Double-Frequency Harmonic Mode Locking in a Pulsed Nd:YAG Laser

Abstract

The harmonic mode locking (ML), i.e. the modulation of resonator losses at a frequency that is an integer multiple N of the fundamental mode spacing, has been a technique intensively studied for achieving minimum pulse duration in low-gain actively mode locked continuos wave (CW) Nd:YAG laser (see refs. [1, 2] and references therein). The experiment we have performed concerns the first demonstration of the harmonic ML on an high-gain QS Nd:YAG laser, operated at 10 Hz repetition rate. To avoid the generation of several pulses per round-trip we implemented the ML by simultaneously operating two phase-locked acousto-optic modulators (AOM) with N=3 (radio frequency: RF=150 MHz) and N=4 (Rf=200 MHz), placed at 1/3 and 1/4 of the resonator length, respectively. By operating a stable cavity in QS ML regime we observed that the short buildup time allowed by the QS was not sufficient to let the ML to evolve to the stationary regime, so that the generated pulses resulted surrounded by a≈1 ns long pedestal and satellites exist placed at 1/3 and 1/4 of the cavity length. To increment the build-up time we realised a simple kind of prelasing by slightly rotating the QS quarter-wave plate and letting the laser starting to oscillate in free running (FR) ML regime. When the first FR spike was developed, a photodiode triggered the QS to allow the seeding pulse to be amplified up to gain saturation. The amplified ML train contains 4-5 pulses whose duration, measured by SH scanning autocorrelation, was =100 ps FWHM. A typical autocorrelation trace is given in Fig. 1. No satellites were present in this regime. With the laser operated with 80% output coupling, the maximum pulse-train energy, set by the damage threshold of resonator optics, was ≈10 mJ. The shot-to-shot energy fluctuations at the fundamental wavelength were below 5%, while below 8% those at the SH, indicating the high energy and temporal stability's of the generated pulses.