GB2115572A - An acousto-optic heterodyne signal processing device - Google Patents

Abstract

An acousto-optic heterodyne signal processing device includes an optical path traversed by first and second substantially identical acousto-optic diffraction means 10, 11, e.g. SAW devices, which act parallel with one another in opposite directions to diffract light from coherent source 14. Diffracted light is brought to a common focus at various parts of the photodetector array 15, according to the frequency components of the r.f. signal which is applied simultaneously to the two diffraction means. The output angle of beams 17, 22 is invariant with r.f. signal frequency. The optical path may be a surface wave guide region in an electro-optic substrate with lenses 12, 13 or alternatively mirrors, fabricated therein, or the device may be implemented using discrete bulk optical components. <IMAGE>

Description

SPECIFICATION An acousto-optic heterodyne signal processing device This invention relates to an acousto-optic heterodyne signal processing device such as may be used for r.f. spectrum analysis and which is suitable for implementation as an integrated acousto-optics device.

The use of acousto-optic techniques for spectrum analysis has provided a relatively new receiver concept, and one which is being extensively researched for solving problems of increasing density and complexity of radar environments. "Spectrum Analysis Using Optical Processing", T. M. Turpin, Proc, IEEE, Vol. 69, Jan., 1981, pp 79-92, presents six selected topics on the subject. In one form of spectrum analyser using an acousto-optic Bragg cell, for example, the cell is illuminated by coherent light and the power spectrum of the r.f. input is provided by a frequency steered deflection of the light onto a photo-diode array. Such an arrangement is illustrated in Fig. 1 of the referenced paper by Turpin.Fig. 5 of the same paper illustrates a form of heterodyne device (an interference rejection filter) in which the coherent light illumination provides, by means of beam splitting devices, the reference signal with which the r.f. modulated light is combined.

Critical in the practical implenetation of any optical heterodyne system is the requirement for waveform matching of signal and local waves at the detector, and for the Bragg cell this is particularly complicated by the angular output beams which have to be handled. As a result, previously considered heterodyne systems have imposed stringent requirements on either beam alignment, detector size, or both, and it has been difficult to configure a practical implementation in pianar integrated optical form. In the present invention a scheme of heterodyne operation is disclosed which removes the wavefront matching problem and provides a solution to implementation in integrated optic form as well as being equally applicable to construction using conventional bulk optics.

According to the present invention there is provided an acousto-optic heterodyne signal processing device including an optical path traversed by first and second substantially identical radio frequency responsive acoustooptic diffraction means acting parallel with one another in opposite directions, the optical path including a source of coherent light, input and output focussing means between which the diffraction means are positioned, and an array of photodetector means whereby light from the source is formed as a collimated beam interacting with both diffraction means and diffracted light from each diffraction means is brought to a common focus at the array.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 a illustrates a basic configuration of an acousto-optic heterodyne signal processing device, Fig. 1 b illustrates details of the acousto-optic interaction geometry of Fig. 1 a, and Fig. 2 illustrates an alternative optical arrangement of the signal processing device.

The device shown in Fig. 1 comprises an acousto-optic cell 1 which is provided with two parallel substantially identical acoustic wave paths. Each path has an electro-acoustic input transducer 10, 11 and an acoustic wave termination 1 Oa, 11 a. The two acoustic paths are arranged in opposite directions to one another and transverse an optical path in the acoustooptic cell extending between input and output focussing lenses 12, 13. A laser source 14 illuminates the input lens 12 from which a collimated coherent beam of light 1 7 propagates in the optical path. An array of photodiodes 1 5 is placed to receive the focussed output from lens 13.

If now a radio frequency signal is applied to transducer 10 such that an acoustic wave 1 6 of frequency m is propagated across the optical path then, assuming the correct geometry has been established, a Bragg diffraction of the optical beam Coc will be effected. The principle of acoustic Bragg diffraction is well documented, see for example "Acousto-Optics-A Review of Fundamentals", A. Korpel, Proc. IEEE, Vol 69, Jan.

1981, pp 48-53. The efficiency of the Bragg acousto-optic interaction can be tailored so that the percentage of incident light that is diffracted can be set to a predetermined value. The above referenced paper by Turpin discusses this apect at page 81. In the present application the interaction is designed, principally on the basis of linearity, so that only a small percentage, say 0.1-1%, of the incident light is diffracted for each frequency component of the input signal. The diffraction produces a second beam 18 of frequency Wc+Wrn The remainder of the primary beam 1 7 continues on its original path.The transducer 11 is energised with the same r.f. signal 69mmas tranducer 10, forming a second acoustic wave 1 9 travelling in a parallel but opposite direction to wave 1 6. This second acoustic wave 1 9 will interact with both the second diffracted beam 18 and the remainder of the primary beam 1 7 from the first acousto-optic interaction.Part of the remaining primary beam 17 will be diffracted to form a third beam 20 which is now parallel to the secondary beam 18 but which will have a frequency a)cWm At the same time part of the second beam 1 8 is diffracted to form a fourth beam 21 parallel to the primary beam 17 and having a frequency w,+2w,. The complex acousto-optic interactions occurring at the second surface acoustic wave are illustrated in greater detail in Fig. 1 b.The diffracted beams 18 and 20 are both focussed by lens 13 onto the photodiode array 1 5. Since the same r.f. signal m is applied in both transducers 10, 11 the beams 18 and 20 will always remain parallel to all angles of diffraction, the angle of diffraction at any given time being dependent on C9m. Thus as varies the two substantially colinear beams 18 and 20 are deflected by equal amounts and brought to a common focus as they scan the array 15, mixing in a photodiode to produce an output current i(t).

Mathematically we can represent the process of conventional optical heterodyne detection (as disclosed by Fig. 5 of the referenced paper by Turpin) by a summation of a reference beam (R) and a frequency shifted signal beam (S). In the case of colinear beams with parallel wavefronts, then, R (t)=A cos wct S (t)=B cos (Wc+Wm)t where Wc is the optical carrier frequency and Corn is the RF input modulation frequency.

The photodetector output current i(t) is given by i(t)=R(t)+S(t). R(t)+S(t)* =A2+B2+2AB cos comet (1) Thus the detector output has a component at the RF input frequency, and at dc. Further, for a constant reference beam amplitude, the RF output current at the photodetector is directly proportional to the input signal beam amplitude and thus the RF conversion is linear in amplitude.

Compared then with the conventional AO spectrum analyser, which is linear in power, the benefit is a doubling (in dB) of the optical dynamic range of the photodetector.

However, because in this invention the reference beam is also frequency shifted from the original light frequency of w the r.f. component in expression (1) above is now at frequency 2s9m.

Amplitudes and phases are balanced to the degree that both beams pass through the same optics and the expression for the photocurrent may now be written as i(t)=2A2 [1 +cos (2cornt)] (2) The double diffracting interaction also produces two beams 17,21 at frequencies Coc and Wc+2Ct)m and these also emerge colinear and can be brought to a focus on a separate photo diode 22. In this case, however, the output angle, and thus spot position, is invariant with RF input frequency and the heterodyned output from the photodiode is the convolution of the RF input signal.Mathematically, i(t)~cos (2c3mt)S(t). S(2t-T)dT (3) Planar devices which make use of surface acoustic waves (SAW) have a greater efficiency than alternative bulk effect devices and a more compact form which make them potentially suitable for fabrication as an integrated sub system. Lithium Niobate (LiNbO3) is a favoured material for the acousto-optic interaction, having superior piezoelectric and electro-optic properties.

Unfortunately, semiconductor lasers and photodiode arrays must be fabricated in other materials by may be combined with LiNbO3 substrates through hybrid integration. In integrated form the structure shown in Fig. 1 would thus combine a body of material such as lithium niobate.

An alternative structure to that of Fig. 1 a is shown in Fig. 2. The difference is that instead of lenses, which admittedly can be fabricated in a planar waveguide structure, beam focussing mirrors are utilised. The laser 14, transducers 10, 11 and photodiodes 15, 22 are the same components as before. Light from the laser 14 is now substantially orthogonal to the optical path through the acousto-optic diffraction region, being re-directed and collimated by a first beam shaping mirror 23. Similarly the emergent and diffracted beams are again re-directed and focussed by a second beam shaping mirror 24.

Such mirrors can be fabricated as integrated optics devices in the surface waveguide region of the planar substrate by ion milling techniques, to provide vertical steps in the substrate surface which are then metallised to complete the reflective surfaces. Such a technique is disclosed in British patent application 8120062 (J. S.

Heeks-C. B. Rogers 30-2).

In order to exploit the performance advantages of optical heterodyne operation, a high speed photodetector element is necessary and this should be compatible with integration into a linear array and ultimately into microwave circuits. It is also required to perform high speed data processing for this as well as conventional implementations of the Bragg spectrum analyser.

These requirements may be considered against a Bragg cell component having a 500 MHz octave bandwidth and a 1 MHz resolution capability, which imposes on the photodetector an operating frequency range of 1-2 GHz with each postdetection channel handling a r.f. signal bandwidth of approximately 2 MHz.

In considering speed and functional complexity at low power it is noted that GaAs is a natural technology base on which these required functions can evolve. A promising approach to high speed photodetection on GaAs utilizes the photoconductive mechanism, and devices based on this have been reported, C. W. Slayman eh al, 'Frequency and Pulse Response of a Novel High Speed Interdigitated Surface Photoconductor', IEEE Electron Device Letters, May 1981, pp 112-114, with bandwidths up to 4 GHz: sensitivities are quoted at comparable levels to the best commercial PIN and APD devices. The device structure can be simple, such as a MESFET which is easy to integrate on GaAs and which has potential for integration with optical waveguides.

The invention is also realisable using discrete bulk optical components, including laser source, lenses (or mirrors), photodetector array and a bulk wave acousto-optic cell providing the diffraction means.

Claims (11)

Claims

1. A acousto-optic heterodyne signal processing device including an optical path traversed by first and second substantially identical radio frequency responsive acoustooptic diffraction means acting parallel with one another in opposite directions, the optical path including a source of coherent light, input and output focussing means between which the diffraction means are positioned, and an array of photodetector means whereby light from the source is formed as collimated beam interacting with both diffraction means and diffracted light from each diffraction means is brought to a common focus at the array.

2. A device according to claim 1 wherein the optical path is defined by a surface waveguide region in a planar electro optic substrate and the diffraction means are provided by acoustic wave transducers aligned to propagate acoustic waves transverse the optical path.

3. A device according to claim 2 wherein the input and output focussing means and lenses fabricated in the surface waveguide region of the electro optic substrate.

4. A device according to claim 2 wherein the input and output focussing means are beam shaping mirrors fabricated in the surface waveguide region of the electro optic substrate.

5. A device according to claim 2, 3 or 4 wherein the source of coherent light is a semiconductor laser affixed to the edge of the planar substrate.

6. A device according to any one of Claims 25 wherein the acousto-optic diffraction means are provided by surface acoustic wave transducers aligned to propagate surface acoustic waves transverse the optical path.

7. A device according to any one of claims 25 wherein the array of photodetector means comprises an array of semiconductor photodiodes.

8. A device according to any preceding claim including an additional photodetector means positioned to receive both undiffracted and colinear doubly diffracted light from the optical path brought to a common focus by the output focussing means.

9. A device according to claim 1 implemented using discrete bulk optical components, including laser source, lenses (or mirrors), photodetector array, and a bulk acousto-optic cell providing the diffraction means.

1 0. An acousto-optic heterodyne signal processing device substantially as described with reference to the accompanying drawings.

11. A method of heterodyning a radio frequency signal comprising applying the signal simultaneously to two identical parallel acoustic optic diffraction means operating in opposite directions transverse an optical path in which is propagated a beam of collimated coherent light and focussing diffracted light from each diffraction means at a common focus which scans an array of photodetectors according to the frequency content of the radio frequency signal.