Higher order modulation and switching of mode-locked states in the Nd:YAG laser

Generation of high repetition rate optical pulses by means of mode-locking lasers for pulse communication systems is of current interest. The repetition rate of the pulses generated is usually equal to the longitudinal mode spacing of the optical cavity. For this fundamental mode operation the maximum PRF is limited by the physical dimension of laser rod, modulator crystal and ancillary optics. Another practical problem is the switching of the phase of the output pulse train relative to the modulator drive signal. In the first part of the paper, experimental results are presented on higher order mode-locking of a cw pumped Nd: YAG laser in which optical pulses are generated at multiples of the c/2L frequency of the laser cavity. Intracavity LiNbO3, orientated for use as a phase modulator, was driven at frequencies up to 1800 MHz, six times the fundamental PRF. In each case the laser mode-locked at the respective drive frequency. The effect of modulation order on pulse width is discussed.