Abstract

A AgGaSe₂ OPO pumped at kilohertz repetition rates has been demonstrated. The pump laser consisted of a diode-pumped oscillator and a double-pass, tungsten-lamp-pumped amplifier. Both the Tm, Ho:YLF oscillator crystal and the Er, Tm, Ho:YLF amplifier crystal were cooled to 77 K with liquid nitrogen. An acousto-optic Q-switch allowed operation throughout the 1–50 kHz range with pulse widths of 15 ns at 2.5 kHz and 30 ns at 5.0 kHz. Average output powers up to 10 W were demonstrated with 1100 W of tungsten lamp power and 2 W of oscillator power. The OPO resonator used a 5 m radius of curvature input mirror coated to be highly transmissive at 2 μm and highly reflective in the 3–5 μm region. The output mirror was piano and 80% reflective in the 3–5 μm region. Physical resonator length was 4.5 cm. Two anti-reflection coated AgGaSe₂ crystals were used: one, 34 mm long with 2.1 μm loss of α = 0.024 cm⁻¹ and a standard coating; the other, 30 mm long with 2.1 μm loss of α = 0.013 cm⁻¹ and an improved polish and coating. At 2.5 kHz, the low-loss, improved-coating crystal produced 740 mW of 4 μm output with 5.4 W of 2 μm input and a 1/e² beam diameter of 2 mm. With a pump beam diameter of 1 mm, 2.3 W of pump power generated 550 mW of OPO output power, a conversion efficiency of 24%. The standard-coating crystal demonstrated only 100 mW of 4 μm output because of damage limitations. In all cases, maximum OPO input power was limited by damage to the AgGaSe₂ crystal surface. However, conversion efficiency was found to peak at pump powers below the damage threshold. This saturation of conversion efficiency was most likely a thermal effect. The improved-coating crystal consistently demonstrated damage levels a factor of two better than those of the standard-coating crystal.

© 1992 Optical Society of America