High-speed system of IR-image conversion in a nonlinear crystal pumped by a thermal source

Abstract

The necessity of registration of superfast optical processes in the infrared region of the spectrum has arisen with the development of pico-and femtosecond laser technology. It is most expedient to convert the image registered in the IR range of the spectrum to the visible range and to investigate it by the standard methods. We note that thermal imaging systems available at present do not allow superfast processes in the IR range of the spectrum to be investigated [1,2].To convert the IR image to the visible range of the spectrum, it makes sense to use nonlinear optical crystals that allow the image to be converted simultaneously in all regions of the crystal with high temporal resolution [3,5]. In this case, the image of the IR-object whose radiation has frequency ω IR is formed in the crystal by the IR objective. Pump radiation with frequency ω p is also sent into the same crystal. Pump radiation is chosen so that ω v = ω IR + ω p , where ω v is the circular frequency of radiation converted to the visible range of the spectrum. The spatial spectrum of the IR-image is transformed by a certain method into the visible range of the spectrum [3][4][5][6][7][8][9].The characteristics of the system of IR-image conversion are mainly determined by pump radiation (narrow band laser radiation or wideband non-laser radiation of thermal sources). The method of its formation in the nonlinear optical crystal is also important (a system with tangent matching is used for a collimated laser beam, a system with critical vector matching is used for a laser beam focused by a cylindrical lens, and homocentric radiation beams are used for thermal non-laser radiation focused by a spherical lens) [3][4][5][6][7][8][9].Systems of IR-image conversion to the visible range of the spectrum in nonlinear crystals pumped by laser radiation sources cannot be used for this purpose. Such systems are too sensitive to the degree of focusing and nonmonochromaticity, they do not allow wideband thermal radiation to be registered, and the converted image is compressed compared to the initial image by ω v /ω IR times in the matching plane [3,4,7].The above-indicated disadvantages can be easily excluded by using wideband non-laser thermal pumping in the form of homocentric radiation beams in conversion systems [6,8,9]. In this case, the increased degree of radiation focusing and the increased spectrum width cause the conversion efficiency to increase. During focusing (homocentric beams), the contribution of vector interactions (intersecting beams) to converted radiation increases by 2-3 orders of magnitude [7].The increased width of the spectrum leads to the fact that the frequencies symmetric about the matching frequency contribute to converted radiation, that is, the wider the spectrum, the higher the conversion efficiency. Such type of matching (interaction) is called multifrequency [6-9].The advantage of this system is the possibility of IR image conversion to the visible range of the spectrum without distorting the geometrical sizes of IR images (du...