The filtering properties of three-phonon acousto-optical Bragg diffraction for increasing the degree of coherence of a partially coherent optical field are investigated by the example of multiphonon acousto-optical interaction in a TeO_2, single crystal. Two possible mechanisms of coherent scattering upon the formation of the highest orders are taken into account. It is shown that the three-phonon interaction is more efficient by at least a factor of two than any mode of one-phonon diffraction realized at the same frequency and the same acoustooptical interaction length. The experimental investigation, using partially coherent radiation of a He--Ne laser ((lambda) = 0.63 um) diffracted from a transverse acoustic wave propagating in a TeO_2 single crystal, on the whole confirmed the basic theoretical foundations.
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