JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 17, SEPTEMBER 1, 2010 2521

All-Optical Clock Recovery for 20 Gb/s Using anAmplified Feedback DFB Laser

Yu Sun, Jiao Qing Pan, Ling Juan Zhao, Weixi Chen, Wei Wang, Li Wang, Xiao Fan Zhao, and Cai Yun Lou

Abstract—We report all optical clock recovery based on a mono-lithic integrated four-section amplified feedback semiconductorlaser (AFL), with the different sections integrated based on thequantum well intermixing (QWI) technique. The beat frequencyof an AFL is continuously tunable in the range of 19.8–26.3 GHzwith an extinction ratio above 8 dB, and the 3-dB linewidthis close to 3 MHz. All- optical clock recovery for 20 Gb/s wasdemonstrated experimentally using the AFL, with a time jitterof 123.9 fs. Degraded signal clock recovery was also successfullydemonstrated using both the dispersion and polarization modedispersion (PMD) degraded signals separately.

Index Terms—All optical clock recovery, distributed feedback(DFB) lasers, injection locking, semiconductor lasers, quantumwell intermixing (QWI), optical signal processing.

I. INTRODUCTION

3R signal regeneration (Re-amplification, Retiming,and Reshaping) is a key function for ultra-long-haul

transmission and for scalable networks with optical switchingnodes. The 3R regeneration which can improve signal in boththe amplitude and the time domains is important for repairingoptical signals that has been degraded from accumulated noisesafter data transmission. Clock recovery is a key componentof 3R regeneration for retiming and reshaping. As high-speedoptical transmission such as all-optical packet routing systemsrequires high-speed operation and data transparency, all-optical3R regeneration approach is preferred compared to electrical3R regeneration. Monolithic InP based self-pulsation laserdevices have demonstrated all optical clock recovery based onthe injection locking mechanism. These devices have manyadvantages such as compactness, low-power consumption,good reliability, low cost, simple biasing circuit, and havebeen studied extensively. Several types of compact InP basedsemiconductor laser based devices have been reported forall-optical clock recovery, such as mode-locked laser diodes

Manuscript received March 26, 2010; revised June 17, 2010; accepted June21, 2010. Date of publication June 28, 2010; date of current version August18, 2010. This work is supported by the National 863 project under Grants2006AA01Z256, 2007AA03Z419, and 2007AA03Z417, in part by the National973 Program of China under Grants 2006CB604901 and 2006CB604902 andthe National Natural Science Foundation of China under Grants 90401025,60736036, 60706009, and 60777021.

Y. Sun, J. Q. Pan, L. J. Zhao, W. Chen, and W. Wang are with the Key Labora-tory of Semiconductor Materials Science, Institute of semiconductors, ChineseAcademy of Science, Beijing 100083, China (e-mail: sunyu2006@semi.ac.cn).

L. Wang, X. F. Zhao, and C. Y. Lou are with the Department of ElectronicEngineering, Tsinghua University, Beijing 100084, China.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2010.2055539

(MLLD) [1], dual-mode laser with two different DFB-sec-tions (TS-DFB) [2] and amplified feedback DFB lasers (AFL)[3]–[5]. For MLLDs the RF oscillating frequency is to firstorder determined by the length of their cavities, the limitationof the fabrication accuracy makes achieving precise frequencycontrol difficult. TS-DFB and AFL designs both include fre-quency tuning sections. The TS-DFB laser is composed oftwo different DFB sections, making the fabrication processrather complicated. The TS-DFB laser operation is based onthe well-balanced interaction of dual lasing modes from thetwo different DFB sections, which requires precise control ofthe device dimensions and biasing conditions. Compared toTS-DFB, AFLs based on a single DFB section has a simplerfabrication process. It consists of a tunable compound cavitywith two longitudinal lasing modes which can be adjusted tohave the same threshold gain.

The previous work had theoretically predicted that compoundcavity semiconductor lasers could generate dual longitudinalmodes [6]. Amplified external cavity feedback lasers had beenfabricated as microwave sources, which could generated 28GHz to 41 GHz tunable frequency microwave [7]. Four-sectionAFL and three-section AFL were fabricated to realize 10 Gbsclock recovery based on passive mode locking mechanism and33 Gbs clock recovery based on compound cavity mode beatingmechanism, respectively [4], [5].

One of the major challenges of AFL device process isachieving correct wavelength detuning between the DFB sec-tion and the phase control section. Previous works [3]–[5],[7] adopted the butt-joint selective area growth (BJ-SAG)technique, which involves the selective removal of waveguidecore material, followed by the regrowth of an alternate wave-guide core using different material composition. This processallows the independent control of individual sections, butthe reproducibility and controllability are poor as it involvescomplex growth steps, excessive processing steps and strictprocess tolerance. Quantum well intermixing (QWI) is anothertechnique which could widen selectively the bandgap of phasecontrol section and without requiring additional material re-growth step [8]. Compared to BJ-SAG, the QWI technique ismuch simpler and reproducible technique. Furthermore, theuse of QWI ensures perfect alignment between the active andpassive sections of the device and results in a negligibly smallinterfacial reflection.

In this paper, we describe the fabrication of a four-sectionAFL using QWI technique, we also present experimental resultsfor the tunable self-pulsation operation near 20 GHz, and 20Gbit/s all-optical clock recovery based on the fabricated device.Section II describes the structure and process flow of an AFL

0733-8724/$26.00 © 2010 IEEE

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Fig. 1. Schematic diagram of AFL.

Fig. 2. (a) The optical spectrum when � � � mA, � � ��� mA, � �

���mA,�� � ����� nm. (b) The RF spectrum analyzer screen. The frequencyspan is 100 MHz, and the beat-frequency is 23.4 GHz with 3-dB linewidth is3 MHz.

device. Section III describes the self-pulsation characteristicsof the four-section AFL. The optical clock recovery experimentsetup and results are given in Section IV.

II. STRUCTURE AND FABRICATION

A schematic illustration of the AFL chip is shown in Fig. 1.The device consists of four separate sections: a 300 m long

Fig. 3. The sample frequency scatter gram with � � � mA and no current ap-plied to transparent section. The dash line shows the continuously tuning trace.

Fig. 4. RF extinction ratio variation with frequency. The extinction ratio isabove 8 dB over the entire frequency range, and the RF with extinction ratioshigher than 10 dB is obtained in the 6-GHz-wide frequency band of 20–26 GHz.

DFB section, a 350 m long phase control section, a 350 mlong amplifier section and a 300 m long transparent section.Shallow etched isolation trenches 20 m in width separate eachadjacent section. The AFL laser operates as a single mode laserwith a short feedback cavity generates RF pulsation with thefrequency determined by the beating frequency of two longitu-dinal compound cavity modes with comparable threshold gains.A gain-coupled grating is included in the DFB section in order toprovide single-mode operation. The phase control, the amplifiersection and the transparent section form an integrated feedbackcavity. The amplifier section and the phase control section allowthe feedback strength and phase to be controlled via current in-jection and, accordingly, the pulsation frequency to be tuned.The transparent section is for the device length, and it worksas a waveguide with no current applied. Because the beat fre-quency range of AFL is almost determined by the length of thefeedback cavity. As the transparent section has the same mate-rial with the phase control section, the transparent section canalso be used as another phase control section.

The device material was grown on an InP substrate by onestep metal-organic chemical vapor deposition (MOCVD). The

SUN et al.: ALL-OPTICAL CLOCK RECOVERY 2523

Fig. 5. Experiment setup of optical clock recovery.

active region consists of a separate graded-index confinementheterostructure with strain-compensated InGaAsP-InGaAsPmultiple quantum wells. The lasing wavelength of DFBsection is 1545.5 nm, and the amplifier section is the samematerial as the DFB section except for the absence of thegrating. The phase control and transparent sections are passiveregions which required low absorption at the lasing wavelength.By using QWI technique, which involved P ion implantationwith a dose of /cm and a rapid thermal annealing(RTA) at 650 C for 120 s, the peak photoluminescence (PL)wavelength of phase control and transparency section are blueshift to 1455 nm. The 90 nm blue shift ensures that there islow absorption and the phase of the longitudinal modes couldstill be controlled. The front and the end facets are as cleavedwith semiconductor-air interface reflectivity of 0.3. Indepen-dent biasing and control currents and are appliedrespectively to DFB section, phase control section, and theamplifier sections.

III. DEVICE CHARACTERISTICS

The lasing threshold of the AFL is mA, and thelasing wavelength is 1545.5 nm. Self-pulsation can be achievedwith and appropriately adjusted. The AFL output is cou-pled into a 40 GHz bandwidth high-speed photodiode and theRF spectrum characterized with a microwave spectrum analyzerand the optical spectrum with an optical spectrum analyzer.Fig. 2(a) shows the optical spectra and Fig. 2(b) shows the RFspectrum (biasing condition: mA, mA and

mA). The frequency span is 100 MHz and 3-dBlinewidth is 3 MHz. The wavelength detuning betweenthe two optical modes is 0.187 nm, and matching the 23.4 GHzbeat signal as shown in Fig. 2(b). The beat frequency changeswith the currents injected into the amplifier section, DFB sec-tion, and phase control section [9]. Fig. 3 shows the sample fre-quency scatter gram with fixed at 3 mA, no current appliedto the transparent section. The color of the points represents thecurrent interval applied to amplifier section, and the dash linedemonstrated the continuously tuning trace. The RF frequencycan be continuously tuned in the range of 19.8–26.3 GHz. Fig. 4shows the variation in the RF extinction ratio with the frequency.The extinction ratio is above 8 dB over the entire frequencyrange, and the RF with extinction ratios higher than 10 dB isobtained in the 6-GHz-wide frequency band of 20–26 GHz.

Fig. 6. (a) 20 Gbit/s RZ PRBS �� � �� optical data, (b) The 20 GHz opticalclock recovered from 20 Gbit/s RZ PRBS �� � �� optical data by AFL.

IV. ALL OPTICAL CLOCK RECOVERY

All optical clock recovery is an important application ofAFLs. 10 Gb/s all optical clock recovery using AFLs hasbeen reported [4]. In this paper, 20 Gb/s all clock recoveryhas been demonstrated, and optical clock was recovered fromdegraded signals. Fig. 5 is the schematic illustration of injectionlocking experiment setup. A 10 GHz, 2 ps optical pulse train at1554 nm was generated using an electroabsorption modulator(EAM) and a two-stage nonlinear compressor that consists ofa section of dispersion shifted fiber (DSF) and a comb-likedispersion profiled fiber (CDPF). Subsequently the 10 GHzpulse train was modulated with a return to zero (RZ) pseudorandom bit sequence (PRBS) of at 10 Gbit/s by aLiNbO Mach–Zehnder modulator (MZM) with a pulse patterngenerator (PPG) and further multiplexed to 20 Gbit/s witha passive fiber multiplexer [10]. The optical band pass filter(OBPF) with a center frequency of 1554 nm and bandwidth of1 nm was employed after the signal was amplified by EDFA.The free-running pulsation from the AFL was set to a centralfrequency of 20 GHz. When the PRBS RZ data are injected intothe DFB section via an optical circulator after its polarizationis controlled, the free-running pulsation locks to the frequencyof 20 GHz, the optical outputs from the optical circulator aremeasured using a sampling oscilloscope and an RF spectrumanalyzer. Fig. 6(a) shows the sampling oscilloscope traces forthe input 20 Gb/s RZ PRBS optical data pattern. Theoptical outputs from the optical circulator are measured using asampling oscilloscope and an RF spectrum analyzer. Fig. 6(b)shows the optical clock recovered from the input data by AFL.

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Fig. 7. The RF spectrum of the recovered 20 Gbs clock.

TABLE ITHE VALUE OF PARAMETERS IN FORMULA (1)

The time jitter can be calculated from the RF spectrum usingformula (1) [10]–[12]

(1)

Here, in this equation, is the harmonic number, and ,are the powers contained in the phase noise side band and thecarrier, respectively, is the -dB bandwidth of the noiseband, RB is the resolution bandwidth of the RF spectrum ana-lyzer, is the pulse frequency, with being the timing jitter.The RF spectrum of recovered 20 Gbs clock and the value ofparameters in formula (1) are shown in Fig. 7 and Table I. Thetime jitter was calculated to be 123.9 fs.

We also performed clock recovery experiment based on de-graded signals. The 20 Gbit/s RZ PRBS optical signalwas degraded by single mode fiber (SMF) or DSF, and thenwent into the circulator, and the clock was regenerated by AFL.Fig. 8 shows the experiment results. Picture (a) shows the 3.3km single mode fiber (SMF) dispersion degraded 20 Gbit/s datasignal. The dispersion parameter of SMF is 17 ps/(nm-km), sothat the dispersion is 56.1 ps/nm. Picture (b) shows the 20 GHzclock recovered using AFL from the 3.3 km SMF dispersion de-graded signal which showed in picture (a). And the time jitter

Fig. 8. (a) The 3.3 km single mode fiber (SMF) dispersion degraded signal.(b) The clock recovered from 3.3 km SMF dispersion degraded signal usingAFL. (c) The PMD degraded signal. (d) The clock recovered from the PMDdegraded signal.

of the recovered clock is 371.7 fs. Picture (c) shows the polar-ization mode dispersion (PMD) degraded 20 Gbit/s signal whichthe differential group delay (DGD) is about 11.2 ps. And picture(d) shows the 20 GHz clock recovered from the PMD degradedsignal showed in picture (c). AFL shows good performance fordispersion degraded signal, and not so good for PMD degradedsignal. That’s because the material of AFL MQW is polarizationsensitive. We are now working on the polarization independentmaterial AFL.

V. CONCLUSION

A four-section amplified feedback laser (AFL) is fabricatedwhich generated 19.8 GHz–26.3 GHz tunable beat-frequencyof optical microwave with extinction ratio above 8 dB over theentire frequency range and 3-dB linewidth about 3 MHz. Andthe RF with extinction ratios higher than 10 dB is obtained in the6-GHz-wide frequency band of 20–26 GHz. 20 Gb/s all clockrecovery for RZ PRBS data has been demonstrated withtime jitter about 123.9 fs, and optical clock was recovered from56.1 ps/nm dispersion degraded signal and 11.2 ps DGD PMDdegraded signal.

ACKNOWLEDGMENT

The authors would like to thank Zhou Fan, Wang Bao-jun,Shu Hui-yun for help with the experiment, and Wang Lu-feng,Bian Jing, An Xin for their support on the device test.

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Yu Sun was born in Heilongjiang, China, in 1984.She received the B.S. degree in material science and engineering from

Tsinghua University, Beijing, China, in 2005. She is currently pursuing thePh.D. degree in microelectronics and solid state electronics from the Instituteof Semiconductors, Chinese Academy of Sciences, Beijing, China.

Her research interests include dual-mode semiconductor lasers, optical mi-crowave source and optical signal processing.

Jiao Qing Pan received the B.Sc. degree in physics from Shandong University,Shandong, China, in 1997, M.Sc. and Ph.D. degrees from Institute of CrystalMaterials Shandong University in 2000 and 2003 respectively.

Since 2003, he has been working with the Key Laboratory of SemiconductorMaterials Science, Institute of Semiconductors, CAS, first as a PostdoctoralFellow, and then became an Associate Professor. His current research interestsare in III-V semiconductor materials growth, monolithically integrated opto-electronic devices, such as DFB lasers, modulators, photodetectors, etc.

Ling Juan Zhao received the B.Sc. and M.Sc. degrees in semiconductors fromTianjin University, Tianjin, China, in 1985 and 1988, respectively.

In March 1988, she joined the Institute of Semiconductors, Chinese Academyof Sciences, Beijing, China, where she worked on the semiconductor lasersand related optoelectronic devices. In 1997–1999, she was a Visiting Scholarat Brookhaven National Lab, Upton, NY, where she was involved in researchand development in semiconductor nuclear detectors, including Silicon drift de-tector, Silicon pixel detector and X-ray detector, etc. Since 2003, she has been aProfessor with the Institute of Semiconductors, Chinese Academy of Sciences.Her current research interests are in monolithically integrated optoelectronic de-vices, such as electro-absorption modulators integrated with DFB lasers, widelytunable DBR lasers, EA modulated tunable DBR lasers, and semiconductor op-tical amplifiers, etc.

Weixi Chen received the B.S. degree in physics from of Peking University,Peking, China, in 1960.

In 1960, she joined the faculty in Department of Physics, PKU after gradu-ation. In 1994, she was promoted to be a professor in PKU. Her major field ofstudy is Physics of Semiconductor. In 1983–1986, she was a Visiting Scholarin ECE Department, UCSD (USA), where she worked on Ion Beam Millingand Non-Alloyed Ohmic Contact to semiconductor. Since 1987 she has beendoing research on the area of vertical microcavity surface emitting semicon-ductor lasers and subpicosecond dynamic behaviors of optoelectronic materialand devices. In 1996–1998 and 2000–1003 she was a Visiting Assistant ProjectScientist in ECE Department, UCSD (USA). In 1999–2000, and since 2004she has been a consultant in National Research Center for Optoelectronic Tech-nology, Institute of Semiconductors, Chinese Academy of Sciences.

Wei Wang received the B.S. degree in physics from of Peking University,Peking, China, in 1960.

In 1960, he joined the Research Division of Semiconductor Materials in theInstitute of Semiconductors, Chinese Academy of Sciences (CAS), Beijing,China. Since 1969, he has been doing research on the semiconductor optoelec-tronic devices. In 1987, he worked in Prof. Suematsu’s Laboratory at TokyoInstitute of Technology, Tokyo, Japan, as a Visiting Scholar, where he was en-gaged in the research of the 1550-nm BIG-DBR lasers. In 1988, he has been aProfessor in the Institute of Semiconductors, CAS. His current research interestsare monolithically integrated optoelectronic devices on InP substrate, such aselectroabsorption modulated DFB lasers, widely tunable DBR lasers, EA mod-ulated tunable DBR lasers, and semiconductor optical amplifiers, etc.

Prof. Wang is an academician of the Chinese Academy of Sciences.

Li Wang was born in Yunnan, China, in 1985. He received the B.S. degree inelectronic engineering from Tsinghua University, Beijing, China, in 2007. Heis currently pursuing the M.S. degree in physical electronics from the State KeyLaboratory of Integrated Optoelectronics, Tsinghua University, Beijing, China.

His research interests include high-speed all-optical clock recovery andTera Hz.

Xiao Fan Zhao was born in Jilin, China, in 1982. He received the B.S. degree inelectronic engineering from Tsinghua University, Beijing, China, in 2005. He iscurrently pursuing the Ph.D. degree in physical electronics from the State KeyLaboratory of Integrated Optoelectronics, Tsinghua University, Beijing, China.

His current research interest is high-speed all-optical signal processing usingsemiconductor optical amplifier.

Cai Yun Lou was born in Tianjin, China, in 1946. She received the B.S degree inelectronic engineering from Tsinghua University, Beijing, China, in 1960. From1960 to 2010, she worked in the electronics department of Tsinghua University,and became an full professor in 2004. Her research interests are in high-speedoptical communication systems, high repetition rate ultra-short pulse generationand measurement, non-linear high-speed optical switch and all-optical signalprocessing.