Tunable acousto-optic filter utilizing internal mode conversion

Claim 1

Apparatus for diffracting light from a first polarization to a second polarization, said apparatus comprising an acoustically anisotropic, optically birefringent crystal, means for passing a light beam through said crystal along a selected axis oriented at a non-zero angle with the optic axis of said crystal; means for exciting a first acoustic wave in said crystal and varying the frequencies of said first acoustic wave; means for directing said first acoustic wave to propagate along a selected direction in said crystal to be incident on an internal reflection surface of said crystal to excite a second acoustic wave such that the acoustic group velocity of the said second acoustic wave is noncollinear with the incident light beam in the said crystal and such that the tangents to the locus of the wave vectors for the incident light and diffracted light are substantially parallel; the orientation of said internal reflection surface being determined by the relation, ##EQU5## wherein "α" is the angle between the normals of said internal reflection surface and said first acoustic wave, V a ' and V a are the phase velocities of said first and second acoustic wave, respectively, and θ a ' and θ a are the polar angles of said first and second acoustic waves, respectively.