A longitude-purification mechanism for tunable fiber laser based on distributed feedback

Advanced applications in optical frequency metrology demand improved tunable lasers with high coherence. Herein, a tunable single-frequency fiber laser based on the longitude-purification induced by the distributed feedback has been demonstrated. The key device in the proposed laser structure is a distributed self-injection feedback structure (DSIFS) which function as a mode selector to have a robust frequency-domain mode suppression and wavelength adaptability. Our work is distinctively focused on the theoretical analysis of the formation process of single-longitudinal-mode (SLM) laser output and the wavelength adaptive characteristics in the DSIFS. In this experiment, the side mode suppression of the resonant longitudinal modes can be achieved in a fiber ring laser without any accurate control of the main cavity. The fiber laser can be tuned over the entire flat gain region wherein a laser linewidth of 855 Hz, the side mode suppression ratio of ~67 dB and frequency noise suppression of 40 dB. In addition, the SLM operation of the laser can also be guaranteed at different pump power and every wavelength channel. Moreover, this method is applicable to any other gain-type lasers, indicating that the longitude-purification mechanism has a broad application prospect in other highly coherent tunable lasers.