Up-conversion of CO2 laser radiation with ZnGeP2

Up-conversion of long, 3-50 μs pulses, imitating input signals of CO₂ laser DAS Lidars was investigated. CW laser was Q-switched by means of in- or external cavity opto-mechanical obturator at frequency of 10 to 200 Hz. Output emission pulses had a peak power 10 W to 3 kW and duration 1 to 20 μs. TEM₀₀ mode structure was formed by means of iris diaphragms and magnetostriction adjustment. Efficiency of 1 to 2% has been achieved when using 1.06-μm Nd:YAG pump emission and 3.9 mm ZnGeP₂ crystal that is characterised by absorption coefficient equal to 2.0 cm^-1 at this wavelength and 2.5 cm^-1 at up-converted 0.9548 μm one. The nonlinear crystal made has an orientation: θ=85° and φ=45°. The emission has been detected with pyroelectric power and energy meter. The pulse time shape has been observed with specially selected PMT with S-1 (Ag-O-Cs) photocathode: quantum efficiency of 2×10^-3% at λ=1.064 μm and 0.1% at the up-converted wavelength 0.9548 μm. When casing a silicon avalanche photodiode operating in photon counting mode operation, 450 times increase in signal-to-noise ratio is realised in comparison with direct detection of the CO₂ laser lidar signals by MCT photodiode