Study of elastic nonlinearity of the crystals used to control optical beams

摘要

When a new element base is created for information and telecommunication systems, the properties of the materials used must be well understood. In particular, a thorough investigation of the elastic and elasto-optical properties is required for materials used in optical telecommunication systems, optical display panels, acousto-electronic information devices, etc. For example, the elastic nonlinearity for various crystals must be known. If the materials under study are optically transparent, acousto-optics can be used to obtain such information. It is known that elastic nonlinearity is one of the most important factors distorting and limiting the performance of acousto-optic information systems. Due to such nonlinearity, the attenuation coefficient of acoustic waves increases; this effect can be strong in some materials and depends on the acoustic power density. Using the coupled-mode method, new solutions describe the spatial distributions of the acoustic wave amplitude, taking into account the initial power of the wave at the generation edge and assuming the square law of the nonlinearity. The current method approximates real distributions more accurately than previous solutions, and we have confirmed this experimentally. The experimental results of a specific nonlinearity in an acoustic resonator are shown. The higher harmonics sharply reduce their amplitudes after reflecting from the rear face of the sample, then the amplitudes steadily increase. The decrease in the amplitude of the higher harmonics is related to the decrease of the attenuation coefficient for the main acoustic mode. Using the measured data, an approximate synthesis has been made for the shape of the acoustic signal deformed by the elastic nonlinearity of the medium. Evaluations have been made for a 500 MHz longitudinal acoustic mode and an initial power level of 200 W/cm~2 at various traveling points of that wave in LiNbO_4 crystal.