Acousto-optic devices for use in laser beam scanners have become viable alternatives to electro-mechanical scanners, the latter having obvious mechanical limitations, including lower scan speeds.
In an acousto-optic device, a traveling ultrasonic wave establishes a thick diffraction grating in an interaction medium, with grating spacing equal to the ultrasonic wavelength .LAMBDA.. In a deflector, or scanner, the result is an angular scan, which can be converted to a linear scan with a lens.
Diffraction from the acoustically generated grating is at an angle .theta. given approximately by EQU .theta..apprxeq.sin .theta.=(.lambda./.lambda.)=(.lambda.f/v)
where .lambda. is optical wavelength measured in air, f is ultrasonic frequency, and v is ultrasonic velocity. Varying the applied acoustic frequency changes the diffraction angle of the beam, with total angular swing .DELTA..theta. produced by the scanner proportional to frequency variation .DELTA.f EQU .DELTA..theta.=(.lambda..DELTA.f/v)
Thus linear frequency modulation (with df/dt constant) produces an angular scan at a constant rate.
The concept of using a stepped (or phased) transducer array to improve the bandwidth or resolution of an acousto-optic deflector has been shown in the prior art. The most important requirement for optimum diffraction efficiency of the laser beam is that the laser beam must be oriented at the Bragg angle with respect to the acoustic wavefronts. As the center frequency is changed to change the deflection angle, the Bragg angle must also change. That is, the angle between the incident laser beam and the acoustic wavefront must vary with frequency. In the phased transducer array deflector this is accomplished by grinding steps on the transducer bonding surface which are one-half wavelength high at the center frequency of operation. Furthermore, each transducer element is electrically 180 degrees out of phase with its neighboring transducer elements.
Another prior art acousto-optic deflector is described in U.S. Pat. No. 3,373,380. The patent discloses a technique for increasing the bandwidth of the transducer utilized in conjunction with interaction medium is to increase its thickness from one side to the other. Either the thickness of the piezoelectric material or that of the sandwiching electrodes may be tapered. The increased mass of the transducer element as a whole toward the one side decreases its resonant frequency. That is, the tapering of the mass across the length of the transducer in the direction of light beam travel enables the transducer to exhibit a much greater width of frequency response. In order to insure an appropriate Bragg relationship over a wide range of frequencies, at least a portion of one of the light and sound wave-fronts is curved and the direction of propagation of the sound waves relative to the light waves is such that the tangents to the curved portion include a tangent which intersects the other of the wavefronts at the Bragg angle .theta. corresponding to the wavelengths of the sound and light. The transducer is designed to produce curved sound wavefronts so as to make a collection of angles available which include the proper Bragg angle. This is achieved at least over a significant frequency range by constructing the transducer so that in operation it assumes a non-planar or warped contour. The warped surface of the transducer produces the curved sound wavefronts. In accordance with another embodiment, a uniform thickness transducer 30 may be selected to have a curved contour of at least its active sound propagating surface, or both its front and back surfaces. The concentric wave-fronts thus produced include many tangents one of which intersects the light wave-fronts at the Bragg angle. In another embodiment, the transducer contour is both tapered and curved. The taper causes the point of maximum vibration to move as the frequency changes. By deliberately also curving the sound wave-front shape, the sound wave-fronts follow a path of maximum power flow from the instantaneously active portion of the transducer. This path changes direction relative to the light wave-fronts with change in sound frequency so that a tangent to the sound wave-fronts intersects the light wave-fronts at the Bragg angle .theta. corresponding to the wavelengths of the sound and the light. That is, the sound energy is concentrated and it is turned as the sound frequency changes.
A serious limitation of the stepped transducer array and the array described in the patent described hereinabove is that they are extremely difficult to fabricate. Regarding the former, each transducer must be individually ground to its final dimension prior to bonding to the substrate. This process is costly and limited to applications where the transducer frequency is not much higher than 150 MHz. Regarding the embodiments described in the patent, tapering of the transducer/electrodes or providing a curved contour transducer increases the number of processing steps required with the attendant increase in cost.