Mode-Locked Fiber Laser With Transverse-Mode Selection Based on a Two-Mode FBG

We demonstrate a passively mode-locked fiber laser that incorporates a two-mode fiber Bragg grating (FBG) for transverse-mode selection. By controlling the polarization state of light in the cavity, the laser can provide a continuous-wave output at the LP₀₁ mode, the LP₁₁ mode, or their mix, which correspond to different reflection wavelengths of the FBG. When the laser is mode-locked, it generates picosecond pulses at a fundamental repetition rate of 6.58 MHz for both the LP₀₁ and LP₁₁ modes with a signal-to-noise ratio higher than 56 dB. At a sufficiently high pump power, the laser can generate irregular multiple-pulse trains and harmonic mode-locked pulses. The laser could find applications in mode-division multiplexed systems.