Acousto-optically mode-locked HF laser

A pulsed HF laser has been actively mode-locked at 40 MHz with a germanium acousto-optic Bragg cell (1) which provides the large modulation depth required to loss modulate the high gain SF6-H2-He gas discharge. The Bragg cell, aligned at Brewster´s angle, exhibited low cavity insertion loss, while providing a large interaction volume for the laser radiation and the acoustic standing waves. The laser itself operates at 5 pps with a 1 meter longitudinally excited discharge with SF6-H2-He partial pressures of 1.5, 0.1, and 15 torr, respectively. Mode-locked laser emission on the 2-1 and 1-0 P-branch transitions near 2.7 microns occurred in a pulse train typically 50 µs long with a total energy of 58 microjoules. The 350 MHz inhomogeneously broadened line should yield an approximately 3 ns wide pulse; observed pulses had a Ge:Au detector limited width (FWHM) of 5 ns with a peak power of 10 watts. Details of the acoustic modulator, operating characteristics of the individual P-branch transitions and attempts at producing shorter pulses with a high-pressure, transversely excited discharge will be presented.