Broadband sum-frequency conversion of multiline Q-switched CO laser emission under its double-pass through AR-coated ZnGeP₂ crystal

Broadband sum-frequency conversion of multiline (~ 60 spectral lines within ~ 5–6 μm wavelength interval) Q-switched CO laser emission in an AR-coated ZnGeP₂ crystal was experimentally studied by application of both single-pass and double-pass optical schemes. The maximum conversion efficiency in the double-pass scheme reached ~ 10% that is 2.5 times higher than one obtained in the same optical scheme with an uncoated ZnGeP₂. The spectrum control of frequency converted radiation by changing angles of incident and reversed CO laser beams in the ZnGeP₂ crystal was implemented within the wavelength range from 2.52 μm up to 2.92 μm. The maximum spectrum width of 0.35 μm was obtained with the double-pass scheme when the angle of the laser beam incident on the crystal and the angle of the deflected reversed laser beam were −4.0° and 1.0°, respectively, the spectral bandwidth being three times broader than one obtained with the single-pass scheme.