IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 5, MAY 2005 989 Lasing Wavelength and Spacing Switchable Multiwavelength Fiber Laser From 1510 to 1620 nm Young-Geun Han, Gilhwan Kim, Ju Han Lee, Sang Hyuck Kim, and Sang Bae Lee Abstract—We experimentally demonstrate a flexibly switchable multiwavelength fiber laser with multiple functionalities like a wide tuning range of lasing wavelength, wavelength spacing, and the number of channels simultaneously. The lasing wavelength is controlled in the range from 1510 to 1620 nm based on the effect of nonlinear gain compression of a semiconductor optical amplifier incorporated with an erbium-doped fiber amplifier. The wavelength spacing and the number of channels in the multiwave- length fiber laser are readily controlled by the effective length of polarization-maintaining fiber (PMF) segments in the intracavity PMF Lyot–Sagnac filter. Index Terms—Multichannel filter, multiwavelength fiber laser, optical interleaver. I. INTRODUCTION M ULTIWAVELENGTH fiber lasers have been signif- icantly advanced in recent years due to their various advantages like multiwavelength operation, simple structure, low cost and insertion loss, and resulted in their versatile applications to dense wavelength-division-multiplexed (WDM) systems, optical fiber sensors, optical instrument testing, spec- troscopy, and so on [1]–[5]. An erbium-doped fiber amplifier (EDFA)-based multiwavelength fiber laser has been widely investigated [1]. However, since EDF is homogenous gain medium, it should use the cooling EDF down to cryogenic tem- perature with liquid nitrogen or the frequency-shifted feedback technique within a laser cavity [2]. These methods, however, have the drawback to suppress homogenous line broadening and unstable gain competition for a stable multiwavelength operation at room temperature. A semiconductor optical am- plifier (SOA) or Raman amplifier-based multiwavelength fiber laser has been investigated since it is stably operated even at room temperature [3]–[5]. To enhance the functionality and flexibility of a multiwavelength fiber laser based on all kinds of gain media, the tunability of the lasing wavelength, the wavelength spacing, and the number of channels should be investigated simultaneously. In this letter, we propose and experimentally demonstrate a functional multiwavelength fiber laser based on an SOA incorporating two EDFAs ( - and -band EDFAs) with a wide tunability of lasing wavelength, wavelength spacing, and the number of channels simultaneously. The lasing wavelength can be controlled in a range from 1510 to 1620 nm based on non- linear gain compression effect of an SOA supported by EDFAs. The tunability of the wavelength spacing and the number of Manuscript received December 20, 2004; revised January 12, 2005. The authors are with the Photonics Research Center, Korea Insti- tute of Science and Technology (KIST), Seoul 136-791, Korea (e-mail: yyghan@kist.re.kr). Digital Object Identifier 10.1109/LPT.2005.846748 Fig. 1. Schematic of the multiwavelength fiber laser based on the hybrid optical gain medium (the SOA and two EDFAs) with the switchable PMF Lyot–Sagnac filter (dashed square). channels can be provided by a switchable polarization-main- taining fiber (PMF) Lyot–Sagnac filter with multiple PMF segments. By controlling the effective length of multiple PMF segments in the intracavity PMF Lyot–Sagnac filter, we achieve 20 lasing output with the switchable wavelength spacing in a range from 0.8 to 4.1 nm at room temperature. The interleaved optical switching performance based on the proposed switch- able multiwavelength fiber laser is also discussed. II. MULTIWAVELENGTH GENERATION BASED ON THE HYBRID GAIN MEDIUM The experimental schematic for the proposed multiwave- length fiber laser is shown in Fig. 1. The principle of the proposed scheme is based on the hybrid gain medium and the switchable PMF Lyot–Sagnac filter. The hybrid gain medium is composed of the SOA and two EDFAs in both - and -band. It is difficult to generate the multiwavelength output when an EDFA is only used due to its intrinsic properties like homogeneous line broadening and gain competition as described previously. However, since the mode competition among the wavelengths is suppressed by the self-saturation effect of the SOA as long as the overall gain is larger than zero, it is possible to achieve a stable multiwavelength operation and a high signal-to-noise ratio in the hybrid gain medium [3]. The distinctive characteristics of the SOA-like nonlinear gain com- pression can also be used to induce the tunability of the lasing wavelength. Since the ring cavity is mainly formed by the use of 90 : 10 coupler, 90% of the optical power is feedback into inside of the ring cavity and the SOA is highly saturated. Conse- quently, the reduction of the carrier density of the SOA induces gain compression of the SOA and the shift of its gain peak into the longer wavelength [6], [7]. Since a variable optical atten- uator (VOA) controls the amount of feedback optical power 1041-1135/$20.00 © 2005 IEEE