【正文】 long PMF based on simulated results. When the lengths of the two segments of the PMFs were 1 m, nm of 3 dB linewidth was observed, as shown in Fig. 8(a).With 5mlength of the long PMF, nm of 3 dB linewidth was observed, shown in Fig. 8(b). The experimental results show that the linewidth will be narrower when the length of the long PMF is increased, without a change in wavelength spacing. Thus, the simulated results [Fig. 3(b)] were verified by the experimental results. With smaller peak linewidth, the fiber laser was more vulnerable to environmental perturbations, but the jitter range was less than nm. Fig. 4. (Color online) Output optical spectrum of the multiwavelength fiber laser. (a) Three wavelength laser spectrum. (b) Repeated scans of the output spectrum in 10 min. Fig. 5. Output multiwavelength laser spectrum. (a) Short wavelength band. (b) Long wavelength band. (c) Whole Cband. Fig. 6. Tunable wavelength spacing in the output laser spectrum. (a) nm of the wavelength spacing. (b) nm of the wavelength spacing. Fig. 7. Spectrum of the output multiwavelength laser with changing length of the short PMF. (a) 1 m length of the short PMF. (b) 2 mlength of the short PMF. Fig. 8. Spectrum of the output multiwavelength laser with changing length of the long PMF. (a) 1mlength of the long PMF. (b) 5mlength of the long PMF. 4. Conclusion A tunable multiwavelength ring EDF laser based on double Sagnac loops has been proposed and demonstrated. By Jones matrix, the b filter characteristics of single and double Sagnac loops were analyzed. The simulation results showed that there are better tunability and controllability of double Sagnac loops than a single Sagnac loop. The linewidth of the output laser was measured as nm at 3 dB, and the SMSR was 50 dB. By adjusting two PCs, the multiwavelength wide tunable fiber laser output was observed and the maximum number of stable output peaks was six wavelengths. By changing the length of the short PMF, the wavelength spacing can be tuned with the linewidth unchanged. In addition, by changing the length of the long PMF, the linewidth can be tuned independently. We gratefully acknowledge the financial support of the National Natural Science Foundation of China under grant 60907020. References 1. A. E. H. Oehler, S. C. Zeller, K. J. Weingarten, and U. Keller, “Broad multiwavelength source with 50 GHz channel spacing for wavelength division multiplexing applications in the tele C band,” Opt. Lett. 33, 2158–2160 (20xx). 2. . Liu, X. Dong, P. Shum, S. Yuan, G. Kai, and X. Dong, “Stable roomtemperature multiwavelength lasing realization in ordinary erbiumdoped fiber loop lasers,” Opt. Express 14, 9293–9298 (20xx). 3. T. Kraetschmer, D. Dagel, and S. T. Sanders, “Simple multiwavelength timedivision multiplexed light source for sensing applications,” Opt. Lett. 33, 738–740 (20xx). 4. Z. Chen, S. Ma, and N. K. Dutta, “Multiwavelength fiber ring laser based on a semiconductor and fiber gain medium,” Opt. Express 17, 1234–1239 (20xx). 5. ,F. Fresi, “Continuously spacingtunable multiwavelength semiconductoropticalamplifierbased fiber ring laser incorporating a superimposed chirped fiber Bragg grating,” Opt. Lett. 32, 1032–1034 (20xx). 6. C. S. Jun and B. Y. Kim, “Modelocking and Qswitching in multiwavelength fiber ring laser using low frequency phase modulation,” Opt. Express 19, 6290–6295 (20xx). 7. . Luo, . Luo, and . Xu, “Multiwavelength picosecond and single wavelength femtosecond pulses emission in a passively modelocked fiber laser using a semiconductor saturable absorber mirror and a contrast ratio tunable b filter,” Appl. Opt. 50, 2831–2835 (20xx). 8. J. Li and L. R. Chen, “Tunable and reconfigurable multiwavelength fiber optical parametric oscillator with 25 GHz spacing,” Opt. Lett. 35, 1872–1874 (20xx). 9. J. Tang, J. Sun, L. Zhao, T. Chen, T. Huang, and Y. Zhou, “Tunable multiwavelength generation based on Brillouinerbium b fiber laser assisted by multiple fourwave mixing processes,” Opt. Express 19, 14682–14689 (20xx). 10. M. H. AlMansoori and M. A. Mahdi, “Reduction of gain depletion and saturation on a Brillouin–erbium fiber laser utilizing a Brillouin pump preamplification technique,” Appl. Opt. 48, 3424–3428 (20xx). 11. Y. G. Shee, M. H. AlMansoori, A. Ismail, S. Hitam, and M. A. Mahdi, “Double Brillouin frequency shift through circulation of oddorder Stokes signal,” Appl. Opt. 49, 3956–3959 (20xx). 12. Y. G. Shee, M. H. AlMansoori, A. Ismail, S. Hitam, and M. A. Mahdi1, “Multiwavelength Brillouinerbium fiber laser with doubleBrillouinfrequency spacing,” Opt. Express 19, 1699–1706 (20xx). 13. M. A. Mirza and G. Stewart, “Theory and design of a simple tunable Sagnac loop filter for multiwavelength fiber lasers,” Appl. Opt. 47, 5242–5252 (20xx). 14. J. Wang, K. Zheng, J. Peng, L. Liu, J. Li, and S. Jian, “Theory and experiment of a fiber loop mirror filter of twostage polarizationmaintaining fibers and polarization controllers for multiwavelength fiber ring laser,” Opt. Express 17, 10573–10583 (20xx). 15. M. A. Ummy, N. Madamopoulos, A. Joyo, M. Kouar, and R. Dorsinville, “Tunable multiwavelength SOA based linear cavity dualoutput port fiber laser using Lyot–Sagnac loop mirror,” Opt. Express 19, 3202–3211 (20xx). 16. A. Gonz225。 revised 21 February 20xx。 參考文獻(xiàn) 1. A. E. H. Oehler, S. C. Zeller, K. J. Weingarten, and U. Keller,“ Broad multiwavelength source with 50 GHz channel spacing for wavelength division multiplexing applications in the tele C band,” Opt. Lett. 33, 2158–2160 (20xx). 2. . Liu, X. Dong, P. Shum, S. Yuan, G. Kai, and X. Dong, “Stable roomtemperature multiwavelen