Abstract

$\pi$-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 6 W of pump power by reducing the phase shift once the laser begins oscillating.

© 2018 Optical Society of America

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