WO1986007451A1

WO1986007451A1 - A prismatic acousto-optic direction-of-arrival interferometer for radio frequency signals

Info

Publication number: WO1986007451A1
Application number: PCT/AU1986/000169
Authority: WO
Inventors: Malcolm Stewart Brown
Original assignee: The Commonwealth Of Australia, Care Of The Secreta
Priority date: 1985-06-12
Filing date: 1986-06-12
Publication date: 1986-12-18

Abstract

A prismatic acousto-optic interferometer in which a light input is split into two parallel beams (2, 3) and the beams are independently modulated and then recombined, characterised by a pair of Kosters prisms (1, 8) placed back-to-back but spaced apart on a common axis to divide a light beam passed into a first Kosters prism into a pair of parallel light beams extending between the prisms, at least one of the parallel light beams (2, 3) being reflected out by a light splitter (6, 7), the light beams being combined in the second Kosters prism (8).

Description

"A PRISMATIC ACOUSTO-OPTIC DIRECTION-OF-ARRIVAL INTER¬ FEROMETER FOR RADIO FREQUENCY SIGNALS"

This invention relates to a prismatic acousto-optic direction-of-arrival interferometer.and in particular it relates to an interferometer whereby an input such as a laser beam can be split into two parallel beams 5. and the beams modulated by radio frequency signals and again combined.

Such a system is described by the Applicant in Patent Application No. PG 8069, relating to a Mach-Zehnder Acousto-Optical Signal Processor for electronic support 10. measures, but this invention is not necessarily limited for use in such an application.

According to the earlier invention a signal is received on an antenna consisting of a two element spaced array, and the signal is applied to Bragg cells

15. in a Mach-Zehnder interferometer to produce a first optical beam defining a signal flow from one of the elements of the array, and a second beam defining a parallel signal flow from the other element of the array, the first beam being passed through a Fourier

20. transformer lens to a linear photodetector array, and the combination of the beams is passed through a second Fourier transformer lens to a linear photo diode array, and the two electrical outputs from the photodetector arrays passing thence to a computer.

25. A double image prism is already known as a Kosters prism which is used to split a beam into two parallel beams which could then be processed by passing the beams through appropriate devices and fed back by means of a mirror to issue from the prism at an opposite

30. face. Such a prism of course resulted in beams which passed through the processor in a forward and then in a reverse direction. The present invention uses this type of prism but the object of the invention is to arrange the prism means to have a single pass only through the processing zones. This is achieved .according to this 5. invention by using a pair of Kosters prisms arranged back-to-back so that the first prism splits the beam to provide two parallel beams which are then recombined in the second Kosters prism.

It is already known to use two prisms to compare 10. the two ends of two gauge rods to determine the relative length of the rods and although two prisms are used they operate independently with the beams of one being independent of the beams of the other.

When the present invention is used, for instance, 15. to provide an interferometer such as in the system referred to in the earlier Commonwealth patent app¬ lication, the two Koster prisms are placed back-to- back to provide the two parallel beams with the prisms spaced apart to include in each beam a Bragg cell 20. and a beam splitting mirror, this then resulting in each beam being processed by a Bragg cell through which the beam passes and fed off to an output as well as passing to the second Kosters prism to provide a beam modified by the differential action of the 25. two Bragg cells.

The method comprises passing a light beam through a first Kosters prism to produce two parallel beams, passing each parallel beam through means to modulate the beam, splitting out part of the modulated signal

30. from at least one modulated light beam, recombining the modulated light beams in a second Kosters prism placed coaxially with the first Kosters prism and spaced therefrom, and utilizing the light signal from both the light splitters and the combined signal from

35. the second Kosters prism. A form of the invention is illustrated in the accompanying drawing designated Fig. 1 which shows diagramatically such an interferometer.

In the form shown a first Kosters double-image 5. prism 1 is used to split the light beam from a laser into two equal intensity beams 2 and 3 travelling close together along parallel paths, each passing through a Bragg cell designated respectively 4 and 5 where radio frequency signals cause diffraction 10. of the light waves. The beams 2 and 3 then pass through flat transparent plates 6 and 7 having a metal surface coating that reflects a portion of the light from each Bragg cell out of the sides of the interferometer.

Although not shown one of the transparent glass 15. plates 6 or 7 is used as an optical micrometer to tune the length of one arm of the interferometer to a length of one quarter of a wavelength of the laser light longer than the other arm.

The angle of this glass plate to the light beam 20. is adjusted by for instance a piezo electric device controlled by an electrical signal derived by sending in-phase radio signals to both Bragg cells and measuring the output from the interferometer.

The diffracted light is recombined in a second 25. Kosters double image prism 8 identical to the first, and interference effects between the two beams cause the intensity of the recombined output beam to vary depending on the relative phase of the two radio signals. In the form described above the device is used to determine the direction of radio signals as received by a plurality of radio antennas each antenna being coupled to one of the Bragg cells, hut it will be 5. realised that the invention can have wide application where it is necessary to obtain a combined output beam from a single input beam where a pair of beams so generated are required to be differently processed before again combining the two beams.

10. The device is mechanically robust as it is necessary only to carefully align the two Kosters prisms and this leads to an extremely stable interferometer output.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A prismatic acousto-optic interferometer in which a light input is split into two parallel beams (2,3) and the beams are independently modulated and then recombined, characterised by a pair of Kosters prisms

5. (1,8) placed back-to-back but spaced apart on a common axis whereby a light beam projected into a first of the pair of Kosters prisms (1) is divided into a pair of parallel light beams between the spaced apart Kosters prisms (2,3), means (4,5) in the path of each parallel

10. light beam (2,3) to modulate the light beam, a light splitter (6,7) in at least one modulated light beam (2,3), but optionally both, to reflect out part of the modulated beam, whereby the two modulated light beams are combined by passing them through the second Kosters prism (8).

2. A prismatic acousto-optic interferometer wherein the means (4,5) to modulate the light beams are Bragg cells energised by radio-frequency signals.

3. The method of splitting and recombining a light beam in a prismatic acousto-optic interferometer and modulating the split light beams, characterised by passing a light beam through a first Kosters prism

5. to produce two parallel beams, passing each parallel beam through means to modulate the beam, splitting out part of the modulated signal from at least one modulated light beam, but optionally both, recombining the modulated light beams in a second Kosters prism placed

10. coaxially with the first Kosters prism and spaced therefrom, and utilizing the light signal from both the light splitters and the combined signal from the second Kosters prism.

4. The method of claim 3 wherein the two light beams from the first Kosters prism are modulated by passing the light beams through Bragg cells.^■

5. A prismatic acousto-optic interferometer utilizing a pair of Kosters prisms placed back-to-back on the same axis, substantially as described and illustrated with reference to the accompanying drawings.