Tunable multi-wavelength fiber laser based on nonlinear polarization rotation

A tunable multi-wavelength erbium-doped fiber laser with up to 17 wavelengths was proposed. The intensity-dependent inhomogeneous loss induced by nonlinear polarization rotation (NPR) was applied to mitigate the mode competition caused by the homogeneous gain medium of erbium-doped fiber. Therefore, stable multi-wavelength output was generated at room temperature. In this experiment, a polarization-maintaining fiber(PMF) with a polarization-dependent isolator was used to form an equivalent Lyot birefringent fiber filter. The wavelength spacing could be altered by selecting proper length of the birefringent fiber. In the meantime, the fine tuning of wavelength could be achieved through adjusting polarization controllers(PC). A 10 m length of PMF was inserted into the laser cavity, which resulted in 17 wavelengths output with 0.35 nm wavelength spacing. In addition, a 4 nm tuning range was realized through PCs fine tuning. Three different coupling ratio couplers were used. And the laser was output from the small ratio port. The maximum numbers of the output wavelengths were 13, 14 and 17 with 7 dB,10 dB and 13 dB fluctuation for 50:50, 30:70 and 10:90 couplers, respectively. Besides, the corresponding maximum output power were 0 dBm、-3 dBm and -7 dBm. The results indicate that the output wavelength number is reduced, wavelength spectrum becomes smoother and output power is higher while the output rate of the output coupler is increasing.