Demonstration of a Wavelength Tunable Q-Switched Fiber Laser Manas Srivastava, Deepa Venkitesh, Balaji Srinivasan Department of Electrical Engineering, Indian Institute of Technology, Madras, Chennai-600036, India. ee10s044@ee.itm.ac.in , deepa@ee.iitm.ac.in , balajis@ee.iitm.ac.in Abstract: We have Q-switched an Erbium doped fiber laser cavity in the presence of a tunable band pass filter and have compared the results with those of a filter-less cavity. OCIS codes: (140.3500); (140.3510); (140.3540) 1. Introduction Fiber lasers producing high energy, nanosecond pulses with variable repetition rates are finding wide-ranging applications in material processing, laser based ranging, distributed sensing and as a pump for optical parametric amplifiers/oscillators [1-4]. Q-switching is a commonly used technique to generate such laser pulses by introducing a suitable loss modulator in the laser cavity. In a conventional fiber laser cavity, the lasing wavelength is typically decided by the total loss in the cavity. However, for applications such as optical parametric amplification/oscillation and second harmonic generation, it is desirable to limit the spectral width of the output and to have a wavelength tunable feature [5]. This can be achieved by introducing a suitable filter in the cavity [6]. In this paper, we describe the experimental results for Q-switching a wavelength-tunable erbium doped fiber (EDF) laser using a Fabry-Perot etalon as the tuning element and an acousto-optic modulator as the switching element. We compare such results with respect to a filter-less cavity and qualitatively explain the pulse evolution in either case. 2. Experimental Setup and Conditions Fig. 1 shows the schematic of our Q-switched fiber laser in a ring configuration. A 1.5 m long EDF is used as the gain medium. Pump power of 180 mW provided by a semiconductor laser diode is coupled into the EDF using a wavelength division multiplexer (WDM). The isolator ensures unidirectional propagation of power inside the cavity. The output of the cavity is extracted through a 3-dB coupler. A tunable band pass filter (TBPF, Santec) is included in the cavity for selecting the laser wavelength. This filter can be tuned over the wavelength range of 1533-1565 nm with a bandwidth of 1 nm, has an insertion loss of 2 dB at the centre wavelength, and an out-of-band extinction of >30 dB. In order to demonstrate the tunability of the laser cavity, we initially operated it in the continuous wave (CW) regime by turning off the RF power applied to the acousto-optic modulator (AOM, Brimrose). Figure 2 clearly illustrates the laser spectrum being tuned over the entire C-band of the EDF gain spectrum. Fig.1. Experimental setup of the Q-switched erbium-doped fiber laser incorporating an intra-cavity tunable filter. Fig. 2. Ouput laser spectrum of the CW fiber laser illustrating the wavelength tunablility using the tunable band pass filter The AOM is driven by a 100 MHz RF signal generator, whose output is modulated using pulses of variable repetition rates using a CMOS switch to enable the Q-switching of the fiber laser cavity. The modulated RF signal is