GB2119578A

GB2119578A - Antenna array with correlators

Info

Publication number: GB2119578A
Application number: GB08204674A
Authority: GB
Inventors: Alan Martin Smith
Original assignee: PA Management Consultants Ltd
Current assignee: PA Management Consultants Ltd
Priority date: 1982-02-17
Filing date: 1982-02-17
Publication date: 1983-11-16
Also published as: GB2119578B

Abstract

An adaptive antenna array is described having least two antennae 26 associated with at last one acousto-optic time-integrating correlator 28 having an optical input and an acoustic input. The optical input is modulated in accordance with the signal received by one antenna of the array and the acoustic input is modulated in accordance with the signal received by another antenna of the array, whereby the correlator produces an output representative of the time delay between the signals received by the respective antennae. A processor 30 records the time delays between the individual signals received by the antennae 26 and programs time delay units 29 so that all the return signals are combined in phase. <IMAGE>

Description

SPECIFICATION An optical processing system for adaptive and beam forming antenna arrays Field of invention This invention relates to the implementation of integrated optical devices in a system for adaptive and beam forming antenna arrays. Such a system may be used in radar and sonar applications.

Background to the invention In many radar and sonar applications arrays of antennae are used to transmit and receive electromagnetic or acoustic signals. The relative amplitudes and phases of the signals transmitted by individual elements of the array may be modified in order to transmit a beam with particular spatial characteristics or to compensate for the known non-ideal response of the array.

Similarly, the received signals may be phase delayed or differentially amplified prior to being mixed together to give the composite received signal.

Existing work in the area of adaptive sonar arrays is given, for example, in the paper 'Adaptive Beam Forming in an active Doppler Sonar by J. N. Maksym. Existing work in the area of adaptive radar arrays is given, for example, in the paper 'Adaptive Array Processing Experiments at HF' by L. J. Griffiths. Both papers were published in the proceedings of the NATO Advanced Study Institute on 'Aspects of Signal Processing' held at Portoveneve, La Spezia, Italy from 30th August to 1 itch September, 1976.

For sonar applications where relatively low frequencies are used, fast sampling of the received signals may be carried out and techniques such as Fast Fourier Transformation may be used to process the signals. At the higher frequencies used in radar systems, fast sampling becomes more difficult and variable frequency mixing techniques have been used.

A variety of techniques have been proposed for carrying out signal processing using optical wave propagation in guiding substrates. In particular, lithium niobate substrates with titanium diffused surface guiding layers have been used to construct a number of devices. Of relevance to the current invention is the acousto-optic time integrating correlator described, for example, by I.

W. Yao and C. S. Tsai in 'A time-integrating correlator using guided-wave acousto-optic interactions', published in the proceedings of the Ultrasonics Symposium held at Cherry Hill, New Jersey, U.S.A. from 25th-27th September, 1978. The device allows interaction of a modulated light beam and a surface acoustic wave. Light diffracted by the surface acoustic wave is detected using a photo sensitive array.

The signals which are to be correlated are applied to the light source and acoustic wave generating transducer, whilst the correlation output is extracted by monitoring the elements of the photo sensitive array.

Summary of the invention An object of the present invention is to provide a system for rapid analysis of radar and sonar signals received by an array of antennae and to use the results of the signal analysis to adapt the characteristics of the transmitting and/or receiving arrays.

According to one aspect of the invention an adaptive antenna array having at least two antennae is associated with at least one acoustooptic time-integrating correlator having an optical input and an acoustic input, the optical input being modulated in accordance with the signal received by one antenna of the array and the acoustic input being modulated in accordance with the signal received by another antenna of the array, whereby the correlator produces an output representative of the time delay between the signal received by said one antenna and the signal received by said another antenna, and means responsive to the output of the correlator for modifying the signals received by said one antenna and said another antenna in dependence upon the output of the correlator.

Said means responsive to the output of the correlator preferably combine in phase the signals received by said one antenna and said another antenna. The combined signals represent an overall output indicative of the range and position of the object of interest.

According to another aspect of the invention a beam forming antenna array having at least two transmitting and receiving antennae is associated with at least one acousto-optic time-integrating correlator having an optical input and an acoustic input, the optical input being modulated in accordance with the signal received by one antenna of the array and the acoustic input being modulated in accordance with the signal received by another antenna of the array, whereby the correlator produces an output representative of the time delay between the signals received by said one antenna and said another antenna, and means responsive to the output of the correlator for modifying the signals transmitted by the antenna array.

Whilst the inventive adaptive or beam forming antenna array may have only a pair of antennae with one correlator, it would be usual to have more than two antennae and a corresponding increase in the number of correlators when looking at a small section of space in a selective way. In the general case of n antennae and n-1 correlators, the first correlator is connected across the first and second antennae, the second correlator is connected across the second and third antennae, and so on.

In the preferred construction of acousto-optic time-integrating correlator, the light from the optical input is directed in a collimated beam in a direction generally perpendicular to a surface acoustic wave generated by electrodes to which the acoustic input is connected. In the area of interaction between the light beam and the acoustic wave, a small proportion of the light is diffracted and this diffracted light is focused onto a photodetector array. The pattern of light distribution across the photodetector array is representative of the time delay between the two signals used to modulate the optical input and the acoustic input.

Each correlator preferably has a substrate on which there is an optical guiding layer. Preferred materials for the substrate include lithium niobate and lithium tantalate. The guiding layer may then be formed by titanium in-diffusion or lithium outdiffusion. Alternatively, the substrate may be a crystalline semiconductor material formed from two materials from groups Ill and V of the periodic table, such as gallium arsenide. Where a Ill-V crystalline semiconductor compound constitutes the substrate, the guiding layer may be formed by an alloy of two or more elements which are selected from the following, namely: indium, gallium, arsenic, phosphorus and aluminium.

Alternatively, the substrate may be formed from a glass-like material. Where such a material is used, the guiding layer may be produced either by indiffusion of silver or out-diffusion of sodium.

An advantage of the invention is that it can provide means for very rapid optimisation of a radar or sonar system compared with current systems. The transmitted signal beam may be spatially directed at the field of interest, (for example, a particular moving object) and the receiving array may be optimised to receive signals coming from specific regions. In this way, the signal to noise ratio from objects of interest will be increased and the effect of spurious signals from other objects, or resulting from fixed characteristics of the transmitting or receiving arrays may be reduced substantially or eliminated.

The preferred system thus comprises two arrays of antennae. One array transmits a radar or sonar signal whilst the second array receives reflected or scattered signals from objects in the field of view of the transmitted signal. In some systems the same array may be used for both transmission and reception. A single output signal is produced by a signal generator and is fed to the transmitting array. The amplitude and phase of the signals output by each element of the array is modified prior to transmission by suitable electronic circuitry.

Signals received by elements of the receiver array may be passed through variable gain and phase response circuitry prior to being combined into a final signal.

The signals received by the elements of the receiver are cross-correlated using one or a number of acousto-optic time-integrating correlators. The output signals from the correlators indicate the relative phase and amplitude of the return signals from the object of interest and this information is used to modify the phase and amplitude response of the transmitting and receiving arrays in order to optimise the final output signal.

Brief description of the drawings Figure 1 is a block circuit diagram of a beam forming antenna array forming the preferred embodiment of the invention, and Figure 2, is a diagrammatic view of one of a number of correlators of the array of Figure 1.

Detailed description of the drawings The array 20 of Figure 1 has a signal generator 21 which produces a single signal. For radar applications the signal will be in the frequency range of 100 MHz to 10 GHz or more, whilst for sonar applications frequencies up to 1 MHz will typically be used. The signal is divided into a number of channels corresponding to the number of antenna, 23, in the transmitting array, 24. Each of these signals passes through an electronic circuit, 22, where the amplitude and phase of the signal is varied according to the magnitude of an error signal, or signals, 32, generated as explained below. Each of the signals leaving the circuits 22, is fed to an antenna, 23, and transmitted.The spatial variation of the magnitude of the total transmitted electro-magnetic or acoustic disturbance will be directly related to the relative phases and amplitude of the individual transmitted signals. The field strength will be the vector summation of the product of the transmitted field intensity of each antenna and the propagation delay resulting from the distance between the transmitting antenna and the point in the field.

Objects in the field of transmission will in general reflect or scatter energy. This energy is received by a receiving array of antennae, 25, which may be identical to the transmitting array or may be entirely separate. The signal received by each antenna, 26 may be amplified using an amplifier 27 prior to any signal processing. The amplified signals R,(t) (where i takes the values 1, 2, 3,... N, N being the number of receiving antennae) are compared using (N-1) acoustooptic time-integrating correlators 28. The amplified signals from the antennae n and n+1 form the two inputs of the nth correlator 28. The detailed operation of the correlators 28 may be understood by considering Figure 2.

Each correlator 28 consists of a substrate 36 of lithium niobate on which has been produced an optical guiding layer, (a few optical wavelengths in thickness) by titanium in-diffusion of lithium out-diffusion. Three lens structures 37, 38, 39 are on the substrate and are used to collimate and focus the light produced by a laser 35 which is butt-coupled to the edge of the substrate. A pair (or several pairs) of inter-digitated electrodes 40 is deposited on the substrate and produce, when suitably excited, a surface acoustic wave 43 propagating in a transverse direction relative to the optical beam. Hence the laser 35 forms an optical input and the electrodes 40 an acoustic input for the correlator 28.

The beam of light produced by the laser 35 diverges in the substrate and is coliimated by the lens 37 in order to give a parallel beam through the area of interaction with the acoustic wave 43.

In the area of interaction, some light is diffracted by the surface acoustic wave and some light remains in the original beam. The undiffracted light is focused by the lens 38 onto a beam blocking screen 41 where it is absorbed. The diffracted light is also focused by the lens 38 but passes through an aperture in the screen 41 and is imaged onto a photo-detector array 42 using the lens 39.

The two input signals to the correlator 28 are respectively fed to the laser device 35 and the interdigitated transducers 40. In order to offset propagation delay effects in the correlator 28, it may be necessary to pass one of the input signals through a fixed time delay circuit 34. In Figure 2 this circuit 34 is shown in the signal path to the laser 35 but it is understood that in some circumstances it may be necessary to delay the signal reaching the inter-digitated transducers 40.

The laser 35 is modulated directly by feeding the input signal into the current drive circuitry, or by using external electro-optic modulation.

The pattern of light which falls on the photodetector array 42 is directly related to the cross correlation function of the two input signals.

Each element of the array 42 represents a different time delay between the two signals. A correlation maximum corresponds to a bright spot of light on the array 42 whilst a correlation minimum is a relatively dark region. When the signals from two antennae 26 are correlated a maximum will occur at the element of the photodetector array corresponding to the time difference between the signals returned from the object of interest, collected by the two antennae.

The signals 33 from the photo-detector arrays 42 of the correlators 28 are read out into a processor unit 30 which records the time delays between the individual signals received by the antennae 26. The processor 30 uses this information to program time delay units, 29 so that all the return signals from the antennae 26 are combined in phase to give an overall output 43. Information on the time delays required is available as an output signal 44 and may be used to compute the position of the object of interest.

The processor unit 30 may also output signals 32 to the transmitter amplitude and phase modification circuits 22 which allow the transmitted energy to be directed into a relatively narrow beam directed at the object of interest.

Claims

1. An adaptive antenna array having at least two antennae associated with at least one acousto-optic time-integrating correlator having an optical input and an acoustic input, the optical input being modulated in accordance with the signal received by one antenna of the array and the acoustic input being modulated in accordance with the signal received by another antenna of the array, whereby the correlator produces an output representative of the time delay between the signal received by said one antenna and the signal received by said another antenna, and means responsive to the output of the correlator for modifying the signals received by said one antenna and said another antenna in dependence upon the output of the correlator.

2. An antenna array according to claim 1, wherein said means responsive to the output of the correlator combines in phase the signals received.by said one antenna and said another antenna.

3. A beam forming antenna array having at least two transmitting and receiving antennae associated with at least one acousto-optic time integrating correlator having an optical input being modulated in accordance with the signal received by one antenna of the array and the acoustic input being modulated in accordance with the signal received by another antenna of the array, whereby the correlator produces an output representative of the time delay between the signals received by said one antenna and said another antenna, and means responsive to the output of the correlator for modifying the signals transmitted by the antenna array.

4. An antenna array according to claim 3, wherein the same array is used for transmission and reception.

5. An antenna array according to claim 3, wherein separate arrays are used for transmission and reception.

6. An antenna array according to any of the preceding claims, wherein there are n antennae and n-1 correlators, n being more than two, the first correlator being connected across the first and second antennae, the second correlator being connected across the second and third antennae, and so on.

7. An antenna array according to any of the preceding claims, wherein in the or each acoustooptic time-integrating correlator, the light from the optical input is directed in a collimated beam in a direction substantially perpendicular to a surface acoustic wave generated by-electrodes to which the acoustic input is connected, and wherein in the area of the interaction between the light beam and the acoustic wave, a small proportion of the light is diffracted and this diffracted light is focused on to a photodetector array, the pattern of light distribution across the photodetector array being representative of the time delay between the two signals used to modulate the optical input and the acoustic input of the correlator.

8. An antenna array according to claim 7, wherein the or each correlator has a substrate on which there is an optical guiding layer, the substrate being lithium niobate or lithium tantalate.

9. An antenna array according to claim 8, wherein the guiding layer is formed by titanium in-diffusion or lithium out-diffusion.

1 0. An antenna array according to claim 8, wherein the substrate is a crystalline semiconductor material formed from two materials from groups Ill and V of the periodic table, such as gallium arsenide.

11. An antenna array constructed and arranged substantially as herein particularly described with reference to the accompanying drawings.