GB2250134A

GB2250134A - Local oscillator arrangement for imaging radar

Info

Publication number: GB2250134A
Application number: GB8317782A
Authority: GB
Inventors: John Stuart Heeks; Peter Kenneth Blair
Original assignee: STC PLC; Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1983-06-30
Filing date: 1983-06-30
Publication date: 1992-05-27
Anticipated expiration: 2003-06-30
Also published as: GB2250134B

Abstract

An imaging radar comprises a microwave lens 1 arranged to focus a forward scene onto a silicon slice 2 on the rear of which is formed a monolithic array 3 of semiconductor antennas, receivers and photodetectors. A laser 4 provides, via a modulator 6, an intensity modulated optical local oscillator signal which is directed onto the photodetectors in the array. Each array receiver receives image signals from an associated one of the antennas and a local oscillator signal from an associated one of the photodetectors. <IMAGE>

Description

LOCAL OSCILLATOR ARRANGEMENT FOR WAGING RADAR This invention relates to a local oscillator arrangement for an imaging radar.

In its most widespread form a radar consists essentially of a serial system in which target reflections in the beam are received by a passive antenna and processed in a single channel electronic receiver.

In such equipments rapid scanning in two dimensions leads to very high information processing rates. However, for rapid scanning purposes the mechanical scanned antenna is inconvenient. Phased array radars eliminate the moving antenna and are able to form simultaneous multiple beams in space, each beam in effect operating with its own receiver channel. Phase array radars tend, however, to be complex with individual phase control of the local oscillator (or RF, IF, or baseband) being necessary for each element of the array.

Imaging radars, in which microwave passive lens images the forward scene onto retina like image plane, provide a conceptually simple answer to these problems. For the case of 'nm-wave systems it is likely that the multiple front end receivers in the image plane can be realised monolithically on a semiconductor chip giving rise to extremely compact and potentially cheap systems (in large numbers). An inherent feature of imaging radars is the spatial separation of jammer and target, i.e. a jammer subtending a different angle than the target is focussed onto a different receiver than that picking up the target. Depending on the sidelobe levels of the microwave image, this provides a built in margin of, say, 20 dB or so against the jammer.

The Royal Signals Research Establishment, Malvern, have proposed an imaging radar system which comprises essentially a microwave lens imaging the radar scene through the back of a silicon slice onto a monolithic array of antennas and associated receivers on the outside surface of the semiconductor. Connections are made to the chip for the provision of power, for control and for output signals. Two schemes have been considered for the provision of the local oscillator (L.O.) signal. Most conventionally the L.O. can be distributed to the individual input mixers via transmission lines on the silicon chip but this would be difficult to implement in view of the already complex circuit of multiple antennas and receivers on the silicon surface. Further, a major problem would arise in ensuring phase sychronism over the area of the receiver array.

Another possibility is to bring a suitably 'focusse' L.O. microwave beam via a dicbroic mirror into the input of the system. Methods of isolating signal and L.O. waves have been proposed based on orthogonal polarizations and balanced mixer/antenna configuration.

Problems in this scheme arise from: (i) The L.O. 'optical' arrangements.

(ii) Signal loss in the dichroic mirror.

(iii) Added complexity of on chip antennas for the L.O.

(iv) Relatively high power requirement of L.O.

source, i.e. fractional mw/receiver x no. of receivers x collection efficiency x mirror efficiency. This L.O. power can compare with or exceed that required for transmission! According to the present invention there is provides an imaging radar comprising a semiconductor planar array of microwave antennas and associated receivers,- a microwave lens arranged to focus a forward scene onto said array, and means for providing a local oscillator to the array receivers characterised in that the semiconductor planar array includes a plurality of photodetectors associated with each receiver, and means for transmitting to the photodetector array an intensity modulated optical wave to provide the local oscillator frequency for each receiver.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 illustrates an arrangement for supplying an optical local oscillator signal to the receivers of an imaging radar, Fig. 2 illustrates an alternative arrangement to that of Fig. 1, Fig. 3 illustrates an arrangement for providing an intensity modulated beam of light from a single laser, and Fig. 4 illustrates an arrangement for providing an intensity modulated beam of light from G pair of lasers.

In the arrangement shown in Fig. 1 a microwave lens 1 focusses a radar scene onto a silicon slice 2 on the back of which there is a monolithic array 3 of microwave antennas, optical photodetectors and receivers. Each receiver is associated with respective one of the antennas and photodetectors. Connections (not shown) are made to the chip for the provision of power, for control and for output signals. The local oscillator reference frequency, or subharmonic, is transmitted to the image array 3 as a 1006 intensity modulated beam of light. Light from laser source 4 is focussed by lens 5 onto a modulator 6. The modulated light is then directed onto the array 3 via a suitable lens 7.This single laser direct modulation arrangement is suitable for use with modulation frequencies of up to about 5 GEZ.

For higher modulation frequencies a two-laser beat frequency source is more suitable. In the schematic arrangement shown in Fig. 2 light from two laser sources 4(1), 4(2) is directed onto the image array 3 by means of suitable optics. The two lasers, most conveniently solid state lasers, are locked together in a phase locked loop arrangement with a frequency separation equal to the required microwave frequency or its subharmonic.

Fig. 3 shows an integrated optics implementation of the modulation arrangement of Fig. 1. At the input end light, propagating in a lithium niobate integrated optics waveguide 20 is launched into a two-way splitter 21, one of whose branches feeds a second two-way splitter 22. One branch of this second splitter passes through phase retarding elements 23 and 24, while the other branch passes through a phase retarding element 25, and then the two branches are combined in a recombiner 26.

The output of this recombiner feeds one input branch of a second recombiner 27 whose other branch is fed by the other output of the first two-way splitter 21 having first passed through a phase retarding element 28. Phase retarding elements 24 and 25 are driven from a microwave source (not shown in Figure 2) with a 900 microwave phase shifter 29 in one of the drive paths so that the drives are in phase quadrature. This makes that part of the circuitry from the two-way splitter 22 to the recombiner 26 a straight optical analogue of a conventional single-side-band modulator, with the phase retardation of element 23 being set to a nominal 900.

0 Any departure from an exact 90 compensates for any phase inbalances occurring in the paths of the 555 interferometer in the absence of any modulation applied to elements 2' and 25. The phase of the microwave modulation on the optical carrier emerging from recombiner 27 is controlled by the optical phase retaraation introduced by phase retarding element 28, it an optical frequency phase change of xO at element o 28 producing an equivalent phase change of x in the -microwave-frequency modulation of the output from recombiner 27.

Figure g illustrates an implementation of the alternative phase locked loop controlled microwave modulation system. An injection laser 30 is driven by frequency stabilisation control circuitry 31 to provide a frequency stabilised optical output at a frequency fl, which is launched into an optical waveguide 32. This light is heterodyned in a directional coupler 33 with light from a second Injection laser 34 operating at a frequency f2. The difference frequency (f1 - f2) is the frequency of the required microwave modulation to be impressed on the optical carrier.For this purpose the second laser 34 is driven by frequency control circuitry 35 which derives its control signal from a phase sensitive detector 36 fed with a first signal from a microwave frequency local oscillator 37 operating at the desired modulation frequency and a second microwave frequency signal derive by detecting a portion of the modulated optical carrier. For this purpose a second directional coupler 38 is used to tap off a small proportion of the optical power. This is fed to a detector 39 whose output is fed to the phase sensitive detector 36.

The output of waveguide 32 forms the source for the collimated space beam used to illuminate the photodetectors associated with each receiver circuit.

The phase front of the modulation envelope can be arranged to be planar or convergent/diverent (i.e.

for aberration correction of the microwave lens). The option also exists for the insertion of an optical hologram to convert the space beam to separately focussed per element beams to improve the optical detection efficiency.

For the photodetectors in the array two implementations may be considered. At modulation frequencies of above, say, 60 GHz direct injection locking of an Impatt diode with the optical input. This approach requires hybrid technology of monolithic Impatt diodes and electronic circuit integration. An alternative arrangement, compatible with gallium arsenide integrated circuit technology, comprises a FET type photodetector capable of operation at perhaps 30 GHz with a mixer driven in a subharmonic mode.

Claims

CLAIMS:

1. An imaging radar comprising a semiconductor planar array of microwave antennas and associated receivers, a microwave lens arranged to focus a forward scene onto said array, and means for providing a local oscillator to the array receivers characterised in that the semiconductor planar array includes a plurality of photodetectors associated with each receiver, and means for transmitting to the photodetector array an intensity modulated optical wave to provide the local oscillator frequency for each receiver.

2. An imaging radar according to claim 1 further characterised in that the optical transmitting means comprises a laser source, means for intensity modulating the laser output, and means for focussing the modulated output onto the planar array.

3. An imaging radar according to claim 2 further characterised in that the modulating means comprises an integrated optics waveguide structure wherein a microwave signal is impressed on the guided optical signal in a single sideband (SSB) modulator and the output of the SSB modulator is mixed with an unmodulated optical signal, the waveguide structure including means for imposing phase shifts of the optical signals in one or more branches of the waveguide structure relative to the optical signals in other branches of the structure to compensate for optical path length differences in the structure.

4. An imaging radar according to claim 1 further characterised in that the optical transmission means comprises two lasers having a frequency operating in a phase locked loop configuration, wherein one of the lasers is driven by frequency control circuitry including a phase sensitive detector to which the loop modulation frequency is applied together with a modulation frequency derived from a microwave local oscillator to produce a control signal for said laser.

5. An imaging radar according to claim 5 characterised in that the optical transmission means is formed in an integrated optics structure.

6. An imaging radar substantially as described with reference to the-accompanying drawings.

7. A method of operating an imaging radar wherein a radar scene is imaged onto a planar array of antennas each associated with a receiver, characterised in that the local oscillator signal is an optical signal transmitted to an array of photodetectors each associated with a receiver of the planar array.

Amendments to the claims have been filed ss follows I. An imaging radar comprising a semiconductor planar array of microwave antennas and associated receivers, a microwave lens arranged to focus a forward scene onto said array, and means for providing a local oscillator to the array receivers characterised in that the semiconductor planar array includes a plurality of photodetectors associated with each receiver, and means for transmitting to the photodetector array an intensity modulated optical wave to provide the local oscillator frequency for each receiver.

4. An imaging radar according to claim 1 further characterised in that the optical transmitting means comprises two lasers having a frequency operating in a phase locked loop configuration, wherein one of the lasers is driven by frequency control circuitry including a phase sensitive detector Co which the loop modulation frequency is applied together witn a modulation frequency derived from a microwave local oscillator to produce a control signal for said laser.

5. An imaging radar according to claim 1, 2 or 3, characterised'in that the optical transmitting means is formed in an integrated optics structure.

6. An imaging radar substantially as described with reference to the accompanying drawings.