Continuously bandwidth-tunable nonlinear polarization rotation mode-locked fiber laser

Abstract

Developing ultrafast lasers with controllable flexible pulses, such as wavelength and pulsewidth tunable lasers are desired for various applications like fiber telecommunication and optical sensing. To achieve wavelength or/and pulsewidth tunability in saturable absorber (SA) based mode-locked fiber lasers, some technologies including tunable band pass filter, fiber Bragg grating, diffraction grating mirror, 45°tilted fiber grating, and tunable Lyot filter are proposed to provide the direct spectral manipulation of the pulses. In this paper, we demonstrate a simple bandwidth-tunable ultrashort pulse generation scheme for a nonlinear polarization rotation mode-locked laser. By utilizing a polarization-maintaining-fiber-pigtailed inline polarizer and a polarization controller, a nonlinear polarization evolution (NPE) mode-locking effect as well as a bandwidth-tunable Lyot filter are enabled in the laser cavity when the intracavity polarization settings adjusted. The Lyot filter formed by the inline polarizer and its birefringence of the two fiber pigtails with lengths of 0.3 m could introduce a spectral filtering effect. By only tuning the intracavity polarization controller, the spectral bandwidth is continuously tuned in the range of 7.8 to 3.5 nm. We attribute the lower limit of the spectral bandwidth to the nonlinear self-phase modulation requiring narrow pulses in nonlinear polarization rotation mode-locked lasers. These results provide a simple way for generating subpicosecond pulse with variable spectral bandwith or pulse duration without using a saturable absorber.