Electronic Compensation For Intramodal Dispersion
International Journal of Engineering Research & Technology (IJERT)ISSN: 2278-0181Vol. 3 Issue 4, April - 2014Electronic Compensation for IntramodalDispersionArchana CProf. Helen MascreenProfessor, Opto Electronics and Communication SystemsDept. of Electronics and Communication Engg,TKM Institute of Technology, Kollam, Kerala, India.Abstract- Optical fiber communication is a way of transmitting theinformation from one place to another by modulating the lightsignal with the information signal. Optical fiber communicationsystems primarily operate at wavelengths near 1.55 μm in orderto coincide with the minimum loss point of optical fiber andthereby maximizing the transmission distance. But, at thiswavelength, there is a significant amount of group velocitydispersion (GVD) that limits the achievable propagation distance.Dispersion in a single mode fiber (SMF) is known as intramodaldispersion or chromatic dispersion (CD) which results in pulsespreading and causes intersymbol interference (ISI). Coherentdetection employing multilevel modulation formats has becomeone of the most promising technologies for next generation highspeed transmission systems due to the high power and spectralefficiencies. Using the powerful digital signal processing (DSP),coherent optical receivers allow the significant equalization ofintramodal dispersion, Intramodal dispersion only causes ISI, butit also introduces a power penalty, which can cause degradation ofthe system's SNR. In this paper, electronic dispersioncompensation (EDC) achieved with the help of DSP. Thesimulation has to be carried out using optisystem v 12Keywords: Dispersion, intersymbol interference, group velocitydispersion, intra modal dispersion, single mode fiber, signal tonoise ratio, electronic dispersion compensationtelecommunications carriers prefer to increase the capacity oftheir existing fiber links by using dense wavelength-divisionmultiplexing (DWDM) systems and/or higher bit ratessystems. Most of the installed optical fibers are old and exhibitphysical characteristics that may limit their ability to transmithigh-speed signals. An information signal becomes distorteddue to attenuation and dispersion as it travels in an opticalfiber. Dispersion is the spreading in the time domain of asignal pulse as it travels through the fiber. Both attenuation anddispersion affect repeater spacing in a long distance fiber-opticcommunication system. Dispersion affects the bandwidth ofthe system, hence maintaining low dispersion is of equalimportance for ensuring increased system informationcapacity, versatility and cost effectivenessDifferent types of optical fibers have differentdispersions. For a single-mode optical fiber, the only source ofdispersion is due to group velocity dispersion (GVD). CD isthe destructive forces for pulse propagation in ultra high-bitrate optical transmission system and cause power penalty. Inorder to improve overall system performance influenced by thedispersion, several dispersion compensation technologies wereproposed. Intramodal dispersion compensation in optical fiberCommunication systems is an open issue. In the case ofdispersion-uncompensated metropolitan and regional networks,transmission over standard single mode Fiber (SMF) at 10Gb/s is strongly limited to about 80–100 km .For conventionalnon return-to-zero (NRZ) intensity-modulated, Direct-detected(IM-DD) signals. Dispersion compensation can be generallyachieved by either optical or electrical techniques. Opticaltechniques, such as the use of dispersion-compensating fibers(DCF) or chirped fiber Bragg gratings, are generally expensiveand not easily reconfigurable for varying dispersion conditions.In this project, Intra modal dispersion compensated with thehelp of dsp. More compact electronic components should beused in order to increase system performance and achievebetter integrationIJERTPG Scholar, Opto Electronics and Communication Systems,Dept. of Electronics and Communication Engg,TKM Institute of Technology, Kollam, Kerala, India.I.INTRODUCTIONNowadays communication systems use optical wavesas carriers; hence, the bit rate-distance product BL can beimproved several orders of magnitude compared to othersystem. The most appropriate media that are used as channelsin these systems are optical fibers. The first-encounteredproblem was that available optical fibers during that time hadextremely high loss, exceeding 1000 dB/km. This problemchallenged the researchers and engineers to find processes bylow-loss optical fibers could be fabricated. Finally, solved in1970 when optical fibers having acceptable attenuation werefirst made. The combination of low-loss optical fibers and theadvance in semiconductor technology made optical fibercommunication systems practically possible.In 1978, the first systems were commerciallydeployed. Their operating wavelength was at 0.8 μm, and theywere capable of carrying data at the bit rate of 50–100 Mb/swith a repeater spacing of approximately 10 km. As thetechnologies in optical-fiber fabrication as well as systemcomponents progressed, the BL product was increasedcontinually. Telecommunications service providers have toface continuously growing bandwidth demands in all networksareas, from long haul to access. Because installing the newcommunication links would require huge investments,IJERTV3IS040252II.SYSTEM ARCHITECTUREThe system can be divided into five main parts: DP-QPSKTransmitter, Transmission Link, Coherent Receiver, DigitalSignal Processing, and Detection& Decoding. The blockdiagram of dsp based intramodal dispersion compensation isgiven belowwww.ijert.org153
International Journal of Engineering Research & Technology (IJERT)ISSN: 2278-0181Vol. 3 Issue 4, April - 2014Detection&Decodingcoherent receiver, these functions are implemented in theelectrical domain leading to a dramatic reduction incomplexity. Furthermore, since coherent detection maps theentire optical field within the receiver bandwidth into theelectrical domain it maximizes the efficacy of the signalprocessing. This allows impairments, which have traditionallylimited 40Gbit/s systems to be overcome, since both chromaticdispersion and polarization mode dispersionTransmission linkTransmitterPRBSDSPCoherent receiverververRECEIVERFigure 1 Block diagram of dsp based EDCThe most advanced detection method is coherentdetection where the receiver computes decision variables basedon the recovery of the full electric field, which contains bothamplitude and phase information. Coherent detection thusallows the greatest flexibility in modulation formats, asinformation can be encoded in amplitude and phase, or both inphase (I) and quadrature (Q) components of a carrier. Coherentdetection requires the receiver to have knowledge of the carrierphase, as the received signal is demodulated by a LO thatserves as an absolute phase reference In direct detection asshown in Figure (2), in an opt electrical photo detector (aphotodiode) the light intensity 𝐸 2 is converted in an electricalsignal and the phase information is totally lost.III.DSP FOR DP-QPSKThis is used for linear impairment of the fiber for 16 QAMmodulation format through the following modulation format Analog to digital conversion(down sampling) Dispersion compensationAnalog to digital conversion is a down sampling process. Herewe have select two bit per sampling. However, the samplingrate can be changed. For intramodal dispersion in the absenceof nonlinearity the optical fiber can be modulated as a phaseonly filter with the following transfer function𝐷𝜆 2 𝑍G (𝑧, 𝜔) exp (-j4𝜋𝐶𝜔2 𝑗𝑆 𝜆 4 𝜔 3 𝑧24𝜋 2 𝐶 2)Input optical signalIn which the first part is the effect of fiber dispersion and thesecond term is the dispersion slope for multi-channelapplications. To compensate for the dispersion, multiply theoutput field with the inverse of the channel transfer function(FIR filter). The order of the filter increases as the amount ofdispersion (length of the propagation) increases.PBSIJERTPhoto detectorPhoto detectorFigure 2 Direct detectionAn alternative way to detect the optical signal is coherentdetection, in which the received signal is mixed with local laserbeing detected in the photodiode, and two detectors and properphase delays are used, both amplitude and phase can bepreserved as shown in Figure (3)Input optical signal𝐸𝑥 ,𝐸𝑦PBSsRe (𝐸𝑥 )PHOTO DIODEPHOTO DIODEopticallocaloscillatorPHOTO DIODEEt.𝑜PHOTO DIODERXH(ω)Figure 4 DSP for DP-QPSKTXH(ω)IV.𝐻 1 (𝜔)SIMULATION OF ELECTRONICCOMPENSATIONThe system set up is established using optisystem V12 software. Simulation diagram is shown below. Systemdivided in to five section .transmitter, optical link, receiver,amplification and filtering unit, decoding and detecting unitIm𝐸𝑋Re(𝐸𝑥 )Im𝐸𝑦Figure 3 Coherent detectionWhile coherent detection was experimentally demonstrated asearly as 1979, its use in commercial systems has been hinderedby the additional complexity, due to the need to track the phaseand the polarization of the incoming signal. In a digitalIJERTV3IS040252www.ijert.orgFigure 5 Simulation of DSP based EDC154
International Journal of Engineering Research & Technology (IJERT)ISSN: 2278-0181Vol. 3 Issue 4, April - 201440 Gbps PRBS signal is passed in to a serial to parallelconverter. There the input sequence at bit rate R into twooutput sequence R/2 bit rate and Modulated into twoorthogonally polarized QPSK optical signals by two QPSKmodulators, Bit rate is 40 Gb/s, Sample rate is 1.28 1012 Hz,and Wavelength is 1550 nm .Figure 6 shows the QPSKmodulator.Figure 8 Structure of QPSK receiverIJERTFigure 6 QPSK modulator.The four output signals form Optical Coherent DP-QPSKReceiver are I and Q of the two polarizations(X,Y), whichhave the full information of transmitted signal can berepresented as Output X-I, Output X-Q, Output Y-I andOutput Y-Q. These received electrical signals are thenamplified with a set of four electrical amplifier having gain 15dB .After amplification the signals are passed throughLow Pass Gaussian filters for eliminating the frequenciesabove required band. After the four signals are amplifiedand filtered, they passed in to the DSP unit for intramodaldispersion compensation. The four signals enter the DSPfirst they are converted to digital domain for processingthen the fiber dispersion is compensated using a simpletransversal digital filter. In the absence of fibernonlinearity, the fiber optic could modeled as a filter withthe transfer function as given in EquationOutput from the serial to parallel converter fed into phase shiftkeying generator. There m ary is generated. The bandwidthrequired for transmission of binary digital waveforms may bevery large. In particular, for a channel of bandwidth B Hz theNyquist rate is 1/T 2B symbols per second. In the case ofbinary signaling each symbol carries one bit of information, sothe information rate is limited to 2B bits per second. Clearlyone can increase the information rate through a channel byincreasing the bandwidth and the associated symbol rate.However, if the channel bandwidth is to remain fixed, then theonly option is to increase the amount of information encodedin a symbol. M-ary signaling provides a means of achievingthis:G (𝑧, 𝜔) exp(-j𝐷𝜆 2 𝑍4𝜋𝐶𝜔2 𝑗𝑆 𝜆 4 𝜔 3 𝑧24𝜋 2 𝐶 2)After the digital signal processing is completed, the signalis sent to the detector and decoder unit and output isanalyze with the help of a eye diagram analyzer andspectrum analyzer.Figure 6 M-ary signalingV.After that, it passes through M-ary Pulsesignal ismodulated by Lithium Niobate Mach-Zehnder Modulator andcombined Together to form the QPSK signal.This QPSK feedinto optical transmission link. This link consists of 60 km fiber,amplifier and optical filter. After passing 60 km the signal, getdistorted due to intramodal dispersion .after that the signal isreceived with the help of a QPSK receiver. Figure 8 shows thestructure of QPSK receiverIJERTV3IS040252RESULTS AND DISCUSSIONFor analyzing intramodal dispersion compensationusing EDC with the help of DSP, first 8-bit data transmitwith the help of PRBS. After passing through the opticalfiber (60 km –dispersion length) the data are distorted anddelay occurs. After that, the signal is passed into DSP unitand compensated output is obtained with the help ofwww.ijert.org155
International Journal of Engineering Research & Technology (IJERT)ISSN: 2278-0181Vol. 3 Issue 4, April - 2014spectrum analyzer. Transmitted and received signal shownin figure 9In optical communication systems, optical signal to noise ratio(OSNR) could not accurately measure the system performance.Typically, as a quality factor, Q is one of the importantindicators to measure the optical performance by which tocharacterize the BER. The OSNR is the most importantparameter that is associated with a given optical signal. It is ameasurable (practical) quantity for a given network, and it canbe calculated from the given system parameters. Thelogarithmic value of Q (in dB) related to the OSNRFigure 9 (a) Transmitted signal𝑄𝑑𝐵 20 log 𝑂𝑆𝑁𝑅𝐵𝑂𝐵𝐶Figure 11 shows the eye diagram of compensated output afterpassing 60 km maximum Q factor is 7.47986, minimum biterror rate is 3.70247 10 14 and eye height 0.00503885 isobtained .Figure 9 (b) signal after passing 60 km fiberFigure 9 (c) signal after compensationIJERTFigure 10 shows the optical power spectrum of thetransmitted QPSK signals .Figure 11 Eye diagram of dispersion compensation using DSP at 60kmFigure 12 shows the eye diagram of compensated output afterpassing 80 km maximum Q factor is 7.21986, minimum biterror rate is 2.09997 10 13 and eye height 0.000491224 isobtainedFigure 10(a ) optical power spectrum of the transmitted QPSK signalsI polarizationFigure 10 (b) optical power spectrum of the transmitted QPSK signalsQ polarizationIJERTV3IS040252Figure 11 Eye diagram of dispersion compensation using DSP at 80kmwww.ijert.org156
International Journal of Engineering Research & Technology (IJERT)ISSN: 2278-0181Vol. 3 Issue 4, April - 2014Table: 1 Result obtained from the simulation of microwave tunable dispersioncompensator with various distanceMaximumQfactorMinimumBit error rateEye heightThresholdDecision instant[12]60 km80 km[13]7.479867.21986[14]3.70247 10 142.09997 10 791170.46875[16][17]VI. ][10][11]IJERTIn fiber optical high bit rate long-haul transmission systems,dispersion compensation is one of the most important items tobe considered for design. Variety of linear and nonlineartransmission impairments are suffered by optical networks thisproject concentrated in intramodal dispersion .This directlyaffect the Bit Error Rate (BER) performance of the system andthat support higher data rates and large number of channels. Bysuitable system simulations, demonstrated 40 Gbpspropagation with acceptable penalties, achieved by theexperimented electronic compensator. This compensator canbe applied to compensate fiber intramodal dispersion inMetro/Regional networks characterized by distances betweennodes from 10 to 400 kmHaiqing Wei “Optimal packaging of dispersion-compensatingfibers for matched nonlinear compensation and reduced opticalnoise” optics letters / vol. 30, no. 18 / September 15, 2005Oded Raz, Ruth Rotman, “Implementation of Photonic True TimeDelay UsingHigh-Order-Mode Dispersion Compensating Fibers”IEEE photonics technology letters, vol. 16, no. 5, may 2004Tony Antony, Ashwin Gumaste “WDM Network Design”.SampleChapter is provided courtesy of Cisco Press. Date: Feb 7, 2003.R.S. Kaler,Ajay K.Sharma, T.S. Kamala “A Comparison of pre-,post- and symmetrical-dispersion compensation schemes for 10Gb/s NRZ links using standard and dispersion compensated fibers”accepted 14 May 2002.J.Mora “dynamic optical transversal filters based on a tunabledispersion fiber Bragg grating” Instituto de Ciencia de losMateriales, Universidad de Valencia 2000A. Ghatak and K. Thyagarajan, “Introduction to Fiber Optics”.Cambridge University Press,1998G. P. Agrawal, “Fiber Optic Communication Systems”. New York:John Wiley & son Inc, 1997Jing Zhang “,Optical Gain and Laser Characteristics of InGaNQuantum Wells on Ternary InGaN Substrates” Volume 5, Number2, April 2013.Xu Zhang Digital Signal Processing for Optical CoherentCommunication Systems Department of Photonics EngineeringTechnical University of Denmark. April 27th 2012Tianhua Xu DSP based Chromatic Dispersion Equalization andCarrier Phase Estimation in High Speed Coherent OpticalTransmission Systems Optics & Photonics School of Informationand Communication Technology 2012Smita S. Dabhade & Savita Bhosale “Fiber Bragg Grating AndPhase ConjugatorAs Dispersion Compensator” InternationalJournal on Advanced Electrical and Electronics Engineering,(IJAEEE), ISSN (Print): 2278-8948, Volume-1, Issue-1, 2012.MTE Kahn and ABO Mubinya, “Types of dispersions that affectsdata transmission in optical fiber cables”. Canadian Journal onElectrical and Electronics Engineering Vol. 3, No. 2, rsioninopticalCommunications” Volume VII July 2012.Yong Zhang “Development of Millimeter-Wave Radio-over-FiberTechnology” journal of electronic science and technology, vol. 9,no. 1, march 2011.Bo-ning HU!, Wang Jing! “Analysis on Dispersion Compensationwith DCF based on Optisystem”. 2nd International Conference onIndustrial and Information Systems 2010.Christophe Caucheteur, “All-fiber tunable optical delay line” opticsexpress 3093 February 2010Qingjiang Chang, Tong Ye, and Yikai Su”Generation of opticalcarrier suppressed differential phase shift keying (OCS-DPSK)format using one dual-parallel Mach-Zehnder modulator in radioover fiber systems”. Optical Society of America 2008.Leonardo Ranzani, “Microwave-Domain Analog PredistortionBased on Chirped Delay lines for dispersion compensation of 10gb/s optical communication signals”. journal of lightwavetechnology, vol. 26, no. 15, august 1, 2008.IJERTV3IS040252www.ijert.org157
PG Scholar, Opto Electronics and Communication Systems, Dept. of Electronics and Communication Engg, TKM Institute of Technology, Kollam, Kerala, India. Prof. Helen Mascreen . Professor, Opto Electronics and Communication Systems Dept. of Electronics and Communication
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Dispersion is generally divided into two individual contributions, known as material dispersion and waveguide dispersion [2]. Material dispersion is related to the wavelength dependency of the refractive index of silica (SiO2) glass, which is the host material of optical flbres used for transmission purposes. Waveguide dispersion, on the other 1
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