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Generation and Steering of Bulk Acoustic Waves by Surface Electrode Array

IP.com Disclosure Number: IPCOM000079410D
Original Publication Date: 1973-Jun-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 3 page(s) / 58K

Publishing Venue

IBM

Related People

Tseng, SC: AUTHOR

Abstract

It is well known that a light beam can be diffracted strongly by an acoustic beam, if the Bragg condition as expressed sin Theta = (1 over 2)( Lambda over Lambda). is satisfied, where Lambda is the wavelength of light in the elastic medium, Lambda is the wavelength of the acoustic wave, and Theta is the incident and the diffracted angles of the light beam with respect to the acoustic wave front. Since the diffracted angle Theta depends on the acoustic wavelength, the angle of diffraction can be changed to a certain degree by varying the acoustic wavelength (or equivalently varying the frequency). Such device has been used for light deflectors.

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Generation and Steering of Bulk Acoustic Waves by Surface Electrode Array

It is well known that a light beam can be diffracted strongly by an acoustic beam, if the Bragg condition as expressed sin Theta = (1 over 2)( Lambda over Lambda). is satisfied, where Lambda is the wavelength of light in the elastic medium, Lambda is the wavelength of the acoustic wave, and Theta is the incident and the diffracted angles of the light beam with respect to the acoustic wave front. Since the diffracted angle Theta depends on the acoustic wavelength, the angle of diffraction can be changed to a certain degree by varying the acoustic wavelength (or equivalently varying the frequency). Such device has been used for light deflectors. The angle of diffraction, however, can be changed only in the limited range as follows: (-1 over 2) (Lambda over W) </= (1 over 2) (Lambda over W) where W is the width of the acoustic beam. If the frequency is changed so much that the diffraction angle exceeds the range of limitation set forth above, the Bragg condition is no longer satisfied and the intensity of the diffracted light will be weak. This is because the incident angle (fixed) of the light beam and the diffracted angle (varying with frequency) are no longer symmetrical about the acoustic beam. In order to restore the Bragg condition, the acoustic beam direction must be changed to recover the symmetry of the incident and the diffracted light beams about the acoustic beam. By so doing, the range of frequency variation (the frequency bandwidth) can be increased.

The change in the direction of the acoustic beam by electronic means, without mechanical motion, can be accomplished by the use of an acoustic wave transducer array, properly fed in different phases to form a frequency steering acoustic wave beam. The principle is known in antenna array theory, only to be applied to the acoustic wave case. In such application, the transducers are placed in a common plane. When the electrical excitation fed into adjacent transducers are out-of-phase by 180 degrees, two steerable acoustic beams will be created. Only one of the beams is useful and the power to the other acoustic wave beam will be wasted.

In order to improve the situation, transducers can be placed in a stepped surface rather than on a common plane. Such arrangement can generate only one acoustic wave beam instead of two. Instead of using a stepped arrangement so as to reduce the extra acoustic wave beam, it is also possible to keep the transducers in the same common plane, but to use a phase array of more than two-phase structures. It is known in the antenna theory that in an array of more than two-phase structures, only one beam will be created if the distance between the adjacent transducer is no larger than one-half wavelength; otherwise the second beam will not be completely eliminated.

Although transducer arrays can be placed in a common plane, which has advantages of avoiding the fabrication diffic...