Investigations of fast processes in transparent media are being successfully performed by means of coherent optics methods such as interference, correlation, laser Doppler anemometry (LDA), etc. All this methods require splitting and further bringing together laser beams, and following processing of a useful signal. It is very difficult to fulfill these requirements with the help of traditional optical means, especially for many beams with complex wave fronts. In these cases one has to use optical schemes with a great number of elements, which results in great losses at reflection, in depolarization of the beams, in worsening of the device vibration stability, and in climatic changes. It is possible to get rid of all these disadvantages by using holographic optical elements in such devices.Use of volume holograms with thickness of one millimeter or more is most promising.The volume registering media, e.g. photorefractive crystals such as LiNbO3, reoksan and alkali halides crystals, make it possible to obtain high quality volume holograms with wide aperture. Only one diffraction order is observed when light is diffracted by such holograms and their diffraction efficiency reaches 80%.The remarkable feature of the volume holograms is their high spectral and angular selectivity. The simplest hologram of such a type is the volume diffraction grating obtained at interference of two light waves in the volume of the light-sensitive medium. The angular selectivity of such gratings is determined by the hologram thickness and it can be reduced to a few angular minutes while the intensity of the diffracted wave changes by 100%. In this case, in contrast to thin holograms, the direction of radiating of the diffracted beam remains constant, making these holograms very convenient for using them in transmitting channels of optical systems for splitting of the light waves with a desired ratio of the intensities of the divided beams [1].Our investigations show that the usage of such a beam-splitter in LDA made it possible to create in the exploring volume a high-contrast interference pattern and two independent channels for the radiation receivers. It ensured the increase of signal-to-noise ratio in more than 3 times if compared to ordinary LDA. The error of measurements in the speed range 0.1 -30 m/s was less than 1.5%. The device is practically insensitive to vibrations. The employment of the more complex hologram light splitters obtained by recording three or more beams [2] makes it possible to realize multi-component LDA. The use of the volume holograms in the receiving channels is promising for analysis of informational wave phase modulation. In this case the volume diffraction grating acts a mixer of the waves, or as a diffraction pattern analyzer or a phaseamplitude converter. The possibilities of such a method for dynamic volume holografic gratings are examined in [3]. In present work we propose to use the stationary volume diffraction grating as a mixing element.
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