All-optical clock recovery using erbium-doped fiber ring laser

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					OECC 2006 7C1-2-1




      All-optical clock recovery using erbium-doped
   fiber ring laser incorporating an electro-absorption
         modulator and a linear optical amplifier
                                        Lixin Xu,1,2 L. F. K. Lui,1 P. K. A. Wai,1 and H. Y. Tam,3
                      1
                       Photonics Research Centre and Department of Electronic and Information Engineering,
                                   The Hong Kong Polytechnic University, Hung Hom, Hong Kong
                            Phone: +852 2766-6231, fax: +852 2362-8439, email: enwai@polyu.edu.hk
                2
                    Department of Physics, University of Science and Technology of China, Hefei, 230026, China
                              3
                                  Photonics Research Centre and Department of Electrical Engineering,
                                   The Hong Kong Polytechnic University, Hung Hom, Hong Kong


   Abstract                                                            recovery which composes of an EAM, an LOA, a 12
   We demonstrated 10-GHz all-optical clock recovery using             meter long erbium-doped fiber, a bandpass filter, a
   an     erbium-doped       fiber     laser     incorporating   an    circulator, two isolators, a 980 nm/1550 nm WDM
   electro-absorption modulator and linear optical amplifier.          coupler, a 1550 nm band 90:10 coupler, and a polarization
   Stable clock pulses with peak power of 200 mW and                   controller (PC). The EAM is biased at a DC voltage of
   pulsewidth of 6 ps are obtained.                                    −1.015 V. A 10 Gb/s data stream is injected into the
                                                                       EAM through the circulator (C) in the direction opposite
   1 Introduction                                                      to the circulating direction of the laser cavity. The fiber
        All-optical clock recovery circuit is a key component          laser could be mode-locked and output the optical clock
   for system synchronization in all-optical communication             of the data stream if the parameters of the laser are chosen
   systems.    Several technologies including tank circuit,            properly. The data stream is generated by modulating a
   injection locking, all-optical phase-locked loop have been          10 GHz mode-locked laser source, which has a
   proposed to address this issue [1]. Among these                     pulsewidth of ~1.8 ps, using an M-Z modulator. The laser
   technologies, injection locking in a fiber ring laser is a          gain is provided by the LOA and EDFA.             The LOA
   promising approach because of its capability to generate            operates at a current of 233 mA with a gain of ~13.5 dB,
   high-intensity ultrashort optical pulses [2]. Recently, we          and a saturation output power of 13 dBm. The linearity
   have implemented a novel 10 GHz mode-locked fiber                   of the LOA gain renders the laser system less susceptible
   laser that incorporates an electro-absorption modulator             to the transients due to the variations in the ambient
   (EAM), a linear optical amplifier (LOA), and an Er-doped            conditions.    Amplitude jitter of the mode-locked fiber
   fiber amplifier (EDFA) [3].        The mode-locked fiber laser      ring laser output is therefore reduced. The EDFA was
   can have stable output pulses at very high peak power               pumped at 980 nm through the 980 nm/1550 nm WDM
   with pulse duration of 2.4 ps [3].          In this paper, we use   coupler. The bandpass filter is centered at 1555 nm with
   the fiber laser demonstrated in [3] to implement ultra-fast         a bandwidth of 6 nm and side mode suppression of 20 dB.
   all-optical clock recovery. Stable clock pulse at 10 GHz            The PC is used to optimize the polarization states of the
   with peak power of 200 mW and pulsewidth of 6 ps are                cavity modes because the EAM has a small polarization
   obtained. The timing jitter is less than 1 ps.                      dependent loss. Ten percent of the cavity energy was
                                                                       coupled to output through the 90:10 coupler.
   2 Experimental results and discussion                                    The      output   pulses   were   measured   using   a
   Figure 1 shows the configuration for all-optical clock              YOKOGAWA Optical Sampling Oscilloscope (OSO)
OECC 2006 7C1-2-2




   (model AQ7750). Figure 2(a) shows the 10 Gb/s input                  an LOA. Stable pulses with peak power of 200 mW and
   data pattern 1001001010. Figure 2(b) shows a 10 Gb/s                 pulsewidth of 6 ps are obtained. The timing jitter is less
   pseudorandom binary sequence (PRBS)                  input signal.   than 1 ps. The external data stream is used to optically
   Figure 2(c) is the recovered 10 GHz optical clock when               drive the EAM and actively mode-locked the fiber laser.
   the input data is the fixed data pattern shown in Figure             Since no electronic components are involved, our
   2(a). The output peak power and the pulsewidth of the                configuration can potentially operate at clock rate beyond
   optical clock are 150 mW (1.5 W inside the cavity) and 10            10 GHz.
   ps respectively. Figure 2(d) shows the 10 GHz recovered
   optical clock coresponding to the PRBS input data shown
                                                                         a.                                              b.
   in Figure 2(b).           The output peak power and the
   pulsewidth of the optical clock are 200 mW (2 W inside
   the cavity) and 6 ps respectively. Figure 2 show that the
   optical clock is recovered successfully. The timing jitter            c.                                              d.
   of the output clock is measured to be less than 1 ps.
   Stable optical clock output can still be observed when the
   input data rate varies within 3 MHz (60% of the
                                                                         Fig. 2. Timing diagrams at resolution of 100ps/div. (a) 10 Gb/s
   foundamental frequency which is about 5 MHz in our
                                                                         input data pattern; (b) the 10 Gb/s PRBS input signal. (c)
   setup).     Figure 3(a) is the output spectrum when the               recovered 10 GHz optical clock using the fixed data pattern in (a);
                                                                         (d) recovered 10 GHz optical clock using PRBS as shown in (b).
   input data rate is 9.9580 GHz. The wavelength of the
   optical clock is 1556.19 nm while the wavelength of the
                                                                                                                           0
   input data stream is 1546.64 nm. The bandwidth                  of
                                                                              a.                                          -20
                                                                                                                                   b.
   the optical spectrum is measured to be 1.87 nm. The                                                                    -40




                                                                                                            Power(dBm)
   pulsewidth measured from the OSO is 6 ps. The output                                                                   -60


                                                                                                                          -80

   clock pulses were significantly chirped.               Figure 3(b)                                                    -100


   gives the corresponding RF spectrum with a resolution                                                                   9.950        9.955         9.960
                                                                                                                                         RF frequency(GHz)
                                                                                                                                                              9.965




   bandwidth of 10 kHz. The sidemode suppression ratio is                Fig. 3 Optical Output spectrum and RF output Spectrum. Output
   beyond 48 dB.                                                         RF spectrum with the resolution bandwidth 10 kHz


                                   Pump                                 4     References
                                          12 m
                                                   I1
                            LOA                                         1.     L. Poti, M. Luise and G. Prati, “Ultrafast optical clock
                                   WDM Er Fiber
             10 Gb/s                                                           recovery:   towards      a                 system           perspective,”          IEE
              data
                        C                                  F
                                                                               Proc.-Circuits Devices Syst., Vol. 150, pp. 506-511, 2003.
                                            I2    90:10                 2.     H. Kurita, T. Shimizu, and H.Yokoyama, “All-optical clock
                             EAM
                                    PC
                                                                               extraction at bit rates up to 80 Gbit/s with monolithic
                ∼      AMP                        10%
                                                  Output                       mode-locked laser diodes”. Digest of CLEO’97, Vol. 11,
                                                                               pp.96-96, 1997.
   Fig. 1. Experimental setup of the all-optical clock recovery         3.     Lixin Xu, L. F. K. Lui, P. K. A. Wai, H. Y. Tam, and M. S.
   circuit; F: band pass filter; I1, I2: isolator; EAM:
   electroabsorption modulator; C: circulator; LOA: linear                     Demokan, “10 GHz actively mode-locked erbium-doped
   optical amplifier.                                                          fiber ring laser using an electro-absorption modulator and a
                                                                               linear optical amplifier” Proc. of OFC’2006, paper OWI27,
   3 Summary                                                                   2006.
   We demonstrated 10-GHz all-optical clock recovery using
   an erbium-doped fiber laser that incorporates an EAM and

				
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