Feedback-controlled prelasing: a technique for pulse amplitude and frequency stabilisation of Q-switched lasers

Abstract

A reliable single-frequency Q-switched laser with high pulse repetition frequency (PRF) and good pulse to pulse stability is desirable for many applications, including those where a high average power is required. One popular approach for obtaining single-frequency operation of Q-switched lasers is to use the technique of prelase Q-switching, where a low power single-frequency oscillation is established prior to opening the Q-switch. In practice this technique has been limited to low PRFs (typically < or = 1kHz) since the prelase usually begins with strong spiking behaviour followed by relaxation oscillations. These take a time to decay to the steady prelase required for reliable single-frequency operation and high pulse to pulse stability. At high PRFs, when the Q-switched pulse builds up from prelase spikes, large fluctuations in pulse amplitude and excessive timing jitter occur. In this paper we report a new technique involving actively controlling the diffraction loss of the Q-switch, so as to damp out spiking and relaxation oscillation. We have applied this technique to a Nd:YAG ring laser, pumped by a 1.2W high-brightness diode, in which a single acousto-optic modulator enforces unidirectional operation and is used to Q-switch the laser.

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