GB2327001A

GB2327001A - Radio frequency direction finding apparatus

Info

Publication number: GB2327001A
Application number: GB9415193A
Authority: GB
Inventors: Bradley Richard Firth
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1994-07-28
Filing date: 1994-07-28
Publication date: 1999-01-06
Anticipated expiration: 2014-07-28
Also published as: GB9415193D0; GB2327001B

Description

IMPROVEMENTS IN OR RELATING TO RADIO FREQUENCY DIRECTION FINDING APPARATUS This invention relates to radio frequency (rf) direction finding (df) apparatus, and more especially it relates to acousto-optic homodyne interferometer (AOHI) direction finding apparatus.

Acousto-optic homodyne interferometer apparatus for radio frequency direction of arrival measurements is known, and in this connection attention is directed to an article entitled 'A Wasters Prism Acousto-optic Interferometer For Radio Frequency Direction Of Arrival Measurements' by M S Brown published in J Phys. E: Sci Instrum. 20 (1987). Such known apparatus is especially useful for determining the direction of arrival of frequency hopping signals which are arranged to change frequency at intervals within a predetermined relatively wide rf band. An important advantage of the known apparatus is the ability to intercept simultaneously many coincident signals which differ in frequency.

As hereinafter explained, Bragg cells are used in the known apparatus for converting wideband rf signals into corresponding optical signals which are appropriately processed to determine the direction from which the radio signals of interest are received.

One of the problems with this known kind of df apparatus is that angular ambiguities can occur.

It is an object therefore of the present invention to provide rf direction finding apparatus suitable for use with frequency hopping signals which will provide an indication of the direction from which the frequency hopped signals originate without angular bearing ambiguity.

According to the present invention, radio frequency signal direction finding apparatus comprises a radio receiver affording two rf output channel signals in dependence upon which direction of rf signal arrival is determinable, acousto-optic homodyne interferometer apparatus, including a pair of Bragg cells, one for each output channel, effective to provide two optical signals, one from each Bragg cell in dependence upon which the power spectrum of each channel is determined, polarising beam splitter/combiner means effective to produce from the two optical signals two phase quadrature related optical signals corresponding to the power spectrum of the summation of the signals from each channel, means for detecting the optical signals thus produced so as to provide corresponding electrical signals, and signal processor means responsive to the electrical signals for providing in dependence thereon, an unambiguous indication of direction of arrival of the rf signal.

By using polarising beam splitter/combiner techniques, to produce I and Q signals which may be processed to produce an unambiguous output signal indicative of direction of rf signal arrival, the fabrication of apparatus which is compact, cost effective and reliable in operation is facilitated.

The acousto-optic homodyne interferometer apparatus may comprise a laser, first beam splitter means effective to split a beam from the laser into two beams, which are applied to respective Bragg cells of the pair, one for each channel, which Bragg cells serve to deflect the two beams in dependence upon the radio frequency spectrum in each channel, polarising beam splitter/combiner means responsive to the deflected beams and effective to produce four optical signals, two of which correspond to the power spectrum in one of the two channels, and the power spectrum in the other of the two channels respectively and the other two of which correspond to the power spectrum in each of two phase quadrature related I and Q signals respectively, which are indicative of the power spectrum of the summation of the signals from each channel, and four linear detector arrays which define the said means for detecting the optical signals and to which the four optical signals are applied, one signal to each array, thereby to produce the said corresponding electrical signals.

In accordance with one aspect of the present invention the polarising beam splitter/combiner means may comprise a half wave plate to which a beam from one Bragg cell of the pair is directed, a quarter wave plate to which a beam from the other Bragg cell of the pair is directed, a polarising beam splitter/combiner prism which serves to split each beam from the wave plates so that one portion of the beam from the half wave plate is used to provide an optical signal indicative of the power spectrum in one channel, and so that one portion of the beam from the quarter wave plate is used to provide an optical signal indicative of the power spectrum in the other channel, the other portions of each beam being combined in the said prism and split to provide light signals which are each fed via a half wave plate and a polarising beam splitter to provide the said phase quadrature related I and Q signals which correspond to the power spectrum of the summation of the signals from each channel.

In accordance with an alternative aspect of the present invention, the polarising beam splitter/combiner means may comprise a pair of beam splitters which serve to split the beams from each Bragg cell of the said pair of Bragg cells, into two portions, one portion of one beam being used to provide the optical signal indicative of the power spectrum in one channel, and one portion of the other beam being used to provide the optical signal indicative of the power spectrum in the other channel, the other portions of each beam being fed via a pair of half wave plates and a polarising beam splitter/combiner prism thereby to provide a pair of output optical signals, which are each directed via a half wave plate and a polarising beam splitter so as to produce the two phase quadrature related I and Q signals corresponding to the power spectrum of the summation of the signals from each channel.

The output light from the Bragg cells of the pair should be focused via a transform lens, which may be a lens hereinafter described as the Fourier transform lens, so that the deflected beams are focused on the detector arrays.

Some embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings, in which, FIGURE 1 is a generally schematic block diagram of known acousto-optic homodyne interferometer apparatus which is used for direction finding purposes; FIGURE 2 is a waveform diagram appertaining to the apparatus shown in Figure 1; FIGURE 3 is a block schematic diagram of an acousto-optic homodyne interferometer direction finding apparatus in accordance with one embodiment of the present invention; FIGURE 4 is a waveform diagram illustrating operation of the apparatus shown in Figure 3; FIGURE 5 is a somewhat schematic block diagram of an acousto-optic homodyne interferometer direction finding apparatus according to an alternative embodiment of the present invention, and, FIGURE 6 is a schematic block diagram of an rf - df system.

Referring now to Figure 1, a known acousto-optic homodyne interferometer which can be used for radio direction finding purposes, comprises a single mode laser 1 and a beam splitter 2.

A light beam from the laser 1 is directed towards the beam splitter 2 via a collimator 3 comprising elements 4, 5 and 6, a half wave plate 7, an anamorphic prism pair 8a and 8b and a focusing cylindrical lens 9 which is used to cause the beam to converge in one dimension (this assembly is termed the laser, collimation and conditioning optics). The beam splitter 2 is arranged to produce two beams which are directed towards a pair of Bragg cells 10 and 11, one beam being directed to the Bragg cell 10 and the other beam being directed to the Bragg 11, so that each beam forms a line focus inside the Bragg cell to which it is directed. The rf channel signals 1 and 2 produced in an rf direction finding radio receiver (not shown) are fed to the Bragg cells 10 and 11 respectively. The Bragg cells 10, 11 operate to deflect a fraction of the beams from the beam splitter 2 through an angle which is proportional to the frequency of the applied rf channel signal (NB: Deflection is 'out of the plane of the page in Figure 1). The operation of Bragg cells is very well known and will therefore not be further discussed herein. The light beams from the Bragg cells 10, 11 are fed via a Fourier transform lens 14 (arranged such that the Fourier transform of the Bragg cell aperture is formed at the focal plane of the lens 14 which coincides with the positions of the detector arrays 16, 17 and 18) and a beam splitter/combiner 15 to optical detector arrays 16, 17 and 18. Beam splitting is effected by means of a partially reflective beam splitter coating in the splitter/combiner 15. The detector arrays 16, 17, 18 each comprise a linear array of detector elements. One fractional component of each beam is directed onto the arrays 16 and 17 respectively, the remaining fraction being combined by the splitter/combiner 15 to provide two outputs, one output of which is directed onto the array 18, the other output being 'stopped' because it contains the same information in anti-phase. It will be appreciated that the element in each array on which the beam is incident is determined by the angular deflection occasioned by the Bragg cells. The beam splitter/combiner 15 thus operates so that the light signal incident on the detector array 16 originates from channel 1, the light signal incident on the detector array 17 originates from channel 2 and the light signal incident on the detector array 18 originates from both channels. The position of the focused beams on the detector arrays 16, 17 and 18 corresponds to the frequency of the rf signals in the Bragg cells 10 and 11. Measurement of the light intensity on the detector arrays 16 and 17 determines the power spectrum of the rf signals in the channels 1 and 2 respectively, and in order to correct for path length differences, optical path compensators 19a, 19b are provided. Measurement of the coherent sum of the two Bragg cell outputs on the detector array 18 permits the relative phase between the rf signals in the two channels to be determined. This coherent summation is formed by the apparatus working as an optical interferometer. The basic interferometer operation is described in the prior art reference hereinbefore referred to and has three output channels, the optical intensities of the signals in the three channels are denoted I1, I2 and 13. The II and I2 signals are proportional to the rf powers in the two Bragg cell channels, whilst I3 is proportional to the power spectrum of the coherent sum and is given by

wherein the term e in the equation relates to the mixing efficiency of the interferometer. If the three signal levels I1, I2 and I3 are measured, the phase angle can be calculated from the following equation.

The phase angle thus measured is however ambiguous since both positive and negative angles have the same cosine. And additionally the sensitivity changes with angle as can be see from Figure 2. It is desirable therefore to provide a system wherein this ambiguity is resolved and the sensitivity remains substantially constant with phase angle.

Some embodiments of the present invention will now be described with reference to Figures 3, 4 and 5 wherein a quadrature channel is added which enables the ambiguity to be resolved and the sensitivity to remain substantially constant with phase angle. The phase of the additional channel output is offset from an in-phase output by the quadrature phase angle q as shown in the following equations where 14 is the quadrature output, the phase angle being calculated accordingly without ambiguity.

Referring now to Figure 3, light from a laser 20 is directed via a beam splitter 21 onto Bragg cells 22 and 23. The Bragg cells 22 and 23 operate as herein before described on beams produced by the beam splitter 21, in accordance with the frequency of rf channel signals applied to the Bragg cells via lines 24 and 25.

Light emerging from the Bragg cells 22 and 23 is deflected in dependence upon the frequency of the rf signals in channel 2 and channel 1 respectively. The light which is linearly polarised and contains the phase information from the rf signals, is directed towards a Fourier transform lens 26. In channel 1, light from the Fourier transform lens 26 is directed onto a half wave plate 27, used to cause the polarisation to rotate by 45O so that half of the power is in the s-polarisation, and the rest of the power is in the p-polarisation. In channel 2, light from the Fourier transform lens 26 is directed towards a quarter wave plate 28 which is used to cause the beam to be split equally between the s and p-polarisations. This also has the effect of introducing a phase difference of 900 between the s and p-polarisations. This 900 phase difference is a required phase shift between the in-phase and quadrature channels as will hereinafter be explained. Light from the half wave plate 27 and the quarter wave plate 28 is directed towards a polarising beam splitter/combiner 29. A proportion of the light output from each of the Bragg cells 22 and 23 is reflected by a partially reflecting coating on the polarising beam splitter/combiner 29 so that the beam in channel 1 is incident on a detector array 30 and the beam in the channel 2 is incident on a detector array 32, path compensation means 30a and 32a being included to compensate for path differences. The remainder of each Bragg cell light output is recombined by the polarising splitter/combiner 29. In the beam splitter/combiner 29, the s-polarised beams are reflected and the p-polarised beams are transmitted by the coating applied to an interface. This mechanism serves to mix the signals from the two Bragg cells, the s-polarisation from one Bragg cell being mixed with the p-polarisation from the other Bragg cell and vice-versa. The effect of this is to produce I and Q beams which emerge from the polarising beam splitter/combiner 29 in phase quadrature. The I and Q beams each contain two orthogonally polarised parts which must be resolved along a common axis and split in order for the phase information to be measured. In order to do this, the I and Q beams are fed via cylindrical lenses 33 and 34 respectively to half wave plates 35 and 36 respectively. The polarisation of output light from the half wave plates 35 and 36 and beam splitters 35a, 36a, is rotated so that half of each polarisation is summed and the other halves are subtracted, thereby to provide light output signals for detectors 37 and 38, and light output signals in paths 39 and 40 which are 'stopped', or not used as shown in Figure 4.

Referring now to Figure 5, a simplified version is shown, comprising a laser 41, a collimating and focusing system 42, a beam splitter 43 and Bragg cells 44 and 45. A beam from the laser 41 is split by the beam splitter 43 to provide two beams which are directed to the Bragg cells 44 and 45 respectively.

Beams from the Bragg cells 44 and 45 are directed via Fourier transform lens 46 to a beam splitter 47. Light from the Bragg cell 44 corresponding to one rf channel is directed to a detector array 48 thereby to provide a signal corresponding to the power spectrum of channel 1. Light from the Bragg cell 45 corresponding to channel 2, is directed via the beam splitter 47 to a detector array 49 so as to produce an output signal corresponding to the power spectrum of the channel 2, compensators 48a and 49a being included as shown. Beams from the Bragg cells 44 and 45 are also fed via the beam splitter 47 to half wave plates 50 and 51 respectively and a polarising beam combiner 52. Light from the polarising beam combiner 52 is directed to half wave plates 53 and 54 which feed detectors 55 and 56 via polarising beam splitters 57 and 58 respectively. In Figure 5, the polarisations of the optical beams are indicated by means of arrows, and the beams emerging from the Bragg cells 44 and 45 are scanned out of the plane of the page. Thus, in Figure 5, the height of the optical beams (above the page) on the detector arrays 48, 49, 55 and 56 corresponds to frequency. The light emerging from the Bragg cells is linearly polarised and contains phase information appertaining to the rf channel signals.

The polarisation of the beams from each of the Bragg cells 44 and 45 is rotated by 450 so that half of the output of each channel is now in the s-polarisation and the rest is in the p-polarisation, this being achieved by using half wave plates 50 and 51. A proportion of each Bragg cell output is reflected from the beam splitter 47 and measured on linear detector arrays 48 and 49, one for each channel. The remainder of each Bragg cell output enters the polarising beam splitter/combiner 52. In the beam splitter/ combiner 52, the beams are subject to total internal reflection, which imparts a phase shift between the two polarisations. The beams are in fact incident on a polarising beam splitter coating on an interface between two halves of a prism which defines the splitter/combiner 52 and here the s-polarised beams are reflected and the p-polarised beams are transmitted. This mechanism mixes the signals, the s-polarisation from one Bragg cell being mixed with the p-polarisation from the other Bragg cell and visaversa. The beams emerging from the polarising beam splitter/combiner 52 are thus approximately in phase quadrature.

The two beams which emerge from the beam splitter/combiner 52 in-phase quadrature each contain two orthogonal polarisations which must be resolved along a common axis and split in order for the phase information to be measured. This is done by rotating the output polarisation by means of half wave plates 53 and 54, so that half of each polarisation is summed and the other halves are subtracted, thereby to provide a total of four output phases.

The acousto-optic homodyne interferometer apparatus hereinbefore described, and in particular as shown in Figures 3 and 5, is used to process the signals from an rf direction finding receiver as shown in Figure 6, thereby to provide an indication of direction of rf signal arrival.

Referring now to Figure 6, an rf receiver 60, is provided with a suitable df antenna 61, whereby output signals are provided on lines 62 and 63 which may be processed to determine direction of rf signal arrival. The signals on the lines 62 and 63 are directed to an acousto-optic homodyne interferometer apparatus 64 of the kind described with reference to Figure 3 or Figure 5, wherein they are applied to Bragg cells as shown. Output signals on lines 65, 66, 67 and 68 are provided as shown in the arrangement described with reference to Figures 3 and 5, which are then fed to a signal processor 69 which operates as hereinbefore described to produce an unambiguous output signal on a line 70 indicative of the direction(s) of arrival and frequency(s) of the rf signal(s).

Various modifications may be made to the embodiments herein described without departing from the scope of this invention and for example, additional phase shifts can be introduced between the I and Q channels by using electro-optic phase modulators or liquid crystal phase modulators introduced into the optical train after the beam combiner. These active phase modulators may be used in a closed feedback loop to control the phase difference between the I and Q channels in the presence of drift or vibration.

Claims

1. Radio frequency signal direction finding apparatus comprising a radio receiver affording two rf output channel signals in dependence upon which direction of rf signal arrival is determinable, acousto-optic homodyne interferometer apparatus, including a pair of Bragg cells, one for each channel, effective to provide two optical signals, one from each Bragg cell in dependence upon which the power spectrum of each channel is determined, polarising beam splitter/combiner means effective to produce from the two optical signals two phase quadrature related optical signals corresponding to the power spectrum of the summation of the signals from each channel, means for detecting the optical signals thus produced so as to provide corresponding electrical signals, and signal processor means responsive to the electrical signals for providing in dependence thereon, an unambiguous indication of direction of arrival of the rf signal.

2. For use in the apparatus as claimed in Claim 1, an acoustooptic homodyne interferometer apparatus comprising a laser, first beam splitter means effective to split a beam from the laser into two beams, which are applied to respective Bragg cells of the pair, one for each channel, which Bragg cells serve to deflect the two beams in dependence upon the radio frequency spectrum in each channel, polarising beam splitter/combiner means responsive to the deflected beams and effective to produce four optical signals, two of which correspond to the power spectrum in one of the two channels, and the power spectrum in the other of the two channels respectively and the other two of which correspond to the power spectrum in each of two phase quadrature related I and Q signals respectively, which are indicative of the power spectrum of the summation of the signals from each channel, and four linear detector arrays which define the said means for detecting the optical signals and to which the four optical signals are applied, one signal to each array, thereby to produce the said corresponding electrical signals.

3. Apparatus as claimed in Claim 2, wherein the polarising beam splitter/combiner means comprises a half wave plate to which a beam from one Bragg cell of the pair is directed, a quarter wave plate to which a beam from the other Bragg cell of the pair is directed, a polarising beam splitter/combiner prism which serves to split each beam from the wave plates so that one portion of the beam from the half wave plate is used to provide an optical signal indicative of the power spectrum in one channel, and so that one portion of the beam from the quarter wave plate is used to provide an optical signal indicative of the power spectrum in the other channel, the other portions of each beam being combined in the said prism and split to provide light signals which are each fed via a half wave plate and a polarising beam splitter to provide the said phase quadrature related I and Q signals which correspond to the power spectrum of the summation of the signals from each channel.

4. Apparatus as claimed in Claim 2, wherein the polarising beam splitter/combiner means comprises a pair of beam splitters which serve to split the beams from each Bragg cell of the said pair of Bragg cells, into two portions, one portion of one beam being used to provide the optical signal indicative of the power spectrum in one channel, and one portion of the other beam being used to provide the optical signal indicative of the power spectrum in the other channel, the other portions of each beam being fed via a pair of half wave plates and a polarising beam splitter/combiner prism thereby to provide a pair of output optical signals, which are each directed via a half wave plate and a polarising beam splitter so as to produce the two phase quadrature related I and Q signals corresponding to the power spectrum of the summation of the signals from each channel, the quadrature phase shift being introduced by total internal reflections inside the polarising beam splitter/combiner prism.

5. Apparatus as claimed in any preceding claim, wherein output light from the Bragg cells of the pair is directed via a Fourier transform lens.

6. Apparatus as claimed in any of Claims 2 to 5, wherein means are provided for introducing a predetermined phase shift between signals representing the I and Q channels, thereby to compensate for phase drift.

7. Apparatus as claimed in Claim 6, wherein the said means for providing the phase shift is connected in a closed feedback loop whereby unwanted phase differences between the I and Q channels are automatically compensated for.

8. Radio frequency signal direction finding apparatus substantially as hereinbefore described with reference to Figures 3 to 6 of the accompanying drawings.

9. For use in radio frequency signal direction finding apparatus acousto-optic homodyne interferometer apparatus substantially as herein before described with reference to Figures 3 to 5 of the accompanying drawings.

9. For use in radio frequency signal direction finding apparatus acousto-optic homodyne interferometer apparatus substantially as hereinbefore described with reference to Figures 3 to 5 of the accompanying drawings.

Amendments to the claims have been filed as follows

1. Radio frequency signal direction finding apparatus comprising a radio receiver affording two rf output channel signals in dependence upon which rrrm > +ron of rf Lu w Vx w lrec lon o r signal arrival is determinable, acousto-optic homodyne interferometer apparatus for producing optical signals corresponding to the power spectrum of the rf signal, means for detecting the optical signals thus produced so as to provide corresponding electrical signals, and signal processor means responsive to the electrical signals for providing in dependence thereon, an unambiguous indication of direction of arrival of the rf signal.

2. Radio frequency signal direction finding apparatus as claimed in Claim 1, wherein said acousto-optic homodyne interferometer apparatus comprises a laser, first beam splitter means effective to split a beam from the laser into two beams, which are incident upon respectively a pair of Bragg cells, one for each rf output channel, which Bragg cells serve to deflect the two beams in dependence upon said radio frequency spectrum in each channel, polarising beam splitter/combiner means responsive to the deflected beams and effective to produce four optical signals, two of which correspond to the power spectrum in one of the two channels, and the power spectrum in the other of the two channels respectively and the other two of which correspond to the power spectrum in each of two phase quadrature related I and Q signals respectively, which are indicative of the power spectrum of the summation of the signals from each channel, and wherein said means for detecting optical signals comprises four linear detector arrays to which the four optical signals are applied, one signal to each array, thereby to produce the said corresponding electrical signals.

3. Apparatus as claimed in Claim 2, wherein the polarising beam splitter/combiner means comprises a half wave plate to which a beam from one Brag cell of the pair is directed, a quarter wave plate to which a beam from the other Bragg cell of the pair is directed, a polarising beam splitter/combiner prism which serves to split each beam from the wave plates so that one portion of the beam from the half wave plate is used to provide an optical signal indicative of the power spectrum in one channel, and so that one portion of the beam from the quarter wave plate is used to provide an optical signal indicative of the power spectrum in the other channel, the other portions of each beam being combined in the said prism and split to provide light signals which are each fed via a half wave plate and a polarising beam splitter to provide the said phase quadrature related I and Q signals which correspond to the power spectrum of the summation of the signals from each channel 4. Apparatus as claimed in Claim 2, wherein the polarising beam splitter/combiner means comprises a pair of beam splitters which serve to split the beams from each Bragg cell of the said pair of Bragg cells, into two portions, one portion of one beam being used to provide the optical signal indicative of the power spectrum in one channel, and one portion of the other beam being used to provide the optical signal indicative of the power spectrum in the other channel, the other portions of each beam being fed via a pair of half wave plates and a polarising beam splitter/combiner prism thereby to provide a pair of output optical signals, which are each directed via a half wave plate and a polarising beam splitter so as to produce the two phase quadrature related I and Q signals corresponding to the power spectrum of the summation of the signals from each channel, the quadrature phase shift being introduced by total internal reflections inside the polarising beam splitter/combiner prism.

5. Apparatus as claimed in Claims 2 to 4, wherein output light from the Bragg cells of the pair is directed via a Fourier transform lens.