IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 3, MARCH 2001 191 Wavelength-Switched Optical Pulse Generation in a Fiber Ring Laser with a Fabry–Perot Semiconductor Modulator and a Sampled Fiber Bragg Grating Donghui Zhao, Kam Tai Chan, Y. Liu, L. Zhang, and I. Bennion Abstract—A simple mode-locked fiber ring laser for generating wavelength-switched picosecond pulses was developed in which a laser diode played the role of both an intensity modulator and a tunable filter. A serially cascaded sampled fiber Bragg grating predefined the set of four allowable output wavelengths spaced 1.6 nm apart. Wavelength switching between the longest and shortest wavelengths required only a temperature change of the laser diode by 7.63 C. Index Terms—F–P LD modulator, sampled fiber Bragg grating, wavelength-switched mode-locked ring laser. I. INTRODUCTION T HE GENERATION of picosecond optical pulses that can be switched among a multitude of wavelengths is highly desirable for the implementation of the wavelength division multiplexed (WDM) communication systems [1]. Actively mode-locked fiber ring lasers are attractive candidates to realize such pulse sources. Generally, fiber ring lasers are actively mode-locked by several kinds of modulators such as Mach–Zehnder modulator and electroabsorption modulator. On the other hand, wavelength tuning is normally achieved by additional elements, like the tunable fiber Bragg grating (FBG), diffraction grating or Fabry–Perot (F–P) filter [2]–[4]. Recently, a laser scheme to generate wavelength-tunable pulses using an F–P or DFB semiconductor laser diode (LD) modulator as both an intensity mode locker and a tunable filter was presented [5], [6]. However, although such a scheme is excellent for fine-tuning the lasing wavelength, its tuning range is small with a relatively large temperature change. In this letter, we report a modified scheme by the inclusion of a sampled FBG (SFBG) that enables us to achieve switching of four wavelengths over a range of 4.8 nm by merely changing the F–P LD temperature from 12.02 C to 19.65 C. II. EXPERIMENTAL SETUP AND RESULTS The experimental setup is shown in Fig. 1. The gain of the fiber laser was provided by an erbium-doped fiber, which is 1 m long and doped at about 1000 ppm, and a 980 nm-laser Manuscript received April 3, 2000; revised November 28, 2000. This work was supported by the HKSAR Government under Grant RGC-CUHK 4173/97E. D. Zhao and K. T. Chan are with the Department of Electronic Engineering, The Chinese University of Hong Kong, New Territories, Hong Kong. Y. Liu, L. Zhang, and I. Bennion are with the Department of Electronic En- gineering and Applied Physics, Aston University, Birmingham B4 7ET, U.K. Publisher Item Identifier S 1041-1135(01)01972-3. Fig. 1. Schematic diagram of the wavelength-switched mode-locked fiber ring laser. Fig. 2. Transmission spectrum of the sampled fiber grating. diode pumped at 30-mW. Two optical isolators were used to en- sure unidirectional operation of the ring laser. The 1550 nm F–P laser diode connected to the circulator has a threshold current of 4.5 mA at 20 C and a mode spacing of 1.28 nm. It was biased below threshold and modulated by a radio frequency (RF) sinu- soidal signal to achieve intensity modulation [5]. The SFBG was directly cascaded in the ring cavity. Fig. 2 shows the transmis- sion spectrum of the SFBG, which contains four transmission peaks at the wavelengths nm, nm, nm, and nm. Each peak has a spec- tral width 0.25 nm in the comb-like spectral region. The peak spacing is 1.6 nm and the insertion loss varies from 2.66 dB to 1.51 dB for 1546.64 nm to 1551.44 nm [7]. In this configuration, the wavelength-switching element is composed of the F–P LD and the SFBG. As the laser tended to oscillate at the wavelength with the highest gain, pulse gen- eration should occur at that wavelength where an F–P LD cavity mode and a transmission peak of the SFBG overlapped. In our 1041–1135/01$10.00 © 2001 IEEE