Sodium guide star laser pulsed at Larmor frequency

Abstract

589 nm lasers pulsed at Larmor frequency, several hundreds of kilohertz, can increase the brightness of a sodium guide star and are required in remote magnetometry with mesospheric sodium. By amplification of a continuous-wave single-frequency 1178 nm laser in a pulse-pumped Raman fiber amplifier and frequency doubling in an external cavity, high-power pulsed 589 nm laser at Larmor frequency is obtained for the first time, to the best of our knowledge. The pulse format is mainly determined by the 1120 nm Raman pump laser, whose pulse repetition rate and duty cycle are adjustable. Active pulse shaping is applied to minimize the relaxation spike at the leading edge of the pulses. A reduction in pulse width and conversion efficiency from 1120 to 1178 nm is observed in the backwardly pumped Raman fiber amplifier due to the pump pulse transition effect. A 589 nm laser pulsed at a 350 kHz repetition rate and 20% duty cycle with average power up to 17 W is demonstrated as an operation example intended for a geomagnetic field of 0.5 G.