GB2113862A

GB2113862A - Demodulator for encoded RF signals

Info

Publication number: GB2113862A
Application number: GB08200613A
Authority: GB
Inventors: R C Wainwright; A M Smith
Original assignee: PA Management Consultants Ltd
Current assignee: PA Management Consultants Ltd
Priority date: 1982-01-09
Filing date: 1982-01-09
Publication date: 1983-08-10
Also published as: GB2113862B

Abstract

A demodulator for decoding encoded RF signals comprises a coherent light source such as a laser diode (10) a collimating lens (12) a transducer (16) which produces a transversely extending surface acoustic wave which interacts with the collimated optical wave from the lens (12). One of the waves is modulated by the incoming signal and the other wave is modulated by a reference signal. A second lens (18) focuses the undiffracted light which is blocked by a stop (20) and diffracted light passing through the lens (18) falls on an array of photodetectors (22). An output signal is obtained by scanning the array of photodetectors (22). A further lens may be employed to focus the diffracted light to form a magnified image onto the photodetector array so as to allow the latter to be positioned remote from the stop (20) and thereby improve the signal to noise ratio of the device. <IMAGE>

Description

SPECIFICATION Demodulator for encoded RF signals such as are employed in communication systems Field of invention This invention concerns demodulators for demodulating encoded RF signals and in particular relates to the use of acousto-optic time integrating correlators (AOTIC's), to provide demodulation of an RF carrier up to gigahertz frequencies, to give baseband data out, in a single process. Such demodulators are of particular application in the field of spread spectrum communications systems.

Background of the invention Spread spectrum transmission is an establisbed technique in communication systems and involves the transmission of a signal over a large bandwidth, rather than the utilisation of a narrow bandwidth, as is more common in other types of communication system.

Encoding is achieved in a number of ways including direct sequence modulation, frequency hopping and combinations of these with timed transmissions.

Direct sequence modulation basically takes an RF carrier and mixes it with a pseudo random binary sequence (PRBS) to produce an RF signal spread over a band whose width is twice that of the PRBS clock frequency. Data can be added either to the RF carrier before spreading or by modifying the code in some way.

Frequency hopping systems transmit a series of specific frequencies defined by the PRBS and modified by the addition of data. These systems require an agile frequency synthesiser.

By careful choice of the PRBS type, length, and clock frequency a number of advantages are available to potential users. These include selective addressing and a multiple-user capability by code diversity; a transmitted signal with low power density allowing it to be concealed below the noise level; protection against casual eavesdropping; and good interference rejection.

In order to receive a spread spectrum coded transmission the receiver must synchronise its local code generator with that of the transmitter, which then allows the use of a double balanced mixer either in-line, or more usually with heterodyne techniques, to recover the modulated carrier. Baseband data is then recovered by the method appropriate to the particular modulation used, be it FSK, PSK, or whatever. The problem of synchronisation of the local generator is one of the difficulties most commonly associated with this type of communication. A short code may be easy to identify but more susceptible to noise whereas a long code may take longer to synchronise but may be more stable once this have been achieved. Obviously correlation between incoming signals and a locally generated code in the form of a reference signal must be performed.

Summary of the invention According to one aspect of the present invention a decoder for encoded RF communication signals comprises a correlator device which permits transverse interaction of a guided optical wave and an acoustic surface wave, in which one of the interacting signals is modulated with a reference coding signal and the other is modulated by the received encoded signal.

Such a decoder is or particular application for decoding a spread spectrum signal in a spread spectrum communication system.

The optical wave may be modulated by the reference coding signal and the acoustic surface wave may be modulated by the encoded signal.

Alternatively the acoustic surface wave may be modulated by the reference coding signal and the optical wave may be modulated by the incoming encoded signal.

The correlator device may be formed from a ferro-electric crystalline substrate.

Preferred materials for the substrate comprise lithium niobate and lithium tantalate.

A guiding layer may be produced in the surface of the substrate by infusion of titanium or out diffusion of lithium.

The substrate may alternatively be a crystalline semiconductor material formed from two materials from groups Ill and V of the periodic table such as gallium arsenic.

Where a Ill-V crystalline semi-conductor compound constitutes the substrate, a guiding layer may be formed by an alloy of two or more elements which may be selected from the following namely indium, gallium, arsenic, phosphorus, aluminium.

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

Since lenses are required for focusing and collimating the optical wave, lenses may e formed by deforming regions of the substrate so as to produce an optical lens effect in those regions.

In another arrangement a zone-plate type lens is formed from selective overlayed films.

A device according to the invention may be operated in a manner such that an acoustic wave is launched in the substrate material.

Alternatively the surface acoustic wave may be launched in material overlaying the basic substrate.

In a preferred embodiment of the invention, a laser diode and means responsive to the reference signal for modulating the current through the diode (or for modulating the light produced by the diode for example by an electro-optic modulator) is employed to produce a modulated optical wave and electrodes (typically in the form of interdigitated electrodes) are formed in the substrate and means is provided for supplying the encoded RF signal thereto to produce a modulated acoustic surface wave and a demodulated output signal is obtained from an array of photo detectors which are arranged so as to be responsive to the diffracted light emerging from the region of interaction between the two waves.

In another embodiment a laser diode is arranged together with means responsive to the encoded RF signal for modulating the current through the diode in response to the encoded signal to produce a modulated guided optical wave and electrodes (typically in the form of inter digitated electrodes) are formed in the substrate and means is provided for applying the reference signal thereto, to produce a surface acoustic wave and a demodulated output signal is obtained from an array of photodetectors which are responsive to the diffracted light emerging from the region of interaction between the two waves.

According to another aspect of the invention a method of decoding an encoded RF signal comprises the steps of: 1. producing a guided optical wave, 2. producing an acoustic surface wave, 3. modulating one of the waves by the encoded signal, 4. modulating the other wave by a reference signal, 5. detecting the diffracted light emerging from the region of interaction between the two waves, and 6. generating from the detection an electrical output signal corresponding to a decoding of the encoded signal.

The method is particularly applicable to the demodulation of a spread spectrum communication system signal.

In one arrangement the encoded radio frequency signal modulates the current flowing through a laser diode (or the light produced by the diode), to produce a modulated optical wave and the reference signal is supplied to electrodes in the substrate to produce the acoustic surface wave.

In another arrangement the radio frequency signal is supplied to electrodes in the substrate to produce the acoustic surface wave and the reference signal is supplied to control the current flowing through a laser diode (or the light produced by the diode) to produce the modulated optical wave.

Description of the drawings The invention will now be described by way of example with reference to the accompanying drawings, in which Figure 1 is a schematic diagram of a demodulator embodying the invention, and Figure 2 shows a part of the demodulator of Figure 1.

Detailed description of the figures In Fig. 1 an incoming RF signal Sl(t) is amplified by an RF amplifier 6 and applied to the input of a laser modulator circuit 8 for controlling the current flowing through a laser diode 1 0.

Light from the latter is collimated by a condensing lens 12 before passing across the surface region 13 of the device 14 in which a plurality of interdigitated comb electrodes form a transverse surface acoustic wave under the influence of a transducer 1 6 powered by a modulator circuit 1 7 supplied from a local reference signal generator 1 9 producing a reference signal S2(t).

Diffracted light is focused by a lens 1 8 onto an array of photodetectors 22. Undiffracted light is also brought to a point of focus by the lens 1 8 at which point is located a screen or stop 20 to reduce the possibility of any of this light reaching the photodetectors. The instantaneous light intensity of the light falling on the array is proportional to the instantaneous light intensity from the laser source and the instantaneous acoustic wave amplitude in the region of interaction.

In order to improve inter alia the signal to noise ratio of the signal from the array 22, a third lens 24 may be provided as shown in Fig. 2 together with an aperture 26 to produce an appropriately magnified and focussed image of the diffracted light on an array 22', which array can as a consequence be located at a distance from the stop 20.

If Sl(t) and S2(t) have a strong cross correlation function, there will be a peak in the light intensity at some point along the array.

The positTon of thrs peak is determined by the time delay between Sl(t) and S2(t). For a peak to be detected this delay must be less than the acoustic transit time.

The expression "transverse to the direction of propagation" is intended herein to mean "directed across the direction of propagation" and is not intended to be limited to cases where the two interacting directions are actually perpendicular. Thus in the context of this application a transverse acoustic surface wave need not be perpendicular to a guided optical wave.

In one arrangement the device is formed by integrating the SAW transducer, optical waveguide, and lenses, onto a lithium niobate substrate having a titanium diffused surface region. The light source is a gallium aluminium arsenide double heterostructure laser diode and the detector array is formed from chargecoupled devices. These latter two components are butt coupled to the lithium niobate substrate.

As a development of this embodiment, the laser diode and the detector array may be integrated into a single substrate such as one of gallium aluminium arsenide.

Both the laser diode and the SAW transducer can operate in the gigahertz region and the integration period of the correlator is limited only by the scan time of the array which may be several milliseconds. Therefore it can be seen that this device is excellently adapted for the wide bandwidths of spread spectrum systems and is able to cope with relatively long code sequences.

Demodulation is achieved by applying the incoming RF signal to modulate the light source and applying a locally generated reference signal to a set of interdigitated electrodes in the substrate to set up the modulated surface acoustic wave.

By the use of suitable modulation techniques, baseband data can be obtained directly from such an RF signal using such a demodulator thus eliminating much of the traditional RF and all of the IF receiver circuitry associated with normal spread spectrum receivers.

Synchronisation and tracking are achieved by using conventional techniques such as 'sliding' correlation and 'delay-lock' tracking. However if the photodetector array is scanned at the code repetition rate and the spreading sequence is keyed ON and OFF according to the data bit to be sent, then the output from the array will contain a correlation peak, or no peak, as appropriate.

Taken sequentially in time, using a peak detector clocked at the data rate, these reconstitute the original data.

Alternatively information may be sent by varying the ratio of time for which the spreading sequence is ON to the time for which it is OFF, over a fixed number of code cycles, which is the scan rate oithe detector array. This results in a variation in the amplitude of the correlation peak produced.

Another possibility is to change the phase of the spreading sequence which will result in a change in the position of the correlation peak produced by the photodetector array. However this must be handled carefully since the same effect is produced by a small loss of synchronisation in the receiver.

A preferred method of modulation is one which combines the data bit to be sent with the spreading code by "modulo 2" addition. This results in two possible codes which are the complement of each other. Therefore one of these will result in a correlation peak at the output of the photodetector and the other will result in a correlation trough i.e. absence of light. (These are both relative to the background level produced by noise). This scheme has the advantage that a pulse of one form of the other is always produced each time the array is scanned (for better tracking). In addition the system employs a simple digital data modulation technique (i.e.

modulo 2 addition of data to the code sequence).

In another embodiment of the invention a demodulator comprises a crystalline or glassy substrate of materials such as for example, lithium niobate, lithium tantalate, gallium arsenide or indium phosphide or soda glass on the surface of which a higher refractive index layer has been formed. In the case of ferroelectric crystals this layer may be produced for example, by out-diffusion of lithium or in-diffusion of titanium.

For Ill-V semi-conductor substrates the refractive index change may be produced by laying down layers of, for example, gallium aluminium arsenide or gallium aluminium arsenide phosphide alloys using techniques such as liquid phase epitaxy, vapour phase epitaxy, molecular beam epitaxy or metal organic chemical vapour deposition.

For glassy substrates in-diffusion of silver ions, for example, can produce a layer of the required refractive index.

In all cases, the guiding layer is only a few optical wavelengths in depth and the light which propagates in the device is substantially confined to this layer and the region immediately around it.

Additional layers may overlay the guiding layer.

These may provide, for example, insulation of the substrate from external perturbing influences, modification of the light path in the device, or a medium for acoustic wave propagation.

A complete demodulator has seven basic sections: 1. A laser or other high radiance light source, 2. A guiding region into which the light from the source is coupled and in which the light diverges, 3. A collimating lens structure, 4. A guiding region where the collimated light interacts with the transversely transmitted surface acoustic wave, 5. A second lens which focusses the transmitted and diffracted light, 6. A guiding region where the light converges towards the edge of the substrate, and 7. A detector or array of detectors which monitor the intensity and spatial variation of the diffracted light.

Alignment of the light source (which may produce light of visible or near infra-red wavelengths) and the detectors, with the edge of the guiding layer, may be achieved using precise micropositioning devices. In principle, if a semiconductor substrate is used, both the source and a detector may be integrated on the basic substrate.

The lenses may be produced by a variety of techniques.

In one technique a depression is milled out of the substrate material using, for example, single point diamond turning or ultrasonic grinding. This produces a so-called geodesic lens and these are formed prior to the formation of the guiding layer.

Another technique uses overlay films of material above the guiding layer, to modify the phase velocity of the guided wave in an analogous manner to a zone plate. It is also possible to produce lensing action by electric field interactions. However formed, the lenses typically have a diameter of about 20 mm.

The surface acoustic wave is produced by supplying a signal to interdigitated comb electrodes. Several pairs of electrodes are used in order to provide a collimated acoustic wave over a wide range of frequencies. The electrodes may have different spacings and/or orientations and/or may be arranged as a chirp structure.

If a substrate has large piezoelectric coefficients, as is the case for example with lithium niobate, then the surface acoustic wave may be launched directly along the substrate material.

If the piezoelectric coefficients of the substrate are small, however, it may be necessary to use a thin layer of piezoelectric material above the guiding layer to transmit the acoustic wave.

Propagation of the wave in this layer will produce stress-induced refractive index variations in the guiding layer. This type of approach may be necessary when, for example, devices are constructed on glass-like substrates.

In order for the device to work correctly, careful design is required to ensure that the focal lengths and positions of the lenses are correct, that the surface finish of the substrate and guiding layer are very smooth so as to reduce light scattering loss and crosstalk and that the acoustic wave is adequately absorbed after passing across the substrate so that standing waves are not produced.

Claims

1. A decoder for decoding encoded RF communication signals comprising a correlating device which permits transverse interaction of a guided optical wave and an acoustic surface wave and in which one of the interacting signals is modulated with reference coding signal and the other is modulated by the received encoded signal.

2. A decoder as claimed in claim 1 in which the communication signal is a spread spectrum signal.

3. A decoder as claimed in claim 1 or 2 in which the optical wave is modulated by the reference coding signal and the acoustic surface wave is modulated by the encoded signal.

4. A decoder as claimed in claim 1 or 2 in which the optical wave is modulated by the incoming encoded signal and the acoustic surface wave is modulated by the reference coding signal.

5. A device as claimed in claim 1 in which the correlated device is formed from a ferroelectric crystalline substrate.

6. A device as claimed in claim 5 in which the substrate is lithium niobate.

7. A device as claimed in claim 5 in which the substrate is lithium tantalate.

8. A device as claimed in any of claims 5, 6 or 7 in which a guiding layer is produced by indiffusion of titanium.

9. A device as claimed in 5, 6 or 7 in which the guiding layer is produced by outdiffusion of lithium.

10. A device as claimed in claim 1 in which the correlator device is formed from a crystalline semi-conductor substrate of two materials from groups III and V of the periodic table, such as gallium arsenide.

1 A device as claimed in claim 10 in which the guiding layer is formed by an alloy of two or more elements.

12. A device as claimed in claim 11 in which the alloy contains only two or more of the following elements namely indium, gallium, arsenic, phosphorus and aluminium.

13. A device as claimed in claim 1 in which the correlator device is formed from a substrate of a glass-like material.

14. A device as claimed in claim 13 in which a guiding layer is produced by either indiffusion of silver or outdiffusion of sodium.

1 5. A device as claimed in claim 5, 10 or 13 in which deformed regions of the substrate produce a lens action.

16. A device as claimed in claim 5, 10 or 13 in which a zone-plate type lens is formed from selected overlayed films.

17. A device as claimed in claim 15 or 16 where a surface acoustic wave is launched in the substrate material.

18. A device as claimed in 15 or 16 where a surface acoustic wave is launched in material overlaying the basic substrate.

19. A decoder as claimed in claim 3 further comprising a laser diode, means responsive to the reference signal for modulating the current through the diode (or light therefrom) to produce a modulated optical wave, electrodes (typically in the form of interdigited electrodes) formed in the substrate, means for supplying the encoded signal thereto to produce a modulated acoustic surface wave, and an array of photodetectors which are responsive to the diffracted light emerging from the region of interaction between the two waves.

20. A decoder as claimed in claim 4 further comprising a laser diode, means responsive to the encoded signal for modulating the current through the diode (or light therefrom) to produce a modulated guided optical wave, electrodes (typically in the form of interdigited electrodes) formed in the substrate, means for applying the reference signal thereto, to produce a surface acoustic wave, and an array of photodetectors which is responsive to the diffracted light emerging from the region of interaction between the two waves.

21. A method of decoding an encoded radio frequency signal comprising the steps of:

1. producing a guided optical wave,

2. producing an acoustic surface wave,

3. modulating one of the waves by the encoded signal,

4. modulating the other wave by a reference signal,

5. detecting the diffracted light emerging from the region of interaction between the two waves, and

6. Generating from the said detection an electrical output signal corresponding to a decoding of the encoded signal.

22. A method as claimed in claim 21 in which the encoded radio frequency signal is a spread spectrum communication system signal.

23. A method as claimed in claim 21 or 22 in which the encoded radio frequency signal modulates the current flowing through a laser diode to produce a modulated optical wave and the reference signal is supplied to electrode in the substrate to produce the acoustic surface wave.

24. A method as claimed in claim 21 or 22 in which the encoded radio frequency signal is supplied to electrodes in the substrate to produce the acoustic surface wave and the reference signal is supplied to control the current flowing through a laser diode to produce the modulated optical wave.

24. A demodulator for decoding an encoded RF signal particularly a spread spectrum communication signal constructed arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawing.

25. A method of decoding an encoded RF signal substantially as herein described with reference to and as illustrated in the accompanying drawing.