GB2096401A - Improvements in or relating to electro-optical systems - Google Patents

Abstract

A printed circuit board capable of handling electrical and optical signals has electrical conductors (14) formed in known manner in one or more layers and optical fibres (6) extending within the thickness of the board. Electro-optical transducers (18) are positioned in the thickness of the board in optical mating relationship with the optical fibres. The fibres may terminate at a surface of the board or extend (22) beyond the board or may optically connect transducer devices in the board. <IMAGE>

Description

SPECIFICATION Improvements in or relating to electro-optical systems This invention relates to electro-optical systems. It is increasingly common to use optical fibres as a communications medium. In the normal arrangement the optical fibre transmits signals in the form of variations in light intensity. At the transmitter end the optical signal is produced by a light source driven by an electrical signal. At the receiver end there is a photo-detectorwhich converts the optical signal back to an electrical signal. The object of this invention is to provide novel apparatus allowing conversion from an electrical to an optical signal and/or vice versa.

The invention provides a printed circuit board including a substrate; electrical conductors supported on said substrate; an optical fibre extending within the thickness of the circuit board and having a first end located in the circuit board; and a converter device for converting between optical signals and electrical signals contained within the thickness of the circuit board; said device being optically mated to said first end of the optical fibre and electrically connected to said electrical conductors.

Preferably the board is a multilayer printed circuit board formed by bonding together a plurality of sheets of insulating material each carrying conductive material on at least one surface, the optic fibre and converter device being incorporated in one of the said layers before bonding. It may be incorporated by making a groove or slot in one of the layers.

Preferably the layer in which it is incorporated is not one of the outer layers of the board.

The optic fibre may lie wholly beneath the surface of the board, ending in the plane of an edge-face of the board.

An embodiment of the invention will now be described in greater detail with reference to the accompanying drawings, in which: Figure 1 is a plan view of a printed circuit board and optical fibre terminators; Figure 2 is an exploded view of part of the printed circuit board; Figure 3 is a more detailed view of a part of Figure 2; Figure 4 is a view similar to Figure 3 showing a modified construction of a circuit board; Figure 5 is a view similar to Figure 3 showing another modified construction of a circuit board.

Referring to Figure 1 a printed circuit board 1 is constructed to receive and/or produce optical signals and (when assembled with electronic devices) to process electric signals to or from which the optical signals are converted.

The optical signals are carried along optical fibres gathered into a cable. Two such cables are shown in Figure 1, a cable 2 and cable 3. The cables end in terminator units 4 and 5 respectively mechanically latched to opposite edges of the board 1. In the terminator units 4 and 3 the optical fibres are spread out so that their end faces lie in one or more parallel rows and mated with the end faces of optical fibres 6 (not shown in Figure 1) which lie wholly within the board 1.

Referring to Figure 2, the board in the form of a multilayer printed circuit board is shown exploded. It is formed out of a number of sheets of insulating material 10 to 13 each carrying conductive material on both surfaces, for example, tracks 14 interconnecting pads 15. The sheets 10 to 13 are bonded together by interleaved sheets 16, commonly known as pre-preg, which prior to the bonding operation are in the form of a glass-fibre layer impregnated with partially cured epoxy resin.

The inner sheets 11 and 12 of insulating material incorporate the optical fibres 6. One such fibre is shown in more detail in Figure 3: a slot 17 is made in the sheet concerned, say the sheet 11, and the fibre laid within it. The end of the fibre is optically mated with a converter 18 positioned in the slot 17 and electrically connected by laid-on joints 19 and 20 to the pads 15 of the electrical circuitry. The converter 18 may either produce optical signals from received electrical signals for transmission along the optical fibre 6 and then the appropriate optical cable 2 or 3, or, may respond to optical signal from the fibre 6 to generate an electric signal. The converter may for example be a light-emitting diode or a laser for conversion of electric signals to optical signals.Where it is desired to convert optical signals to electric signals, the converter may for example be a photo-diode.

To manufacture the board the internal sheets 11 and 12 are formed with their conductive patterns using conventional printed circuit techniques, for example a subtractive method such as etching or an additive method such as plating, or a combination of the two. The insulating sheets themselves may be of any suitable material, for example paper fibre, glass epoxy or ceramic. The slots such as the slot 17 are then formed by mechanical routing or chemical milling and the fibres 6 and converters 18 inserted and embedded in epoxy resin 21. The layers are then stacked together with the sheets 16 and bonded. The conductive patterns on surfaces of the sheets 10 to 13 which will be internal in the finished board are formed before bonding and the conductive patterns on outer surfaces of the board may be formed before or after bonding as desired.Inter-layer connections are made, for example, in the known way by platedthrough-holes passing through the pads 15. The plated-through-holes are drilled and plated either through the stack as a whole after bonding or in the internal sheets before bonding.

The optical fibres 6 may be embedded in grooves rather than slots. They may also be inserted in the appropriate sheets before the conductive pattern is applied. The pattern may then be applied by an additive process, or by laminating conductive sheets onto the base material after the optic fibres have been inserted in it. The conductive sheets are then etched. Using these methods a track may cross over an optical fibre inserted in the sheet carrying it.

Inserting the optical fibres in the inner sheets 11 and 12 is a convenient way of ensuring that the optical fibres lie wholly beneath the outer surfaces of the board. However, if desired they may be inserted in the outer sheets, or in the single sheet of a single or double-sided board.

Figure 1 is purely illustrative in showing two optical cables at opposite ends; there may instead, for instance, be only one cable, or more than one but clamped to the same edge. And the end parts of the optic fibres remote from the converters 17 may bend within the board so that the end faces lie in one or other of the major outer surfaces of the board, in which case the terminator unit of the optic cable will be normal to that surface. In another possible arrangement shown in Figure 4 the optical fibres 6 of the board emerge from and extend beyond the board as shown by reference 22 so that they make their optical mating with the fibres of the external cable clear of the board or mate with optical fibres in another board. Optical fibres in a given cable may either all transmit signals in one direction, ortransmit in both directions.

In the general case, optical signals received by the board will be converted to electric signals and processed by electric components mounted on the board. Conversely electric signals will be converted to optical signals and output from the board. For example, signals received from the cable 2 may be regenerated by the electric circuitry and output onto the cable 3. However, the board may be used with one direction of transmission only.

In suitable circumstances, an optical signal may by-pass the electric circuitry, one end being optically mated to an incoming fibre of an optical cable and the other end being optically mated to an outgoing fibre of an optical cable.

Referring to Figure 5 it will be seen that optical fibre 23 of the board is not optically mated to an external cable, but has converters 24,25, mated respectively to each end. The fibre 21 thus transmits optical signals from one piece of electric circuitry on the board to another, functioning as an alternative to a conductive track. A number of signals may be multiplexed along the same fibre, thus allowing a number of conductive tracks to be replaced, simplifying the board and possibly allowing the number or size of the layers to be reduced.

It is likely that the board will have to be electrically connected to external equipment. That may be done in any convenient way, for example by providing an edge region with conductive fingers connected to the circuitry of the board. This edge region is then inserted into an edge connector, or a two-part connector may be used, with one part mounted on the board.

Although not shown in Figure 2, ground or power planes may be present.

The board includes optical fibre runs incorporated during its manufacture. The result may be handled as an ordinary printed circuit, and is both more compact and less expensive than equipment such as has been previously proposed in which special converters for optical signals are mounted on a printed circuit board.

Claims (14)

1. A printed circuit board including a substrate; electrical conductors supported on said substrate; an optical fibre extending within the thickness of the circuit board and having a first end located in the circuit board; and a converter device for converting between optical signals and electrical signals contained within the thickness of the circuit board, said device being optically mated to said first end of the optical fibre and electrically connected to said electrical conductors.

2. A printed circuit board as claimed in Claim 1 in which the optical fibre has a second end, remote from said first end, located in said circuit board.

3. A printed circuit board as claimed in Claim 2 including a second converter device contained within the thickness of the circuit board, said second device being optically mated to said second end of the optical fibre and electrically connected to further electrical conductors supported on said substrate.

4. A printed circuit board as claimed in Claim 2 in which the second end of the optical fibre is located in an edge surface of the circuit board.

5. A printed circuit board as claimed in Claim 2 or 4 provided with means to locate an end of a free optical fibre in optically mating relationship with the second end of the optical fibre extending within the circuit board.

6. A printed circuit board as claimed in Claim 1 in which the optical fibre has a part thereof remote from the first end extending freely beyond the circuit board.

7. A printed circuit board as claimed in any preceding claim including a plurality of electrical conductors arranged in a plurality of layers separated by layers of insulating material.

8. A printed circuit board as claimed in Claim 2 including a plurality of optical fibres each having a second end located in a surface of the board and provided with means to locate corresponding ends of a plurality of free optical fibres in optically mating relationship with the second ends respectively.

9. A method of manufacturing a printed circuit board as claimed in any preceding claim including the step of incorporating a converter device and an optical fibre in optically mating relationship in a first layer and subsequently bonding one or more second layers to the first layer, at least some of said layers carrying electrical conductors thereon.

10. A method of manufacturing a printed circuit board as claimed in Claim 9 including the steps of forming a recess in the first layer to receive the converter device and the optical fibre.

11. A method as claimed in Claim 10 in which the recess is formed as a slot open to opposite faces of the first layer.

12. A method as claimed in Claim 10 in which the recess is closed by subsequent bonding of the second layer or layers to the first layer.

13. A printed circuit board constructed and arranged substantially as hereinbefore described with reference to Figures 1, 2 and 3 or as modified by Figure 4 or Figure 5 of the drawings.

14. A method of manufacturing a printed circuit board substantially as hereinbefore described.