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EP-4042241-B1 - ACOUSTO-OPTIC DEVICE AND METHOD

EP4042241B1EP 4042241 B1EP4042241 B1EP 4042241B1EP-4042241-B1

Inventors

  • DAWSON, CHRISTOPHER JOSEPH

Dates

Publication Date
20260506
Application Date
20201005

Claims (18)

  1. An acousto-optic device (200) operable to deflect a beam of light, the device comprising: an optically transmissive element (210) to receive the beam of light at an input surface, the beam being received from a direction defining an optical axis; a set of acoustic transducers (220) spaced around the element; and a controller (230) operable to activate the transducers (220) to generate an acoustic wave travelling through the device to intersect with the optical axis; the controller being further operable to selectively activate a first subset (221, 223, 225, 227, 229) of the set of transducers such that the acoustic wave propagates in a selected direction (250); and wherein each of the set of acoustic transducers (220) is differently orientated such that the selected direction is controllable through the selection of first subset of transducers from the set of transducers, the device thereby being operable to control the direction in which the beam of light is deflected in two dimensions, and the device being characterised in that the controller is further operable to activate a second subset (221', 223', 225', 227', 229') of the set of transducers, at a delay after the first subset of the set of transducers is activated, the delay, amplitude, and relative phase at which the second subset of the transducers is activated being selected so as to generally cancel the acoustic wave after it has intersected with the optical axis.
  2. An acousto-optic device (200) as claimed in claim 1 wherein the controller (230) is operable to control the frequency, relative phase, and amplitude at which the first subset (221, 223, 225, 227, 229) of acoustic transducers are activated such that the generated acoustic wave exhibits a generally planar wavefront where it intersects with the optical axis.
  3. An acousto-optic device (400) as claimed in claim 1 or claim 2, comprising a plurality of sets of acoustic transducers (421, 422, 423, 424, 425) spaced around the element, each of the plurality of sets of acoustic transducers being spaced apart along the optical axis of the element.
  4. An acousto-optic device (400) as claimed in claim 3 wherein the controller is operable to activate the transducers in each of the plurality of sets (421, 422, 423, 424, 425) of transducers at relative phases and delays selected such that the angle at which the beam intersects the acoustic wavefront remains at or near the Bragg angle as the beam propagates through the element.
  5. An acousto-optic device (200) as claimed in any one of the preceding claims wherein the optically transmissive element (210) has a surface that is at least partly curved, on which surface the set of transducers are mounted.
  6. An acousto-optic device (200) as claimed in any one of the preceding claims wherein the optically transmissive element (210) is generally cylindrical.
  7. An acousto-optic device (200) as claimed in any one of the preceding claims wherein the or each set of acoustic transducers are distributed around a loop on the optically transmissive element (210).
  8. An acousto-optic device (200) as claimed in any one of the preceding claims wherein the beam is received substantially normally to the input surface of the optical element (210).
  9. An acousto-optic device (200) as claimed in any one of the preceding claims wherein the element (210) is formed of a glass.
  10. An acousto-optic device (200) as claimed in any one of claims 1 to 9 wherein the element is formed of a crystal (210).
  11. An acousto-optic device (200) as claimed in any one of the preceding claims wherein the set of acoustic transducers (220) comprises at least 5 acoustic transducers.
  12. An acousto-optic device (200) as claimed in any one of the preceding claims wherein the set of acoustic transducers (220) comprises at least 10 acoustic transducers.
  13. An acousto-optic device (200) as claimed in any one of the preceding claims wherein the set of acoustic transducers (220) comprises less than 100 acoustic transducers.
  14. A LADAR system comprising an acousto-optic device (200) as claimed in any one of the preceding claims.
  15. A guided missile comprising the LADAR system as claimed in claim 15.
  16. A method of operating an acousto-optic device (200) to deflect a beam of light, the device comprising: an optically transmissive element (210) having an input surface; a set of acoustic transducers (220) spaced around the element, each of the set of acoustic transducers being differently orientated; the method comprising the steps of: a) receiving the beam at the input surface from a direction defining an optical axis; and b) selectively activating a first subset (221, 223, 225, 227, 229) of the set of transducers to generate an acoustic wave travelling through the device in a selected direction to intersect with the optical axis; the selected direction being controllable through the selection of first subset of transducers from the set of transducers; and characterised in that the method further comprises the step of c) activating a second subset (221', 223', 225', 227', 229') of the set of transducers, at a delay after the first subset of the set of transducers is activated, and selecting the delay, amplitude, and relative phase at which the second subset of the transducers is activated so as to generally cancel the acoustic wave after it has intersected with the optical axis.
  17. A method as claimed in claim 16, wherein the step of selectively activating a first subset (221, 223, 225, 227, 229) of the set of transducers comprises controlling the frequency, relative phase, and amplitude at which the first subset of acoustic transducers are activated such that the generated acoustic wave exhibits a generally planar wavefront where it intersects with the optical axis.
  18. A method as claimed in claim 16 or claim 17 wherein the beam is received substantially normally to the input surface of the optical element (210).

Description

FIELD OF THE INVENTION This invention relates to an acousto-optic device, and to a method of operating such a device. More particularly the invention relates to an acousto-optic device that is operable to deflect an input optical beam in a controllable direction. BACKGROUND Acousto-optic devices use acoustic waves passing through an optical medium to modulate or deflect light passing through the medium. Acoustic waves result in a series of alternating regions of compression and rarefaction in the optical medium, which in turn modulate the refractive index of the medium. This modulation of the refractive index creates an effect similar to a diffraction grating, the properties of which can be varied through control of the frequency and power of the acoustic wave. US2015338718 discloses an acousto-optic deflector with multiple transducers. US8300303 discloses an acoustically focussed lens. US2002141035 discloses an acousto-optic scanning system which relies on two counter propagating acoustic waves. US5088659 discloses a projectile equipped with an infrared target seeking system having at least one deflection device for scanning the target area. US2002102064 discloses an optical switch utilizing one-dimensional, two dimensional and multi-channel acousto-optic devices optically coupled to optical fibers. WO2005121889 discloses acousto-optic modulation techniques for improving performance of laser systems. EP0867744 discloses a multichannel acousto-optic modulator using a crystal with a plurality of mounting faces. US3633995 discloses an acousto-optic light deflection system including a crystal substrate having a reflective coating on its surface which functions as a mirror, and at least one surface acoustic wave transducer to propagate surface waves on the crystal thereby producing a periodic deformation on the mirror surface. SUMMARY OF THE INVENTION An acousto-optic device operable to deflect a beam of light, the device comprising: an optically transmissive element to receive the beam of light at an input surface, the beam being received from a direction defining an optical axis; a set of acoustic transducers spaced around the element; and a controller operable to activate the transducers to generate an acoustic wave travelling through the device to intersect with the optical axis; the controller being further operable to selectively activate a first subset of the set of transducers such that the acoustic wave propagates in a selected direction; and wherein each of the set of acoustic transducers is differently orientated such that the selected direction is controllable through the selection of first subset of transducers from the set of transducers, the device thereby being operable to control the direction in which the beam of light is deflected in two dimensions. Whilst typical acousto-optic devices can alter the direction of deflection in one dimension by varying the frequency of the acoustic wave interacting with the beam of light, essentially changing the magnitude of the deflection, the present acousto-optic device can provide greater flexibility by enabling control the direction of deflection in two dimensions by altering which of a set of transducers are activated. The controller is further operable to activate a second subset of the set of transducers, at a delay after the first subset of the set of transducers is activated, the delay, amplitude, and relative phase at which the second subset of the transducers is activated being selected so as to generally cancel the acoustic wave after it has intersected with the optical axis. This mitigates potential problems caused by reflection of the acoustic wave within the element, without necessitating the use of an acoustic absorber. The second subset of the set of transducers may comprise a plurality of the set of transducers. For example, the second subset may comprise approximately half of the set of transducers. The second subset of the set of transducers may be positioned adjacent one another on the element. The controller may be operable to control the frequency, relative phase, and amplitude at which the first subset of acoustic transducers are activated such that the generated acoustic wave exhibits a generally planar wavefront where it intersects with the optical axis. The wavefront may be generally planar in the plane perpendicular to the optical axis. Suitable control of the relative phase and amplitude of the activation is expected to enable control of the direction of deflection to within a few degrees in example embodiments described below. The first subset of the set of transducers may comprise a plurality of the set of transducers. For example, the first subset may comprise approximately half of the set of transducers. The first subset of the set of transducers may be positioned adjacent one another on the element. The transducers may be arranged on the optically transmissive element such that individual activation of one of the transducers