Characteristic mode analysis of distributed feedback fiber lasers

Based on the analysis of longitudinal modes of distributed feedback (DFB) fiber lasers, the polarization properties are investigated extensively using the conventional coupled wave theory. The relationships between polarization states, coupling coefficient and phase-shift dispersion of dual polarization states are reported. It shows that when lasers operate at the fundamental mode, the states of polarization are determined by both coupling coefficient and phase shift dispersion of dual polarization states. Single polarization output can be obtained by increasing the phase-shift dispersion under a certain coupling coefficient, or by decreasing the coupling coefficient under a certain phase-shift dispersion. DFB fiber lasers are fabricated in hydrogen-loaded erbium-doped fibers, the output laser has dual polarization states due to large coupling coefficient and extremely low birefringence.