IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 9, May 1, 2007 635 Dual Orthogonal Polarization States in an Active Mode-Locked Birefringent Fiber Ring Laser Huy Quoc Lam, P. Shum, Le Nguyen Binh, Y. D. Gong, and Ming Tang Abstract—We report the generation of dual amplitude pulses in an active mode-locked fiber laser within a birefringent cavity. Different to normal mode-locked pulses with identical amplitude and polarization state, and pulses polarized on both x- and y-axes simultaneously exist in the output pulse train. The two orthog- onal pulse sequences have different amplitudes and lase at different wavelengths. Dual wavelengths are the result of red shift and blue shift of the x- and y-polarization states of the generated pulses, re- spectively, due to the detuning phenomena. Locking to individual x- or y-polarized pulse is also obtained by adjusting polarization controllers. Index Terms—Birefringence, duality, fiber laser, laser, mode-locked lasers, polarization. I. INTRODUCTION A CTIVE mode-locked fiber lasers are very attractive for the generation of ultrashort optical pulses at multiple wave- lengths with multiple gigabits-per-second repetition rate [1], [2]. Although tremendous investigations have been performed to explore mode-locking mechanisms and applications for var- ious mode-locked fiber lasers [3], [4], dynamic behavior of the active mode-locked fiber laser is still not fully understood. Recently, we reported a polarization-switching phenomenon of an active mode-locked fiber laser when tuning the modu- lation frequency [5]. In this letter, we further investigate the dynamic property of the active mode-locked fiber laser with a highly birefringent cavity. Dual-wavelength (dual-mode) pulses corresponding to orthogonal polarization states are generated si- multaneously in the single cavity with different amplitude and pulse properties. By carefully adjusting the polarization state, we also achieve stable mode-locking of two modes separately. The demonstrated phenomenon is useful for understanding the polarization dynamic of the active mode-locked fiber laser and for developing highly stable light source for future optical net- works or sensor applications. Manuscript received December 28, 2006; revised February 7, 2007. This work was supported in part by the Agency for Science, Technology and Re- search (A*STAR), Singapore, and in part by the Open Fund of Key Laboratory of Optical Communication and Lightwave Technologies, Beijing University of Posts and Telecommunications, Ministry of Education, China. H. Q. Lam, P. Shum, and M. Tang are with the Network Technology Re- search Centre, Nanyang Technological University, Singapore 637553, Singa- pore (e-mail: huylq@pmail.ntu.edu.sg). L. N. Binh is with the Electrical and Electronic Engineering De- partment, Monash University, Melbourne VIC3800, Australia (e-mail: le.nguyen.binh@eng.monash.edu.au). Y. D. Gong is with the Institute for InfoComm Research, Singapore 119613, Singapore (e-mail: gongyd@i2r.a-star.edu.sg). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2007.894943 Fig. 1. Active mode-locked fiber laser with birefringent cavity. BPF: bandpass filter. MZIM: Mach–Zehnder intensity modulator. Pol: Polarizer. PD: photo- diode. OSC: oscilloscope. OSA: optical spectrum analyzer. Sig Gen: signal gen- erator. II. EXPERIMENTAL SETUP Fig. 1 shows the experimental setup of the active mode-locked fiber laser incorporating a birefringent cavity. The laser has a ring configuration with 15 m of polarization-main- taining erbium-doped fiber (PM-EDF) serving as a gain medium. This PM-EDF also causes the laser cavity to become birefringent. The PM-EDF is pumped by a 980-nm laser diode through a wavelength-division-multiplexing (WDM) coupler. An isolator integrated in the WDM ensures unidirectional lasing. A thin-film 1.2-nm tunable optical bandpass filter is used to tune the lasing wavelength. A LiNbO Mach–Zehnder intensity modulator is incorporated in the cavity to stimulate mode-locking through the amplitude modulation process. The modulator is driven by a microwave signal extracted from a signal generator. The polarization controller (PC) is used to adjust the polarization state of the lightwave signal traveling in the ring. The optical signal in the ring is coupled to the output port through a 70 : 30 coupler. The output signal is monitored by a high-speed sampling os- cilloscope (50-GHz 3-dB bandwidth) proceeded by a photo- diode (45-GHz 3-dB bandwidth). A polarizer is also inserted in front of the photodiode when the polarization state of the output signal is analyzed. An optical spectrum analyzer with a resolu- tion of 0.01 nm is used to record the pulse spectrum. III. RESULTS AND DISCUSSION The total length of the cavity is about 29.5 m which cor- responds to a fundamental frequency of 6.923 MHz. The 1041-1135/$25.00 © 2007 IEEE