IMPACT OF COHERENT AND INCOHERENT CROSSTALKS AND POWER PENALTY ON THE OPTICAL CROSSCONNECTS

Transcription

1 ISSN: (LINE) ICTACT JOURNAL COMMUNICATI TECHNOLOGY, DECEMBER 011, VOLUME: 0, ISSUE: 04 DOI: /ict IMACT OF COHERENT AND INCOHERENT CROSSTALKS AND OWER ENALTY THE OTICAL CROSSCNECTS Suvarna S. atil 1, Bharat S. Chaudhari and Baoun Li 3 1 Bharati Vidyapeeth Univerity College of Engineering, une, India chorage@gmail.com International Intitute of Information Technology, une, India bharatc@iquareit.edu.in 3 State Key Laboratory of Optoelectronic Material and Technologie, Sun Yat-Sen Univerity, Guangzhou 51075, China Abtract Optical cro-connect are one of the mot important component in the dene wavelength diviion multiplexer baed optical networ. The croconnect uffer from crotal due to the different wavelength light path channel during the witching proce leading to the deterioration in bit error rate (BER) and hence in the ytem performance. Thi paper preent the tudy of impact of coherent and incoherent crotal and power penalty on the optical cro-connect in WDM Networ. The effect of accumulation of coherent crotal at different tage of croconnect ha been alo invetigated and analyzed for the blocing probabilitie. Reult of coherent and incoherent crotal are compared to identify their impact on the woring of the cro-connect. The reult how that the crotal increae with increae in either the number of wavelength per fiber or the number of input fiber. The reult alo illutrate decreae in the interference penalty by correlating the crotal contribution with each other at the appropriate phae angle. We how that an acceptable blocing probability due to crotal i achievable for active wavelength in the WDM networ. The preent tudy can be ued to model the poible number of routing tage in uch networ. Keyword: Coherent Crotal, Incoherent Crotal, Optical Cro-Connect, Blocing robabilitie, ower enalty 1. INTRODUCTI Optical networ are conidered a a promiing olution for the next generation optical networ fulfilling the increaing demand of bandwidth for the application with high Quality of Service (QoS) requirement. Optical networ proce the ignal in optical domain enabling the fater and reliable communication through low power and compact optical integrated circuit (OIC) [1]. Switching and routing of the light pacet i important proce in optical networ and ha to enure correct delivery of pacet to the appropriate port without incorporating any error. In optical networ, optical croconnect (OXC) i reponible for witching and routing of the light pacet entirely in the optical domain []. The maor impairment found in optical networ include ASE (Amplifier Spontaneou Emiion) noie from optical amplifier, crotal from OXC node a well a attenuation, diperion and nonlinear effect from the optical fiber. A typical wavelength diviion multiplexed (WDM) OXC i compoed of wavelength-elective and witching element to route individual wavelength channel from everal input to everal output. While traveling through an optical cro-connect node, an optical ignal experience optical crotal due to narrow pacing between the light carrying waveguide. The crotal i contributed by the adacent input-output WDM channel and delayed verion of the deired ignal that travel through different optical path inide the croconnect. Crotal in OXC can be claified on the bai of Interferometric delay time. If the Interferometric delay time i horter than the light ource coherence time then the crotal i treated a coherent crotal, while Interferometric delay time i longer than the light ource coherence time, the crotal i treated a incoherent crotal [3]. In earlier tudie [4], the crotal analyi i done for the tatic wavelength router tructure. The tudy how that the interference power penalty depend on the linewidth of the laer ource. The parameter lie extinction ratio, input power, bit error rate (BER) are not the part of analyi. While conidering the coherent crotal in optical croconnect it i neceary to conider the phae relation amongt all the interfering ignal. The crotal pecification requirement will increae dratically a the noie power increae linearly with number of tage of OXC. Here in our tudy we howed how the ignal to interference ratio varie with the number of tage of optical croconnect. In [5], variou topologie of OXC are tudied. OXC baed on pace witch i one of the topology conidered there. The calability of the OXC i tudied in function of the number of wavelength channel. It how that the crotal increae with increaing number of channel and optimal performance for a certain throughput i obtained if the number of fiber equal the number of wavelength. In our wor, the performance analyi of OXC i carried out for coherent and incoherent crotal by conidering phae, the coherence time and the linewidth of the laer ource. For computation of crotal power penalty, 1-dB power penalty criterion i conidered. We have tudied the traffic carrying capacity of the OXC node to achieve required BER for thee crotal. The organization of the paper comprie of four ection. Section 1 give introduction to the type of crotal in OXC. The parameter uch a ource linewidth, the input power, BER and non-zero extinction ratio contribute to exceive power penalty. The baic OXC tructure and the impact of thee parameter on crotal are tudied in the Section. Analytical reult are dicued in Section 3. Finally the concluion i dicued in Section 4.. ANALYSIS OF CROSSTALK Optical in-band crotal occur when a ignal and interferer have cloe value wavelength. A a conequence, the ignal and interferer are within the pa band of practical optical filter. Generally uch a filter i located at the front end of 47

2 SUVARNA S ATIL et. al.: IMACT OF COHERENT AND INCOHERENT CROSSTALKS AND OWER ENALTY THE OTICAL CROSSCNECTS the receiver and interference caue a eriou degradation in the ytem performance. A the interference i not mitigated, optical in-band crotal will propagate with the dene WDM (DWDM) channel. The detructive effect of thi type of crotal accumulate in the optical node. In uch cae, the deired ignal and the lea ignal have an identical wavelength. If the light ource, are loced to a wea external laer line, then they have coinciding wavelength but till be individually uncorrelated with repect to phae noie proce. There will be another ituation where the channel of WDM ytem are haring the ame ource but the difference of propagation delay between adacent channel i greater than the laer coherence time. The frequency pacing in uch cae i negligible or almot zero, therefore the power penalty i caued due to phae-to-intenity noie converion. At the output of the cro-connect, the multiplexer collect all wavelength channel together and the reulting output channel uffer from the crotal caued due to the demultiplexer and the pace witch located at the output demultiplexer. The detructive impact on the deired channel i enhanced by the fact that both the ignal have almot ame wavelength and the reulting beat term are pectrally located within the receiver bandwidth. If everal cro-connect are connected in a cacaded configuration in the networ then the in-band crotal grow dramatically cauing more eriou degradation in the ytem performance. Further, the coherent nature of the crotal degrade the performance. The mot general cae in waveguide array baed optical communication where, each of the channel i operated with an independent laer ource or the deired ignal i generated by a ingle ource but the deired ignal i delayed by much longer period than the laer coherence length while witching at the OXC, the beating product will have incoherent nature. In integrated optical croconnect the circuit configuration can be choen uch that the amount of crotal i minimized and that the dominant crotal contribution are in the incoherent regime. Optical out-band crotal arie from inadequately uppreed neighboring wavelength channel in the demultiplexer, which alo contribute to the crotal [6]. To tudy the crotal, 4 4 OXC tructure coniting four fiber at the input and four fiber at the output i conidered a hown in Fig.1. The OXC i implemented by connecting the output of a WDM de-multiplexer to the input of a WDM multiplexer through pace witche. A pecified wavelength channel can be paed to a deired output by activating the witch either in cro or in bar-tate. During the witching, the fraction of interfering ignal get leaed into the other pace witche. For M wavelength per fiber and for N fiber at the input of OXC, interference with the deired ignal can be written a M-1+N-1=M+N-. In the preent tudy we have conidered M = N, where the calability i poible in term of number of node a well a number of wavelength and input fiber. Thee contribution can be coherent or incoherent depending upon whether they combine with the deired ignal within the coherence time of the ource or not. We have analyzed thee contribution for both, coherent a well a incoherent cae, which wa beyond the cope of earlier tudie..1 CROSSTALK MODELING λ 11, λ 1, λ 13 λ 14 λ 1, λ, λ 3 λ 4 Input Fiber λ 31, λ 3, λ 33 λ 34 λ 41, λ 4, λ 43 λ 44 Optical Demultiplexe r Fig.1. Structure of 4 4 Optical Croconnect (OXC), the dotted line how the crotal ignal lea Let an optical ignal of pea power i fed into an optical fiber with power level, and depending upon logical E and ZERO preent in the input data equence. The power level can be related to the average input power av, and an extinction ratio r=0 [7], a Output Fiber r av ; r 1 r 1 av (1) The optical field of the deired ignal in the fiber correponding to the laer ource will be decribed a the complex form [8], E r exp[ i t i ] () where, r expree the tate of polarization, the optical angular frequency, (t) the intantaneou optical phae and i the initial phae of the laer where denote the deired ignal. Thi deired ignal could be interfered by crotal contribution from -tranmitted ignal. The total interfering field Ex(t) can be written a, t E E x ( ) t d (3) where, E (t) i the field correponding to interfering ignal and d denote the Interferometric delay time. The total optical field, compriing the deired and the interfering field (M+N- contribution), incident upon a photodetector i given a, Space Switch Optical Multiplexer 48

3 ISSN: (LINE) ICTACT JOURNAL COMMUNICATI TECHNOLOGY, DECEMBER 011, VOLUME: 0, ISSUE: 04 E p h E Ex r b exp[ i t i ] X 1 c N X l1 i ( t ) ] r l c b ( t )exp[ i ( t ) i ( t ) ] r b ( t )exp[ i ( t ) where, C i the crotal parameter, given by the ratio of crotal power to the input power.b (t) and b (t) are the binary data equence with the bit interval T of the deired ignal and crotal ignal, repectively. and l are the crotal contribution from other wavelength channel in that fiber and other input fiber, repectively. r and r are the tate of polarization in the repective fiber. X[1,M-1] i the number of crotal ignal contributed by the different fiber in the each pace witch. Thee contribution are generally treated a the delayed verion of the deired ignal and the ignal leaed from other witche while propagating through the croconnect. and are the phae difference between the ignal with it delayed verion and the number of beating term generated due to contribution from other witche, repectively, where a K and are the correponding propagation delay difference. Eq.(4) mainly comprie of three term, firt term i the deired ignal, the econd term i the beating effect between the deired ignal and the interfering crotal which can be the delayed deired ignal itelf called a elf crotal, and the third term i the interfering ignal leaed from other ignal at the ame wavelength from ame port or from the other input fiber port. The latter i called a co-channel or neighboring crotal [9]. The pot detection filter wa aumed to be an ideal integrator over the time interval [0, T].The photocurrent at the output of the photodetector i given by i R Eph( t) R E( t) Ex( t) (5) ph where R=e/(hv)=1 i the photodetector reponitivity. While detecting thi combined ignal at the receiver the dominant noie term involved are the ignal crotal beat noie, hot noie and thermal noie. Therefore the total noie power in the receiver bandwidth, B e =40GH Z i given a [10] (4) Be N hot R R F S N ( f ) (6) Be where, N i noie power at the output of the receiver, i the ummation of quantum or hot noie, thermal noie, input amplifier noie current ource and the input amplifier noie voltage ource. R F i the feedbac reitance ued in amplifier circuit. Thi noie power decide the minimum detectable ignal power at the receiver. The third term in Eq.(6) S N (f) i the twoided noie pectral denity including the ignal induced hot noie and amplified pontaneou emiion (ASE) noie of the photodetector current. Wavelength and linewidth ( ν) for the laer ource i 1550nm and 10 GHz repectively. When two or more crotal contribution from M+N- are coherent with the deired ignal, the phae relation amongt the contribution determine the magnitude of each compoite crotal. When the propagation delay difference i much le than a bit interval, it get added to the ignal amplitude. When the propagation delay time of the interfering ignal i greater than the coherence time, the crotal channel are all incoherent. It i een from the reult that the power penalty i more for incoherent channel reulting in more BER. The adacent channel in a WDM ytem caue more crotal a compared to the farthet channel from the deired ignal in term of the frequency eparation [7].. BER CALCULATI The Q - factor i calculated at the receiver, [1] which i related to the BER being the error probability. It i given by, I Q I, CTK,, CTK and,, CTK r 1 av r 1, CTK (7) are ignal and noie power correponding to tate and tate repectively. Signal power fed in the deired ignal i conidering all E. We have conidered the unequal powered cae of interfere [11]. BER e 1 exp Q The power penalty i calculated by taing the logarithmic ratio of the minimum received ignal level to achieve pecific BER with the crotal contribution to that of the minimum detected power to achieve the ame BER without crotal contribution, Q REC (8) ower enalty db log 10 (9) R where, REC i the minimum detected ignal along with the crotal term [1]. The received and the input power are the function of linewidth to bandwidth ratio, extinction ratio, propagation time and initial phae of the ignal, generated by the ource. Therefore the reult obtained are baed on thoe parameter included in REC..3 TRAFFIC BEHAVIOR AND BLOCKING ROBABILITY CALCULATI By auming maximum M wavelength out of which only K active wavelength, for the oion proce with mean γ for the wavelength arrival requet, the holding time of the active wavelength with an exponential ditribution and mean μ, the traffic load behavior baed on Erlang model [13], 0 K ( ),! 0,1, K (10) where K (K) i the DF of K, and 0 i obtained by conidering the normalized condition K (K)=1 a follow: 49

4 SUVARNA S ATIL et. al.: IMACT OF COHERENT AND INCOHERENT CROSSTALKS AND OWER ENALTY THE OTICAL CROSSCNECTS 1 0 K 1 0! 1 K 1 0! (11) wherea S =/ denote the traffic load carried by the wavelength channel. The blocing probability due to crotal can be calculated a, b K 1 K( ) e( ) (1) where, e () i the error probability due to the active wavelength. 3. RESULTS AND DISCUSSI The plot of crotal veru power penalty for different value of phae angle are hown in Fig.. It can be een from the reult that the amount of crotal induced power penalty i higher if all the crotal contribution are cauing due to incoherent crotal. The coherent contribution are analyzed for the variou phae angle condition varying between [0, ]. The coherent contribution with the phae angle 180 are aid to be maximally uncorrelated and called a incoherent. It i een from the reult that with the 1-dB power penalty criterion, crotal of -0 db i tolerated in which the deired ignal added to the delayed verion of itelf with zero correlation (with phae hift of 180 ). For coherent crotal, the power penalty i reduced down to much lower level and crotal can be tolerated up to 13 db to atify 1-dB power penalty criterion. Smaller value of linewidth increae the coherence time, and more number of crotal contribution may fall in coherence with the deired ignal, with the power addition effect coherent crotal will be le harmful to the ytem performance a een in Fig.3. The effect of the coherent interference in optical networ can be fairly characterized by multipath fading effect in time cale of econd to minute. The error probability will be greatly affected when conidering the incoherent crotal contribution. lot of extinction ratio veru log of error probability for different crotal level are hown in Fig.4. BER of 10-9 i achieved with the input power level of 38 dbm with the tolerable crotal of 30 db at each OXC tage. Le than 10-9 BER i obtained at 30 dbm. Fig.3. The coherent crotal and incoherent crotal plotted againt BER Fig.4. Extinction ratio V BER when crotal level C i varying between 35 db to 0 db With the higher value of extinction ratio, larger tolerance toward power penaltie i achieved. At maller extinction ratio, average power at the input alo reduce, cauing more crotal and increaed BER. Fig.. Coherent crotal veru power penalty Fig.5. Input power veru error probability for different Crotal level 430

5 ISSN: (LINE) ICTACT JOURNAL COMMUNICATI TECHNOLOGY, DECEMBER 011, VOLUME: 0, ISSUE: 04 By increaing the input power to the fiber channel, one can achieve the BER below Increaing the input power can caue aturation for the optical amplifier if that are ued at the input of the receiver. Fig.5 how the error probability i almot contant beyond certain level of input power. 3.1 SCALABILITY OF OXC To tudy the calability of the optical croconnect (OXC), we have conidered L tage. For multitage OXC, the total crotal contribution grow dratically. For L tage OXC, the number of contribution goe to L(M+N-). If all thee contribution are coherent, it degrade the overall ytem performance. The reult of the number of tage veru Optical SNR for the different value of crotal are hown in Fig.6. A fair value of Optical SNR i achieved for a networ with 0 tage OXC but atifie 1-dB power penalty for crotal level of 0 db. Thi i in good agreement with the reult reported in [11]. the number of input fiber (M=N=4) a the crotal varie from 30 db to 0 db and the BER can be obtained le than The reult a hown in Fig.8 illutrate that the blocing probability varie with the number of active wavelength and will be le than 0.00 for 15 active wavelength with traffic load of 300 Erlang. Under the ame etting, we achieve the blocing probability below for maximum value of. Fig.8. Blocing probability plotted againt traffic load Fig.9 how the reult for number of active uer plotted againt the BER. Increaing the active uer reduce the power per input channel and hence increae crotal. If a crotal level i 30 db then acceptable BER i achieved even for large number of imultaneou tranmitter or uer. Fig.6. Optical SNR degrade with the amount of crotal and number of tage in cacade Fig.9. Number of Active Uer V BER Fig.7. Variation of BER with crotal while conidering the wavelength caling Reult reported in Fig.7, how that the bet performance in cacade can be obtained when the number of wavelength equal 4. CCLUSI We have analyzed and compared the performance of optical croconnect by conidering coherent and incoherent crotal. The effect of incoherent crotal can be reduced if the interfering channel are in maximum correlation. An extinction ratio of 0 i required to maintain lower limit of power penalty. A BER of 10-9 can be achieved with the input power of 38 dbm 431

6 SUVARNA S ATIL et. al.: IMACT OF COHERENT AND INCOHERENT CROSSTALKS AND OWER ENALTY THE OTICAL CROSSCNECTS at the cot of crotal of 30 db. Optical SNR reduce a the ignal propagate from one tage to the other of OXC, and a minimum of 1 db ratio will be maintained if crotal i 0 db. The calability of OXC i tudied by conidering the number of wavelength channel carried by the fiber at the input of OXC. The traffic model how that a blocing probability of i achievable for 0 active wavelength. With the help of preent tudy, the poible number of croconnect baed routing tage in WDM optical networ can be modeled for tolerable threhold of crotal, BER and power penalty. REFERENCES [1] Chretien G.. Herben, Xaveer J.M. Leiten, Crotal erformance of Integrated Optical Cro- Connect, IEEE Journal of Lightwave Technology, Vol. 17, No. 7, pp , [] H.J.S. Dorren, H. de Waardt, and I. Tafur Monroy, Statitical Analyi of Crotal Accumulation in WDM Networ, IEEE Journal of Lightwave Technology, Vol. 17, No. 1, pp , [3] Y. Shen, Keie Lu and Wanyi Gu, Coherent and Incoherent Crotal in WDM Optical Networ, IEEE Journal of Lightwave Technology, Vol. 17, No. 5, pp , [4] Chung-Sheng Li, and Fran Tong, Crotal and Interference enalty in All Optical Networ Uing Static Wavelength Router, IEEE Journal of Lightwave Technology, Vol. 14, No. 6, pp , [5] Tim Gyeling, Geert Morthier, and Roel Baet. Crotal Analyi of Multiwavelength Optical CroConnect, IEEE Journal of Lightwave Technology, Vol. 17, No. 8, pp , [6] Eugenio Iannone, Roberto Sabella, Monica Avattaneo, Gabriele De aoli, Modeling of In-Band Crotal in WDM Optical Networ, IEEE Journal of Lightwave Technology, Vol. 17, No. 7, pp , [7] C.S.Li, Michael Olen, and David G. Meerchmitt, Analyi of Crotal penalty in dene optical chip interconnect uing ingle-mode waveguide, IEEE Journal of Lightwave Technology, Vol. 9, No. 1, pp , [8] Eduward Tangdiongga, Tafur Monroy, Crotal in WDM Communication networ, Kluwer Academic ubliher, London, 00. [9] Tao Deng, Sureh Subramaniam and Jinghao Xu. Crotal-Aware Wavelength Aignment in Dynamic Wavelength-Routed Optical Networ, roceeding of International Conference on Broadband Newtor, pp. 140, 004. [10] Gerd Keier, Optical Fiber Communication, McGraw Hill, International Edition, 000. [11] J.C. Attard, J.E. Mitchell, Member, C.J. Ramuen, erformance analyi of Interferometric Noie Due to Unequally owered Interferer in Optical Networ, IEEE Journal of Lightwave Technology, Vol. 3, No. 4, pp , 005. [1] Bart Baeelandt, C. Melange, J. Bauwelinc,eter Oieur, T. Ridder, Xing Qiu, J.Vandewege, OSNR enalty Impoed by Linear In-Band Crotal Caued by Interburt Reidual ower in Multipoint-To-oint Networ, Journal of hotonic Tech. Letter, Vol. 0, No. 8, pp , 008. [13] Hamzeh Berranvand and Jawad A. Salehi. Multiervice roviioning and Quality of Service Guarantee in WDM Optical Code Switched GMLS Core Networ, IEEE Journal of Lightwave Technology, Vol. 7, No. 1, pp ,