1 The Characterization of a Simple, Smoothly Wavelength- Tunable Harmonically Mode-Locked Fiber Ring Laser Lingze Duan † , James Jones, and Julius Goldhar Department of Electrical and Computer Engineering, University of Maryland, College Park, MD 20742 † Currently with the Research Laboratory of Electronics, MIT, 77 Massachusetts Ave, MA 02139 Tel: 617-253-8514, Fax: 617-253-9611, e-mail address: dlz@mit.edu Abstract: A very simple configuration of smoothly wavelength-tunable mode-locked fiber ring laser is characterized. It can directly generate 11-ps near transform-limited pulses with 1.7% amplitude noise and 0.7-ps timing jitter and achieve 12-nm smooth wavelength tuning. 2002 Optical Society of America OCIS codes: (140.4050) Mode-locked lasers, (140.3510) Fiber lasers, (250.5980) Semiconductor optical amplifiers 1. Introduction Harmonically mode-locked fiber ring lasers (HMLFRLs) are able to generate high-speed, wavelength-tunable picosecond pulses with simple implementation. However, most configurations reported so far can not achieve wavelength tuning without interrupting the stable pulsing due to the multi-parameter nature of their tuning mechanisms [1, 2]. We have demonstrated a stable and smoothly wavelength-tunable HMLFRL by combining dispersion tuning and an intracavity SOA [2]. However, the laser requires external pulse compression and appears to produce strong asymmetric pulses with large nonlinear chirp due to the nonlinearity of the SOA. In this paper, we report a simplified but improved design, which properly combines the effects of dispersion and SOA to achieve better pulse quality while maintaining the unique features of smooth wavelength tuning and low amplitude noise. 2. Experiment Fig. 1 shows the laser configuration. The EDFA is the major gain medium. Unlike the previous system, in which a LiNbO 3 modulator was used as the mode-locker and the SOA was used to suppress supermode noise, the SOA is directly modulated to achieve active mode locking so that the modulator can be removed. The intracavity group velocity dispersion is provided by 25-m dispersion compensating fiber, which has a normal dispersion parameter of - 88 ps/(km⋅nm) at 1560 nm. Fig. 1. Schematic of the laser configuration. The experiment was conducted at repetition frequencies around 6 GHz. This value was limited by the RF coupling to the SOA. Stable pulsing was realized as evident from the oscilloscope trace (inset of Fig. 2 (a)). A FWHM pulsewidth of 11.4 ps and a 3-dB spectral width of 0.25 nm were measured at 1558 nm, leading to a time-bandwidth product of 0.35 with the assumption of Sech 2 pulses, which indicates that the pulses are near transform-limited even without pulse compression. Fig. 2 shows the normalized autocorrelation trace of the pulses and the optical spectrum. EDFA SOA Isolator 25-m DCF Synthesizer 10% Output Isolator © 2003 OSA/CLEO 2003