OPTICS

Rena J. Kasumova

Baku State University, Azerbaijan, Baku

 

Redoubling of optical frequency in CGA and ZGP

 

          The CGA and ZGP crystals present a great interest for application in nonlinear optical frequncy converters of laser radiation of İR-range [1-2]. The most efficient way of frequency conversion is performed by using generation of second harmonic. It is explained by high nonlinearity of second order in crystals –converters in comparison with nonlinearity of the third and etc. orders.

          Creation of frequency converters with tuneable frequency of radiation is one of the actual tasks of nonlinear optics. It is, in particular important for IR –range of spectrum where two windows of atmosphere transparency exist. There are a number of the perspective crystals transparent in the IR –range of spectrum. These are CdGeAs2 (CGA), AgGaSe2, AgGaS2, ZnGeP2 (ZGP), AgGaxIn1-xSe2 and others [3-5]. But a basic drawback of the listed crystals was their considerable absorption, worse thermal properties what restricts an efficiency of frequency conversion in these crystals. However, owing to the achievements of modern technology of crystal growth, it became possible to create crystals with the improved optical characteristics.

          The researches in this direction are being continued. It is possible to study nonlinear interaction of optical waves by the direct numerous account of related equations. However, development of the analytical method permits to receive the concrete analytical expressions and determine the optimum parameters of the task for the purpose of obtaining maximum efficiency of conversion.

          The present work considers an efficiency of frequency conversion in CGA and ZGP crystals. An analysis has been made in the constant –intensity approximation [6-7] enabling to take into account the reverse influence of excited wave on exciting one. There has been made comparison of the obtained results with the experimental ones.

          The task is solved with the use of the well –known system of the reduced equations with corresponding boundary conditions reflecting the process of second harmonic generation [8-10]. In Figs. 1 and 2 the numerical solution of analytical expressions received in the constant –intensity approximation in experimentally realized conditions for case of CGA and ZGP crystals is shown.

Fig. 1. Dependences of conversion efficiency of radiation energy of pump wave (l=9.55 mcm) to energy of wave of second harmonic h2 on lengths of CGA crystal l calculated in the constant-intensity approximation (curves 3 and 4) and accurate counting (dotted curves 1 and 2) for d1,2=0.05 cm-1 [2], D=0,0028 cm-1 and pump intensity of I10= 30 MW/cm2 [6] (curves 2 and 3) and 160 MW/cm2 [6] (curves 1 and 4).

           

          In Fig. 1 dependences of frequency conversion efficiency in CGA on crystal length  are displayed. There were considered 2 versions of conversion differing by pump intensity of CO2 laser radiating on wavelength in 9.55 mcm: 30 MW/cm2 (curve 3) and 160 MW/cm2 (curve 4). It follows from behavior of curves that there exists an expressed maximum, i.e. the optimum value of crystal length at which conversion efficiency is maximum. As far as pump intensity increases the maximum conversion is reached at lesser lengths of crystal, i.e. with an increase in pump intensity the coherent length of crystal decreases. Just here for comparison of the corresponding dependences  received at the numerical account for the system of reduced equation in relative units [2].

          In Fig.2 there are cited the analogous dependences calculated in CIA (curves 3 and 4) and obtained experimentally [2] (curves 1 and 2) in case of conversion in ZGP crystal. With this an examination was carried out for two values of pump intensity: crystal breakdown value in ZGP (intensity of optical breakdown is equal to 142 MW/cm2 [2, 11] (curves 1 and 4) and for MW/cm2 (curves 2 and 3).

Fig. 2. Dependences of conversion efficiency of radiation energy of pump wave (l=9.55 mcm) to energy of wave of second harmonic h2 on lengths of ZGP crystal l calculated in the constant-intensity approximation (curves 3 and 4) and accurate counting (curves 1 and 2) for d1,2=0.05 cm-1 [2], D=0,00065 cm-1 and pump intensity of I10= 30 MW/cm2 [6] (dotted curves 2 and 3) and 160 MW/cm2 [6] (solid curves 1 and 4).

 

 

From comparison of cited dependences it follows that qualitative behavior of curves received in the constant –intensity approximation and by the direct accurate calculation, coincides.

          Dependences in Fig.1 have the pronounced maxima with similar values of the optimum length for CGA crystal calculated in the given approximation and at the precise account. In case of ZGP crystal (Fig.2) the values of coherent lengths for two versions of the account are differed several times. This fact may be explained by the lack of detailed information on conditions of the experimental researches, spread of different parameters values of the task what was also noted by the authors [11].

          The advantage of the analytical method of research consists in that it is possible to calculate the optimum parameters of conversion proceeding from the dimensions of the concrete crystals –converters. And namely at the given values of crystal –converter length it is possible to calculate an optimum value of pump intensity, as well as to calculate the coherent length of a crystal –converter at chosen pump intensity of laser radiator. The analytical method permits also to estimate expected conversion efficiency at different wavelengths of laser radiation.

 

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