Pattern compensation in SOA-based gates. Article (peer-reviewed)

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1 Tile Paern compensaion in SOA-based gaes Auhor(s) Webb, Rod P.; Dailey, James M.; Manning, Rober J. Publicaion dae 21 Original ciaion Type of publicaion Link o publisher's version Righs Webb, R.P., Dailey, J.M., Manning, R.J. (21) 'Paern Compensaion in SOA-Based Gaes'. Opics Express, 18 (13): Aricle (peer-reviewed) hp:// hp://dx.doi.org/1.1364/oe Access o he full ex of he published version may require a subscripion. 21 Opical Sociey of America. This paper was published in Opics Express and is made available as an elecronic reprin wih he permission of OSA. The paper can be found a he following URL on he OSA websie: hp:// Sysemaic or muliple reproducion or disribuion o muliple locaions via elecronic or oher means is prohibied and is subjec o penalies under law Iem downloaded hp://hdl.handle.ne/1468/41 from Downloaded on T1:54:22Z

2 This paper was published in Opics Express and is made available as an elecronic reprin wih he permission of OSA. The paper can be found a he following URL on he OSA websie: hp:// Sysemaic or muliple reproducion or disribuion o muliple locaions via elecronic or oher means is prohibied and is subjec o penalies under law.

3 Paern compensaion in SOA-based gaes R.P. Webb, J.M. Dailey and R.J. Manning Tyndall Naional Insiue & Deparmen of Physics, Universiy College Cork, Lee Malings, Cork, Ireland Absrac: We propose a novel scheme employing complemenary daa inpus o overcome he paerning normally associaed wih semiconducor opical amplifier based gaes and demonsrae he scheme experimenally a 42.6Gb/s. The scheme no only avoids inroducing paerning during swiching, bu also compensaes for much of he paerning presen on he inpu daa. A novel gae was developed for he experimen o provide he complemenary signals required for he scheme. 21 Opical Sociey of America OCIS codes : (2.376) Logic-based opical processing; (2.615) Signal regeneraion, (25.314) Inegraed opoelecronic circuis.. References and links 1. Y. Ueno e al., "Nonlinear phase shifs induced by semiconducor opical amplifiers wih conrol pulses a repeiion frequencies in he 4 16-GHz range for use in ulrahigh-speed all-opical signal processing", J. Op. Soc. Am. B, Vol. 19, No. 11, pp , OSA, Y. Liu e al., "Error-Free All-Opical Wavelengh Conversion a 16 Gb/s Using a Semiconducor Opical Amplifier and an Opical Bandpass Filer Journal of Lighwave Technology, Vol. 24, No. 1, pp 23-6, IEEE, R. J. Manning e al., The 'Turbo-Swich' - a Novel Technique o Increase he High-speed Response of SOAs for Wavelengh Conversion, Conference on Opical Fibre Communicaions (OFC), OWS8, Anaheim, CA, USA, R. P. Webb and R. J. Manning, Compensaion for Paerning in SOA-Based Swiches, Phoonics in Swiching, WeI3-6, Pisa, Ialy, S. Bischoff, M. L. Nielsen, and J. Mørk, Improving he All-Opical Response of SOAs Using a Modulaed Holding Signal, Journal of Lighwave Technology, Vol. 22, No. 5, pp 133-8, IEEE, I. Kang e al: "All-opical XOR and XNOR operaions a 86.4 Gb/s using a pair of semiconducor opical amplifier Mach-Zehnder inerferomeers", Opics Express, Vol. 17, No. 21, pp , OSA, G. Conesabile e al: "Opical Reshaping of 4-Gb/s NRZ and RZ Signals Wihou Wavelengh Conversion", Phoonics Technology Leers, Vol. 2, No. 13, pp , IEEE, G. Talli and M. J. Adams, Gain dynamics of semiconducor opical amplifiers and hree-wavelengh devices, J. Quanum Elecronics, Vol. 39, No. 1, pp IEEE, G. D. Maxwell, Hybrid Inegraion Technology for High Funcional Devices for Fuure Opical Communicaions, Conference on Opical Fibre Communicaions (OFC), OWI3, San Diego, CA, USA, J. M. Dailey and T. L. Koch, Simple Rules for Opimizing Asymmeries in SOA-Based Mach Zehnder Wavelengh Converers, Journal of Lighwave Technology, Vol. 27, No. 11, pp 148-8, IEEE, Inroducion Inerferomeers incorporaing semiconducor opical amplifiers (SOAs) are widely regarded as he mos pracical form of all-opical gae for elecommunicaions applicaions, offering wha many consider o be he bes combinaion of speed, size and power consumpion. Differenial or push-pull operaion allows swiching by reurn-o-zero (RZ) signals a raes beyond he limiaion of he SOA recovery ime [1], bu alhough swiching windows of a few

4 picoseconds duraion can be precisely defined in ime, he size of he phase difference creaed in he inerferomeer sill varies from pulse o pulse. Because SOAs canno fully recover beween consecuive pulses, he firs pulse in a rain induces a larger phase difference han do subsequen pulses and consequenly swiches a larger proporion of he probe power, resuling in he familiar paerned oupu characerisic of SOA-based gaes operaing a raes of 4Gb/s and above. One approach o he problem is o increase he effecive speed of he gae wih an ensuing componen, which may be eiher an offse filer afer he inerferomeer [2] or an addiional SOA in he Turboswich configuraion [3]. Here, however, as firs repored in [4], we make use of an invered inpu signal in order o presen boh SOAs in a Mach-Zehnder gae wih near consan inpu energy in every bi period, hus removing he iniial cause of paerning. A complemenary pair of signals was generaed by a modified form of Mach-Zehnder gae developed for he purpose and, wih hese signals, he paern compensaion scheme was implemened in a second gae. The probe o he firs gae can be eiher a sream of pulses, in which case he wo complemenary signals boh have an RZ forma, or a CW inpu, in which case he invered signal has a varying level and resembles he modulaed holding beam used in a previously repored paern compensaion scheme [5]. Anoher scheme, applicable o XOR gaes, ha employed complemenary inpu signals o reduce paerning has achieved 86.4Gb/s operaion [6]. A regeneraor based on wo sages of cross-gain modulaion has also made use of complemenary signals o avoid paerning by ensuring ha he power in he second sage remained consan [7]. However, by removing he power variaions wihin each bi period, his sysem also cancels he phase changes ha would be needed in an inerferomeer-based gae. The scheme described here uses he invered signal o provide paern compensaion wihou swiching he gae. As a resul, i is applicable o a variey of gaes exploiing he cross-phase modulaion induced by an RZ signal in a Mach-Zehnder inerferomeer (MZI). In his paper, we presen numerical simulaions ha elucidae he operaing mechanism ogeher wih new experimenal resuls. 2. Principle Inpu daa, 1 Daa complemen probe, 2 Delay = SOA 1 SOA 2 P ou, Paern compensaion Fig. 1. Paern compensaion scheme applied o an SOA-MZI gae. Paerning arises in an SOA-MZI gae because he SOAs do no receive he same inpu energy in each bi period when hey are being swiched by an arbirary daa signal. However, if he logical complemen of he daa is also applied o each SOA, hen he oal inpu energy per bi period can be made consan. The daa complemen mus be prevened from also swiching he device and his may be achieved by applying i o boh SOAs simulaneously, e.g. hrough an inpu o he inerferomeer (Fig. 1). When he inpu daa is a 1, he firs SOA receives a push pulse and he second SOA receives a pull pulse afer a shor delay, causing he device o swich for he iner-pulse inerval in he normal way. When he inpu daa is a, boh SOAs receive a pulse simulaneously and no swiching occurs (Fig. 2). Thus boh SOAs receive he same pulse energy in every bi period wih jus a small variaion in arrival ime. The SOA gains are herefore close o he same values a every pulse arrival and paerning is

5 subsanially reduced. Push-pull operaion is a necessary par of he compensaion scheme and so he inpu daa mus have an RZ forma. Daa Daa complemen Inpu o SOA 1 Inpu o SOA 2 Delay = Fig. 2. Inpus received by SOA 1 and SOA 2. An anicipaed addiional benefi of his scheme is ha i will become less imporan o minimise he recovery ime of he SOAs by operaing hem a high bias currens, which offers he prospec of SOA-based gaes wih lower power consumpion. 2.2 Derivaion of complemenary signals A prerequisie for he implemenaion of his scheme is a complemenary pair of inpu signals and here are various means by which hese may be obained. The invered signal could be generaed from he daa using cross-gain modulaion in an SOA [5]. Alernaively, complemenary signals may be available from he device preceding he gae in he sysem. For example, when differenial phase-shif keyed (DPSK) signals are received, hey are decoded wih he aid of an inerferomeer which generaes complemenary ampliude-modulaed signals a is wo oupus. Logic gaes employing inerferomeers can also produce complemenary oupus. We have designed a novel form of he SOA-MZI gae ha allows independen opimisaion of he daa and daa complemen oupus (Fig. 3). SOAs Phase adjusers Daa in Probe Push Pull Daa ou Daa ou Fig. 3. SOA-MZI gae wih independenly opimised oupus. Comparison wih he convenional form of he gae (as in Fig. 1), shows ha he modified gae has a second MZI parly overlaid on he firs. The phase differences beween he arms in he wo inerferomeers can be adjused separaely o obain he bes exincion raios for he daa and is complemen. Proper complemenary oupus can only be obained from a single inerferomeer when he phase difference induced during swiching is exacly π radians for every daa pulse. 3. Numerical simulaion In order o illusrae he operaion of he compensaion scheme, a simulaion was carried ou using a rae-equaion based SOA model similar o ha presened in [8]. Firs he gae was simulaed wihou paern compensaion. A shor sequence of 4Gb/s daa consising of 3ps pulses wih mean power 3.5dBm served as he inpu daa and a -1dBm CW probe was used. The spli of he inpu power beween SOA 1 and 2 and he phase difference beween he arms of he inerferomeer were adjused o give an oupu wih good exincion raio and minimum

6 paerning. The delay beween he push and pull inpus was se o 6ps. Boh SOAs were biased a 3mA. SOA 1 SOA 2 Daa inpu Probe a SOA oupu 25ps SOA 1 SOA 2 Probe phase a SOA oupu SOA 1 SOA 2 π Phase difference MZI oupu Fig. 4. Simulaion of a convenional SOA-MZI gae showing paerned SOA responses and oupu. The ampliudes and phases of he amplified probe signals a he oupus of SOA 1 and 2 are shown in Fig. 4. From hese he phase difference and he resulan signal a he MZI oupu were calculaed. A his bi rae, he probe ampliude and phase could only recover o seadysae levels when here were hree or more consecuive zeros, so he response o a daa pulse depended on he values of he previous few bis. In paricular, a large phase change was induced by he firs bi in a rain of pulses bu he subsequen phase changes became progressively smaller. The phase difference creaed beween he MZI arms during each swiching window herefore varied, despie he good cancellaion beween swiching windows. Consequenly he oupu pulse heighs showed significan paerning. The simulaion was repeaed wih a 3dBm mean power daa complemen signal, also comprising 3ps pulses, spli equally beween he wo SOA inpus (Fig. 5). The SOAs now received a pulse in every bi period and as a resul underwen almos he same gain and phase excursions in every period. During a 1, he push-pull delay resuled in a phase difference beween he wo SOA oupus for he duraion of he swiching window, bu he phase changes during a were simulaneous. Now he phase differences creaed by he daa pulses in each swiching window were almos idenical, ye he phase variaions induced by he daa complemen pulses were accuraely cancelled. As a resul, he oupu of he MZI showed negligible paerning.

7 SOA 1 SOA 2 Daa inpu Probe a SOA oupu 25ps SOA 1 SOA 2 Probe phase a SOA oupu SOA 1 SOA 2 π Phase difference MZI oupu Fig. 5. Simulaion of an SOA-MZI gae wih paern compensaion. (Daa complemen shown in red wih daa inpus.) 4. Experimenal sysem 4.1 Reurn-o-zero compensaion signal A pair of SOA-MZI gaes having he configuraion described in Secion 2.2 was fabricaed by CIP Technologies using heir hybrid-inegraion process [9]. The firs gae provided complemenary oupu signals wih which paern compensaion was implemened in he second gae (Fig. 6). The inpus o he firs gae were obained from a pseudo-random bi sream (PRBS) of 3ps pulses a 42.6Gb/s, which was divided ino push and pull inpus. A pair of opical aenuaors a he inpu ogeher wih he inegraed phase adjusers enabled he oupu signal o be opimised [1]. Signal powers and wavelenghs are shown in Table 1. The probe inpu was a rain of 2ps clock pulses a 42.6GHz. The currens for he SOAs in he push and pull arms were 4mA and 36mA respecively. One of he inerferomeers in he firs gae was phase-biased o give a non-invered oupu which was amplified and spli o provide push and pull inpus for he second gae.

8 Aenuaors PRBS Clock CW probe (experimen 4.2) Daa CW probe Variable delays SOAs Push Pull Push Phase adjusers Linear SOAs Oupu Daa Pull Hybrid inegraed circui Fig. 6. Experimenal sysem. The second inerferomeer of he firs gae was biased o produce an invered daa oupu o ac as he compensaion signal. Being composed of swiched clock pulses, i was an RZ signal like he non-invered daa oupu. This signal was also amplified and conneced o he second gae hrough an inerferomeer inpu o ensure ha i reached boh SOAs simulaneously. The delay beween he push and pull pulses in he second gae was 1ps and he compensaion signal was synchronised wih he cenre of his inerval. A CW source was used as he probe. Boh SOA currens were se o 4mA and heir 1/e recovery imes were esimaed a 4ps under he experimenal condiions. The final oupu was aken from one of he inerferomeers in he second gae, which was biased o give a non-invered signal wih a mean power of -6.dBm. Gae 1 Table 1: Signal powers and wavelenghs for RZ compensaion signal experimen. Wavelengh (nm) Power (dbm) Gae 2 Wavelengh (nm) Power (dbm) Push inpu Push inpu Pull inpu -4. Pull inpu -3.5 Clock Compensaion inpu -.6 Daa ou.3 CW probe Daa complemen Daa ou Reurn-o-on compensaion signal In he experimen jus described, he compensaion signal had an RZ forma only because he probe in he firs gae consised of a rain of clock pulses, bu i is ofen more convenien o use a CW probe. In ha case, alhough he daa oupu from he firs gae would remain in he RZ forma, he invered oupu would ake he form of a reurn-o-on signal. Tha is, is oupu power would be nulled during he swiching window and have a varying on-level beween swiching windows as he SOA gains recovered. Accordingly, a second experimen was performed in order o esablish wheher such a signal could ac as a compensaion signal. A CW probe was applied o he second inerferomeer inpu of he firs gae, in addiion o he clock probe already conneced (Fig. 6). Two probes were used in order o generae he daa and compensaion signals a differen wavelenghs, hus prevening inerference in he second gae in he even of incomplee exincion of eiher signal. (Boh probes could have been CW inpus, bu i was

9 experimenally expedien o reain he clock.) The filers a he invering oupu of gae 1 were reuned o he CW wavelengh (1553nm) and hus passed he reurn-o-on oupu o gae 2 as he compensaion signal. The filers a he non-invering oupu were lef unchanged and coninued o pass he swiched clock pulses as before. The signal powers and wavelenghs employed in his experimen are lised in Table 2. Table 2: Signal powers and wavelenghs for reurn-o-on compensaion signal experimen. Gae 1 Wavelengh (nm) Power (dbm) Gae 2 Wavelengh (nm) Power (dbm) Push inpu Push inpu Pull inpu -6.7 Pull inpu -3.8 Clock Compensaion inpu CW probe CW probe Daa ou Daa ou Resuls 5.1 Reurn-o-zero compensaion signal The oupu of gae 2 was recorded wih boh an opical sampling oscilloscope and wih a 7GHz elecronic oscilloscope (Fig. 7). Before he compensaion signal was conneced, he oupu showed a variaion beween he maximum and minimum pulse ampliudes of 3.2dB, a degree of paerning ha migh be expeced afer he passage of he signal hrough wo successive gaes. Connecing he compensaion signal reduced he variaion o 1.dB. Gae 2 oupu: no compensaion signal , ps 1ps/div Gae 2 oupu wih paern compensaion , ps Fig. 7. Waveforms (opical sampling oscilloscope) and eye diagrams (elecronic oscilloscope) wih no compensaion and wih an RZ compensaion signal. 5.2 Reurn-o-on compensaion signal For his experimen, he oupus of boh gaes were recorded (Fig. 8). The variaion in pulse ampliude afer gae 1 was 2.4dB. Wihou compensaion, his was lile changed by passing

10 hrough gae 2, bu i is noiceable ha he pulse ampliude hen fell o is minimum value even for he second pulse in a rain. The compensaion signal produced by gae 1 shows clear nulls during he swiching windows, bu a widely varying power level beween windows. Neverheless, when conneced o gae 2 i caused a subsanial reducion in he oupu paerning. Alhough he variaion in pulse ampliude was sill 1.7dB, i was less han ha on he oupu from he firs gae. 15 Gae 1 oupu Gae 2 oupu. No compensaion. 1 2, ps , ps , ps , ps , ps , ps Compensaion signal 1 from gae 1. 5 Gae 2 oupu wih compensaion , ps 3 4 Fig. 8. Waveforms (opical sampling oscilloscope) and eye diagrams (elecronic oscilloscope) wih no compensaion and wih a reurn-o-on compensaion signal. 6. Conclusions We have proposed a novel scheme for prevening he oupu paerning ha normally occurs in SOA-based gaes. I is applicable o he widely used SOA-MZI configuraion and makes use of he complemen of he inpu daa o equalise he pulse energies received by SOAs in each bi period wihou causing unwaned swiching. Nohing in he scheme reduces he operaing speed of he gae and i should herefore be applicable a he highes speeds of which his ype of gae is capable. The scheme has been experimenally demonsraed a 42.6Gb/s wih boh RZ and reurno-on compensaion signals ha were obained from a preceding gae wih a novel srucure designed for he purpose. The RZ compensaion signal gave he beer performance, bu in boh cases, a subsanial reducion in paerning was observed when he compensaion signal was conneced. Paerning was reduced o a level below ha on he signal received from he uncompensaed firs gae, showing ha he scheme no only prevened he second gae from causing addiional paerning, bu also compensaed for he paerning already inroduced. Acknowledgemen This work was suppored by Science Foundaion Ireland under gran 6/IN/I969.