A Television Display Using Acoustic Deflection and Modulation of Coherent Light

Abstract

Bragg reflection of laser light by ultrasonic waves in water produces the horizontal deflection in a television display. The ultrasonic waves are frequency-modulated with a sawtooth function. Deflection angles are small but there are 200 resolvable positions; the constant rate of angular change which characterizes a television scan permits the use of a wide optical aperture, leading to a small spot size. Conventional optical magnification follows the horizontal deflection, rendering a 3 MHz video signal visible on the screen. Bragg reflection requires the acoustic wave front to be symmetrical with respect to the incident and diffracted light rays. Thus, as the Bragg angle is altered, the acoustic wavefront should rotate. This is accomplished by a phased array of transducer strips whose combined wavefront rotates as the frequency changes, providing excellent correction over a wide band (19 to 35 MHz in this experiment, corresponding to a +/-30 percent change in Bragg angle). Broadband electrical and acoustical matching techniques make it possible to diffract all the incident light with about one watt of electrical input. A second acoustic diffraction cell intensity-modulates the light. In an early experiment, the laser beam was constricted to a very small diameter before entering the modulator cell; even so, the finite beam size caused a significant loss of high-frequency response. An improved version uses an old principle (Scophony, 1939): the laser beam traversing the cell is made wide enough to encompass several picture elements, all traveling across the beam at sound velocity; the horizontal deflection system nullifies the apparent motion of these elements making them stand still on the screen while a fan of light sweeps over them. With this modulation system, spatial coherence is needed only across the vertical dimension of the laser. The tolerance on the orientation of the acoustic wavefronts, the improvement brought about by the phased array, and the amount of power needed to drive the diffraction cell are calculated and the results confirmed by measurement. There is also good agreement between the experimentally observed optical resolution (spatial frequency response) and the theoretical expectation based on the computed far-field intensity pattern.