GB2119533A

GB2119533A - A Bragg cell

Info

Publication number: GB2119533A
Application number: GB08211193A
Authority: GB
Inventors: Stephen Cecil Gratze
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1982-04-17
Filing date: 1982-04-17
Publication date: 1983-11-16
Also published as: GB2119533B

Abstract

Acoustic surface skimming bulk waves (SSBW) interact with electromagnetic waves so that the SSBW diffract the electromagnetic waves through a wavelength dependent angle. The SSBW have good interaction efficiency with the optical waves together with a fairly broad bandwidth. The SSBW are generated by transducers 2 and 3 according to a signal source 4 e.g. a radar receiver, the frequency spectrum of which it is desired to analyse, and are guided along a layer 1B to a region 7 where they interact with optical radiation from a monochromatic laser 5. The optical radiation undergoes Bragg diffraction and is received by an array 11 of detectors, the detector or detectors illuminated depending on the frequency components of the signal source 4. Separate guides may be used for the optical and acoustic waves instead of the single guide layer 1B and in some cases the acoustic guide layer may be omitted, the waves then remaining close to the surface due to their surface skimming nature. <IMAGE>

Description

SPECIFICATION A Bragg cell This invention relates to a Bragg cell, which is defined for the purpose of this Specification as a device in which the effect of mechanical waves is used to diffract electromagnetic waves through an angle dependent on the wavelength of the mechanical and electromagnetic waves. Bragg cells are used as spectrum analysers. In such a spectrum analyser a signal to be analysed is used to generate the aforementioned mechanical waves, e.g., surface acoustic waves, and the angle of diffraction of the electromagnetic waves, e.g., light or infra-red, is measured to give an indication of the frequency or of different frequency components of the incoming signal.

Bragg cells employing bulk waves have the advantage of operating over a wide bandwidth but do not have a very good interaction efficiency between the bulk waves and the electromagnetic waves. The use of surface acoustic waves instead of bulk waves gives a higher interaction efficiency but only a limited bandwidth.

This invention provides a device comprising means for directing surface skimming bulk waves and electromagnetic waves through a medium to a region where they interact in such a way that the surface skimming bulk waves diffract the electromagnetic waves through an angle dependent on the wavelengths of the surface skimming bulk waves and electromagnetic waves.

By employing surface skimming bulk waves it is believed that it is possible to obtain both a reasonably high interaction efficiency and a reasonably wide bandwidth.

A surface skimming bulk wave is defined for the purpose of this Specification as a shear wave propagated through a medium at an acute angle to the surface thereof from electrodes spaced on that surface. The surface skimming bulk waves are preferably generated by applying a varying potential difference across interdigitated sets of electrodes.

One way in which the invention may be performed will now be described with reference to the accompanying drawing which is a schematic perspective view of a Bragg cell spectrum analyser constructed in accordance with the invention.

A plate 1 consists of a lithium niobate substrate 1 A on which an arsenic trisulphide guide 1 B is deposited. The arsenic trisulphide has a higher refractive index than the lithium niobate and a lower acoustic velocity. Hence the layer 1 B is able to behave as an acoustic and an optical waveguide. The lithium niobate is 64" rotated Y cut with the X direction orientated as shown. Two transducers 2 and 3 each has at least one pair of electrodes as shown spaced in the X direction and separated by a distance such that, at the desired range of operating frequencies, they serve to launch surface skimming bulk waves through the plate 1 in a direction inclined slightly with respect to the X direction so that the Bragg diffraction condition between the acoustic and electromagnetic waves is fulfilled.The spacing of the electrodes of transducers 2 and 3, whilst similar, is slightly different so that transducer 2 operates more efficiently within one part of the band of operating frequencies whilst the transducer 3 operates more efficiently within another part of that band.

The signal to be analysed is derived from a source 4.which is indicated only schematically in the drawing, but which could, for example, be a radar receiver. The signal is applied simultaneously to both transducers 2 and 3.

A monochromatic laser 5 directs a beam of coherent light onto a prism 6 which couples the light into the optical guide constituted by the layer 1 B. At a region 7 the optical beam interacts with the surface skimming bulk wave and is diffracted through an angle dependent on the wavelength of those waves. The optical beam is thus diffracted through an angle dependent on the wavelength of the surface skimming bulk wave or, if the incoming signal contains two or more different frequency components, it is split into separate beams, e.g., the two beams 8 and 9 shown in the drawing. After diffraction, the optical beams 8 and 9 are coupled out of the guide 1 B by a second prism 10 and are incident on an array 11 of detectors arranged so that different detectors are illuminated for corresponding different deflections of the optical beam.Thus the outputs from the detectors give an indication of the frequency components of the incoming signal received from 4.

The purpose of including two different transducers 2 and 3 are arranged at different angles and designed for operating at different frequencies is to ensure that the Bragg angle at the region 7 is correct for these different frequencies. It may be desirable to include more than two transducers in alternative embodiments of the invention.

In another possible embodiment of the invention, instead of using a singleguide such as shown at 1 B to guide both the optical and acoustic waves, it would be possible to use separate guides for these two purposes though, of course, there would have to be sufficient diffusion of the light into the acoustic guide or vice versa to obtain the desired interaction in a region corresponding to the region 7 shown in the drawing.

In other embodiments different known Bragg cell transducer arrangements can be used. It is also pointed out that an acoustic guide need not be employed since it is possible to rely on the surface skimming nature of the waves to confine them in a region close to the surface. Instead of forming the optical guide by depositing a layer onto a substrate it can be formed by diffusing a material such as titanium into a substrate such as lithium niobate.

Claims

1. A device comprising means for directing

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION A Bragg cell This invention relates to a Bragg cell, which is defined for the purpose of this Specification as a device in which the effect of mechanical waves is used to diffract electromagnetic waves through an angle dependent on the wavelength of the mechanical and electromagnetic waves. Bragg cells are used as spectrum analysers. In such a spectrum analyser a signal to be analysed is used to generate the aforementioned mechanical waves, e.g., surface acoustic waves, and the angle of diffraction of the electromagnetic waves, e.g., light or infra-red, is measured to give an indication of the frequency or of different frequency components of the incoming signal. Bragg cells employing bulk waves have the advantage of operating over a wide bandwidth but do not have a very good interaction efficiency between the bulk waves and the electromagnetic waves. The use of surface acoustic waves instead of bulk waves gives a higher interaction efficiency but only a limited bandwidth. This invention provides a device comprising means for directing surface skimming bulk waves and electromagnetic waves through a medium to a region where they interact in such a way that the surface skimming bulk waves diffract the electromagnetic waves through an angle dependent on the wavelengths of the surface skimming bulk waves and electromagnetic waves. By employing surface skimming bulk waves it is believed that it is possible to obtain both a reasonably high interaction efficiency and a reasonably wide bandwidth. A surface skimming bulk wave is defined for the purpose of this Specification as a shear wave propagated through a medium at an acute angle to the surface thereof from electrodes spaced on that surface. The surface skimming bulk waves are preferably generated by applying a varying potential difference across interdigitated sets of electrodes. One way in which the invention may be performed will now be described with reference to the accompanying drawing which is a schematic perspective view of a Bragg cell spectrum analyser constructed in accordance with the invention. A plate 1 consists of a lithium niobate substrate 1 A on which an arsenic trisulphide guide 1 B is deposited. The arsenic trisulphide has a higher refractive index than the lithium niobate and a lower acoustic velocity. Hence the layer 1 B is able to behave as an acoustic and an optical waveguide. The lithium niobate is 64" rotated Y cut with the X direction orientated as shown. Two transducers 2 and 3 each has at least one pair of electrodes as shown spaced in the X direction and separated by a distance such that, at the desired range of operating frequencies, they serve to launch surface skimming bulk waves through the plate 1 in a direction inclined slightly with respect to the X direction so that the Bragg diffraction condition between the acoustic and electromagnetic waves is fulfilled.The spacing of the electrodes of transducers 2 and 3, whilst similar, is slightly different so that transducer 2 operates more efficiently within one part of the band of operating frequencies whilst the transducer 3 operates more efficiently within another part of that band. The signal to be analysed is derived from a source 4.which is indicated only schematically in the drawing, but which could, for example, be a radar receiver. The signal is applied simultaneously to both transducers 2 and 3. A monochromatic laser 5 directs a beam of coherent light onto a prism 6 which couples the light into the optical guide constituted by the layer 1 B. At a region 7 the optical beam interacts with the surface skimming bulk wave and is diffracted through an angle dependent on the wavelength of those waves. The optical beam is thus diffracted through an angle dependent on the wavelength of the surface skimming bulk wave or, if the incoming signal contains two or more different frequency components, it is split into separate beams, e.g., the two beams 8 and 9 shown in the drawing. After diffraction, the optical beams 8 and 9 are coupled out of the guide 1 B by a second prism 10 and are incident on an array 11 of detectors arranged so that different detectors are illuminated for corresponding different deflections of the optical beam.Thus the outputs from the detectors give an indication of the frequency components of the incoming signal received from 4. The purpose of including two different transducers 2 and 3 are arranged at different angles and designed for operating at different frequencies is to ensure that the Bragg angle at the region 7 is correct for these different frequencies. It may be desirable to include more than two transducers in alternative embodiments of the invention. In another possible embodiment of the invention, instead of using a singleguide such as shown at 1 B to guide both the optical and acoustic waves, it would be possible to use separate guides for these two purposes though, of course, there would have to be sufficient diffusion of the light into the acoustic guide or vice versa to obtain the desired interaction in a region corresponding to the region 7 shown in the drawing. In other embodiments different known Bragg cell transducer arrangements can be used. It is also pointed out that an acoustic guide need not be employed since it is possible to rely on the surface skimming nature of the waves to confine them in a region close to the surface. Instead of forming the optical guide by depositing a layer onto a substrate it can be formed by diffusing a material such as titanium into a substrate such as lithium niobate. Claims

1. A device comprising means for directing surface skimming bulk waves and electromagnetic waves through a medium to a region where they interact in such a way that the surface skimming bulk waves diffract the electromagnetic waves through an angle dependent on the wavelengths of the surface skimming bulk waves and electromagnetic waves.

2. A device according to Claim 1 comprising a substrate and a layer formed on the substrate and acting as a guide for the surface skimming bulk waves and the optical waves.

3. A device according to Claim 2 in which the guide is arsenic trisulphide formed on a lithium niobate substrate.

4. A device according to any preceding Claim including two transducers for generating the surface skimming bulk waves, the two transducers being designed to operate at different frequencies and being arranged at different angles so that surface skimming bulk waves generated by them are directed at respective different angles thereby ensuring that the Bragg angles are correct for these different frequencies.

5. A device according to any preceding Claim in which the electromagnetic waves are light or infra-red.

6. A device according to any preceding Claim including an array of detectors arranged so that the electromagnetic waves illuminate a different detector for different angles of diffraction.

7. A device substantially as described with reference to the accompanying drawing and substantially as illustrated therein.