Frequency conversion of CO laser radiation was experimentally studied in ${\mathrm{ZnGeP}}_2$ and GaSe nonlinear crystals. To enhance conversion efficiency, we applied a CO laser system “master oscillator power amplifier” with concurrent mode locking and $Q$ switching. The laser system emitted $\sim 1$-μs train of ns pulses. Internal efficiency of frequency doubling reached 37% in ${\mathrm{ZnGeP}}_2$ crystal and 5% in GaSe crystal. Two-stage frequency conversion (including second-harmonic and difference-frequency generation) in ${\mathrm{ZnGeP}}_2$ crystal under optical pumping by a selective two-spectral line CO laser was studied.

A carbon monoxide laser emitting on the highest ever observed vibrational transition with a wavelength of 8.7 μm was for the first time launched. An influence of gas mixture content on the CO laser spectrum and factors limiting the longest CO laser wavelength are discussed.

Collinear cascaded mid-IR frequency conversion in a single nonlinear optical crystal was accomplished. Concurrent collinear generation of the sum frequency of multiline fundamental band carbon monoxide laser radiation as a first frequency conversion cascade resulted in collinear difference frequency generation within the 4.3 to 4.9 μm spectral range when mixing this sum frequency radiation with the fundamental one as the second cascade in the same ${\mathrm{Zn}\mathrm{G}\mathrm{eP}}_2$ nonlinear optical crystal.