IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 21, NOVEMBER 1, 2011 1627 Stable Multiwavelength Semiconductor Laser Using FWM and SBS-Assisted Filter C. H. Yeh, Member, IEEE, C. W. Chow, Member, IEEE, Y. F.Wu, Student Member, IEEE, F. Y. Shih, Student Member, IEEE, J. H. Chen, Member, IEEE, and C. L. Pan, Senior Member, IEEE Abstract—A stable and flattened quadruple-wavelength laser system is proposed and experimentally investigated. The laser consists of an external injection Fabry–Pérot laser diode (FP-LD) and a stimulated-Brillouin-scattering (SBS)-assisted filter. In the measurement, the power difference of the lasing quadruple-wavelength can be enhanced within 2.0 dB by SBS-as- sisted filtering after 22-km single-mode fiber (SMF) propagation. In addition, we can change the two injected wavelengths for tuning the quadruple-wavelength. Besides, the output stability of the proposed laser has been studied and analyzed. Index Terms—Fabry–Pérot laser diode (FP-LD), four-wave mixing (FWM), stimulated Brillouin scattering (SBS). I. INTRODUCTION R ECENTLY, stable and tunable erbium-doped fiber (EDF) ring lasers with single- or multiwavelength operations are desirable for wavelength division multiplexed (WDM) transmission systems, optical device testing, fiber-optic sensor systems, and precise spectroscopy. However, owing to the homogeneous gain broadening characteristic of the EDF, the maximum number of generated lasing wavelength could only be two or three [1]. Thus, to solve the limitation by the ho- mogeneous broadening of EDF at room temperature, several techniques have been proposed and investigated [2]–[6]. Fur- thermore, to improve the stability of the multiwavelength EDF lasers, many different techniques for reducing mode competi- tion have been proposed and investigated, such as cooling the EDF at 77 K [7], introducing active overlapping linear cavities [8], or using the polarization hole-burning [6]. In this study, we propose and experimentally investigate an external-injection Fabry–Pérot laser diode (FP-LD) to achieve the flattened quadruple-wavelength output by using four-wave mixing (FWM) and stimulated-Brillouin-scattering (SBS)-as- sisted filter. Here, using two external-injected wavelengths into the FP-LD can achieve dual-wavelength lasing and produce two sidemodes due to the FWM. Then, the SBS-assisted filter can Manuscript received February 26, 2011; revised August 02, 2011; accepted August 14, 2011. Date of current version October 19, 2011. C. H. Yeh, Y. F. Wu, and F. Y. Shih are with the Information and Communi- cations Research Laboratories, Industrial Technology Research Institute (ITRI), Hsinchu 31040, Taiwan (e-mail: yeh1974@gmail.com). C. W. Chow is with the Department of Photonics and Institute of Electro- Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan. J. H. Chen is with the Department of Photonics, Feng Chia University, Taichung 40724, Taiwan. C. L. Pan is with the Department of Physics and Institute of Photonics Tech- nologies, National Tsing Hua University, Hsinchu 30013, Taiwan. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2011.2165836 be used to suppress the output powers of dual-wavelength to 20 dB after 22 km single-mode fiber (SMF) transmission. In addition, we can change the two injected-wavelengths for the quadruple-wavelength tuning in the -band. The experimental results show that the proposed laser not only presents the flat- tened quadruple-wavelength output with wavelength tunabiltiy, but also has good output stability. II. EXPERIMENT AND RESULTS Fig. 1 shows the experimental setup of the proposed quadruple-wavelength laser configuration. The proposed laser scheme was constructed by a FP-LD, two distributed feed- back laser diodes (DFB-LDs), an optical circulator (OC), a polarization controller PC , and a 1 2 and 50:50 optical coupler (CP). Here, the mode-spacing and threshold current of multilongitudinal-mode (MLM) FP-LD were 1.11 nm and 9.5 mA, respectively. In the experiment, the bias current of the FP-LD was operated at 25 mA under the temperature of 25 C. Fig. 2(a) presents the output spectrum of the MLM FP-LD before optical-injection, and the power of the center wavelength was about 5 dBm. The optical spectrum was measured by an optical spectrum analyzer (OSA) with a resolution of 0.05 nm. Two DFB-LDs were connected to the CP, OC and PC, and then to the FP-LD, as shown in Fig. 1. The wavelengths of the two DFB-LDs were 1545.10 and 1546.21 nm. Their output powers and linewidth were 2 dBm and 100 kHz respectively. The total insertion losses were: CP ( 3.2 dB), OC ( 0.7 dB) and PC ( 1.2 dB). Therefore, the actually continuous-wave (CW) injected power into the FP-LD was nearly 7 dBm after passing through these three passive components. The two CW injection lights could be regarded as the pump lights for the FWM. The PC in the experiment could be used to adjust the proper polarization to maximum the effect of FWM. In the experiment, the FP-LD (the master laser) is temperature stabilized. By adjusting the wavelengths and the polarizations of the two pumps, FWM can be observed at the output port of the FP-LD. When the phase matching condition of the injected pump signals are satisfied, FWM can be observed in semicon- ductor materials such as in semiconductor optical amplifier (SOA) and FP-LD [9]. Detail discussion has also been reported in [9]. Fig. 2(b) shows the output spectrum of dual-wavelength in the proposed laser scheme when the two CW lights were used to inject into FP-LD, observed at point “a” in Fig. 1. Due to the FWM effect, two sidemodes were also produced at 1543.98 and 1547.34 nm with 28.5 and 30.7 dBm peak powers, re- spectively. Hence, four wavelengths of the proposed quadruple- 1041-1135/$26.00 © 2011 IEEE