Laser Physics, Nonlinear Optics

 

Rena J. Kasumova, Sh.A. Shamilova

Baku State University, Azerbaijan, Baku, rkasumova@azdata.net

 

Second  harmonic  generation  of  laser radiation  in AgGa(Se1-xSx)2  crystal

 

In addition to the short-wave region of spectrum, for IR-region of the spectrum, where as is known, there are two atmospheric windows (which is very important in application) coherent radiation, tunable according to frequency, has a wide range of application. For instance, on the basis of parametric frequency converters, LIDARs, which are used to investigate the Earth and atmosphere are constructed. Despite the importance of applications, nonlinear crystals, which provide for smooth tuning of laser radiation in an entire optical diapason has not been elaborated yet. For frequency conversion of laser radiation in IR-region of spectrum CGA crystals are more often used, which is distinguished among other transparent crystals in this region of spectrum, for its extremely high quadratic nonlinearity in 236 pm/V. Recently the increased interest in CGA crystals is because of their achievements in its technological elaboration, which is related to get the same media with improved optical qualities, at bigger dimensions and resistant to higher radiations [1-9].

Search of the prospect materials for the tasks of modern nonlinear optics goes on. Nonlinear crystals of the IR-range of spectrum take a particular place [1-7]. Recent years the nonlinear crystals of mixed type have turned to be the subject of the researches. It is connected with that realization of the efficient tuning of radiation in a broad range of spectrum needs in the nonlinear crystals for which the condition of uncritical phase matching is kept in the chosen range of spectrum. In compounds of mixed type on the account of increasing the content of one element and decrease of the other’s content, the possibility of elaboration of crystals with uncritical phase matching on the chosen radiation wavelength is experimentally shown [10-12]. Among the crystals of mixed type we can cite Zn1−xMgx Se [10], AgGaxIn1−xSe2 [11] and AgGa(Se1−xSx)2 [12].

For the analysis of nonlinear process the use of the direct numerical account of reduced equations is possible. However, the development of the analytical method will allow one to obtain the concrete analytical expressions and determine the optimum parameters of the task with the aim of obtaining maximum conversion efficiency. The simultaneous account for changes of phases and losses of interacting waves works well in the constant –intensity approximation [13] taking into regard the reverse reaction of excited wave to pump wave.

In the present work the results of investigation of pump intensity impact on conversion efficiency in AgGa(Se1-xSx)2 crystal in conditions of existing experiment have been considered. Comparison has been made of the received results on conversion efficiency with the analogous results obtained in the experiment. The applied analytical method permits to calculate the optimum parameters of both crystal –converter and a source of radiation. For example, optimal crystal length at the given losses and pump intensity what makes possible an estimation of expected efficiency of conversion.

In the experiment for real frequency converters, it is impossible to ensure a condition of phase matching (Δ = 0). An error that is followed from the condition of phase matching determines its width. Phase mismatching is affected by spectral width of pump radiation line, deviation from phase matching angle which is caused by divergence of laser radiation and instability of temperature for a crystal converter. Then the information that we have, particularly on angular width of phase matching will make possible to calculate the maximum divergence of light beam for pumping.

          Let’s determine deviation angle from the direction of the phase matching  for mixed type AgGaxIn1-xSe2 (for x=0.6 reflecting indium content in crystal) in which second harmonic generation of CO2 laser radiation occurs on pump wavelength of 9.64 mcm (scalar phase matching of the first type for îî®å interaction). For this crystal, the calculation was carried out using coefficients in the relation of Sellmeier for the main values of refractive indices cited in [4, 10].

          The result of calculation for angular dispersion coefficient of the second order is equal to 19.4 10-6 cm-1ang.min.-2.

It is shown that with increasing the concentration of indium in the mixed crystal from 1 to 0.6, the dependence  becomes more flat. It testifies the transition to the regime of uncritical character of crystal towards following condition of phase matching.

Thus, the results of studying nonlinear interaction of waves with regard for changes of interacting waves phases in the process of second harmonic generation in AgGa(Se1-xSx)2 crystals permit to state the following. By a choice of the optimum values of nonlinear medium length, pump intensity, phase match and with the account for the influence of linear losses in a medium it is possible to increase efficiency of conversion to second harmonic in these crystals of mixed type and to select conditions for fulfilling or increasing a degree of uncritical angular phase matching.

 

References

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