase modulation [8].
3.4.5 3.4.5
Stimulat Stimulated ed Rama Raman n Sc Scatte attering ring (SRS) (SRS)
SRS occurs when the pump power increases increases beyond the threshold, threshold, however SRS can happen in either direction, forward and backward. The molecular oscillations set in at the beat frequency and the amplitude of the scattering increases with the oscillations. The equations that govern the feedback process are [8]:
dI p dz
dI s z
g R I p Is p I p .............................................................................. ..............................................................................(3.9) (3.9)
g R I p Is s I s ................................................................................ 3.10) ( 3.10)
where gR is is the SRS gain. I p and Is are intensities of Pump and stokes field. In case of the threshold power, the Pth is given by [8], Pth 16 ( w ) / g R .................................................................................(3.11) 2
2
where πw is the effective area of the fiber core and w is the spot size.
Even though there are some detrimental effects posed by these two effects, SBS and SRS can also be used in a positive way. Since both deal with transferring energy to the signal from a pump, they can be used to amplify the optical signal. Raman gain is also used in compensating losses in the fiber transmission. Table 3.1 shows comparison of property behavior under the influence of SBS and SRS.
Table 3.1
Comparison between SBS and SRS
Property
SBS
SRS
Direction of
Only in backward
In both forward and backward backward
scatter
direction
direction
Frequency shift
About 10 GHz
About 13 THz
Spectrum width
Narrow width
Broad spectrum width
29
CHAPTER 4
METHODOLOGY
4.1
Introduction
This chapter highlights the techniques and methods employed to study the nonlinear effects of FWM in WDM for RoF as well as to analyze the modelling results obtained. obtained. Details of the methods will be given in the proceeding proceeding sections. sections.
4.2 4.2
Simu Simula lati tion on usin using g Op Opti tisy syst stem em Soft Softwa ware re
OptiSystem software is a numerical simulation enables users to plan, test and simulate almost every type of optical link in the physical layer across the broad spectrum spectrum of optical optical networks. networks. Algorithms are included included for dispersion map design, design, bit error rate calculation, system penalty estimations, and link budget calculations.
Each layout can have certain component parameters assigned to be in sweep mode. The number of sweep iterations to be performed on the selected parameters could be defined. The value of the parameter changes through each sweep iterations; which which produc produces es a series series of different different calcula calculatio tion n results results,, based based on the paramet parameter er values. These processing parameters effect on the results are channel pacing, input power, effective effec tive area and dispersion disper sion of the fiber
30 4.3 4.3
Thee Simul Th imulat atio ion n Model del
There There are two two techn technol olog ogie iess for for modu modulat latio ion n, dire direct ct or with withou outt exte extern rnal al modulation as shown in Figure 4.1 which the RF signal directly varies the bias of a semiconductor laser diode
Figure 4.1
Direct modulation
The other technology technology is the external modulators are typically typically either integrated integrated Mach-Zehnder interferometers or electroabsorption modulators as shown in Figure 4.2 4.2 whic which h the the cons consta tant nt wave wave (CW) (CW) lase laserr (alw (alway ayss on brig bright ht)), and and the the ligh lightt is modu modulat lated ed by an exter externa nall lith lithiu ium-n m-nio ioba bate te electr electroo-op opti ticc modu modula lato tor. r. Exte Extern rnal al modulation is currently preferred over any other form of modulation because it has best performance, in spite of high cost.
Figure 4.2
External modulation
31 Usin Using g Optis Optisys ystem tem softw softwar are, e, two two type typess of simu simula latio tion n mode models ls have have been been developed developed to study FWM effects. The two models are with external modulated modulated signal and and witho without ut exte extern rnal al modu modula late ted d sign signal al as show shown n in the the Figu Figure re 4.3 4.3 and and 4.4, 4.4, respectively.
The frequency of the phase modulator modulator drive signal signal was kept at 2.4 GHz. The phase m odulator has been bee n used to sweep the optical frequency, it was nec essary to first integrate the drive signal [11]. .
Figure 4.3
Simulation model with external modulated signal
32
Figure 4.4
Simulation model without external modulated signal
The simulation models were modified according to the related parameters or components for different types of simulation process as given below
i.
Effec ffectt of chan chann nel spac spacin ing g.
ii.
Effect Effect of differe different nt Power Power Level Level of the signals signals Sources Sources
iii. iii.
Effe Effect ct of incre increase ase disp dispers ersio ion n of the the Fibe Fiberr Opti Opticc
iv. iv.
Effect Effect of Increas Increasee Effectiv Effectivee Area Area of the Fiber Fiber optic optic
4.4
Simula Simulatio tion n of the Four Four Wave Wave Mixing Mixing effec effectt
Each component in both simulation models, shown in Figures 4.3 and 4.4, has its own role, to play in the process.
The Pseudo Random Bit Sequence Generator is a device or algorithm, which outputs a sequence of statistically independent and unbiased binary digits.
33 NRZ Pulse Generator (non-return-to (non-return-to-zero) -zero) refers to a form of digital data transmission in which the binary low and high states, represented by numerals 0 and 1, are transmitted by specific and constant DC (direct-current) voltages. In positive-logic NRZ, the low state is represented by the more negative or less positive voltage, and the high state is represented by the less negative or more positive voltage. In negative-logic NRZ, the low state is represented by the more positive or less negative voltage, and the high state is represented by the less positive or more negative negative voltage. voltage. The The cont contin inue uess wave wave (CW) (CW) Gene Generat rator or is a gene generat rator or of cont contin inuo uous us-w -wav avee millimeter-wave optical signals. The spectral linewidth of the generated millimeterwave signals is 2 kHz. The power of the measured cw millimeter-wave signals is almost in proportion proportion to the power multiplicatio multiplication n of the two input optical signals. The Mach-Zehnder Mach-Zehnder Modulator, Modulator, is a modulator, modulator, which has two inputs, one for the laser diode and the other for the data from the channels.
The The WDM WDM Multi Multipl plex exer er is a meth method od of trans transmi mitt ttin ing g data data from from diffe differen rentt sources over the same fiber optic link at the same time whereby each data channel is carried on its own unique wavelength.
The Optical Fiber is a component, used in the simulation is a single mode fiber (SMF-28), where the dispersive and nonlinear effects are taken into account by a direct numerical integration of the modified nonlinear Scrödinger (NLS) equation.
Besides the above components components there are three types of components, components, which used for visualizing purposes: i.
Opti Optica call Powe Powerr Mete Meterr Visu Visual aliz izer er
ii. ii.
Optica ical Spectr ctrum Analy alysis sis
iii.
WDM analyzer
Below are the tables for parameters setting. Table 4.1 shows the set of the global parameters; and Table 4.2 shows the parameters, set for the CW laser sources. The parameters set in the WDM MUX are shown in Table 4.3. There are many tabs
34 for the optical fiber parameter settings, where Table 4.4 gives the setting for the main and the dispersion tabs, Table 4.5 gives the setting for the nonlinear tab, and Table 4.6 gives the setting for the numerical and PMD tabs in optical fiber respectively. respectively.
Table 4.1
Table 4.2
Global parameters
CW Laser sources parameters
35
Table 4.3
Table 4.4
WDM 2x1 multiplexer parameters
Main tab and and dispersio dispersion n tab parameters parameters are are set for optical optical fiber
Table 4.4: Main tab and Dispersion Dispersion tab Parameters for Optical Fibers
36
Tables 4.5
Table 4.6
Nonlinear Nonlinear tab parameters parameters for for optical optical fiber fiber
Numerical Numerical tab and PMD tab parameters parameters for optical optical fiber fiber
37
4.5
Simula Simulatio tion n of FWM FWM for higher higher number number of channe channels ls
Sources in the simulation model were increased to three or four channels. Figures Figures 4.5 and 4.6 show the sources increased in the new simulation simulation model based on direct modulation [22].
Figure 4.5
Simulation model with three channels
Figure 4.6
Simulation model with four channels
38 4.6
Effect Eff ect of Differ Different ent Power Power Level Level of of the the Signal Signalss Sourc Sources es
The main requir requirem ement ent from from a wireles wirelesss commun communicat ication ion system system is that that the transmitted transmitted electro magnetic (EM) wave must reach the receiver receiver with ample power to allow the receiver to distinguish distinguish the wave from the background background noise.
Another common property used to describe signal strength is the S/N ratio. The The S/N S/N ratio ratio does does not not desc describ ribee the the abso absolu lute te powe powerr in the the signa signal, l, but but inst instead ead describes the power of the signal in comparison to the power of the background noise. The higher the S/N ratio, the better or more powerful the signal. Since the S/N ratio accounts for the level of background noise, it is a very valuable and widely used indicator of signal strength.
In the simulation process, the power at the simulation model sources was vari varied ed from from 20 dBm to -10 dBm with with step step of -10 -10 dBm dBm to in orde orderr try try diff differe erent nt simulations.
4.7
Effect Eff ect of Increa Increase se dispers dispersion ion of the the Fiber Fiber Opt Optic ic
Wavelen Wavelength gth disper dispersio sion, n, is a signal signal disper dispersion sion,, which which occurr occurrss primar primarily ily in single-mode fiber. A significant amount of the light launched into the fiber is leaked into the cladding. This leaked amount is wavelength dependent and also influences the speed of propagation. High volume communication lines have carefully timed spacin spacings gs between between indivi individua duall signal signals. s. Fortun Fortunatel ately, y, wavelen wavelength gth dispersi dispersion on can be minimized minimized by careful designation of fiber refractive refractive index. index. The The disp dispers ersio ion n param paramet eter er of the the fiber fiber opti opticc in the the simul simulat atio ion n mode modell was was varied from 1 ps/nm/km to 16.75 ps/nm/km. This has been done in order to compare the results with different dispersion parameters and the power level of sources set at 0 dBm.
39 4.8
Effect Eff ect of Increa Increase se Eff Effec ectiv tivee Area Area of the Fiber Fiber optic optic
The effe effect ctiv ivee area area (Aeff ) of the sin single-mode fiber is an imp importa rtant measurement parameter. It is the area of the cross section of the beam arrived into the the fibe fiber. r. The The effect effectiv ivee area area eval evalua uatio tion n requi requires res the the measu measure remen mentt of the the field field distribution in the fundamental mode
The effective area parameter of the fiber optic in the simulation model has 2
2
been changed from 64 μ m to 76.5 μm , in order to compare the results with different effective area parameters as the power level of sources set at 0 dBm. .
4.9 4.9
Mode Modellling ling the the Effe Effect ct of FWM FWM
Matlab program is used to develop the analytical model of the effect of FWM in WDM for RoF. The modelling is meant to study the nonlinear effects due to the FWM in WDM for RoF when the light passing through the medium. Figure 4.6 shows the steps that will be followed followed in the modeling process. process.
The total total polariz polarizati ation on P P is nonl nonlin inea earr with with respe respect ct to the the electr electric ic field field E, however, it can be written as: P 0 . (1)
( 2)
..
(3 )
. .. ......... ...................... ........................(4.1) ......... (4.1)
j) where ε0 is the vacuum permittivity and χ ( j) ( j = j = 1,2,…) is j is jth th order susceptibility.
When light propagates in a transparent medium, its electric field causes some amount of polarization in the medium. While at low light intensities the polarization is linear linear with with the electric electric field, field, nonlin nonlinear ear contrib contributi utions ons become become import important ant at high high optic optical al inte intens nsiti ities, es,
so the the polar polariza izatio tion n equa equatio tion n cons consist istss linear linear terms terms as well well as (1)
nonline nonlinear ar terms. terms. The first first order order suscept susceptibi ibility lity χ
represen represents ts the linear linear term, term, and
nonlineariti nonlinearities es can have strong effects in fibers at the third order susceptibility susceptibility χ χ (3). So, only only the the nonlinear effe effects cts in the optica opticall fibers fibers,, which which origin originate ate from from the third-or third-order der (3) willl , wil
susceptibility χ
be cons consid ider ered ed and and the the othe otherr term termss will will be negl neglec ected ted.. The The
40 programming will start from the third-order susceptibility χ (3) . Thus the electric field of the signal can be written as [6]: N
....................................... ............................................. ........................... .....(4.2) r t Ei cos i t i z ................... i 1
Where β is the propagation constant, and ω is angular frequencies
Substituting Equation 4.2 into Equation 4.1, and if only the term of the third order susceptibility is taken into account, the nonlinear dielectric polarization ( P NL r , t ) can be written written as [6]: [6]:
P NL r , t o
3
n
n
n
E cos t z E i
i
i1 j 1 k 1
3 o
3
4
o
3
4
3 o 4
n
E n
n
3 o 4
2 i
E i 1 j 1
3
n
cos 3i t 3 i z ....................................................... ......... .( erm )
i 1 j 1
3
3 i
i 1
4
n
n
2 i
E j cos 2i t 3i z t 2 i j z ................... .....(term3) E j cos 2i t 3i z t 2 i j z ....................... .(term 4)
n
E E E i 1 j i k j
cos i
cos j t j z Ek cos k t k z
n 2 E 2 E E .......... .(term1) i j Ei cos i t i z .............................. i i 1 j 1
E
3
j
n
n
3 o
i
j
i
j
k
k t cos i j k z .................................... ........................................ .... ....( erm5
cos i j k t cos i j k z ......................... ............... ...(ter m6 ) cos i j k t cos i j k z ......................... ............... ...(ter m7 ) cos i j k t cos i j k z ...................... ............ ......(te rm 8) (4.3)
The nonlinear susceptibility of the optical fiber generates new waves at the angular frequencies ωr ± ωs ± ωt (r , s, t = = 1, 2,…). Term 1, in the above equation represents represents the effects of SPM and XPM.
41 Terms 2, 4 and 5 can be neglected, due to lack of phase matching. The remaining terms can satisf satisfy y the phase phase match matching ing conditio condition. n. The power transfer transferred red due to the FW FWM M to new frequencies after light has propagated distance L in the fiber can be estimated from equation 4.4 [6]:
2
i jkjk d i jkjk 3 2 Pijk .............................................. .............................................. ................................... ............ .......... 4. 4) PP i j P k L ....................... 8 Ae ffff ne ffff c
where n where n eff is the effective index, A index, A eff is is the effective area, P i, P j j and P k k are the input powers at ω i, ω j and ωk . The factor d d ijk ijk depends on the number of channels affecting the FWM
The efficien efficiency cy of FWM and noise noise perform performance ance are analyz analyzed, ed, taking taking into into accou account nt the the effect effectss of diff differe erenc ncee chan channe nell spaci spacing ng.. Equa Equatio tion n 4.5 4.5 is prese present nted ed to evaluate evaluate the efficiency efficiency of the FWM [23].
n2 2 Aeff D( )
2
(4.5)
Equation Equation 4.6 is used to investigate investigate the relationship between the efficiency and the power of the FWM [23].
2 2 2 P k dijk pi p j pk exp( L )Leff 9
(4.6)
where Leff is effective length, which can be calculated by using Equation 4.7.
l
Leff
1 e
(4.7)
where ω is the Angular frequency, d is the degeneracy factor, (3) is the third order susceptibility, Aeff is the effective Area, n2 is the nonlinear reflective index, c is the speed speed of ligh light, t, D is the the disp dispers ersio ion, n, coefficient and L is total fiber length.
λ
is the channel space,
α is the fiber loss
42 The The third third orde orderr susc suscep eptt ibility χ (3) , which includ includes es self-ph self-phase ase modula modulatio tion n (SPM) (SPM) and cross-p cross-phas hasee modula modulatio tion n (XPM) (XPM) as well well as four four-wave -wave mixing mixing (FWM). (FWM). Therefore, the SPM and XPM will be considered as zero, thus, their effects on FWM modeling are neglected. Term1 representing XPM and SPM will be considered as of zero effect and will be neglected too.
The four-wave mixing, require the phase matching to be efficient. Essentially this is mean to ensure a proper phase relationship between the interacting waves. FWM FW M will will be a peak peak at the the phase phase match matchin ing g spect spectru rum. m. Equatio Equation n 4.8 4.8 satisf satisfies ies the the condition of phase matching: β = β(ω1)+ β(ω2)- β(ω3)- β(ω4)
Where
(4.8)
β j is the propagation constant. If β = 0 the phase matching condition is
satisfied, otherwise mismatching occurs.
The model in this study will use only two wavelengths, therefore the phase matching condition will be
β
= β(ω2) - 2 β (ω1) =0 in order order to satisfy satisfy the phase phase matchin matching g
requirement as shown in Figure 4.7.
β1
β1
β
β2 Figure 4.7
The phase matching condition of two different wavelengths [8]
Term2, Term2, term4, and term5 in the polarization polarization Equation Equation 4.3 are considered considered as mismatching mismatching terms. After neglecting neglecting the terms representing representing the effects of SPM, XPM that lack phase matching, matching, the remaining remaining terms in the nonlinear equation, equation, which satisfy the phase matching condition, condition, will be used later to model the FWM.
43
CHAPTER 5
RESULTS AND DISCUSSIONS
5.1 5.1
Intro ntrodu duct ctio ion n
This chapter presents and discusses the results obtained from the simulation mode modell by usin using g Opti Optisy syste stem m as nume numeric rical al simul simulat atio ion n and and Matlab Matlab as analy analytic tical al simulation. The numerical simulation is simulated accordingly as mentioned in the previous chapter, with and a nd without external modulated laser.
5.2
Simula Simulatio tion n of the Four Four Wave Wave Mixing Mixing Eff Effec ectt
In the FWM simulation simulation model layout, two types of visualiser visualiser tools have been used. The optical spectrum analyzer analyzer and the WDM analyzer were fixed after MUX and at the end of the fiber optic. The results obtained after the multiplexer are same as the input power level shown before the nonlinear nonlinear effect. The nonlinear nonlinear effect occurs only during the propagation of signals through the fiber. The optical spectrum analyzer analyzer has been used to show the waveform whereby the WDM analyzer has been used to display signal power (dBm), noise power (dBm) and OSNR (dB).
44 5.3
Simula Simulatio tion n Results Results without without the Exter External nal Modulate Modulated d Signal Signal
In this this simu imulati latio on two two CW lase lasers rs were ere used sed as sign signal alss sour source ces, s, the the frequencies were set at 1550 and 1550.1 nm, where as the power was set at 0 dBm. The linewidth has been set at 0, due to the interest in measuring only the total power of the sideband frequencies, where the shape of the spectrum is not required. The input signals have propagated through 25 km of nonlinear fiber.
5.3.1 5.3.1
Effect Effe ct of Chann Channel el Spacing Spacing variation variation
Figure 5.1 shows the signal at the input channel when the channel spacing is set at 0.1 nm.
Figure 5.1
Optical Optical spectrum spectrum at the the input input of the the fiber when channel channel spacing spacing is set at 0.1 nm
The result obtained from the simulation is depicted in Figure 5.2. From this figu figure re,, the the FW FWM M effe effect ct is obvi obviou ousl sl beca becaus usee the the simu simula lati tion on with withou outt exte extern rnal al modu modula late ted d lase laserr is simp simple lerr comp compar ared ed to the the simu simula lati tion on mode modell with with exte extern rnal al modula modulated ted laser. laser. The interfe interferin ring g wavelen wavelength gthss generat generated ed around around the origin original al two
45 wavelength systems are 1549.9 nm and 1550.2 nm, thereby the power of the each FWM sideband is approximately -59 dBm
Figure 5.2
Optical Optical spectrum spectrum at the the output output of the fiber fiber when when channel channel spacing spacing is set set at 0.1 nm
Figure 5.3 shows the signal at the input channel when the channel spacing is set at 0.2 nm.
Figure 5.3
Optical Optical spectrum spectrum at the the input input of the the fiber fiber when when channel channel spacing spacing is set at 0.2 nm
When the channel spacing is increased to 0.2 nm, the result obtained from the simulation is depicted in Figure 5.4. The interfering wavelengths generated around
47 the original two wavelength system are 1549.8 1549.8 nm and 1550.4 nm, thereby the power of the each FWM sideband is approximate approximately ly -61 dBm.
Figure 5.4
Optical Optical spectrum spectrum at the the output output of the fiber fiber when when channel channel spacing spacing is set set at 0.2 nm
Similarly, Figures 5.5 shows the signal at the input channel when the channel spacing spacing is increased increased to 0.5 nm.
Figure 5.5
Optical Optical spectrum spectrum at the the input input of the the fiber when channel channel spacing spacing is set at 0.5 nm
Figure 5.6 shows the interfering wavelengths generated around the original two wavelength system of 1549.5 nm and 1551 nm; thereby the power of each FWM sideband is approximately -71 dBm.
48
Figure 5.6
Optical Optical spectrum spectrum at the the output output of the fiber fiber when when channel channel spacing spacing is set set at 0.5 nm
Therefore, Therefore, as the spacing between channels channels is increased the effect of the FWM is decreased decreased
5.3.2 5.3.2
Effect Effe ct of Differ Different ent Power Power Level Level of the Signals Signals Sources Sources
In the following process, the power level of the input sources was varied from 20 dBm to -10 dBm with step -10 dBm while other parameters such as the dispersion and the effective area were kept unchanged.
The result obtained from the simulation when the input source power is set at 20 dBm is depicted depicted in Figure 5.7.
49
Figure 5.7
Optical Optical spectrum spectrum at the the output output of the fiber fiber when when input input power power is set at at 20 dBm
The result obtained from the simulation when the input source power is set at 10 dBm is depicted depicted in Figure 5.8.
Figures 5.8
Optical Optical spectrum at at the output output of the fiber when when input input power is set at 10 dBm
The result obtained from the simulation when the input source power is set at -10 dBm is depicted depicted in Figure Figure 5.9.
50
Figures 5.9
Optical Optical spectrum spectrum at the output output of the the fiber when input input power power is set at -10 dBm
From the the results, given it is clear that when the the power level is increased increased to 20 dBm the effect of the FWM becomes very severe as shown in the Figure 5.7. As the power level of the signal sources is decreased to -10 dBm the FWM becomes less effectiv effective, e, as shown shown in the Figure Figure 5.9, 5.9, therefo therefore, re, the FWM becomes becomes signif significan icantly tly effective effective at high optical power levels.
5.3.3 5.3.3
Effect Effe ct of Increase Increase Dispersion ispersion of the Fiber Fiber Optic
The disper dispersio sion n paramet parameter er of fiber fiber optic optic was change changed d from from 1.0 ps/nm/ ps/nm/km km to16.75 ps/nm/km, at input power of 0 dBm. The results were taken at the end of the fiber optic.
Simulation Simulation results at dispersion dispersion of 16.75 ps/nm/km ps/nm/km at input input power of 0 dBm is shown in Figures 5.10.
51
Figure 5.10
Optical Optical spectrum at at the output output of the optical optical when the dispersio dispersion n of fiber optic is set at 16.75 16.75 ps/nm/km
The results obtained at the end of the fiber when the power level is set at 0 dBm and the dispersion is set at 16.75 ps/nm/km as shown in Figure 5.10, was comp compare ared d with with the the resul resultt obtai obtaine ned d at the the same same powe powerr leve levell and and disp dispers ersio ion n of 1 ps/nm/km as shown in Figure 5.4, these result show that the FWM products were reduced when the dispersion parameter is increased. It is important to mention that the the disp dispers ersio ion n param paramet eter er can not be set set at too too high high valu valuee becau because se it does does brin bring g limitation in bandwidth in the WDM model.
5.4
Simulation Simulation Results Results with with the External External Modulat Modulated ed Signal
In this this sim simulat ulatio ion n two two CW lase lasers rs were were used sed as sig signals nals sou sources rces,, the the frequencies were set at 1550 and 1550.1 nm, as shown in Figure 4.1, where as the power was set at 0 dBm, due to the interest in measuring only the total power of the sideband frequencies, where the shape of the spectrum is not required. The input signals have propagated through 25 km of nonlinear fiber.
52
5.4.1 5.4.1
Effect Effe ct of Channel Channel Spacing Spacing varia variation tion
Figure 5.11 shows the signal at the input channel when the channel spacing is set at 0.1 nm.
Figure 5.11
Optical Optical spectrum at at the input input of the fiber when when the channel channel spacing is is set at 0.1 nm
The result obtained from the simulation is depicted in Figure 5.12. The FWM effect is not quite obvious because the external modulation produce sideband.
53
Figure 5.12
Optical Optical spectrum at at the output output of the fiber when when the channel channel spacing spacing is set at 0.1 nm
Figure 5.13 shows the signal at the input channel when the channel spacing is set at 0.2 nm.
Figure 5.13
Optical Optical spectrum at at the input of the the fiber when when the channel channel spacing spacing is set at 0.2 nm
From From Figu Figures res 5.14 5.14,, the the FW FWM M effect effect is quite quite obvi obviou ouss when when the the chan channe nell spacing is increased to 0.2. The power of the FWM sideband is approximately -72 dBm
54
Figure 5.14
Optical Optical spectrum spectrum at the output output of the fiber fiber when the the channel spacing spacing is set at 0.2 nm
Figure 5.15 shows the signal at the input channel when the channel spacing is set at 0.5 nm.
Figure 5.15
Optical Optical spectrum at at the input input of the fiber when when the channel channel spacing spacing is set at 0.5 nm
Also in Figures 5.16, the FWM effect is quite obvious when the channel spacing is increased to 0.5 nm. The power of the FWM sideband is approximately 87 dBm
55
Figure 5.16
Optical Optical spectrum at at the output output of the fiber when when the channel channel spacing is set at 0.5 nm
Therefore, Therefore, as the spacing between channels channels is increased the effect of the FWM is decreased decreased
5.4.2 5.4.2
Effect Effe ct of Differe Different nt Power Power Level Level of the Signals Signals Sourc Sources es
In the following process, the power level of the input sources was varied from 20 dBm to -10 dBm with step -10 dBm while other parameters such as the dispersion and the effective effective area were kept unchanged. unchanged.
The result obtained from the simulation when the input source power is set at 20 dBm is depicted in Figure 5.17.
56
Figure 5.17
Optical Optical spectrum at at the output output of the fiber when when input input power is set at 20 dBm
The result obtained from the simulation when the input source power is set at 10 dBm is depicted in Figure 5.18.
Figure 5.18
Optical Optical spectrum spectrum at the output output of the fiber fiber when input input power power is set at 10 dBm
The result obtained from the simulation when the input source power is set at -10 dBm is depicted depicted in Figure Figure 5.19.
57
Figure 5.19
Optical Optical spectrum at at the output output of the fiber fiber when input input power power is set at 10 dBm
From the the results, given it is clear that when the the power level is increased increased to 20 dBm the effect of the FWM becomes very severe as shown in the Figure 5.17. As the power level of the signal sources is decreased to -10 dBm the FWM becomes less effectiv effective, e, as shown shown in the Figure Figure 5.19, 5.19, therefo therefore, re, the FWM become becomess signif significan icantly tly effective effective at high optical power levels.
The new generated mixing products have high possibilities of falling directly on the original signal, which produce crosstalk.
5.4.3 5.4.3
Effect Effe ct of Increase Increase Dispersion ispersion of the Fiber Fiber Optic
Simulation Simulation results with the use of the external external modulated laser at dispersion dispersion of 16.75 ps/nm/km ps/nm/km at input power of 0 dBm is shown in Figures 5.20.
58
Figure 5.20
Optical Optical spectrum at at the output output of the fiber when when input input power is set at 0 dBm
The results obtained at the end of the fiber when the power level is set at 0 dBm and the dispersion is set at 16.75 ps/nm/km as shown in Figures from 5.20. were compared compared with the result obtained obtained at the same power level and dispersion dispersion of 1 ps/nm/km as shown in Figure 5.12, these result show that the FWM products were reduced when the dispersion parameter is increased. It is important to mention that the the disp dispers ersio ion n param paramet eter er can not be set set at too too high high valu valuee beca becaus usee it does does brin bring g limitation in bandwidth in the WDM model.
5.4.4 5.4.4
Effect Effe ct of of Incre Increase ase Effect Effective ive Area of the Fiber Fiber Optic
Simulation Simulation results with the use of the external modulated modulated laser at effective effective area of 76.5 μm 2 at input input power of 0 dBm are shown in Figure 5.21.
59
Figure 5.21
Optical Optical spectrum at at the output output of the fiber when when the effective effective area of 2
the fiber optic is set at 76.5 μ m
Results obtained at the end of fiber where the power level is set at 0 dBm, and 2
the effective area is increased to 76.5μ m is shown in Figure 5.21 is compared with Figure 5.12 which the effective area is set at 64 μ m2. It is found that the increasing of the effective area can reduce the FWM effect.
5.5
Simulation Simulation of Four Wave Wave Mixing Mixing for Higher Higher Number Number of Channels Channels
This section presents presents the simulation simulation results as the number of channels is increased increased to four in the simulation simulation model, with or without without the use of external external modulated laser.
60 5.5.1 5.5.1
Simulation Simulation Results Results for for Four Signal Signal Source Source witho without ut External External Modulated Modulated Signal
The simulation results for four channels, without use of external modulated laser, Figure 5.22 shows input signal when number of channels is increased to four and the channel spacing is set at 0.1 nm.
Figure 5.22
Four optical optical spectrum spectrum at the intput intput of the fiber fiber when the the channel spacing is set at 0.1 nm
The The resul resultt obta obtain ined ed from from the the simul simulati ation on when when the the numb number er of chan channe nell is increased increased is depicted depicted in Figure Figure 5.23. The number number of FWM also is increased increased
61
Figure 5.23
Four output output optical spectrum spectrum channe channels ls when the the channel channel spacing spacing is set at 0.1 nm
The The resul resultt obta obtain ined ed from from the the simul simulati ation on when when the the numb number er of chan channe nell is increased and the channel spacing is set at 0.5 nm is depicted in Figure 5.24. The number number of FWM also is increased
Figure 5.24
Four output output optical optical spectrum spectrum channels channels when when the the channel channel spacing spacing is set at 0.5 nm
62 5.5.2 5.5.2
Simulation Simulation Results Results for for Four Signal Signal Source Source with with External External Modulate Modulated d Signal
The simulation results for four channels, when using External modulated Laser, at different channel spacing..
Figure 5.25 shows input signal when number of channels is increased to four and the channel spacing is set at 0.1 nm.
Figure 5.25
Four Input Input optical optical spectrum spectrum channels channels when when the channel channel spacing is set at 0.1 nm
The The resul resultt obta obtain ined ed from from the the simul simulati ation on when when the the numb number er of chan channe nell is increased and the channel spacing is set at 0.1 nm is depicted in Figure 5.26. The number number of FWM is also increased.
63
Figure 5.26
Four output output optical optical spectrum channels channels when the the channel channel spacing spacing is is set at 0.1 nm
The The resul resultt obta obtain ined ed from from the the simul simulati ation on when when the the numb number er of chan channe nell is increased and the channel spacing is set at 0.5 nm is depicted in Figure 5.27. The number number of FWM is also increased but with less effect.
Figure 5.27
Four output output optical spectrum spectrum channe channels ls when the the channel channel spacing spacing is set at 0.5 nm
64 5.6 5.6
Dis Discuss cussio ion ns
Based on the results presented, The FWM effects increase as the number of chan channe nels ls is incre increase ased. d. The The numb number er of spurio spurious us signa signals ls due due to FW FWM M incre increase ase geometrically and given by
3
2
M= (N -N )/2
(5.1)
where N is the number of channels and M is the number of the newly generated sidebands. sidebands. The new generated mixing products have high possibilities possibilities fall directly on the original signal, this could produce crosstalk.
Therefore, as the spacing between channels is reduced or remained equal the effect of the crosstalk is found to become greater. When the spacing between the channels is unequal, showed that the mixing products have low power level and highly possible not to falls on the original signal, which makes them easy to be filtered, filtered, and in turn improve improve the system performance.
Results obtained at the end of fiber where the power level is set at 0 dBm, and 2
the effective area is increased to 76.5μm are shown in Figures 5.10. It is found that the OSNR obtained is better than before increasing the effective area as shown in Figure 5.4.
As general, the increase of the effective area can reduce the FWM effect and give higher OSNR value compared to the simulation simulation result obtained with the same power level.
The The effect ffectiv ivee area area refer referss to the the equi equiva vale lent nt area area of the the fiber fiber in whic which h the the optic optical al powe powerr is trans transmi mitte tted. d. In the the case case of sing single le mode mode fiber, fiber, this this is roug roughl hly y proportional to the core area. a rea. Fiber with a large effective area offers reduced optical power density, which raises ra ises the power threshold for the FWM penalties. penalt ies. In addition, addition, the effective effective area parameter and the dispersion parameter can be used to calculate calculate the FWM efficiency as follows: follows:
65 η = n2 /(A eff x D x λ 2 )
(5.2)
where η is the FWM efficiency, n 2 is the nonlinear nonlinear index coefficient, coefficient, Aeff is the effective area, D is the dispersion and λ is the spectral width.
5.7 5.7
Anal Analyt ytic ical al Mode Modell llin ing g
Matlab based program program has been developed developed using Equations 4.4 to 4.6 in order to design analytical model (Appendix A), which assists to predict the expected FWM power in different channel spacing. The designed model can give the expectation value of the FWM power in different different input signal signal power level. The analytical results have been compared compared to the results obtained obtained from the numerical simulation, as shown in Figures 5.28 and 5.29.
-90
-95
-100 ) m B -105 d ( r e w o p M-110 W F -115
-120
-125
5
10
Figure 5.28
15
20
25
30
Power per channel channel vs. FWM FWM power power
35
40
66
-5 5 A na ly ti c al s im u la ti o n n u m e r i c a l s i m u la la t i o n -6 0
) m -6 5 B d ( r e w o p M -7 0 W F
-7 5
-8 0 0.1
0.15
0.2
Figure 5.29
0.25
0.3
0.35 0.4 channel spacing
0.45
0.5
0. 55
0.6
Channel spacing versus FWM power
These results show that when power per channel is increased the spurious power increase, too. The power of the FWM produced is found to be inversely proportional to the square of the channel spacing, when all channels have the same input power. Furthermore, Furthermore, the FWM effects increase exponentially exponentially as the level of the optical optical power from the signal sources sources is increased, as shown shown in the Figure Figure 5.28
Based on results presented, it is clear that when the channel spacing is smaller the FWM effect becomes more significant due to the phase matching, as shown in Figure 5.29.
5.8 5.8
Four Four Wave Wave Mixi Mixing ng Redu Reduct ctio ion n
One way to combat the FWM process is to use unequal channel spacing, so that the mixing products do not coincide with signal frequency, and to use low input power, or high effective ef fective area. Fiber dispersion management is a very effective e ffective way, helpful helpful not for FWM but also is the case of other nonlinear nonlinear phenomena, phenomena, that degrade degrade
67 transmission transmission performance performance in the fiber, also FWM can be mitigated by increasing the effective area of the fiber [19].
5.8.1 5.8.1
Effect Effe ct of Unequal Unequal Channels Channels
Figure Figure 5.30 shows input signal when the channel spacing is unequal. unequal.
Figure 5.30
Optical Optical spectrum at at the input input of the fiber when when the the channel spacing spacing is unequal
When the spacing between the channels is unequal, showed that the mixing products have low power level and highly possible not to falls on the original signal, which makes them easy to be filtered, and in turn improve the system performance. As shown Figure 5.31.
68
Figure 5.31
Optical Optical spectrum at at the output output of the fiber with with unequal unequal channel channel spacing
5.8.2 5.8.2
Effect Effe ct of of Incre Increase ase Effect Effective ive Area of the Fiber Fiber Optic
2
The effective area parameter of fiber optic has been changed from 64 μ m to 76.5 μm 2 at the the power level level set at 0 dBm. The result resultss were taken taken at the end of the fiber optic. Simulation results at effective area of 76.5 μm2 at input power of 0 dBm are shown in Figures 5.32.
Figure 5.32
Optical Optical spectrum at at the output output of the fiber when the the effective area of of the fiber optic is set at 76.5 μ m2
69 Results obtained at the end of fiber where the power level is set at 0 dBm, and 2
the effective area is increased to 76.5μ m s shown in Figure 5.32. It is found that the increasing increasing of the effective effective area can reduce the FWM effect.
70
CHAPTER 6
CONCLUSIONS AND RECOMMENDATIONS
6.1
Conclusion ion
Future Future wireles wirelesss systems systems it will will be targetin targeting g towards towards provid providing ing broadb broadband and access and personal area multimedia services to large number of subscribers. Radio over over fiber fiber (RoF) (RoF) networ network k accompa accompanie nied d with with wavelen wavelength gth divisi division on multip multiplex lexing ing (WDM (W DM)) can prov provid idee a simpl simplee topo topolo logy gy,, easier easier netw networ ork k mana manage geme ment nt,, and and an increased capacity by allocating different wavelengths to individual remote nodes. The The perf perfor orma manc ncee of WDM WDM netw networ orks ks is stro strong ngly ly infl influe uenc nced ed by nonl nonlin inea eari rity ty characteristic inside the fiber. Therefore the nonlinearity effects of fiber optics pose additional limitation in WDM systems.
It is well known that FWM in WDM for RoF signals are mostly generated by non-degenera non-degenerate te FWM process regardless regardless of the number number of input signals. signals. In this study only two and four input signals were launched launched into the optical fiber. The FWM effect has been investigated investigated analytical analytically ly and numerically numerically simulated. Simple equations to determine the spectral linewidth, the FWM power due to channel spacing and the power of the FWM components due to the input power have been deduced.
The The nume numeri rica call simu simula lati tion on resu result ltss obta obtain ined ed have have show shown n the the spect pectra rall char charac acte teri rist stic icss of the the FW FWM M in WDM WDM for for RoF RoF where where the the effe effect ctss of FW FWM M are are pronounced with decreased channel spacing of wavelengths or at high signal power levels.
71 The numerical simulation model results and the analytical model results were compared. The numerical simulated results clearly demonstrate that the degradation due to FWM can be minimized by ensuring that the phase matching does not occur. This This has been been achiev achieved ed by increas increasing ing the channe channell separat separation ion and supply supplying ing low signal power level. The high effective area is also found to the decrease FWM effect. It is noticed that the FWM also causes inter-channel cross talk for equally spaced WDM channels. Thus, FWM can be mitigated using unequal channel spacing.
It could be concluded concluded that results obtained obtained from this study will provide useful info inform rmat atio ion n for for iden identif tifyi ying ng the the fund fundam amen enta tall limit limit of the the capa capacit city y of the the WDM WDM systems.
6.2
Recomm Recommend endati ations ons for Future Future Work Work
FWM FW M in WDM WDM for for RoF RoF effect effectss are likely likely to beco become me the the main main sour source ce of performance degradation in contemporary and future fiber optical communications, therefore future studies in attempt to overcome such problems, the following could be recommended. r ecommended.
Inve Invest stig igati ation on of FW FWM M effec effectt usin using g more more than than eigh eightt sour sources ces is essen essenti tial al because most technologies nowadays use DWDM in order to meet the huge capacity demands
Crosstalk Crosstalk is the transfer of power from one channel to another, can occurs due to nonlinear effect. FWM can produce crosstalk between wavelength channels. This crosstalk is strongly dependent dependent on channel separation separation and optical optical power. Therefore it is important to estimate how large the cross talk is.
72 7.
References:
[1]
Hamed Al-Raweshidy, “ Radio over Fiber Technologies for Mobile Communications Networks” Networks ” , Artech House, 2002.
[2] [2]
Guo, Guo, Y., Y., Kao Kao,, C. C. K. K. and and Chia Chiang ng,, K. K. S. S. “ Nonlinear “ Nonlinear photonics: nonlinearities in optics, optoelectronics and fiber communications .” Springer-Verlag, 2002. Berlin, Germany.
[3]
Govind P.Agrawal, “ Fiber-Optic communication system” system” McGraw McGraw-Hi -Hill ll December 2001.
[4]
Robert , C.
E. “ Opti Optica call
Netw Networ orki king ng A
begi begine ner’ r’ss
guid guidee”
McG McGraw raw-
Hill/Osborn Hill/Osbornee 2004 [5] [5]
Yann Yannis, is, L. G. G. “New optic optical al Micro Microwa wave ve Up-C Up-Con onve versi rsion on Solu Solutio tion n in Radio Radio over Fiber Network” Journal of lightwave technology, technology , 24 (3) (2006) 12771282
[6]
Antti Lamminpää, “ Measurement of nonlinearity of optical fiber.” fiber. ” Master thesis, thesis, Helsinki University of Technology, Technology, 2003.
[7]
D. Marcuse, Marcuse, “Effect “Effect of Fiber Fiber Nonlin Nonlineari earity ty on Long Long-Distan -Distance ce Transm Transmissi ission” on”,, Journal of lightwave technology, technology, 9 (1) (1991) 121-128. 121-128.
[8] [8]
Remi Remig gius ius Zeng Zenger erle le,, “ Modeling of Nonlinear Phenomena in Optical Multichannel channel Trasmission Trasmission system” system” Master Master thesis, thesis, Univer University sity of Kaisers Kaiserslau lautern tern,, 2005.
[9]
A. V. V. Yulin Yulin,, D. V., V., and and P. St. J. Russe Russell ll “Four “Four-wa -wave ve mixi mixing ng of of linear linear waves waves and soliton solitonss in fibers fibers with with higher higher-or -order der disper dispersio sion” n” Journal Journal of lightw lightwave ave technology, technology, 29 (3) (3) (2004) (2004) 950-955. 950-955.
[10 [10]
Aju Ajung Kim, Kim, You Young Hun Hun Joo, Joo, and Yun Yungso gsoo Kim, im,
“60G “60GHz Hz Wire Wirele less ss
Communicatio Communication n Systems Systems with Radio-over-Fiber Radio-over-Fiber Links for Indoor Indoor Wireless LANs” Journal Journal of lightwave lightwave technology, technology, 20, (4), (2004 ( 2004)) 517-521. 517-521. [11] [11]
Anth Anthon ong g No’ No’om oma, a, “ Radio over Fiber Technology for Broadband wireless communication systems” systems ” Master thesis, Eindhoven University of Technology, 2005.
[12] [12]
Takuo Takuo Tanemur Tanemuraa and Kazuro Kazuro Kikuch Kikuchi, i, “Unifie “Unified d analysis analysis of modulatio modulational nal instability induced by cross-phase modulation in optical fibers” Journal of lightwave technology, technology, 20 (12) (2003) 2502-2513.
73 [13] [13]
Caiqin Caiqin Wu and and Xiupu Xiupu Zhang, Zhang, “Imp “Impact act of Nonli Nonlinear near Disto Distortio rtion n in Radio Radio Over Over Fibe Fiberr
Syst System emss
Subcar carrie rier
With With
Sing Single le-S -Sid ideb eban and d
and and
Tand Tandem em
Sing Single le-S -Sid ideb eban and d
Modulat lation ions ” Journal of lightwave technology, technology , 24 (5) (2006)
2076 – 2090. [14]
M. J. Ablowi Ablowitz tz and G. Biondin Biondini, i, “Four-wave “Four-wave mixing mixing in wavelen wavelength gth– – division division multiplexed soliton systems: ideal fibers”, Journal of Optical . Soc. Am. Am. 14 (7) (1997) 1788 – 1794.
[15]
Hiroshi Hiroshi F., F., Koji Koji Y., Tetsufum Tetsufumii S, and Sei-ichi Sei-ichi Itabashi Itabashi “Four-wav “Four-wavee mixing mixing in silicon wire Waveguides Waveguides ” Optics Express 13 Express 13 (12) (2005) (2005) 4629-4637 4629-4637..
[16]
J. Zhou, Zhou, R. R. Hui, Hui, and N. Caponio, Caponio, “Spectral “Spectral Linewid Linewidth th and and Frequency Frequency Chirp of Four-Wave Mixing Components in Optical Fibers”, Photonics Technology Lelters, Lelters, V 6, (3) (1994) (1994) 434-436. 434-436.
[17]
Y. S. S. Jang and Y. Y. C. Chung, Chung, “Four “Four-Wave -Wave Mixing Mixing of Incohe Incoherent rent Light Light in a Dispersion-Shifted Fiber Using a Spectrum-Sliced Fiber Amplifier Light Source” Photonics Source” Photonics Technology Letters, Letters , 10 (2) (1998) 218-220.
[18]
S. Betti, Betti, M. M. Giaconi, Giaconi, and M. M. Nardini, Nardini, “Effect “Effect of Four-Wave Four-Wave Mixing Mixing on WDM WDM Optical Systems: A Statistical Analysis”, Photonics Analysis”, Photonics Technology Letters, Letters , 15 (8) (2003) 1079-1081.
[19]
Vrizlynn Vrizlynn L. L. Thing, Thing, P. Shum, Shum, and and M. K. Rao, Rao, “Bandwi “Bandwidth dth-Effici -Efficient ent WDM WDM Channel Allocation for Four-Wave Mixing-Effect Minimization”, Transactions On Communications , 52 (12) (2004) 2184-2089.
[20] [20]
Ivan Ivan Kami Kamino now, w, and and Ting Tingye ye Li “ Fiber Optic Telecommunication ”, Academic Press 2002.
[21]
M. L. F. Abbade, Abbade, and E. A. A. M. Fagotto Fagotto,, R. S, “Optical “Optical Amplitude Amplitude Multiplexing Through Four-Wave Mixing in Optical Fibers” Photonics Technology Letters, Letters, 17 (1) (2005) (2005) 151-15 151-153. 3.
[22]
Ooi Sock, “ Four Wave Mixing Nonlinearity Effect in Wavelength Division Multiplexing System for Radio Over Fiber.” Fiber.” Bachelo Bachelorr thesis, thesis, Univer University sity Technology Malaysia, 2007.
[23 [23]
A. V. Ramp Rampra rasa sad d, and and M. Meen Meenak aksh shi, i, “Fou “Fourr Wave ave Mix Mixing ing on Den Dense wavelength division Multiplexing Optical system” Academic Open Internet Journal , 17 (1) (1) (2006) (2006) 1-8.
74
APPENDIX A
MATLAB PROGRAM FOR FWM POWER
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % this progr program am is used to compute power per channel versus FWM power % and to compute channe spaing versus FWM channel %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% variables % % X = third order susceptibility susceptibility % lemda = wavele wavelength ngth in vacuum % c = speed og light in vacuum % Aeff= effecive area of the optical fiber % n= nonli nonlinear near reflective inex % alfa = fiber loss % D= degene degeneracy racy factor % eff = FWM efficiency % Leff = effctive length % x=6*10^-15; lemda=0.5*10^-6; c=3*10^8; Aeff=6.4*10^-11; n=1.48;
75 alfa=.0461; eff=.05; Leff=22*10^3; D=3;
k=(32*(pi)^3*x)./(n^2.*lemda*c)*(Leff/Aeff)
P=eff*(D.*k).^2*(1*10^-3)^3*exp(-alfa*75)
Pdb= 10*log10(P/10^-3) 10*log10(P/10^-3)
plot(x,y) hold on hold on y1 = [-59.5 -61.2 -61.2 -65.5 -68 -72.5 -80]; plot(x,y1,'r' plot(x,y1, 'r'))
