PP-crystals Lengths Optimization to Improve the Efficiency of Two-Cascade Nearly-Degenerate DFG of 3µm Radiation from Fiber NIR Lasers

High absorption of the electromagnetic radiation in the spectral range around 3 µm wavelength by water and molecules of organic compounds provide opportunities for its application in biology and medicine [1] . One of the approaches to obtain mid-infrared (mid-IR) radiation is frequency down-conversion process of near infrared (NIR) lasers radiation in nonlinear optical crystals [2] . However, optical resonator schemes, which are commonly used for this purpose, can be more complicated and difficult to tune [3] . Using seed radiation at one of the output wavelengths of nonlinear process can help to improve its conversion efficiency, which is directly proportional to the product of the pump and seed signal intensities. Thereby, a high efficiency of mid-IR radiation generation is possible to achieve in two-stage conversion of pump at 1 um and seed at 1.5 um wavelength [4] .