Efficiency increase of distributed feedback Raman fiber lasers by dynamic control of the phase shift.

Abstract

π-phase-shifted distributed feedback, ultralong fiber Bragg gratings (FBGs) with Raman gain have been shown to be excellent ultranarrow single-frequency lasers that can be operated at any wavelength. However, these lasers have shown unusually low slope efficiency (1%-10%), while theoretical simulations predict a much higher (30%-60%) number. We believe this poor performance is due to a thermally induced phase shift inside the FBG due to absorption of the high intensity of the signal oscillating in the cavity. To compensate for this, a thermally controlled dynamic phase shift is proposed to increase efficiency after lasing first occurs. We show here an increase in the slope efficiency of a factor of 4 and an increase in the total output efficiency by a factor of 6.5 with 6W of pump power by reducing the phase shift once the laser begins oscillating.