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
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