Power-insensitive linewidth narrowing of DFB lasers using an all-pass fiber ring resonator

Self-injection locking is a useful method to achieve an ultra-narrow linewidth laser. However, add-drop configurations provide strong feedback and may cause multi-longitudinal-mode oscillation. To solve this problem, we experimentally demonstrate a narrow linewidth self-injection locking DFB laser based on an all-pass fiber ring resonator. The backward Rayleigh scattering is weak and does not cause multi-longitudinal-mode operation at high drive current. The fiber ring resonator has a Q factor of more than 90 million, which is measured from the transmission spectrum and the ring-down trace, and it operates as a narrow band filter to purify the laser spectrum. The intrinsic linewidth of the DFB laser is reduced from 220 Hz to 19 Hz by the Rayleigh scattering with 44.7 mW output power. By adding the injected power to increase the Rayleigh scattering, a 3 Hz intrinsic linewidth laser output is achieved. Compared to add-drop configurations, the linewidth suppression effect is power-insensitive and the beating spectrum nearly remain the same with increasing the drive current, because of the weak feedback from the Rayleigh scattering.