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I Determination of nonlinear susceptibilities of new materials Creation of highly effective converters of laser frequencies allows to solve numerous tasks of nonlinear crystal optics, laser physics, medical diagnostics, ecology and so on. On the other hand the task of the analysis of interaction of intensive radiation with substance is important. It will allow to define material parameters of substances, first of all nonlinear susceptibilities, for example, fullerene structures. In the given work it is offered to spend the analysis of process of conversion in the constant-intensity approximation. The analysis of frequency conversion process in this aproximation, taking into consideration changes of the phases of interacting waves, allowed one to suggest a new method of determination of nonlinear susceptibility of the second and higher orders. As a result of analysis in this approximation in contrast to the constant-field approximation the minima of intensity of the second-harmonic dependent on the nonlinear susceptibility of a substances. This fact allows simple, reliable way with greater accuracy to define nonlinear susceptibilities of substances. Areas of application A measurement technique and skilled installation in which the companies can become interested, engaged search of new nonlinear materials for development of effective converters the frequencies. The companies working in the optical industry, creating devices of management of laser radiation in the optical communication systems, developing elements of the optical computers, new coherent sources of optical radiation in UV area of a spectrum, The developed pilot (skilled) installation will be offered to the industrial enterprises on manufacturing converters of frequency, optical production, for the decision of problems in spectroscopy and the optical industry. Introduction The methods of nonlinear optics are successfully used for measuring nonlinear optical properties of materials. As a rule the results of measuring don't differ with high accuracy that is connected with the accuracy of registration of power of the basic wave, second-harmonic and space -time distribution of intensity of laser beam. That's why as a rule in most cases non absolute but relative value of nonlinearity is measured. As a rule the determination of nonlinear susceptibility of a matter was made at small efficiency of frequency conversion, analytically described by expressions obtained in the constant-field approximation. However in this approximation the inverse reaction of the excited wave on the pumping wave is not taken into consideration, that's why the information about qualitatively important features of nonlinear process is lost. Description The new way of definition of a nonlinear susceptibility of the optical substances, differing from earlier existing the technique is offered. As to accuracy of spent measurements comparison of results of calculation of first three zero of a curve of synchronism in the constant-intensity approximation the constant-field approximation and the numerical account has been lead. Comparison shows, that results in the constant-intensity approximation will well be agreed with results of the numerical account. Hence expected accuracy in measurement should be above, than in the existing methods of the measurement based on the constant-field approximation. For intensity of the second harmonic it is received I 2 22 I1(0) 1(sin 2 x sh2 y)exp ( 2 21) z , (1) where 21 2 ( 2 21 )2 a b , a 2 , , b 2 4 4 2 2 2 2 2 2 1 2 I1 (0) , b 2 I1 (0) A1 ( z 0) , x z cos , y z sin , arctg . 2 2 a From this it follows that at low absorption coefficients the intensity of a harmonic is a periodic function of the nonlinear interaction length. As the serial number of the period increases, the maximum intensity I 2 decreases. Unlike the results of the constant-field approximation, in the constant-intensity approximation the period of the spatial beats of the amplitude of a harmonic depends on the nonlinear susceptibility . This means that it can be determined by measuring the spatial position of the minima of the intensity of a harmonic. By measuring this intensity, it is also possible to determine the absolute value of for given values of the parameters of the problem. Table: Positions of the first three zeros of the synchronism curve obtained in calculated directly ( c ), in the constant-intensity ( CIA ) and constant-field ( CFA ) approximations. 1 Laser beam 1 3 4 1 5 2 1 ) 3 1 4 ) Fig. Schematic diagram of the apparatus intended for measuring the relative quadratic nonlinear susceptibility of materials: (1) crystal with an unknown and (2) with a known nonlinear susceptibilities; (3) filters, (4) photodetectors; (5) laser beam; the optic axes of the crystals are parallel to the front face which serves as the axis of rotation. Thus the original way of definition of a nonlinear susceptibility of the optical substances, differing from earlier existing the technique is offered. The offered method is more exact, simple and reliable. Measurement both relative, and an absolute susceptibility of substances is expected. And measurement of a relative susceptibility will be with greater accuracy as thus there is no necessity for absolute measurements of characteristics of laser radiation. References: 1. Z.A.Tagiev, and R.J.Kasumova, "Determination of Nonlinear Optical Susceptibility of Substances“, Patent I 2000 0162, Azerbaijan. 2. Z.A.Tagiev and R.Zh.Kasumova,” Determination of Nonlinear High-Order Susceptibilities”, Opt.Spectrosk., 1996, v.80, 941-943 [Opt.Spectrosc. 1996, v.80, 848-850].