2544 IEEE PHOTONICS TECHNOLOGY LETTERS,VOL 1.NO.23,DECEMBEL L,2006 判决再 All-Optical Clock Recovery for NRZ-DPSK Signals Giampicro Contestabile.Maro Presi.Nicola Calabretta.and Emesto Ciaramella,Member IEEE Abstruc-We experimentally demonstrate an opticul dock IL.WORKING PRINCIPLE recovery scheme for moureturprto-zere differential phase shifting 任务：把线 NRZ-DPSK data can he detected using narrow optical fil- tering 31.By using this technique.a Gaussian-shaped filter with traction circuit,we obtain a stable and low jitter 10-GHz optical afull-width at half-maximum (FWHM)of around 60%of the the originl electrical dock in bit-error-rate measurements and bit rate provides demodulation of the incoming DPSK signal, 0 weilloscope triggering opernti性 while also strongly enhancing the signal tolerance to chromatic Iader Terms-Al-eptical dlock recovery.Fabry-Perot dispersion 3]From a physical point of view,the same consec- utive symbols (data persistencel,which are spectrally close to amplifier (SOA). the carrier.pass through the filter unaffected.On the other hand. data transitions,which are aside in the spectrum.are cut.In [31. 对某时隙 L INTRODUCTION this filter was a tiber Bragg grating(FBG)used in reflection to- gether with an optical circulator.We highlight that the part of 码流 HE synchronization issue is a serious challenge in several the input signal transmitted through the FBG is composed by synchronous and asynchronous network architectures all the transitions in the NRZ-DPSK sequence.Hence,it results 判决，恢 under stedy.In future high-speed transparent networks,a reli in a pulse at every phase transition of the incoming data.Con- able optical synchroaization signal could be needed for several seqeently,this sequence contains significant clock components. applications:namely.in receivers,in 3R regenerators and,in Un this RZike signal,cock recoverycan be perfomed.To general.in all optical signal prooessing [11.On the other hand. this aim.we use the circuit introduced in [41.[51.which is ba- phase modulated signals.as differential phse shifted keying sically made of a Fabry-Perot filter(FPF)and an SOA.Thanks (DPSK)modulation fomats,acquire inceasing popularity to the interplay of the memory properties of the high-finesse because of a number of advantages in respect of the usual FPF with the free spectral range (FSR)matching the clock fre- ON-OFF keyed (OOK)format.They show high resilience to quency and the power-limiting capability of the saturated SOA. nonlinear impairments [2]and.when using balanced detection. it is possible to fill the zero slots of an incoming RZdata stream have around 3-dB optical signal-to-noise ratio (OSNR)atvan- with optical pulses.Indeed.even in case of consecutive zero se- tage in respect of OOK.Regarding DPSK.while return-to-zero quences.when the FPF introduces pulses with decaying inten- DPSK (RZ-DPSK)signals show strong clock components in sity (and thus amplitude oscillations arise).the saturation effects both the optical and electrical spectrum.nonreturn-to-zero of the SOA can be used to equalize the pulses amplitde [4]. DPSK (NRZ-DPSK)signals have ideally noclock lines and,in The effectiveness of this circuit in all-optical regenerative appli- practical realizations,only weak clock components.This makes catioas was recently demonstrated in an over-million-kilometer by far more difficult the realization of clock rocovery systems for NRZ-DPSK data.In principle.it could be possible to use transmission experiment with 3R egeneration for both RZsig- nals [61,and NRZ signals [71 established clock recovery schemes after DPSK demodulation and a following peeprocessing stage used to emphasize the weak clock lines.Amy way.a simpler tochnique able to disectly extract the clock would be preferred.Here,we experimentally IIL EXPERIMENT AND DISCUSSION demonstrate,for the first time to our knowledge,an optical cireuit capable of NRZ-DPSK clock reoovery.This circuit is The schematic of the clock extraction circuit is reported in 甘刻” mainly based on proper selective optical fhering and uses a Fig.I.The optical carrier at=11.73 nm is generated by semiconductor optical amplifier (SOA)for signal equalization a tunable laser.The NRZ-DPSK signal is obtained by means The clock-recovered signal shows high quality and stability to of a LiNbO,phase modulator driven by a-1 long PRBS be potentially employed as an altemative to electronic circuits sequence at 10.2 Gh/s.The signal is then amplified by an EDFA 是取准 in a number of high-speed synchronization applications.ie to 13 dBm to account for the circuit loss.Then it is sent via un optoelectronic continoous or burst receivers optical circulator(OC)to the FBG. The FBG has a Gaussian reflectivity profile,with FWHM of 6 GHz and an almost flat phase response with low spurious dis- Manucript received Jaoe 14.2006 rerised Oetober 10.2006.This work wa persion.Combined with the OC.the FBG provides in reflection 卡定， partially sepportedby the Earopean Comnistian FP6p ram (L.P.NOBEL In effective DPSK demodulation [see Fig.2(e)]3].The optical Theare with S Superiore Sant'Ann Pisa laly ga5nhhoeapann spectrum of the input DPSK signal and of the transmitted part foom the FBG are reported in Fig.2(a)and (b).respectively.The transmitted part of the signal is composed by a series of pulses, 收 each one produced at a phase transition [Fig.2(f)]As can be 104115S30,000246EFF 人dlesto时nd松ae1er居k与Dw向dan Mac82s2i:4eeu程风eene中判决再生与时钟提取 任务：把线性通道输出的波形恢复成数字信号 对某时隙的码元进行 判决，恢复原始信息 为精确确定“判决时刻” ， 需要从信号码流中提取准 确的时钟信息作为标定， 以保证与发送端一致