1810 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 8, AUGUST 2004 Rational Harmonic Mode-Locking of Erbium-Doped Fiber Laser at 40 GHz Using a Loss-Modulated Fabry–Pérot Laser Diode Gong-Ru Lin, Member, IEEE, Yung-Cheng Chang, Student Member, IEEE, and Jung-Rung Wu Abstract—Rational harmonic mode-locking of an Er- bium-doped fiber ring laser (EDFL) at repetition frequency of 40 GHz is demonstrated by using a purely loss-modulated Fabry–Pérot laser diode (FPLD) at 1 GHz. The FPLD is neither lasing nor gain-switching, which requires a threshold modula- tion power of 18 dBm to initiate harmonic mode-locking of the EDFL. After chirp compensation, the nearly transform-limited pulsewidth and spectral linewidth of 3 ps and 1.3 nm are ob- tained at repetition frequency of 40 GHz, which corresponds to a time-bandwidth product of 0.31. The EDFL gradually evolves from harmonic mode-locking to injection-locking mode as the FPLD changes from loss-modulation to gain-switching mode by increasing its dc driving current. Index Terms—Erbium-doped fiber laser (EDFL), Fabry–Pérot laser diode (FPLD), harmonic mode-locking, injection-locking, loss modulation, rational harmonic mode-locking. I. INTRODUCTION S HORT-PULSED erbium-doped fiber lasers (EDFLs) have been comprehensively investigated to generate high-bit-rate ( 10 GHz) optical carriers for versatile applications such as wavelength-division-multiplexing (WDM)/time-division-multiplexing transmission in fiber-optic communication networks, sampling and switching in pho- tonic network systems [1], [2]. Harmonic and rational harmonic active mode-locking schemes are currently the main technologies to meet these demands. Typical mode lockers to achieve loss modulation of the EDFL cavity are a Mach-Zehnder integrated-optic modulator [3], a semiconductor multiple quantum-well electroabsorption modulator [4], and a gain-switched Fabry–Pérot laser diode (FPLD) [5], etc. To overcome the difficulty of gain-depletion modulation in the EDFL with extremely long carrier lifetime ( 10 ms), versatile repetition frequency multiplication schemes have recently emerged, such as the fiber dispersion induced pulse split- ting [6], the intracavity fiber Fabry–Pérot filter (FFPF)-based high-repetitive pulse extraction [7], and the optical pulse-injec- tion-induced frequency multiplication [8], [9], etc. In particular, the harmonic mode-locking of EDFL can also be demonstrated using cross-gain-modulated semiconductor optical amplifier Manuscript received July 25, 2003; revised April 27, 2004. This work was supported in part by the National Science Council under Grant NSC92-2215-E- 009-028. The authors are with the Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C. (e-mail: grlin@fac- ulty.nctu.edu.tw). Digital Object Identifier 10.1109/LPT.2004.831057 Fig. 1. Embodiment of an FPLD mode-locked EDFL. Amp: RF power amplifier. PD: high-speed photodetector. RFS: radio-frequency synthesizer. WDM: WDM coupler. [10] or directly modulated laser diode [11]. In this work, the rational harmonic mode-locking of EDFL using a purely loss-modulated FPLD are investigated. The FPLD can be driving at either loss-modulation or gain-switching via the adjustment of dc current and radio-frequency (RF) power. The evolution of the EDFL lasing mechanism from mode-locking to injection-locking is observed as the FPLD changes from loss-modulation to gain-switching mode. The 40-GHz rational harmonic mode-locked EDFL pulse train generated using a loss-modulated FPLD at repetition frequency of 1 GHz is characterized. II. PRINCIPLE AND EXPERIMENTAL Fig. 1 plots the setup of an EDFL rational harmonic mode- locked using a purely loss-modulated FPLD. An Erbium-doped fiber amplifier (EDFA) with maximum gain of 17 dB is con- necting with a 1560-nm FPLD via a 50% optical coupler (OC) to construct an EDFL ring cavity. A polarization controller (PC) before the FPLD is used to adjust the intensity of the injec- tion light. There is no direct feedback from the EDFA output to the EDFA input. A 35% OC is employed to monitor the EDFL output. The EDFL cavity length is 50 m, corresponding to a fun- damental cavity frequency of around 4.48 MHz. The FPLD exhibits a threshold current and a longitudinal mode spacing of 13 mA and 1.2 nm, respectively. To achieve pure loss-modu- lation instead of lasing, the FPLD is un-dc-biased but modu- lated using an RF synthesizer (Rohde & Schwarz, SML01) in connection with a power amplifier, which generates RF power from 0 to 30 dBm. To achieve perfectly gain-switched lasing, 1041-1135/04$20.00 © 2004 IEEE