IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 11, NOVEMBER 2004 2523 40-GHz All-Optical Clock Extraction Using a Semiconductor-Assisted Fabry–Pérot Filte Giampiero Contestabile, Antonio D’Errico, Marco Presi, and Ernesto Ciaramella Abstract—We obtain 40-Gb/s all-optical clock recovery by means of a very simple scheme. We use a high finesse low-loss Fabry–Pérot filter and a semiconductor optical amplifier acting as an amplitude equalizer. The recovered clock signal shows large locking range, low amplitude fluctuation, and limited time jitter. Index Terms—All-optical clock recovery, Fabry–Pérot filter (FPF), optical tank circuit (OTC), semiconductor optical amplifier (SOA), time jitter. I. I NTRODUCTION T HE ALL-OPTICAL clock recovery function will be a key building block for future all-optical networks. Hence, sev- eraltechniques have been demonstrated so far. Among them, mode-locked ring lasers [1], electronic [2] or optical [3] phase- locked loops, and self-pulsating distributed feedback lasers [4] generally suffer from high architectural complexity or use very complex semiconductor-based devices. On the other hand, optical tank circuit (OTC) [5] is a very simple technique with potentially ultrahigh-speed operation and very short capture time. OTC is realized by means of an optical Fabry–Pérot filter (FPF). The FPF extracts the carrier and the timing components from the optical spectrum of a return-to-zero (RZ) intensity modulated signal. In [5], the OTC technique was first proposed and demonstrated at 2 GHz, using an FPF with a finesse F value of 170, without characterizing the recovered clock signal in terms of amplitude and time jitter. Typical FPFs, however, have a limited F value which results in a noisy clock signal. Hence, in [6] and [7], OTC circuits realized with a rela- tively low finesse filter ( 21) were used as preprocessors, i.e., in combination with other clock extraction schemes. As a sim- ilar application, the use of an FPF with a semiconductor optical amplifier (SOA) has been proposed for repetition rate upgrade of optical pulse sources [8]. In that case, because of the poor F value, a double-pass setup was necessary. Here we demonstrate the feasibility of complete clock re- covery operation by using an FPF with a high F value and an SOA acting as an amplitude equalizer in a single pass con- figuration. We report the experimental results of the proposed Manuscript received May 27, 2004; revised July 8, 2004. This work was sup- ported in part by a grant from Marconi Communications SpA. G. Contestabile and E. Ciaramella are with Scuola Superiore Sant’Anna, Pisa 56124, Italy, and also with Photonic Networks National Laboratory, CNIT, Pisa 56124, Italy (e-mail: giampiero.contestabile@cnit.it). A. D’Errico is with Photonic Networks National Laboratory, CNIT,Pisa 56124, Italy. M. Presi is with the Physics Department, Università di Pisa, Pisa 56100, Italy, and also with Photonic Networks National Laboratory, CNIT, Pisa 56124, Italy. Digital Object Identifier 10.1109/LPT.2004.835608 Fig. 1. 40-Gb/s clock recovery experimental setup. SOA-assisted OTC operating with a 40-Gb/s signal and fully characterize the extracted clock signal. II. E XPERIMENT In the spectrum of an RZ signal, the carrier and the cloc could, in principle, be isolated from the information encod the intensity modulation. This could be obtained by using an FPF with high F value and a free spectral range (FSR) exa matching the clock frequency. In the time domain, this ope corresponds to fill the zero slots in the data stream with op pulses. Indeed, the input pulses are reflected forward and ward in the FPF and partially extracted from the output m each round-trip. The effectiveness of this clock recovery pr can be affected by various experimental parameters: name carrier phase noise and frequency drift of the signal, the F and the wavelength stability of the FPF [9]. The experimental layout is reported in Fig. 1. In our dem stration, we use a fiber-based FPF with FSR GHz, F (full-width at half-maximum (FWHM) of about 150 MHz and 4-dB insertion loss. The filter can be wavelength-tuned means of an ultra-fine Peltier cooler. First,we generate a pulse train by externally modulating continuous-wave distributed feedback laser at nm, with a low-chirp electroabsorption modulator (EAM). The frequency of this pulse train is 39.97 GHz, exactly matchin FSR value. Then we produce a 39.97-Gb/s data stream by s perimposing a 9.9925-Gb/s pseudorandom bit sequence (PRBS) using a Mach–Zehnder (MZ) intensity modulator. W modulate the RZ train with a PRBS at exactly a quarter of pulse frequency. This PRBS is synchronized to the 40-GHz pulse train by means of a 10-MHz reference signal. In this we overcome the lack of a 40-Gb/s PRBS generator, produ a sequence of RZ pulses modulated four at a time.This data stream is not actually pseudorandom, but shows, compare a 10-Gb/s PRBS, four times longer sequences of zeros (i.e., the longest sequence is bitlong),hence, it is very suitable for testing and stressing a clock recovery cir 1041-1135/04$20.00 © 2004 IEEE