Conversion of IR laser radiation

Conversion of IR laser radiation into new wavelength ranges by methods of nonlinear optics

The development of laser radiation sources operating in the mid-IR range with a wavelength of 2 to 20 μm is of great interest for spectroscopy, photochemistry, isotope separation and a number of other applications because of atmospheric “transparency windows”. One of the promising ways to master this spectral interval is related to the CO laser frequency conversion by methods of nonlinear optics, in particular, by generating summation and difference frequencies. 
One of the atmospheric transparency windows is within the interval of 4.3 to 4.7 μm; however, this interval is between the generation bands of a CO laser on the basic and overtone vibrational transitions. To master this interval, new laser sources are intensively developed. However, the same problem can be solved by converting the radiation frequency of a CO laser in nonlinear crystals. We proposed to use the generation of the difference frequency (GDF) between lines of the basic and overtone bands of radiation of a CO laser based on the ZnGeP2 crystal (ZGP). Then we proposed to replace the radiation of the overtone band by a radiation obtained by generation of the summation frequencies (GSF) of the lines for the basic band of radiation of the CO laser and, therewith, to implement the GSF and GDF simultaneously in one crystal specimen, thus reducing optical losses and saving the nonlinear optical material. We implemented the two-cascade conversion of frequency of a multi-frequency CO laser  in one ZGP crystal; herewith, GDF radiation overlapped the wavelength interval from 4.3 to 4.9 μm.
 Calculation of the conditions for the GSF and GDF phase synchronism in a ZGP crystal showed that, using the method of two-cascade frequency conversion in one nonlinear crystal specimen, we can produce radiation beyond the limits of the long-wavelength boundary of the pumping CO laser radiation. The topicality of generating radiation of a wavelength greater than 8 μm is due to utilizing the atmospheric transparency window within the wavelength range of 8 to 12 μm.  In our experiments, the radiation spectrum of the pumping CO laser was within the interval of 5 to 7.5 μm and consisted of ~150 lines. The implementation of the two-cascade conversion in a ZGP crystal made it possible to significantly enrich (more than 650 converted lines were registered) and expand the radiation spectrum, which covered the interval of one and a half octaves (the wavelength of 2.5 to 8.3 μm). The use of other nonlinear crystals (for instance, gallium selenide GaSe) enables extending the radiation spectrum into the region of even longer wavelengths.

• Yu.M. Andreyev, A.A. Ionin, I.O. Kinyaevsky, Yu.M. Klimachev, A.Yu. Kozlov, A.A. Kotkov, G.V. Lansky, Conversion of CO laser frequency in a nonlinear ZnGeP₂ crystal. Kratk. Soobshch. Fiz., 1, 19 (2010).
• A.A. Ionin, J. Guo, L.-M. Zhang, J.-J. Xie, Yu.M. Andreev, I.O. Kinyaevsky, Yu.M. Klimachev, A.Yu. Kozlov, A.A. Kotkov, G.V. Lanskii, A.N. Morozov, V.V. Zuev, A.Yu. Gerasimov, S.M. Grigoryants, Mode-locked CO laser frequency doubling in ZnGeP₂ with 25% efficiency. Laser Phys. Lett., 8(10), 723 (2011).
• A. Ionin, I. Kinyaevskiy, Yu. Klimachev, A. Kotkov, A. Kozlov, Novel mode-locked carbon monoxide laser system achieves high accuracy. SPIE News Room (6 January 2012), doi: 10.1117/2.1201112.004016, http://spie.org/x84753.xml.
• A.A. Ionin, I.O. Kinyaevskiy, Yu.M. Klimachev, A.A. Kotkov, A.Yu. Kozlov, Yu.M. Andreev, G.V. Lanskii, A.V. Shaiduko, A.V. Soluyanov, Cascaded carbon monoxide laser frequency conversion into the 4.3–4.9 μm range in a single ZnGeP₂ crystal.Opt. Lett., 37(14), 2838-2840 (2012).
• Yu.M. Andreyev, A.A. Ionin, I.O. Kinyaevsky, Yu.M. Klimachev, A.Yu. Kozlov, A.A. Kotkov, G.V. Lansky, A.V. Shaiduko, Broad-band carbon monoxide laser system operating in the wavelength range of 2.5–8.3 μm. Quantum Electron.,43(2), 139–143 (2013).