Operation-state manipulation of a passively mode-locked erbium-doped fiber laser by polarization control Hsiao-Hua Wu 1 , Kuei-Huei Lin 2 , Jung-Jui Kang 3 , Chao-Kuei Lee 3 and Gong-Ru Lin 4 1 Department of Physics, Tunghai University, Taichung 407, Taiwan Phone: +(886)423590121ext.32121, Fax: +(886)423594643, Email: hhwu@thu.edu.tw 2 Department of Science, Taipei Municipal University of Education, Taipei 100, Taiwan 3 Department of Photonics, National Sun Yat-Sen University, Kaohsiung, 804 Taiwan 4 Graduate Institute of Photonics and Optoelectronics, and Department of Electrical Engineering, National Taiwan University, Taipei, 10617 Taiwan Abstract We demonstrate the manipulation of a passively mode- locked erbium-doped fiber laser among stable Q- switching, Q-switched mode-locking, continuous-wave mode-locking, pulse splitting, and harmonic mode- locking by adjusting a polarization controller without changing the pump power. Introduction Short-pulse lasers with high peak power or high energy are very useful in many practical applications in different areas of technology. Usually, the operation of short- pulse lasers can be classified into three major categories, namely, Q-switching [1-2], continuous-wave mode- locking (CML) [3-4], and Q-switched mode-locking (QML) [5] for the generation of high energy or high peak power pulses. These operation states can also be found in fiber lasers by using various active or passive technologies. Recently, much attention has been focused on the development of passive technologies [2, 4], especially, by using a saturable absorber. Using a resonant semiconductor saturable absorber mirror (SESAM) with high-modulation depth, efficient pulse shortening in Q-switched fiber lasers have been demonstrated [2]. A resonant SESAM with a large change in nonlinear reflectivity also allows reliable start- up of passive mode-locking in a wide range of normal or anomalous cavity dispersion. By using a SESAM in combination with a nonlinear amplifying loop mirror, the laser performance could be further improved. It is demonstrated that such a laser is capable of generating subpicosecond pulses with repetition rates exceeding 1 GHz and time jitter between pulses of < 1 ps [6]. Here, we propose a simple method for obtaining an operation state manipulable erbium-doped fiber laser (EDFL), which uses a SESAM in combination with a nonlinear polarization evolution mechanism and can be operated in CW state or various short-pulse states by simply adjusting an intracavity polarization controller. Q-switching, QML, CML, or even pulse splitting, and harmonic mode-locking (HML) states can be obtained without changing the pump power. The EDFL can generate CML pulses with duration of 800-fs at a repetition rate of 4-MHz. Experimental Setup The experimental setup of the EDFL is schematically shown in Fig. 1. The output port of a C-band erbium- doped fiber amplifier (EDFA) is connected to an input port of a 22 3-dB coupler. One of the output ports of the 3-dB coupler is connected to a SESAM (Batop, SAM 1550-30-10ps), which has an absorption of A = 30% and modulation depth R = 18% at 1550 nm. The reflected beam from the SESAM is coupled back to the input port of the EDFA through a polarizer and a polarization controller to form a ring cavity EDFL. Another port of the 3-dB coupler is used as the EDFL output. The EDFL output is characterized by a power meter, a high speed InGaAs detector that is connected to an oscilloscope, and an optical spectrum analyzer (Ando AQ6317B). A noncollinear autocorrelator (Femtochrome FR-103XL) is used for measuring the duration of mode-locked pulses. EDFA SESAM 50/50 Coupler Output Polarizer Polarization Controller EDFA SESAM 50/50 Coupler Output Polarizer Polarization Controller Fig. 1. Schematic diagram of the erbium-doped fiber laser. Results and Discussion To begin with, the EDFL can be adjusted to operate in the CW state [Fig. 2(a)]. With an injection current of laser diode (LD) at 70 mA, the EDFL output power is 3.3 dBm and the laser wavelength is 1559 nm. By adjusting the polarization controller, the EDFL can then be operated in the Q-switching state [Fig. 2(b)] at a repetition rate increasing with the injection current of LD. With fixed injection current of LD, the repetition rate of Q-switched pulses can be slightly tuned by adjusting the polarization controller. For cavity length of 50 m and an injection current of LD at 70 mA, the repetition rate of 978-1-4244-4103-7/09/$25.0 © 2009 IEEE FA1