Patent GB-1380475-A (GB)
Title: Acoustooptic light deflectors
IPC: C09K3/00
Filed: 1972-12-04
Abstract: 1380475 Deflecting light WESTERN ELECTRIC CO Inc 4 Dec 1972 [6 Dec 1971] 55829/72 Heading H4F In an acousto-optic light deflector using " anisotropic Bragg diffraction ", in which the moduli of the incident and diffracted optical k-vectors are unequal as a result of the optical anisotropy of the elasto-optic medium, whereby the incident and diffracted rays form unequal angles # i  and # d  respectively with the normal to the acoustic propagation direction-see IEEE, J. Quantum mechanics, QE-3, 1967, pp. 85-93, dealing with birefringent media-the optical anisotropy of the medium may be provided by its optical activity. As shown in the above article, when # i  and # d  are plotted against the acoustic frequency f, at a particular frequency f<SP>1</SP> (which is a function of the optical wavelength used), there is a point of inflection in the # i  plot while # d  varies with f in substantially the same manner as in normal Bragg diffraction. That is, compared with the normal case, for a given acousto-optic interaction length L, the acoustic bandwidth #f and the optical deflection range ## may be increased, or for a predetermined #f and ##, L may be increased and the acoustic power reduced, for the same diffraction efficien-. cy. The deflector described and claimed comprises a body of paratellurite tellurium dioxide, means for generating acoustic shear waves for propagating along the [110] direction in the body, means for elliptically or circularly polarizing an optical beam incident on the body and directing it at a small angle to the optic [001] axis of the body, and means for modulating the frequency of the acoustic waves about a centre frequency f<SP>1</SP>, being the inflection point frequency corresponding to the optical wavelength used. The acoustic mode and direction is chosen to give a low acoustic velocity, i.e. a long acoustic-optic interaction time, whereby the resolution of the deflector is increased. Paratellurite is birefringent and optically active, and f<SP>1</SP> for these two forms of anisotropy and using this particular acoustic wave, is 0À45 to 1À17 GHz, and 18-82 MHz respectively, for optical wavelengths of 1À15 to 0À44 Á. Values of f<SP>1</SP> in the former range would give very high acoustic loss. In the preferred embodiment, plane polarized laser light is passed via a #/4 plate or a Soelil- Babinet compensator to a cylindrical lens to given an elliptical beam for greater aperture and resolution. The elliptical beam is at a small non- zero angle γ, preferably less than 20 degrees, to the optic axis of the paratellurite body, and a small angle # i  (e.g. 1-3 degrees) to the normal to the acoustic propagation direction. Increasing γ increases f<SP>1</SP> and reduces the deflection efficiency. The acoustic and optical beams lie in a plane at an angle # to the optic axis, where # is typically less than 20 degrees and may be zero. The acoustic displacement reactor is in the [T10] direction. The deflected beam may be passed to a second deflector, which may be similar to that described or, for example, a galvanometer, or it may be focused by a telescope before use. The paratellurite body is grown by the Czochralski method from 99À9999% pure polycrystalline powder. Two opposed ends form an angle of 2 degrees, one being provided with an x-cut LiNbO 3  electroacoustic transducer and the other with a magnesium acoustic absorbing block. The 2-degree inclination deflects any remaining acoustic energy away from the optical beam. A heat sink is secured to the magnesium block.