GB2176281A

GB2176281A - Optical signal processor

Info

Publication number: GB2176281A
Application number: GB08612557A
Authority: GB
Inventors: Jeffrey Wyn Lewis
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1985-05-22
Filing date: 1986-05-22
Publication date: 1986-12-17
Also published as: GB8512963D0; GB8612557D0; GB2176281B

Abstract

An optical signal processor in which a modulated optical signal from source 1 is applied to a time/space converter in the form of a group of optical fibres 3-7 bunched together at the input end 2 and arranged in a linear array at the output end 8, the fibres being progressively longer along the array to provide a time spread. The output of the linear array is applied to a mask 10 transmitting selectively components of the time-spaced signals in parallel, and the signals passing the mask are concentrated onto detectors 13-17. The processing may be a Fourier transform, Laplace transform, convolution, or cross- or auto correlation. <IMAGE>

Description

SPECIFICATION Optical signal processor The present invention relates to electro-optical signal processors, and relates particularly to the processing of real-time samples of optical signals or their electrical analogues. Electro-optical signal processors have applications in microwave frequency analysers and filters for example.

Electro-optical signal processors can be classified as operating on either coherent or incoherent optical signals. Electro-optical processors of both types have the advantage over conventional electronic processors that they can easily be arranged to process in parallel a two-dimensional array of signal inputs-for example when a light beam is transmitted through a 1000 by 1000 resolution mask, 106 analogue multiplications are performed simultaneously in an interval of the order of 10-17 seconds.

It is frequently desirable that an electro-optical signal processor should be designed to operate on incoherent optical signals, since otherwise precautions must be taken against random optical phase changes which tend to arise as a result of vibration and temperature changes. One such signal processor is disclosed in Proc. SPIE Vol 352 pp 76 to 81 "Charge-coupled device (CCD) based electrooptic processor" by R.H.Patterson et al. The signal processor disclosed comprises an optical mask interposed between a single light-emitting diode (LED) and an area array charge coupled device (CCD). Another light source such as a laser diode or a c.w. laser with a modulator could be used instead of an LED if preferred. The mask comprises an array of apertures in a predetermined distribution over the elements of the CCD array.It is shown that by appropriately summing charges generated by elements in the CCD array, the processor can be arranged to perform. vector-matrix multiplications of the general form:

m= 1,2,3 . . . M where g is the output, f,, is a sequence of analogue samples of the input signal (applied to the LED) and hrnn is the transmittance function of a mask of M rows and N columns. One example of a practically useful multiplication of this type is the Fourier transform. However in this arrangement it is necessary to feed discrete samples of the input waveform to the LED and the CCD array is necessarily complicated and slow-acting in comparison with currently available discrete electro-optic detectors.Thus this arrangement.does not take full advantage of the potential capabilities of optical signal processing.

It is an object of the invention to provide a high speed electro-optic signal processor which can utilise incoherent light and which operates on optical (as opposed to electronic) signals to a greater extent than in currently available electro-optic processors.

According-to the present invention an electro-optic signal processor comprises at a light source arranged to generate optical signals, optical detector means arranged to detect said optical signals, a mask interposed in the optical path between said light source and said optical detector means, said mask being arranged to transmit selectively components of said signals in parallel, and a group of discrete optical waveguides constituting part of said optical path and arranged to selectively delay said components according to the regions of said mask through which said components pass.

The terms light" and "optical" are to be understood to pertain not only to visible radiation but to all radiation which obeys the laws of geometrical optics; thus the invention includes within its scope electro-optic signal processors utilising l.R. radiation for example.

The transmissivity of the mask may vary continuously or discretely across its area. Preferably the mask comprises an array of discrete apertures. Preferably the transmissivity distribution over the mask area is a two-dimensional distribution.

Preferably the optical waveguides define the light path between a common light source and respective regions of the mask.

Preferably the optical waveguides are of differing lengths so as to selectively delay said components of said optical signal accordingly.

The optical waveguides are preferably optic fibres.

Preferably the transmissivity distribution over the mask area is two-dimensional and the optical waveguides direct light onto, or receive light from, respective parallel strips of the mask area and the optical detector means comprises a plurality of optical detector elements arranged to receive light from respective further parallel strips of the mask area, said further parallel strips being transverse to said respective parallel strips.

Preferably the transmissivity distribution over the mask area is two-dimensional, the optical detector means comprises a plurality of optical detector elements, and the processor is arranged to perform operations of the form:

m=1,2. .

where C, is the output of each optical detector element m (there being M optical detector elements in total), an is the delayed signal component associated with each optical waveguide n (there being N optical waveguides in total) and bmn is the transmissivity distribution of the mask in the optical paths between each waveguide n and each detector element m.

Preferably the optical waveguides are coupled to a common source of optical signals and arranged to illuminate said mask with selectively delayed components of said optical signals, and the transmissivity distribution of the mask is a two-dimensional periodic distribution such that the output of said optical detector means is representative of a Fourier transform of said optical signals in real time. The invention includes within its scope a frequency analyser incorporating such a processor.

One embodiment of the invention will now be described by way of example only, with reference to Figs 1 and 2 of the accompanying drawings, of which: Figure 1 is a schematic representation of an electro-optic processor in accordance with the invention, and Figure 2 shows a Fourier transform mask suitable for use in the processor of Fig. 1.

The processor shown in Fig. 1 comprises an L.E.D. 1 which emits an optical signal in response to a microwave or other high frequency electrical signal (typically at a frequency of 1 to 10GHz) applied to it. In order tonsure that the optical output of LED 1 follows negative as well as positive parts of the input waveform, biassing means of known type (not shown) are provided in its input circuit. The optical signals generated by LED 1 are fed into a potted bundle 2 of optical fibre ends, and the optical fibres 3, 4, 5, 6 and 7 conduct components of the optical signals in parallel to a regular linear array 8 at which they terminate. Although only five optical fibres are shown (for the sake of clarity) in practice the processor might incorporate 50 or more fibres between bundle 2 and array 8.The lengths of the optical fibres increase regularly from optical fibre 3 to optical fibre 7, so that the optical signal components emerging from the fibre ends of array 8 are successively delayed. Thus if the output intensity as a function of time of LED 1 is l(t) then the instantaneous spatial variation of intensity l(n,t) of the respective signal components from the n th fibre-end in the array 8 will be:

where I is the length of the shortest fibre, Al is the incremental difference in length between the fibres and C,,b". is the velocity of light in the optical fibres. Typically dl may be 5 to 100mm.It will be apparent that because the optical fibres constitute discrete waveguides, there will be no mutual interference between the optical signal components which they carry i.e. the phase information of the electricalsignal applied to LED 1 is retained but the optical phase information of the LED output is immaterial. Consequently, temperature changes, vibration and other effects which slightly affect the optical phase have no appreciable effect on the sampled optical intensity distribution emerging from array 8.It will be apparent that variable time delays may be imposed on components of the output signal from LED 1 by other means for example waveguides of differing refractive index may be employed and in some cases the waveguides may even incorporate a medium of controllable refractive index, such as a liquid crystal, so that the sampled intensity distribution may be varied by a suitable control signal.

The delayed optical signal components from array 8 are directed onto a mask 10 by anamorphic lens arrangement 9. Mask 10 comprises a 5X5 matrix of discrete apertures, and each row of apertures is illuminated by delayed optical signal components from a respective fibre end in array 8. On the other side of mask 10 an anamorphic lens arrangement 11 concentrates transmitted optical signal components from each column of apertures onto a respective detector element 13, 14, 15, 16 or 17. Mask 10 is shown for convenience as a 5X5 matrix but will preferably have N rows corresponding to the N fibres of the array 8, and M columns corresponding to the M detector elements of the array 12. Thus in general, if the mask has a transmissivity of b,, at the m th column and- n th row, the output C, at the m th detector - element is given by

m=1,2... M where an is the optical intensity of the delayed signal component at the n th optical fibre, the mask 10 having N rows and M columns. In a typical frequency analysis application, N and M may be 50 or greater, and Fig. 2 shows a mask 10 which can be used in the processor of Fig.

1 to perform a Fourier transform on the signal applied to LED 1. The left-hand side of the mask generates the real, and the right hand side the imaginary components of the transform. The outputs of the pair of optical detector elements associated with each pair of real and imaginary components may be squared and added by conventional means (not shown) to evaluate the frequency-power distribution of the signal applied to LED 1.

The expression set out above for the operation performed by the processor of Fig. 1 is a general expression for a number of specific transformations useful in signal processing apart from the Fourier transform. Such transformations include the Laplace transform and the functions of convolution, cross-correlation and autocorrelation for example. Thus the processor described can be adapted for a variety of functions useful in signal processing by using a mask 10 with an appropriate transmissivity distribution.

Claims

1. An electro-optic signal processor comprising a light source arranged to generate optical signals, optical detector means arranged to detect said optical signals, a mask interposed in the optical path between said light source and said optical detector means, said mask being arranged to transmit selectively components of said signals in parallel, and a group of discrete optical waveguides constituting part of said optical path and arranged to selectively delay said components according to the regions of said mask through which said components pass.

2. An electro-optic signal processor according to Claim 1 wherein said mask comprises an array of discrete apertures.

3. An electro-optic signal processor according to Claim 1 or 2 wherein the transmissivity distribution over the mask area is two-dimensional.

4. An electra-optic signal processor according to any preceding claim wherein said optical waveguides define the light path between a common light source and respective regions of the mask.

5. An electra-optic signal processor according to any preceding claim wherein said optical waveguides are of different lengths so as to selectively delay said components of said optical signal accordingly.

6. An electro-optic signal processor according to any preceding claim wherein said optical waveguides are optic fibres.

7. An electro-optic signal processor according to any preceding claim wherein said transmissivity distribution over said mask area is two-dimensional and said optical waveguides direct light onto, or receive light from, respective parallel strips of said mask area, and said optical detector means comprises a plurality of optical detector elements arranged to receive light from respective further parallel strips of the mask area, said further parallel strips being transverse to said respective parallel strips.

8. An electro-optic signal processor according to any preceding claim wherein said transmissivity distribution over said mask area is two-dimensional, said optical detector means comprises a plurality of optical detector elements, and the processor is arranged to perform operations of the form:

m=1,2. .M where C, is the output of each optical detector element m (there being M optical detector elements in total), an is the delayed signal component associated with each optical waveguide n (there being k optical waveguides in total) and b,, is the transmissivity distribution of the mask in the optical paths between each waveguide n and each detector element m.

9. An electro-optic signal processor according to any preceding claim wherein said optical waveguides are coupled to a common source of optical signals and arranged to illuminate said mask with selectively delayed components of said optical signals, and the transmissivity distribution of said mask is a two-dimensional periodic distribution such that the output of said optical detector means is representative of a Fourier transform of said optical signals in real time.

10. A frequency analyser incorporating a processor as claimed in any preceding claim.

11. An electro-optic signal processor substantially as hereinbefore described with reference to the accompanying drawing.