RECENT DEVELOPMENTS IN FIBRE TECHNOLOGY AND ITS APPLICATION WITHIN HIGH SPEED OPTICAL COMMUNICATIONS

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1 RECENT DEVELOPMENTS IN FIBRE TECHNOLOGY AND ITS APPLICATION WITHIN HIGH SPEED OPTICAL COMMUNICATIONS D.J. Richardson, J.Y.Y. Leong, F. Parmigiani, P.J. Almeida, M. Ibsen, P. Peropoulos Opoelecronics Research Cenre Universiy of Souhampon, UK Phone: , Fax ABSTRACT Recen advances in opical fibre echnology, mos noably in he area of microsrucured fibres, high power rare-earh doped fibres, and supersrucured fibre Bragg graings offer a hos of new opporuniies wihin fuure high speed communicaion sysems. Herein we review he increased range of waveguide characerisics ha can now be obained wihin fibre wih an emphasis on boh fibre nonlineariy and dispersion conrol, and describe he laes advances in he design and fabricaion of fibre Bragg graings for he manipulaion and shaping of ulrashor pulses. We hen ouline how such componens can be inegraed ino various all-opical processing devices for high speed opical communicaion sysems focusing in paricular on daa reiming and OTDM add-drop muliplexing. 1. INTRODUCTION The developmen of low loss, dispersion opimised ransmission fibres coupled wih he developmen of he erbium doped fibre amplifier has revoluionized elecommunicaions. Tbi/s daa capaciy ransmission over ransoceanic disances can now be rouinely achieved using DWDM echnology [1], and single channel daa raes of >.5Tbi/s over ~160km have also now been demonsraed []. In order o fully exploi his inheren ransmission capaciy wihin neworks, he use of all-opical processing echniques such as opical regeneraion, wavelengh conversion and rouing will ulimaely be required. I looks likely ha fibre echnology could play a criical role here also, boh in erms of pulse shape conrol and in erms of providing an ulrafas nonlinear opical elemen. In his paper we review recen developmens in nonlinear fibre fabricaion wih a primary emphasis on microsrucured fibre echnology. We hen briefly ouline he sae-of-he-ar in erms of fibre graing for pulse shaping applicaions, and conclude by describing he operaion and performance of wo elemenary pulse processing sysems ha we believe highligh he poenial of combining hese wo enabling echnologies for opical processing applicaions.. NONLINEAR FIBRES The effecive nonlineariy-per-uni-lengh γ of a fibre a a wavelengh λ is given by γ = π n / (λa eff ) where n is he nonlinear refracive index of he glass and A eff is he effecive area of he fibre. In order o conrol γ one can in pracice eiher conrol he value of n hrough maerial choice, and/or change A eff. The exen o which A eff can be conrolled is ulimaely defined by he index conras beween core and cladding maerials, which can be quie limiing in he case of convenional solid fibre fabricaion. Finding suiable hermally and opically mached maerials for core and cladding severely limis he range of high n maerials ha can be used for convenional solid fibres..1 Convenional solid HNLFs Over recen years here has been considerable commercial ineres in developing silica based doped fibres, produced using sandard deposiion echniques, offering boh high nonlineariy wih accuraely conrolled dispersion characerisics. Typical values of nonlineariy per uni lengh γ lie in he range 15-5 W -1 km -1, however i is o be appreciaed ha hese values can be achieved wih only relaively modes compromise in he fibre loss which is ypically 0.5-1dB/km. Highly nonlinear fibres (HNLFs) wih a zero dispersion wavelengh around 1550nm exhibiing boh low hird order dispersion [3], and indeed fourh order dispersion have now been repored [4]. Such fibres are now commercially available and have been used in a range of elecommunicaions applicaions, including opical demuliplexing, wavelengh conversion and parameric amplificaion.

2 By moving o maerials wih a higher inrinsic nonlineariy, e.g. bismuh based glasses, i is possible o achieve far higher values of γ, albei wih increased losses. For example, values as high as γ = 1000 W1 km-1 wih losses ~1dB/m have been achieved in Bismuh doped fibre, and again hese fibres are now commercially available [5]. However, i is o be appreciaed ha i is no possible o achieve zero dispersion wavelengh a 1550nm due o he high maerial dispersion of bismuh based glasses a hese wavelenghs. This is due o he limied index conrass ha can be achieved using suiable combinaions of glass for core and cladding.. Microsrucured fibres Microsrucured fibre echnology allows he possibiliy o benefi from he large linear and nonlinear refracive index conrass beween glass and air o realize fibres wih ruly unique and useful properies. By suiable conrol of he fibre ransverse srucure, and maerial choice, i is possible o envisage fibre designs wih nonlineariies ha are ailorable over more han 7-orders of magniude [6]. This opens up a hos of new device possibiliies ranging from nonlinear elemens for elecommunicaions applicaions hrough o low nonlineariy fibres for high power laser delivery. In he low nonlineariy exreme we esimae ha low loss PBG fibres (see Fig.1a) wih nonlineariies around 1000 imes lower han exising SMF should be possible, by minimizing he amoun of ligh propagaing wihin he solid regions of he fibre, and maximizing he amoun of ligh wihin he low nonlineariy air core. A he oher exreme we can anicipae HFs wih nonlineariies ha are 10,000 imes ha of convenional SMF by making use of boh he igh mode confinemen possible in smallcore, high-na HF and he large maerial nonlineariies ha can be achieved in high index maerials such as chalcogenide glass. Nonlineariies approaching he maximum heoreical values possible in fibre form have now been achieved for several maerials. For example, γ values a 1550nm of 70 W1 km-1and 1860 W-1km-1 have been achieved in pure silica (see Fig.1c) [7] and lead silicae HFs (see Fig.1d) [8] respecively. Noe ha once such high levels of nonlineariy can be achieved, e.g. o he poin ha devices of order 1m become pracical, hen he relaively high losses (~1dB/m) of high-index base maerials cease o be a major limiing issue for many imporan applicaions. Whils his is in iself an impressive achievemen, (a) (c) (b) (d) Fig.1 Some ypical microsrucured fibre ypes fabricaed a ORC: (a) a PBG fibre (b) a large mode area holey fibre (~0 µm core diameer); (c) a small core silica HF which provides high opical nonlineariy; (d) a high nonlineariy HF in SF57 glass produced using an exrusion echnique. obaining suiable dispersion properies is ofen of greaer imporance for many nonlinear device applicaions, necessiaing a rade-off in magniude of nonlineariy for dispersion conrol. The high index conras beween air and glass allows for exreme values of waveguide dispersion and his can be used o conrol he large maerial dispersion associaed wih compound glasses a wavelenghs of ineres in he near IR e.g. a 1550nm for elecommunicaions, or around 1060nm for incorporaion wihin Yb-doped fibre based devices. In Fig. we show a lead silicae HF wih a hexagonal arrangemen of effecively 4 rings of holes (48 holes in oal), produced by sacking ogeher jus 7 srucured elemens [9]. As opposed o our previously exruded suspended-core HFs [8], whose criical properies are deermined primarily by jus one parameer, he core diameer, hese SEST fibres have wo adjusable parameers: he hole-o-hole pich Λ and he hole diameer d (or relaive hole size d/λ). Λ is adjused by he scale of he microsrucured region. Noe ha he core size of he SEST fibres depends on boh d and Λ, and is abou Λ-d. The core size of he HF shown in Fig. was varied beween µm and had a d/λ raio of 0.48 in he ouer cladding region, and 0.55 in he region surrounding he core. The loss of hese early SEST fibres is around 3dB/m, alhough losses below 1dB/m are ulimaely expeced for his glass ype. The grea advanages of SEST HFs hough can be appreciaed when he fibre dispersion is considered.

3 We have performed numerical simulaions using a full vecor model o calculae he group velociy dispersion (GVD) profiles of he 3 SEST fibres so far produced. In Fig.3 we plo he heoreical variaion of dispersion wih wavelengh for our hree fibre samples. HF # and #3 are seen o exhibi anomalous GVD hroughou he C-band due o an excess of waveguide dispersion in his region. However, for HF #1 he effecs of maerial and waveguide dispersion oppose each oher in such a way ha he zerodispersion wavelengh is shifed o he region beween nm, as inended from our design calculaions. We also plo heoreical dispersion profiles of several exruded SF57 suspended-core fibres previously fabricaed wih maximum nonlineariy in mind [8]. The primary zero dispersion wavelengh for hese fibres lies around 1µm. The figure shows ha he SEST designs offer far greaer conrol and flaer dispersion profiles in he 1.55µm region compared o he suspended-core srucures previously fabricaed. Fig.3 also shows he dispersion profile of a heoreical SEST design opimised for flaened dispersion a 1.55µm which highlighs he excellen properies ha should ulimaely be achievable using he SEST approach [10]..3 Pulse shaping using SSFBGs Fibre Bragg graing echnology has now advanced o he poin ha graings wih ruly complex phase and ampliude refleciviy profiles can be reliably designed, and fabricaed [11]. I is hus possible o produce graings designed o reshape opical pulses suied for a given opical communicaions funcion. To dae pulse shaping graings have been mos commonly used for dispersion compensaion applicaions, bu ineres is now growing in using such devices for a hos of opical processing applicaions, including opical code generaion and recogniion, iming jier oleran opical demuliplexing [1] and pulse repeiion rae muliplicaion [13]. In he following secion we presen daa on a pulse reshaping graing designed o generae parabolic shaped pulses from inciden solion pulses as reliably generaed from a modelocked fibre laser. Parabolic pulses are of grea ineres for applicaions such as all-opical Fourier ransformaion, opical regeneraion and high power pulse amplificaion schemes amongs ohers. 3. EXAMPLES OF OPTICAL PROCESSING USING HNLF S AND SSFBG S 3.1 Timing Jier reducion Timing jier reducion is likely o be an essenial funcion wihin fuure high speed opical communicaion sysems. Here we presen a echnique for drasically reducing iming jier induced in shor pulse ransmission sysems based on he use of Cross- Phase Modulaion (XPM) in a HNLF [14]. Fig. SEM image of he dispersion shifed SEST fibre a 1.55µm (HF#1). Dispersion (ps/nm/km) H F# W ave leng h ( um ) H F# 3 H F# um core HNLF-SF um core HNLF-SF57 [ref. 3] SF57 bulk glass O pim is e d S E S T H F1 H F H F3 Fig.3 Comparison of dispersion profiles of our previous work on suspended-core HFs wih differen core sizes and various SEST fibres including an opimised design offering reduced dispersion slope. The nonlineariies of hese hree fibres were 170,50,410 W -1 Km -1 for HF1, HF and HF3 respecively. 4KHz 10GHz ~ FM Gain Swiched DFB λ=1556nm EFRL λ ο=154nm Compression sage Pulse Shaping SSFBG SMF 500m MOD 10Gbi/s 31-1 PRBS Generaor EDFA Delay line HNLF 0m Fig.4 Experimenal se-up of he parabolic pulse reimer. The experimenal se-up and he reiming scheme principle are shown in Fig.4. The echnique is based on he fac ha he frequency-shif impared by an ~ ~ / ~ / λo=1556nm

4 inense conrol pulse on a co-propagaing signal in a Kerr medium is given by: φ U ( T T ) δω( T) = = γpo Leff, T T where γ and L eff are he nonlinear coefficien and he effecive lengh of he HNLF respecively, P o is he conrol peak power, U(T) is he normalized slow varying ampliude of he pulse envelope, and T is he iniial relaive ime delay beween he signal and he conrol pulse. Thus if he conrol signal has a well defined clock rae and comprises pulses wih a parabolic inensiy profile hen we can use his ime derivaive effec o impar a frequency shif on individual pulses wihin an inciden daa sream ha is simply proporional o he emporal offse of he daa pulses from he conrol pulse cenre. There is hus a one-o-one linear mapping of iming jier o frequency jier. By passing he frequency modulaed daa bis hrough a suiably dispersive delay line, i is possible o reime he daa bis on a bi-by bi basis. The key issue wih his echnique is how o produce he conrol pulses wih a sufficienly accurae parabolic pulse shape. Here we show ha his is possible using a simple graing based pulse shaper. Fig.5 (Lef) Experimenal (doed line) and calculaed (dashed-doed line) specrum of he parabolic pulses. Fig.6 (Righ) Inensiy of he shaped parabolic pulses measured using SHG-FROG and is corresponding gradien. A mode-locked erbium fibre ring laser (EFRL) operaing a 154nm is used o generae ~ ps solion pulses a a repeiion rae of 10 GHz. These pulses are used as he inpu o he parabolic pulse shaper. The shaped pulses are designed o have a parabolic envelope (FWHM of ~10 ps) superimposed upon a 5 h order super-gaussian profile which is used o smoohen he pulse edges and reduce is specral exen. The graing was designed and fabricaed wih an appropriae phase and ampliude response o conver he ps solions ino he arge parabolic pulses. The measured opical specrum of he pulses from he SSFBG is shown in Fig.5 (doed line) ogeher wih he heoreically calculaed single parabolic pulse specrum (dashed-doed line). As can be seen, we can accommodae 11 specral lobes wih an excellen agreemen, highlighing he qualiy of our graing wriing process. A deailed assessmen of he qualiy of he parabolic pulses is carried ou before processing wih our reiming experimen. We characerised he pulses refleced off he SSFBG using second harmonic generaion frequency-resolved opical gaing (SHG-FROG) and calculaed he ime derivaive of heir inensiy profile, as seen in Fig.6. As can be seen, his derivaive exhibis a good linear slope across he 10 ps pulsewidh. Oupu Timing Jier RMS [ps] 4 (a1) (a) (a3) 3 1 (a4) (a5) (a6) Inpu Timing Jier RMS [ps] Fig.7 Oupu versus inpu rms iming jier of he daa signal (Inse) Eye diagrams before (a.1-3) and afer (a.4-6) he reiming scheme for hree differen values of induced iming jier: rms~1.8ps (p-p~10.4ps), rms~.9 (pp~15.3ps) and rms~3.5ps (p-p~16.4ps). Time scale: 10ps/div. The daa source, synchronised o he EFRL signal, was a gain-swiched Disribued Feedback (DFB) laser, operaing a 1556nm. The pulses were compressed down o ~ps using 15m of DCF followed by a nonlinear compression sage. The pulses were hen modulaed by a 31-1 pseudorandom bi sequence using a lihium niobae modulaor and fed ino 0m of HNLF (γ ~0W -1 km -1 ) by a splier. The HNLF has a zero-dispersion wavelengh a 1550nm, which ensured minimum walk-off ime beween he wo signals. An opical delay line was used o emporally overlap he wo signals. The daa signal was hen filered using a 5 nm broadband opical filer and hen launched ino 500 m of SMF as required o remove he chirp, and hus o reime he pulses. We arificially inroduce iming jier on he daa pulses by frequency modulaing he 10 GHz RF

5 Fig. 8 Experimenal configuraion of he TADM. drive signal of he laser wih a 4 khz one. The amoun of induced iming jier could be changed by varying he ampliude of he frequency modulaion. We evaluaed he qualiy of our reiming scheme a he sysem oupu for several values of iming jier deliberaely applied o he inciden daa pulses. These measuremens are summarised in Fig.7, where we also show examples of eye diagrams a he inpu and oupu of he reiming scheme, obained for incoming signals which have an rms iming jier of ~1.8 ps ((peak-peak (p-p) of ~10.4 ps), ~.9 ps ( p-p of ~15.3 ps) and ~3.5 ps ( p-p of ~16.4 ps) respecively. The measured jier of he pulses a he oupu of he reiming scheme cases was measured as ~1 ps (RMS value) he resoluion limi of he sampling scope used for our measuremens. 3. Add drop muliplexing Time division add-drop muliplexing (TADM) is likely o become a key funcion in fuure ulra-highspeed opical communicaions. TADM has been demonsraed by various groups based on he dropping of channels direcly wihin he ime domain using a fas opical swich. However he repored values of suppression raio beween he hrough and cleared channels is ypically less han 5-dB and is limied by he finie exincion raio of he swich. This severely limis he cascadabiliy of such devices. Here we demonsrae a TADM approach capable of providing a 35-dB channel suppression raio based on OTDM o WDM forma conversion [15]. Our scheme is shown in Fig.8. The incoming OTDM signal is used o sample a chirped opical pulse hereby generaing a mixed TDM-WDM represenaion of he inciden OTDM signal. Each ribuary channel is mapped ono a separae WDM channel which allows furher processing of he signal wihin he frequency domain. (Addiional ime domain processing can also be performed a his sage if required). Since we are using linearly chirped pulses and a 1:1 mapping exiss beween he ime and wavelengh domains his allows us o use high exincion raio filers, (high precision fibre graings in our case), o obain high exincion raio channel drops when we conver he inermediae TDM-WDM signal back o an OTDM signal a he TADM oupu. The emporal windows of he hrough and drop pors are dependen on he linear chirp rae of he mapping pulses. For our paricular sysem, where we have used mapping pulses wih a linear chirp rae of 6 nm over 100 ps, he 1.5 nm bandwidh of he FBG corresponds o a swiching window of 5 ps (Fig.9). For reference, he specral races shown in he figure were aken wih a resoluion of 0.01 nm which corresponds o a emporal resoluion of 170 fs. These races also show ha he FBG had a minimum side lobe exincion of 4 db in reflecion and a ransmission loss a of 35 db a he peak of he refleciviy curve. The channel suppression is calculaed by inegraing he power wihin he imeslo before and afer he hrough por. In his case we obained 33dB across he full-widh a halfmaximum of he dropped channel. The OTDM daa pulses were generaed using a gainswiched diode laser producing 9-ps pulses a a repeiion rae of 10-GHz and a cenral wavelengh of 1546-nm. They were modulaed exernally wih a 31-1 PRBS and muliplexed o a line rae of 40Gbi/s. Wavelengh (nm) Transmission (5 db/div) 4-dB Through Dropped 35-dB Time delay (ps) Fig. 9 Through and drop por swiching windows. 100

6 W ave le ng h ( nm ) (a) Back-o-back (b) TDM-WDM P o w e r ( d B m ) (c) Through-Channels P o w e r ( d B m ) P o w e r ( d B m ) W a v e l e n g h ( n m ) W a v e l e n g h ( n m ) (d) Dropped-Channel (e) Add-channel W a v e l e n g h ( n m ) Fig. 10 Lef: Opical specrum ( nm) (5 db/div, nm/div, 0.01-nm resoluion). Righ: Eye diagrams. Fig.10(a) shows he corresponding specrum and eye diagram. A he add-drop muliplexer inpu he 40 Gb/s daa was amplified o an average power of 0-dBm and launched ino he conrol por of a highly-nonlinear fibre opical loop mirror (HNLF- NOLM). The signal o he HNLF-NOLM comprised 100-ps linearly-chirped recangular pulses a a 10- GHz repeiion rae and highly fla op bandwidh of 6-nm. A he HNLF-NOLM oupu he conrol pulses were filered ou and a TDM-WDM replica of he original daa was carved ono he chirped pulses, Fig. 10(b). Each channel -3dB bandwidh was 0.7-nm and he spacing beween hem 1.4-nm. The TDM-WDM signal was hen ransmied hrough he unable fibre Bragg graing (FBG) add/drop muliplexer using he circulaor arrangemen shown in Fig.8. The drop por was hen por 3 of C1 and he add por of a new 10- Gb/s channel was por 1 of C. Fig. 10(c) and (d) show he specra and eye diagrams of he hrough and dropped channels respecively. Raher han using a new pulse source we chose o re-injec he dropped signal hrough he add por. Noe ha his case represens he wors case in erms of coheren inerference wih he previously dropped channel. The second HNLF-NOLM was used o conver he mixed TDM-WDM signal back o a single wavelengh 40 Gb/s TDM signal. The polarizer was insered o equalize he SOP of all channels a he inpu of he second NOLM swich. The TDM-WDM signal was hen amplified o an average power of 18- dbm and used as he NOLM conrol o swich a coninuous-wave beam a 1541-nm. A he oupu of he NOLM a 1.3-nm band-pass filer isolaed he swiched signal from he conrol, as shown in Fig. 10(e). The final resuling signal was characerised using a 40 Gb/s receiver consising of a 40:10 Gb/s elecro-absorpion modulaor-based demuliplexer. Error-free operaion was achieved wihin a -db power penaly for all cases, and no degradaion due o crossalk was observed for he added channel. Finally we esed he simulaneous dropping of wo channels in a proof-of-principle demonsraion using he wo graing arrangemen of Fig.11. The eye diagrams ha we obained are shown in he same figure. back-o-back 1 WADM 1 3 TFBG 1 3 TFBG drop ch.1 drop ch. Fig.11 Simulaneous muliple channel drop. 4. CONCLUSIONS Through-Channels In conclusion we have reviewed recen progress in he area of boh nonlinear fibre and SSFBG fabricaion. We have shown ha by combining hese wo enabling echnologies i is possible o perform a range of ulrafas all-opical processing funcions in a relaively simple and sraighforward manner. We consider our resuls o highligh ha his a mos promising approach o manipulaing and conrolling opical signals wihin fuure opical neworks. REFERENCES [1] G. Charle e al., paper PDP 36, OFC 004. [] H.G. Weber e al, Th4.1., Proc. ECOC 005. [3] M. Takahashi e al. paper We4.P019, Proc ECOC 004. [4] M. Hirano e al., PD paper Th4.4.4, Proc ECOC 005. [5] K. Kikuchi, K. Taira, N. Sugimoo, Elecron. Les, 38, pp , 00. [6] D.J. Richardson e al., Proc IEEE WFOPC 005. [7] V. Finazzi, T.M.Monro, D.J.Richardson, J. Op. Soc. Am. B, 0, pp (003). [8] J.Y.Y. Leong e al. Paper PDP, OFC 005. [9] J.Y.Y. Leong e al. PD paper Th4.4.5, proc ECOC 005. [10] F. Polei e al., Op. Exp., 13, pp , 005.[ 11] M. Ibsen e al., IEEE Phoon. Technol. Les, 10, pp.84-4, [1] J.H. Lee e al., IEEE Phoon. Technol. Les, 14, pp.03-5, 00. [13] P. Peropoulos e al., Op. Les., 5, pp.51-3, 001. [14] F. Parmigiani e al., Paper Tu4.6.4, ECOC 005 [15] P. J. Almeida e al., Paper OWW3, OFC 006.