Experimental investigation of wavelength-tunable WADM and OXC devices using strain-tunable fiber Bragg gratings

Transcription

1 1 October 1999 Optcs Communcatons Expermental nvestgaton of wavelength-tunable WADM and OXC devces usng stran-tunable fber Bragg gratngs Shen-Kue Law a,d,), Keang-Po Ho b, Chnlon Ln c, Sen Ch a a Insttute of Electro-Optcal Engneerng, Natonal Chao-Tung UnÕersty, PO Box 75, Yang-Me 326, Hsn-chu, Tawan b Department of Informaton Engneerng, The Chnese UnÕersty of Hong Kong, Shatn, New Terrtores, Hong Kong c Tyco Submarne Systems Labs, Eatontown, NJ 07724, USA d Department of Electrcal Engneerng, Da-Yeh UnÕersty, Chang-Hua, Tawan Receved 29 January 1999; receved n revsed form 20 July 1999; accepted 21 July 1999 Abstract Two reconfgurable wavelength addrdrop multplexer Ž WADM. and optcal cross-connect Ž OXC. devces based on stran-tunable fber Bragg gratngs Ž ST-FBGs. are proposed. The feasblty of WADM s verfed by measurng the bt-error-rate performance of the dropped Ž cross-connect. and pass-through Ž no cross-connect. channels over a 50- and 100-km standard sngle-mode fber for a 2.5 Gbrs system. Compared to back-to-back transmsson, a neglgble power penalty of only 0.2 db s observed. q 1999 Publshed by Elsever Scence B.V. All rghts reserved. Keywords: Wavelength dvson multplexng; Wavelength addrdrop multplexer; Optcal cross-connect; Fber Bragg gratng; Optcal network 1. Introducton Wavelength addrdrop multplexer Ž WADM. and optcal cross-connect Ž OXC. swtches are essental components n the future dense wavelength dvson multplexng Ž WDM. optcal networks, especally n a reconfgurable network topology wx 1. Many sgnfcant efforts have been devoted to the desgn of hgh-capacty, flexble, relable and transparent mulw1,2 x. The WADMs twavelength optcal networks allow the network nodes to access a subset of wavelengths n the optcal networks, reducng the hardware requrements and processng load n ntermed- ) Correspondng author. Tel.: q ; fax: q ; e-mal: u @cc.nctu.edu.tw ate nodes by handlng only pass-through traffc. The OXCs are confgurable on a lnk-by-lnk bass to allow optmzaton of capacty allocaton, management and scalablty of network sze. Combnaton of the WADMs and OXC devces can provde the flexblty to satsfy the reconfgurable requrements and enhance network survvablty wx 3. Conventonal WADMs and OXC devces may use the 1=N demultplexer Ž DMUX. and N = 1 multplexer Ž MUX. par to separate and combne WDM channels. The ablty for reconfguraton s ntroduced by addng space dvson swtches between the DMUXrMUX par. However, addtonal optcal components for space dvson swtches technologes, such as wavewx 4, semconductor optcal gude drectonal couplers amplfers Ž SOAs. wx 5, low-gan erbum-doped fber r99r$ - see front matter q 1999 Publshed by Elsever Scence B.V. All rghts reserved. PII: S

2 76 ( ) S.-K. Law et al.roptcs Communcatons amplfers Ž EDFAs. wx 6 and some other components, are requred. The drawbacks of complcated desgns, controls, expensve cost and ncreased nserton losses of these elements have to be solved for vast applcatons of WADMs or OXCs. Usng arrayed-wavegude gratngs Ž AWGs. and double-gate swtches as the key components of WADMs, the merts of these wavelength addrdrop flters have been addressed by prevous works wx 7. On the other hand, fber-bragg-gratng Ž FBG. -based WADMrOXC archtecture wth the features of loss unformty, hgher contrast rato and low cost s an alternatve canddate when desgn these devces. For the commercal AWG-based OXC devce, the adjacent channel crosstalk s typcally one order hgher than that of FBG-based OXC devce for a 200-GHz channel-spacng WDM system. The larger crosstalk wll degrade the system performance and restrct the applcatons of AWG n dense WDM system for wavelength addrdrop andror cross-connect. Also, the nserton loss s hgh unless SOAs are used for loss compensaton. On the other hand, FBG-based WADMs andror OXC devces have shown many promsng characterstcs. Several approaches have been proposed or expermentally demonstrated, such as those based on fber and array wavegude gratngs wx 8, ntegraton of the WADM wx 9 and OXC devce w10x by usng reflectve FBGs, optcal crculators Ž OCs. andror optcal swtches Ž OSWs.. Another proposal s demonstrated by usng demultplexer and tunng property of FBGs to desgn the OXC devce w11 x. In ths paper, we propose the WADM and the closely related 2 = 2 OXC devce for smultaneously multple-wavelength addrdrop and cross-connect based on stran-tunable FBGs Ž ST-FBGs. and threeport OCs. An experment for the WADM devce s nvestgated to verfy the WADM devce n a 2.5 Gbrs system over 100 km of conventonal snglemode fber. 2. and 8 Ž s4=2. n Ref. w11 x, 256 Žs2 8. and 2 Žs 2 1. for the parallel P type buldng block n Ref. w10 x, but only 8 and 2 n our proposal, respecw10x has the problem that any status change Žbar or tvely. The seres S-type buldng block n Ref. cross-state. of mechancal optcal swtches for some channels may nterrupt the nformaton transmsson of the other Ny1 channels. Also, the dfference n optcal attenuaton between the hgher loss paths and lower loss paths may nduce non-unform loss among channels, and affect the recever senstvty and system sgnal-to-nose rato Ž SNR.. Usng FBGs for wavelength addrdrop and cross-connect, central wavelength-shft control of FBG can be acheved by varyng the envronmental temperature or applyng mechancal stran. However, the response speed for temperature varaton s slow manly due to low temperature coeffcent Ž ; nmr8c. of FBG. Applyng stran, the tunng range of FBG by tensle stress and compressve stress can be up to 10 nm and 32 nm, respectvely w12 x. Applyng stran wll change the gratng ptch and fber ndex. The tunng range as mentoned above s large enough for WDM system wth 200 GHz Ž 1.6 nm. channel spacng. The operatng mechansm of a ST-FBG s depcted n Fg. 1Ž. a and the possble three-port ele- 2. Confguraton of WADM and OXC devces The proposed WADMs as well as OXC devces are composed of a set of ST-FBG chan and two three-port OCs. The number of necessary fber components s reduced tremendously compared to prevous works. For an eght-channel two-fber system, the total requred FBGs and OCs are 32 Žs4=4= Fg. 1. Ž. a Operatng mechansm of the stran-tunable fber Bragg gratngs; and Ž. b two possble canddates of the three-port element.

3 ( ) S.-K. Law et al.roptcs Communcatons ments are shown schematcally n Fg. 1Ž. b. The three-port element could be a three-port OC or a 1 = 2 50:50 coupler combnng two optcal solators to reduce the cost. Fg. 2Ž. a shows the schematc dagram of two dentcal WADMs carryng the counter-propagatng sgnals from the opposte drectons. The Bragg wavelength of ST-FBG s desgned to match the WDM-channel sgnals of l and l X Ž 1FFN.. Before applyng stran to the ST-FBG, the channel sgnal at l Žl X. s reflected and then drops from the same three-port elements contanng the I1 Ž I2. port for the upper Ž lower. WADM devce. At the same tme, a set of new WDM channel sgnals X X X l, l... and l Ž l, l... and l. 1 2 N 1 2 N s added nto the upper Ž lower. WADM devce va port 1 of the same three-port elements contanng the O1 Ž O2. X port, where l and l Ž 1 F F N. have the same wavelength. On the other hand, f the Bragg wavelength of the ST-FBG s shfted about 0.60 nm Ž n our example. away from ts ntal poston by applyng stran, l Žl X. may pass-through the WADM and contnue ts forward propagaton. The crteron of wavelength-shft value depends on stopband bandwdth of the ST-FBG and the channel spacng of the WDM system. Because the proposed WADM structure s seral-based operaton, the possble effect of wavelength-dependent tme delay or dsperson of the FBG should be consdered. Usually, FBG only nduces dsperson to adjacent channels but not chanw13 x. Stran tunng of FBG s faster nels far away than temperature tunng of FBG. Pezoelectrc ceramc transducer Ž PZT. or acousto-optc tunable fl- Ž. Ž. Fg. 2. a Two wavelength-addrdrop multplexers; and b one reflecton type optcal cross-connect devce. Both the WADM and OXC devces consst of two three-port elements and several stran-tunable fber-bragg-gratngs.

4 78 ( ) S.-K. Law et al.roptcs Communcatons ter Ž AOTF. may be used for stranrcompress tunng of the FBGs wth a fast response speed. For example, when FBG s adhered on a PZT rod and several volts are appled to the PZT, the central wavelength of the FBG can easly be shfted about 1 nm away from ts orgnal central wavelength. If a commercalzed PZT wth 10 KHz response speed s used, the reconfguraton tme s suffcently short for most optcal swtchng networks. To fulfll the requrement n even more hgh-speed optcal swtchng networks, AOTF wth swtchng speed n the sub-mcroseconds regme, demonstrated by Rza and Chen w14 x, s the promsng canddate to realze hgh-speed addrdrop functon. Temperature control and lnear feedback crcut are requred to reduce the wavelength fluctuaton due to FBGs nduce ampltude modulaton. It s nterestng to menton that when two WADM devces are connected back to back n a system havng two fber lnks, a reflecton-type 2=2 OXC swtch s realzed. The dropped channels from fber lnk 1 may travel nto added port of the fber lnk 2, whle the dropped channels from fber lnk 2 may travel nto to added port of the fber lnk 1, respectvely. The reflecton-type OXC devce means that any par of WDM channel sgnals Žl and l X. can be cross-connect from I1 to O2 and from I2 to O1 smultaneously f they are reflected by the correspondng ST-FBG. To smplfy the confguraton, another reflecton-type OXC swtch s shown schematcally n Fg. 2Ž. b. One WADM can be used as an OXC devce by approprately choosng the correspondng I1, I2, O1 and O2 ports. There are two nput ports of I1 and I2 as well as two output ports of O1 and O2 n the OXC devce. These two nput ports can operate n the same drecton or opposte drecton e., b-drectonally. only by rearrangng the 3-port elements. Each FBG s de- sgned to match the WDM channel sgnals of l and l X transmttng n the upper and lower fber lnks smultaneously. For example, when all wavelengths requrng nterchangng, none of the FBG s appled stran and each FBG reflects the ndvdual channel sgnal back to port 3 of the same three-port element as ts nput port. In that case, cross-connecton of all channel sgnals s realzed by the ST-FBGs. When only some of the wavelengths are nterchanged, for example, the exchange of l and l wth l X and 2 N 2 l X N. Most of the FBGs except the FBG2 and FBGN are appled mechancal stran. The WDM channels of l, l, l...l and l X l X, l X Ny l Ny1... wll pass through the OXC devce and contnue ther forward propagaton to O1 and O2, respectvely. Meanwhle, both the WDM channels of l and l, 2 N l X and l X 2 N are reflected by the correspondng FBG2 and FBG and then swtched to port O2 Ž O1. of the N three-port element. 3. Expermental setup ST-FBG s the same as a regular FBG n structure. On applyng mechancal stran, ST-FBG can be tuned to longer or shorter wavelength by stretchng or squeezng t. The Bragg wavelength l Ž n nm. B s gven by lb s2nl Ž 1. where L Ž n nm. s the gratng ptch and n s the effectve ndex of the sngle-mode fber. The shft n Bragg wavelength for a longtudnal stran at a constant temperature Ž DT s 0. s gven by the exw15x DlBslBŽ 1yP e. Ž 2. presson 4 P s1r2n 2 P yn Ž P qp. Ž 3. e where Pe s the photoelastc coeffcent and has a numercal value of about 0.22, P12 and P11 represent the components of stran-optc tensor Ž Pockel coeffcents. and n s the Posson rato. The measured stran response at constant temperature s found to be Dl rž l D. s0.78=10 y6m y1 Ž 4. B B where m s 10 y6. Fg. 3 shows the expermental confguraton of one-channel WADM to verfy the system performance. One narrow lnewdth, tunable laser source s tuned to nm and externally modulated by a 2.5 Gbrs non-return-to-zero Ž NRZ. pseudo-random bnary sequence. A polarzaton controller s used to adjust the polarzaton-state of the laser source for matchng to the LNbO3 ntensty modulator. Two spools of 50-km sngle-mode fber, locatng before and after the WADM, are used as the transmsson lnk. Two erbum-doped fber ampl- fers EDFAs wth a saturated output power of

5 ( ) S.-K. Law et al.roptcs Communcatons Fg. 3. Expermental setup: TLS: tunable laser source. MOD: modulator. OBPF: optcal bandpass flter. RX: optcal recever. BERT: bt-error-rate test set. about 11.0 dbm are employed to compensate the fber loss. A PIN FET recever wth a senstvty of y9 y38.6 dbm at a bt-error-rate BER of 10 s used to measure the system performance. The nterport solaton s 50 db and the nserton loss s 0.9 db for the three-port OC. The FBG has a reflectvty of 99.9% and the y10, y20 and y30 db bandwdths of the FBG are 0.20, 0.35 and 0.60 nm, respectvely. Instead of usng a PZT for stran tunng, the FBG s stuck onto two translaton stages for feasble study of wavelength pass-through and addrdrop. The central reflected wavelength of the FBG s nm wthout stran and nm when stran s appled Ž D l s 1.2 nm. STRAIN. The stran appled to FBG s about 1000 m measured by usng a stran gauge for wavelength-shft of 1.2 nm. 4. Results and dscusson The channel assgnment for an N-channel WDM system could be based on the Internatonal Telecommuncaton Unon Ž ITU. proposal of 200-GHz Ž1.6- nm. spacng. Wthout applyng stran, the central reflectve wavelength of FBG could be assgned such as l sl qž y1. =1.6 Ž n nm. 1 for 1FFN. FBG 1, FBG 2... FBGN are located consecutvely, from the left-hand sde to the rght-hand sde, as shown n Fg. 2, between the two three-port elements. Homodyne crosstalk may be nduced from the add port to the drop port, or the nput port to the output port. To suppress the homodyne crosstalk effect, the central reflectve wavelength of FBG may be shfted between l q D l and l y 30dB q1 Dl30dB when stran s appled, where Dl30dB s the y30 db stopband bandwdth of the FBG wth a value of about 0.6 nm n ths experment. Thus, the approprately tunng range for FBG s from 0.6 to 1.0 nm. For example, f the central wavelength of FBG s shfted from l to lq1ydl30db and lq1 s dropped by the FBG at the same tme, y30 db q1 homodyne crosstalk may contamnate the dropped channel of l q1. Nevertheless, the y30 db crosstalk nduce power penalty s nearly 0 db by expermental measurement and only 0.3 db even usng the worst case Gaussan approxmaton w16 x. Based on prevous analyss, the acceptable tunng range s 1.6 nmy2 =Dl30dB n ths example. One extra hgh reflectvty of FBG Ž over 99.9%. was used durng our measurement. Also, the homodyne crosstalk effect due to the 0.1% Ž y30 db. transmsson of optcal power s neglgble. Also, each laser wavelength and ts correspondng FBG were carefully adjusted to avod wavelength msalgnment. Fg. 4 shows the measured BER of the WADM aganst the receved power for the baselne sgnal Ž 0 km., the dropped sgnal Ž50 km. and the pass-through sgnal Ž 100 km.. For the measurement of pass-through channel, the data was measured by tunng the central wavelength of FBG of 1.2 nm away from ts ntal wavelength. Note that the performance of drop channel s even a lttle bt better than that of the back to back condton, due to the fact that the majorty of amplfed stmulated emsson Ž ASE. nose from the EDFA s fltered out when the sgnal s reflected by the FBG. A neglgble power penalty of only 0.2 db for the passng channel

6 80 ( ) S.-K. Law et al.roptcs Communcatons A reconfgurable all-fber WADM and reflecton type OXC devce based on ST-FBGs ntegrated three-port elements are proposed. One-channel addedrdropped experment of the WADM devce s nvestgated over a 50-km and a 100-km standard fber for the dropped channel and the pass-through channel n a 2.5 Gbrs system. Compared to backto-back transmsson, no power penalty s found. The FBG-based WADM as well as the closely related OXC devce wth the features of dynamcally wavelength-selectve functon, hgh sgnal-to-nose contrast rato, potentally low crosstalk, unform and low nserton loss, low cost, hgh scalablty and cascadablty may provde vast applcatons n the optcal networks. Acknowledgements Fg. 4. Measured BER performance of the WADM at nm aganst the receved power for the back-to back Ž 0 km., dropped Ž 50 km. and pass-through Ž 100 km. condtons n a 2.5-Gbrs externally modulated system. over a 100-km transmsson s observed compared to the back-to-back transmsson. A smlar result could be obtaned from the reflecton-type OXC devce except that the ASEs of EDFAs result from dfferent fber lnks wll travel along wth the sgnals. However, the ASEs wll not degrade the system performance f only the SNR of each channel s above a certan level, say, 15 db. Hence, the WADMrOXC archtecture represents a thoroughly optcal transparent ablty. Each wavelength path s represented by ts correspondng FBG wth a roughly rectangular shape and a low sdeband loss. 5. Concluson The authors are ndebted to P.-Y. Chen and K.-Y. Hsu for equpment support and helpful dscusson. The BER measurement of ths paper were conducted n the Lghtwave Communcaton Laboratory, Department of Informaton Engneerng, The Chnese Unversty of Hong Kong, Shatn, Hong Kong. References wx 1 C.A. Brackett, J. Lghtwave Technol. 14 Ž wx 2 Y.D. Jn, M. Kavehrad, IEEE Photon. Technol. Lett. 7 Ž wx 3 Mult-wavelength optcal technology and networks, J. Lghtwave Technol., 14 June, wx 4 R.A. Spanke, IEEE Commun. Mag. 25 Ž wx 5 W.D. Zhong, J.P.R. Lacey, R.S. Tucker, J. Lghtwave Technol. 14 Ž wx 6 Y.-K. Chen, W.I. Way, IEEE Photon. Photon. Lett. 6 Ž wx 7 K. Okamoto, K. Takguch, Y. Ohmor, Electron. Lett. 32 Ž wx 8 N.A. Rza, Optcs n Computng OC 98, vol. 3490, Bruges, Belgum, 1998, p wx 9 H. Okayama, Y. Ozek, T. Kamjoh, C.Q. Xu, I. Asabayash, Electron. Lett. 33 Ž w10x Y.-K. Chen, C.-C. Lee, J. Lghtwave Technol. 16 Ž w11x D.R. Hjelme, H. Storoy, J. Skaar, OFC 98, paper TuJ6, San Jose, CA, USA. w12x G.A. Ball, W.W. Morey, Opt. Lett. 19 Ž w13x B.J. Eggleton, G. Lenz, N. Ltchntzer, D.B. Patterson, R.E. Slusher, IEEE Photon. Technol. Lett. 9 Ž w14x N.A. Rza, J. Chen, Opt. Lett. 23 Ž w15x K.T. Grattan, B.T. Meggtt, Optcal Fber Sensor Technology, 1st ed., Chapman and Hall, 1995 Ž Secton w16x K.-P. Ho, J. Lghtwave Technol. 17 Ž