Broadband three-stage frequency conversion of multiline sub-microsecond CO laser radiation in a single sample of ZnGeP crystal was experimentally and numerically studied for the first time, to the best of our knowledge. As a result, the hybrid laser system emitted more than 200 narrow spectral lines within the 2.4-6.2 μm spectral range. The measured conversion efficiencies of the first, second, and third stages were about 4.8%, 0.4%, and 0.05%, respectively. Our numerical simulation demonstrated that the third stage of this frequency conversion can extend the laser system spectrum toward the shorter wavelength of 2.2 μm.

Non-linear optical characteristics of a new BaGa 2 GeSe 6 crystal were numerically studied and compared with well-known mid-IR nonlinear crystals ZnGeP 2 , GaSe, and AgGaSe 2 .The calculations demonstrated for the new crystal to be probably the most efficient crystal for second harmonic generation of CO-and CO 2 -laser radiation. It was found that a broadband two-stage frequency conversion of multi-line CO-laser radiation in this crystal is possible within 2.5-9.0 μm wavelength range with higher efficiency than in ZnGeP 2 and AgGaSe 2 crystals. IntroductionA number of fundamental and applied problems in the fields of photochemistry, spectroscopy, plasma diagnostics and others need mid-IR laser sources. To obtain mid-IR laser radiation, different nonlinear techniques have been extensively used: optical parametric oscillators and difference frequency generation of short-wave (0.7-3 μm) lasers [1]; sum and difference frequency generation of mid-IR lasers [2]. Nonlinear optical crystals should have following properties: transparency in required spectral range; adequate birefringence for phase-matching; high nonlinear coefficient; high optical damage threshold and some others. The most known and widespread mid-IR crystals are ZnGeP 2 , GaSe, and AgGaSe 2 [3,4]. It is worth noticing that the ZnGeP 2 crystal is usually considered to be the most efficient one and sometimes called the "standard" of mid-IR crystals [4].A new promising uniaxial nonlinear crystal BaGa 2 GeSe 6 (BGGSe) was developed in the Kuban State University [5]. The BGGSe crystal has a high optical damage threshold (110 MW/cm 2 ), a wide spectral transparency range (from 0.5 to 18 μm) and a high nonlinearity (d 11 = 66 ± 15 pm/V) [5], which makes this crystal very attractive for frequency conversion of laser radiation into (or within) the mid-IR range. Therefore, the objective of this paper is estimation of BGGSe crystal efficiency with respect to ZnGeP 2 , GaSe, and AgGaSe 2 by means of frequency conversion simulation for CO-and CO 2 -laser radiation. The calculations under discussion demonstrated that broadband two-stage frequency conversion of multi-line COlaser is quite possible in a BGGSe crystal as well as in ZnGeP 2 [6] and AgGaSe 2 [7].

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