i -1 :JOam-l2:flflpm(lnvited) WO4

Clock recovery is achieved because the intciisity fluctuations of incoming data creatcs a time-dependcnt carricr density fluctuations at tlic clock frequcncy inside the laser cavity, which leads to frequency locking arid drastically-cnliaiiced mutual coherence of the two modcs rcsponsiblc for self-pulsing in the DFB laser. The output intensity of the DFR laser again becomes the rccovered clock signal. We shall call the sccond mode of clock recovery incoherent clock recovery since it rclics on the intensity-induced carrier density tluctuatioiis. As expected, incoherent clock rccovery possesses the advantagc of wavelength (arid to soiiie degree polarization) insensitivity while coherent clock rccovery possesses the advantage of low threshold power (01.

better sensitivity). Sciisitivity Cor

0-7803-5947-W00/$10.0002000 IEEE

RZ Data --b h,

Probe la

e x TSDFB

, 11

e TSDFB

Pigitre 1. Expcrimcntal sctiip for (a) incoherent (b) coherent wavelength-iosctisitivc clock ~ U C O V C I y.

521

As mentioned ;~bovc, the iiiconriiig signal into ihc two-sectioii DIT3 laser tnusi cotitaiii a clock component. Nonlinear dynamics of tlic BOA can also be uscd to exiIa.ciion, and eiihaiice the clock coiuponent arid enhance the clock-to-data suppt-ession ratio to achieve bcticr qudiiy oi' thc racovcred clock. l ' h e inech~unism for clock extixtiou and cnhanceinci1i IbI NI(% data is as ibllows. As N R % data passes through ihc SOA, air overshoot at eacli leading ctigc is gcricrated clue to gain rleplciiori of the SOA [TI. l'he dcplctcd carriel. density increases the rcitaciive inclcx o T tiic SOA thiurigh the I iiiewidtli enhancement IBctor and shifts the sigiral io the i~eti. I'hcrc is i i cori.cspotiding bliic shift associated with the recovery oi' tlic SOA's gain at each tixiling edge. 'l'liesc ovcrslioots have a clock conipnei i t , wliicli was iirrthcr enhaticeti tJy it notch filter curisistirig oi' iiii optical cilu!lator aiid ii iibcr l h g g gixiiiig. 'The basic cnliancernent meclianism is to convei-t ilie fque i i cy cli irps ai tlic leading ccige antl tailing edge to amplittick inodrrlaiion [4]. The }rcqucticncy-to-arnl,liti~tlc clock ciiliancciiiciit incch;inisnt applies lo K% data its well since each I<% pttlsc also experiences scli- phase niodrilation ilirotigh thc SOA. l'he tict clock ciihaiiccmeiit for K% tiepeutls o i i thc bantlwitltli, rcjectioti niic! opxiting poitii oT the notch filter.

The nonliiiear gating inccliaiiisins that we will briefly describe iticludo c~~oss-gaiii, cross-pliasc iiiocldation using SOAs, cross-absoiptioii modulation using electfo-ahsorpliori mocitilation, atid notiliitear directional coupliiig.

Reference

1. B. Sarioritis, C. Dornholdt, S. Bauer, M. Mohrle, 1'. Briiidel antl 0. Leclcrc, "System application of40 GHz all-optical clock recovery in ii 40 (;bit/s optical 3K regenerator," 0I:C 2000 l'ostcleadliiie papci' No. PD- I 1. Wciming Mao, Xinlrong Wang, Mohammed AI-inutnin and Guifang Li, "40 Cib/s all-optical clock recovery using sell-pulsation UP'E lascrs," PIocecdirigs of OlC, 79-80, 2000. tlyuck Jae Lee; Iiae Geun Kim; Jec Yon C h i ; I-lak Kyu I ,cc, "All-optical clock recovery from NKZ t lak with simple NRZ-to-PRZ converter bascd 011 sell-phase nioclulatioii of semiconductor optical ampliticr", Electronics Letters, 35( 12 ), 1999 , pp 989-990( l w ) .

4. M. MeAdains, E P e d , L). I'rovenzano, W. K. Marshal, atid 0. Yariv, Appl. Phy. Lett. 71(7), 1997.

2.

3 .

522