GB2205211A

GB2205211A - Signal processing device

Info

Publication number: GB2205211A
Application number: GB08811129A
Authority: GB
Inventors: David Lovell Switzer; William Alvin Wilby
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1987-05-11
Filing date: 1988-05-11
Publication date: 1988-11-30
Anticipated expiration: 2008-05-11
Also published as: GB2205211B; GB8711108D0; GB8811129D0

Abstract

A signal processing device, simulating a tapped delay line, providing multiple replicas of a radar or like input pulse. Parallel optical fibre paths are coupled together at a succession of junctions 5 to combine and re-share the signals between the two output paths at each position. One of the two branches between successive locations has a minimal delay ??? while the other has a graded delay 2<n> L+??? increasing from stage to stage binarily. The number of paths from input to output increases exponentially with the number of stages, the total delay varying progressively to provide successive replicas of the input pulse. <IMAGE>

Description

Signal Processing Device This invention relates to signal processing devices and in particular to such devices employing fibre-optic signal paths. Signal processing is a necessary feature of, for example, radar signal analysis for the detection and discrimination of radar targets. In such systems it is sometimes desirable to replicate the radar signal returns and for this purpose tapped delay lines may be used each tapping providing a replica of the input signal, with a delay between successive tap outputs. If the radar is a pulsed system and the inter-tap delays exceed the pulse length then the outputs are isolated in time as well as space. With a C.W. system the whole signal will be staggered from tap to tap.

For most purposes it is not essential that the delayed outputs are separated in space and an object of the present invention is to provide a quasi-tapped delay line which provides successive time delays with low losses and minimum equipment.

According to the present invention, a signal processing device comprises parallel fibre-optic paths, a series of fibre optic couplers at intervals along the paths, the couplers being arranged to couple optical signals between the paths and the paths between each pair of successive couplers being of different lengths to provide different delay times, one of said paths at the first of the series of couplers providing an input port and a combination of the paths at the outputs of the last coupler of the series providing an output port, the arrangement being such that an optical signal applied to the input port is replicated many times at the output port.

There may be two parallel fibre-optic paths and each coupler may have a coupling coefficient of 0.5. One of the two paths between each pair of successive couplers preferably has minimal length and corresponding time delay and the other path has a length and corresponding time delay which increases binarily from coupler to coupler.

There may be included an optical source and means for intensity modulating the source in dependence upon an electrical signal, the output port being coupled to an optical detector to produce a series of electrical replicas of the input electrical signal.

A signal processing device for use in a radar processing system will now be described, by way of example, with reference to the accompanying drawing.

An optical source modulator 1 is provided with a modulating signal, this being the R.F signal of interest. As explained above this may be a radar pulse signal the amplitude of which is significant. The optical source, in general a laser, is intensity modulated by the R.F. signal and the resulting optical signal applied to an input port 3. This input port is one of two fibre-optic inputs of a coupler 5 which may be of a kind known as a bi-conical fused taper fibre-optic coupler. It may comprise two fibre-optics 7 & 9 which, without interrupting either of them, are twisted and partially fused together to provide optical coupling from each to the other.

The coupling may be symmetrical, i.e., having a coupling coefficient of 0.5, or may be asymmetrical to any predetermined degree. The coupling coefficient may also be made variable to enable flexibility In operation. For the purposes of the present embodiment the coupling coefficient Is assumed to be fixed at 0.5.

The fibre optics 7 & 9 extend continuously through a series of such couplers 5, in this embodiment all identical.

Between each pair of successive couplers 5 one of the two fibre-optics takes a direct path but the other takes a longer path which may be formed by a loop 11 or other extended form. The length, L, of the first loop is much as to provide the desired delay between the output signal replicas. The actual signal transmission time on this path between the first and second couplers 5 is made equal to L+oc where oc is the minimal delay imposed by the other, direct, path between the couplers.

Subsequent loops 11 are of such length as to provide binarily increasing delays i.e., L+oc, 2L+oc, 4L+oh etc, the delay increasing exponentially with the number of couplers/loops.

A signal applied at the input port 3 will therefore have two path options to the second coupler, four path options to the third coupler, etc.

The final path options are of delays of 32L+otby a loop 11 and oc by a direct route. These two paths are combined in a lossless combiner 13 which has an output fibre core size which is at least twice that of each input fibre.

There are 64 path options between the input port 3 and the output port 15, and all of these paths include a basic minimal delay 60t plus a purely binary content of zero to 63L. Thus the minimum delay is 60c and the maximum 63L+60C , in steps of L.

A photodetector 17 is connected to the output port 15 to convert the optically replicated signals back to electrical signals.

The advantages of using such a fibre-optic incoherent processor in a radar or other RF system where tapped delay lines are required offering many taps and long delays are as follows. Optical fibres and optical fibre components are low loss devices and their perfomance is essentially independent of RF frequency, unlike Surface Acoustic Wave devices or co-axial cable whose losses increase with frequency. The fundamental RF bandwidth of the device will be limited, by the electrical response of the modulated optical source and the photodetector 17 and by dispersion in the optical fibre where very long delays (greater than 10 psec) are required. However, with the appropriate choice of components bandwidths in excess of 10 Gigahertz with these long delays can be achieved. Optical fibres are low cost and low volume and offer immunity to electro-magnetic interference.

The QTDL performs the same function as a fibre-optic tapped delay line or a series of fibre-optic parallel delay lines. It has the advantages of being of lower volume, as it uses less optical fibre; lower cost, again because it uses less fibre and fewer photodetectors; lower insertion loss because it uses fewer complex fibre-optic components and the output is on a single photodetector.

Fusion junctions in the fibres are unnecessary except at the input and output, so reducing connection losses to a minimum.

Claims

1. A signal processing device comprising parallel fibre-optic paths, a series of fibre optic couplers at intervals along the paths, the couplers being arranged to couple optical signals between the paths, and the paths between each pair of successive couplers being of different lengths to provide different delay times, one of said paths at the first of said series of couplers providing an input port and a combination of the paths at the outputs of the last coupler of the series providing an output port, the arrangement being such that an optical signal applied to the input port is replicated many times at the output port.

2. A signal processing device according to Claim 1, comprising two said parallel fibre-optic paths and wherein each coupler has a coupling coefficient of 0.5.

3. A signal processing device according to Claim 2, wherein one of the two paths between each pair of successive couplers has minimal length and corresponding time delay and the other path has a length and corresponding time delay which increases binarily from coupler to coupler.

4. A signal processing device according to any of Claims 1, 2 & BR< 3, further comprising an optical source and means for intensity modulating the source in dependence upon an electrical signal, said output port being coupled to an optical detector to produce a series of electrical replicas of said electrical signal.

5. A radar signal processor including a device according to any preceding claim.

6. A radar signal processing device substantially as hereinbefore described with reference to the accompanying drawing.