GB2191646A

GB2191646A - Fibre optic transversal filter

Info

Publication number: GB2191646A
Application number: GB08614405A
Authority: GB
Inventors: Kevin Christopher Byron
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1986-06-13
Filing date: 1986-06-13
Publication date: 1987-12-16
Anticipated expiration: 2006-06-13
Also published as: GB2191646B; GB8614405D0

Abstract

An optical transversal filter includes a number of signal inputs, e.g. from antennae (A1 to A4) of a radio DF system from which optical versions of the signals are launched into multimode fibres (MF1 to MF4) by lasers (L1 to L4). The fibres are of equal length and each has its other end aimed at a photodetector array (PD1 to PD4), with means to select in each array which detector is to be read to derive the signals to be processed. This enables the delays in each fibre to be suitably chosen. <IMAGE>

Description

SPECIFICATION Fibre optic transversal filter The present invention relates to a transversal filter, usable in the signal processing operations needed in some radio direction finding systems.

According to the present invention, there is provided an optical transversal filter which includes anumberofsignal inputs each applied to a light source arrangement such as a semiconductor laser, asimilarnumberofequal-length optical fibres into the ends of which are launched the signals being received on respective ones ofthe signal in puts, and a similar number of arrays of photo-responsive detectors, which arrays are respectively associated with the fibres, wherein the arrangement of each said fibre and its said photodetector array is such thatthe output ofthe filter is derived from a selection of the photodetectors which includes one such detector from each said array.

Such a transversal filter is usable in the signal processing operations which are needed in radio direction finding systems which use a number of closely-located antennae. The signals recrived on such antennae are processed in order to determine the direction from which the signals reach the antennae.

Therefore, in accordance with the present invention, there is also provided a radio direction finding antenna system which includes a number of similar closely adjacent antennae by which a signal whose direction of origin is to be determined is received, a similar number of light source arrangements such as semiconductor lasers to which the signals received by the antennae are respectively applied, a similar number of equal length multimode optical fibre into the ends of which are launched the opticalised version ofthe signals being received by respective ones ofthe antennae, a similar number of arrays of photo-responsive detection, which arrays are respectively associated with the fibres, means for selecting for each said fibre array combination the photo-responsive detector from which the light isto be obtained, so thatthe signals may besubjected tothe sameor different delays as they pass through the fibres, and means for processing the thus-delayed signals to assessthedirection of origin ofthesignal being received.

An embodiment of the invention will now be described with reference to the accom panying drawing, in which Figure 1 shows schematicallythe arrangement of a transversal filter embodying the invention, while Figure 2 shows schematicallythe arrangement ofthe photo-diode arrays used in the transversal filter of Figure 1.

Although optical transversal filters have applications other than to the signal processing for radio direction finding systems, the invention is described herein in that application. Thus in Figure 1 we see four antennae Al to A4 whose outputs are applied respectivelytofoursemiconductor lasers L1 to L4. This application can be direct if the power received bythe antennae is adequate, or via suitable amplification if not.

Each laser is located with a length of multimode fibre MF1 to MF4, these fibres being of equal length.

There are also a setoffourphoto-diode arrays PD1 to PD4, one perfibre, and each fibre is so terminated that its light output can be directed onto any one of the fibres of its array. This can be done by making the fibre ends each movable so that each such fibre end can be "aimed" directly at one of the diodes ofthe array.

Figure2 shows schematically two of the fibre ends and two of the photo-diode arrays. In this example, each such array includes six photo-diodes in a linear array, although other forms of array are possible. It should also be noted that the arrangement can have any required number of antennae, and with corresponding numbers of optical fibres. These fibres are all ofthesame length so as to introduce constant delays into the signals traversing them, Thus when a radio signal is received it is applied via the transversal filterformed by the optical fibres and associated elements to the photo-diode arrays.

The signal processing equipment selects for each processing operation which of the diodes' outputs is to be used. This can be used, with suitable circuitry associated with the photo-diode arrays, to givethe desired weightings to the signals from the various antennae.

Thus if photodetector 1 is used at each arraythere is no delay difference between the signals as they are passed via the various fibres. Thus in this case we have: Photodetector 1 at output of delay line MF1 Photodetector 1 at output of delay line MF2 Photodetector 1 at output of delay line MF3 and soon.

If we alter the photodetectors' read we can get various delay combinations. Thus Photodetector 1 at output of delay line MF1 Photodetector2atoutputofdelaylineMF2 Photodetector 3 at output of delay line MF3 and so on, gives increasing delaysforthesignals from the respective antennae.

As another example: Photodetector 1 at output of delay line MF1 Photodetector 3 at output of delay line MF2 Photodetector Sat output of delay line MF3 and soon, gives increasing delays largerthan in previous case.

Thusitwill be seen thatvaryingthe photodetectorsfrom which the output of an optical fibre varies the delay experienced by the light traversing that fibre. This increased delay is at least in partduetothefactthat in the normal position each optical fibre is "aimed" directly at photodiode No. 1 of its array of such photodetectors.

Although received power is relatively small, the power actually launched into the fibre is relatively larger in some arrangements. Attenuation may be adjusted in the processing arrangement.

1. An optical transversal filter which includes a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Fibre optic transversal filter The present invention relates to a transversal filter, usable in the signal processing operations needed in some radio direction finding systems. According to the present invention, there is provided an optical transversal filter which includes anumberofsignal inputs each applied to a light source arrangement such as a semiconductor laser, asimilarnumberofequal-length optical fibres into the ends of which are launched the signals being received on respective ones ofthe signal in puts, and a similar number of arrays of photo-responsive detectors, which arrays are respectively associated with the fibres, wherein the arrangement of each said fibre and its said photodetector array is such thatthe output ofthe filter is derived from a selection of the photodetectors which includes one such detector from each said array. Such a transversal filter is usable in the signal processing operations which are needed in radio direction finding systems which use a number of closely-located antennae. The signals recrived on such antennae are processed in order to determine the direction from which the signals reach the antennae. Therefore, in accordance with the present invention, there is also provided a radio direction finding antenna system which includes a number of similar closely adjacent antennae by which a signal whose direction of origin is to be determined is received, a similar number of light source arrangements such as semiconductor lasers to which the signals received by the antennae are respectively applied, a similar number of equal length multimode optical fibre into the ends of which are launched the opticalised version ofthe signals being received by respective ones ofthe antennae, a similar number of arrays of photo-responsive detection, which arrays are respectively associated with the fibres, means for selecting for each said fibre array combination the photo-responsive detector from which the light isto be obtained, so thatthe signals may besubjected tothe sameor different delays as they pass through the fibres, and means for processing the thus-delayed signals to assessthedirection of origin ofthesignal being received. An embodiment of the invention will now be described with reference to the accom panying drawing, in which Figure 1 shows schematicallythe arrangement of a transversal filter embodying the invention, while Figure 2 shows schematicallythe arrangement ofthe photo-diode arrays used in the transversal filter of Figure 1. Although optical transversal filters have applications other than to the signal processing for radio direction finding systems, the invention is described herein in that application. Thus in Figure 1 we see four antennae Al to A4 whose outputs are applied respectivelytofoursemiconductor lasers L1 to L4. This application can be direct if the power received bythe antennae is adequate, or via suitable amplification if not. Each laser is located with a length of multimode fibre MF1 to MF4, these fibres being of equal length. There are also a setoffourphoto-diode arrays PD1 to PD4, one perfibre, and each fibre is so terminated that its light output can be directed onto any one of the fibres of its array. This can be done by making the fibre ends each movable so that each such fibre end can be "aimed" directly at one of the diodes ofthe array. Figure2 shows schematically two of the fibre ends and two of the photo-diode arrays. In this example, each such array includes six photo-diodes in a linear array, although other forms of array are possible. It should also be noted that the arrangement can have any required number of antennae, and with corresponding numbers of optical fibres. These fibres are all ofthesame length so as to introduce constant delays into the signals traversing them, Thus when a radio signal is received it is applied via the transversal filterformed by the optical fibres and associated elements to the photo-diode arrays. The signal processing equipment selects for each processing operation which of the diodes' outputs is to be used. This can be used, with suitable circuitry associated with the photo-diode arrays, to givethe desired weightings to the signals from the various antennae. Thus if photodetector 1 is used at each arraythere is no delay difference between the signals as they are passed via the various fibres. Thus in this case we have: Photodetector 1 at output of delay line MF1 Photodetector 1 at output of delay line MF2 Photodetector 1 at output of delay line MF3 and soon. If we alter the photodetectors' read we can get various delay combinations. Thus Photodetector 1 at output of delay line MF1 Photodetector2atoutputofdelaylineMF2 Photodetector 3 at output of delay line MF3 and so on, gives increasing delaysforthesignals from the respective antennae. As another example: Photodetector 1 at output of delay line MF1 Photodetector 3 at output of delay line MF2 Photodetector Sat output of delay line MF3 and soon, gives increasing delays largerthan in previous case. Thusitwill be seen thatvaryingthe photodetectorsfrom which the output of an optical fibre varies the delay experienced by the light traversing that fibre. This increased delay is at least in partduetothefactthat in the normal position each optical fibre is "aimed" directly at photodiode No. 1 of its array of such photodetectors. Although received power is relatively small, the power actually launched into the fibre is relatively larger in some arrangements. Attenuation may be adjusted in the processing arrangement. CLAIMS

1. An optical transversal filter which includes a number of signal inputs each applied to a light source arrangement such as a semiconductor laser, a similar numberof equal-length optical fibres into the ends of which are launched the signals being received on respective ones ofthe signal inputs, and a similar number of arrays of photo-responsive detectors, which arrays are respectively associated with the fibres, wherein the arrangement of each said fibre and its said photodetector array is such thatthe output of the filter is derived from a selection ofthe photodetectors which includes one such detector from each said array.

2. A radio direction finding antenna system which includes a number of similar closely adjacent antennae by which a signal whose direction of origin is to be determined is received, a similar number of light source arrangements such as semiconductor lasers to which the signals received by the antennae are respectively applied, a similar number of equal length multimode optical fibre into the ends of which are launched the opticalised version ofthe signals being received by respective ones of the antennae, a similar number of arrays of photo-responsive detection, which arrays are respectively associated with the fibres, means for selecting for each said fibre array combination the photo-responsive detectorfrom which the light is to be obtained, so that the signals may be subjected to the same or different delays as they pass through the fibres, and means four processing the thus-delayed signals to assess the direction of origin ofthe signal being received.

3. An optical transversal filter, substantially as described with reference to the accompanying drawing.

4. A radio direction finding antenna system, substantially as described with reference to the accompanying drawing.