Mid-infrared Wavelength Based On Difference-frequency Generation In Nonlinear Optical Crystals AgGaS₂ And AgGaSe₂

Optical parametric oscillator (OPO) has widely tunable range, high efficiency, simple structure and high reliability. It's one of the most important ways to get widely tunable and highly coherent radiate source, and the new wave band laser systems. Especially in recent years, with the invention, maturation and mass application of the new nonlinear crystals, and the rapid development of the technologies for the nonlinear optical frequency conversion and tunable lasers, many important breakthroughs have been made in the researching field of optical parametric oscillators and their applications. Recently, laser spectrum technology has become to be the most advanced way of detecting material structure. Especially optical remote sensing plays an important role, which is applied to evaluate the changes of earth atmosphere. Almost all of molecules have strong absorption bands in the mid-IR. Most of the toxic gases have unique strong absorption features ranges of 3.3 - 4.2μm and 8 - 12μm atmospheric window. New nonlinear crystals have turned to be significant crystals to generate mid-IR radiation, such as AgGaSe₂, AgGaS₂, which is attracted in the most interests. To achieve wider tunable range, it is suggested that combined tunable technology has its advantage. It is typical that optical parameter oscillator difference frequency technology (OPO– DFG) has attracted most interests. The applications of the mid-IR wave band achieved have included environmental monitoring, remote sensing, medical diagnoses and treatment, laser spectroscopy, material handling, data communication, optical-electronic measures, laser ranging, laser radar and infrared countermeasures.We demonstrate continuous tunable mid-IR radiation of DFG in AgGaSe₂ / AgGaS₂ crystal and output of LiNbO3-OPO in type- I by means of employing a diagram of efficient mixing of the signal and idler, which is pumped by 1030nm/1064nm with extraordinary light. The main research work is summarized as follows.1. The studying status at present and the developing trends in the future of the mid-infrared OPO have been widely investigated and discussed. The emphasis and the developing direction of our own research work have been definited. The development of combined tunable technology, which is optical parameter oscillator difference frequency technology, is summarized.2. Studies on the Yb:YAG solid laser which is the pump source of combined tunable technology OPO-DFG have been carried out. The laser performances of Yb:YAG laser are introduced systematically in this paper, based on the crystal structure and properties. Different characteristics of the laser have been presented. The output characteristics of Yb:YAG all-solid-state laser in the case of stable condition have been obtained by establishing the theoretical model of quasi-three-level laser system. The effect on output power of distribution of space variation, crystal length and transmissivity has been discussed in detail. The numerical simulated calculations for output characteristics have been studied by using MATLAB program language. Furthermore, the results of numerical simulation have been comprehensively analyzed.3. The optical characteristics of nonlinear optical crystals, especially several common mid-IR nonlinear crystals have been introduced in the research of nonlinear frequency conversion crystals. Studies on phase-matching of three wave interaction of uniaxial have been discussed in detail and the expressions of walk-off angle and acceptance angle have been deduced in theory. Second order efficient nonlinear coefficients of each class of crystals have been analyzed based on nonlinear optical polarizability theory.4. A different frequency generation of nonlinear optical crystals of AgGaSe₂ and AgGaS₂ has been analyzed based on the three-wave coupled equation and energy conservation both in the phase-matching of type-and type-II. The theoretical curves of tunable wavelengths of different frequency generation with phase-matching angle have been obtained by numerical simulation. As walk-off angle and acceptance angle of beam limit the conversion efficiency and the tunable wavelength range of difference frequency generation, research include effective nonlinear coefficient, walk-off angle and acceptance angle as functions of wavelength, the relationship between conversion efficiency and acceptance angle, and the impact of crystals length on the process of frequency conversion.