WO1993008494A1

WO1993008494A1 - Opto-electronic combiner

Info

Publication number: WO1993008494A1
Application number: PCT/AU1992/000576
Authority: WO
Inventors: Gavin John Semple
Original assignee: Telstra Corporation Limited
Priority date: 1991-10-24
Filing date: 1992-10-26
Publication date: 1993-04-29

Abstract

An opto-electronic combiner (32) for converting a plurality of discrete optical signals into a combined electrical signal. The ends of a bundle of optical fibres (42a) are arranged such that discrete optical signals emitted therefrom are incident upon discrete areas of a photosensitive surface of a single photodetector (40). The optical signals are converted to electrical signals by the photodetector (40), and an electrical signal comprising a combination of the individual optical signals is available at the electrical output of the photodetector (40).

Description

OPTO ELECTRONIC COMBINER

This invention relates to a opto-electronic combiner, and a telecommunications network including an opto-electronic combiner.

It is widely accepted by telecommunication network providers that the copper cable pairs used to connect customer premises to local exchanges will inevitably be replaced by optical fibres. The advantages of optical fibre have been well documented, and include the wide potential bandwidth available, the small size and weight of the fibres, greater immunity to interference and cross talk, greater signal security, potentially lower transmission losses, better ruggedness and flexibility, and improved system reliability and ease of maintenance. A fuller discussion of these advantages may be found, for example, in the publication "Optical Fiber Communications Principles and Practicέ' by John Senior, published 1985 by Prentice /Hall International Inc., London. Until now, however, high cost levels, particularly those associated with opto-electronic components such as laser diodes used to provide the carrier signals, have provided a barrier to the wide scale application of optical fibre in customer access networks. Consequently, although the ultimate goal is fibre-to-the-home (called "FTTH") increased attention is being given to so called fibre-to-the-curb ("FTTC") and fibre-to-the-pedestal ("FTTP") systems which enable sharing of the cost of opto-electronic components between a number of customers. These systems do not utilise completely optical networks. For example, a hybrid optical system might be utilised, employing, in some parts of the system, communication links where signal information is carried by electrical signals. It is, however, preferable that any system which is not completely optical should not only provide satisfactory service at reasonable cost, but still be readily upgradable later when the cost of relevant optical components needed to fully implement the optical system becomes competitive. This involves that, even if part electronic, the system should still from the beginning be largely of FTTH type.

Principal among the items contributing to the present cost disadvantage of fully optical systems are high power 1300nm laser diodes required to provide the optical carrier signal. These are particularly expensive at this time. On the other hand, less powerful light emitting diodes or laser diodes, such as 850nm LEDs or laser diodes or lower power 1300nm laser diodes are available at relatively low cost. Of course, the cost of a source which is employed at a primary node of a network and which serves many end terminals is less than the cost of the many sources utilised at the end terminals.

This invention is basically directed to the provision of means whereby it is possible to, if so desired, retain a passive optical system from an exchange to customer premises, by use of high power signal sources at the exchanges but which, at the same time, permits the use of relatively low cost or low power sources at the customer premises, for outward transmissions.

In one aspect, the invention provides an opto-electronic combiner comprising a bundle of optical fibres for respective transmission therethrough of optical signals, said fibres terminating at ends which are at or adjacent a single photodetector effective in use to produce electrical signals in correspondence with the optical signals incident thereon, the fibre ends being so disposed relative to the photodetector that areas of the photodetector on which the optical signals from the respective fibre ends in use impinge are substantially discrete.

The invention also provides an opto-electronic combiner comprising a single photodetector for producing electrical signals in correspondence with optical signals incident thereon, and a plurality of optical fibres having ends arranged for respective transmission of discrete optical signals therethrough so as in use to be incident on the photodetector substantially without optical interference between the discrete optical signals.

In one form, end portions of the optical fibres, at which said ends are defined, are mechanically held in adjacent closely spaced positions. The ends may occupy, for example, a substantially circular or rectangular area.

The end portions may be tapered so that the cross-sectional area of the ends of the fibres is less than the cross-sectional area of the fibres at locations 5 away from the end portions.

In an alternative construction, the end portions are fused together in a fashion still inhibiting cross-talk as between optical signals carried by the respective fibres. In this case, the end portions may, such as by a process of 10 hot drawing, be tapered in the direction towards the ends thereof.

There is also provided, in accordance with the invention, a method of forming an opto-electronic combiner comprising forming a bundle of optical fibres and securing end portions thereof in close adjacent relation, cutting said 15 end portions such that the end surfaces of the optical fibres lie in a single transverse plane, and securing an optical sensing surface of a photodetector closely adjacent and substantially parallel to said transverse plane such that optical signals emitted from the ends of the fibres is incident on said sensing surface. 20.

In another aspect, the invention provides an optical communications network having a first terminal coupled by optical communications links to a plurality of second terminals and wherein the optical links include a first optical fibre link for transfer of optical signal from the first terminal to an 5 optical coupler and respective second optical fibre links coupled to the second terminals, the optical coupler being coupled to in use transfer signal from the first link to the second links, the links further including a plurality of third optical fibre links coupling from the second terminals, to an opto-electronic regenerator and a fourth optical fibre link, the opto-electronic regenerator 0 being coupled to receive signal from the third links, to convert the optical signal so received to a corresponding electrical signal, the electrical signal representing the combined input optical signals, and to convert the electrical signal to a combined optical signal for transfer via the fourth link to the first terminal, wherein the opto-electronic regenerator includes an opto-electronic combiner comprising end portions of optical fibres forming said third links, a photodetector, and an optical source, the ends of the fibre portions being arranged so that optical signal therefrom are in use incident on the photodetector at respective substantially discrete areas thereon, the photodetector being arranged to generate said electrical signal pursuant to incidence of the optical signals thereon and the optical source being coupled to in use receive said electrical signal and generate therefrom said combined optical signal for transmission of the combined optical signal along said fourth link to said first terminal.

The invention is further described by way of example only with reference to the accompanying drawings in which :-

Figure 1 is a diagram of a communications network constructed in accordance with the invention;

Figure 2 is a diagrammatic representation of an opto-electronic combiner constructed in accordance with the invention and incorporated into the network of Figure 1; Figure 3 is axial cross-section of another form of opto-electronic combiner constructed in accordance with the invention; Figure 4 is a view on the line A-A in Figure 3; Figure 5 is a diagrammatic representation of a further form of the invention; Figure 6 is an axial cross-section of the embodiment of Figure 5; and

Figure 7 is a view on the line B-B in Figure 6.

Figure 1 shows a first terminal or, in this instance, telephone exchange 10 connected via various optical fibre links to a number of second terminals, in this instance in the form of customer premises 12.

The optical fibre links include a first optical fibre link 14 which provides connection to a passive optical coupler 16, this in turn being connected via a plurality of second optical fibre links 18 to respective ones of the customer premises 12. A plurality of third optical fibre links 20 interconnect respective customer premises 12 to a single opto-electronic combiner /regenerator 22 constructed in accordance with this invention, the output of which combiner /regenerator 22 is interconnected with the exchange 10 via a fourth optical link 24.

The exchange 10 includes a suitable laser source 26 which is interconnected with the link 14 in order to direct optical signals down the link 14 to the optical coupler 16. The information is transmitted down the link 14 by suitable modulation of the source 26 such as to produce digitised or otherwise encoded optical signals which are so passed down the link. The modulation may be effected by a suitable control 30, which may be arranged to time division multiplex input signals so that signals directed down the link 14 are so multiplexed.

Received optical signals from link 24 are manipulated and/or demultiplexed and/or decoded in the exchange 10, in known fashion, for example first being converted to electrical form by a photodetector 35.

The coupler 16 is arranged to split the signal on the link 14 to direct it to the various customer premises 12. The coupler may be a passive coupler arranged to direct signal on the link 14 to all of the premises 12, but it would be possible to arrange the coupler 16 so that signals were selectively directed, such as by a demultiplexing process, to selected ones of the premises 12. Such arrangements may employ known opto-electronic devices, or optical switches or optical wavelength division multiplexers /demultiplexers may be employed. Reference may be had, for example, to the aforementioned publication by Senior, for an explanation of optical switching principles.

The opto-electronic combiner/regenerator 22 comprises an opto¬ electronic combiner 32 coupled to circuitry which generates an electrical output to modulate a laser light source 34. Incoming signals, such as digitised signals on the links 20 from the premises 12 are combined in the combiner to generate the required electrical signal. In one arrangement, the signals on links 20 are digital signals and the timing of arrival of signal packets is regulated by a suitable timing arrangement such that the signals are not simultaneously present on more than one of the links 20 at any one time. That is, optical time division multiplexing is employed. The signals arriving at different times are converted into electrical signals which are manipulated using high gain amplifiers and logic gates. The resulting electrical signals are converted into optical signals which are transmitted on link 24 to the exchange where the signals may, for example, be demultiplexed and/or decoded or otherwise manipulated as required. The.signal may first be converted to an electrical form by a photodetector 35.

The optical signals from the premises 12 are generated by suitable lasers 36, the outputs of which are modulated in known fashion to generate the signals on die links 20. Similarly, at the premises 12, incoming signals from the links 18 are, if necessary, demodulated and then manipulated as required by the customer. For example, a photodetector circuit arrangement 38 may be employed to decode the optical signals and produce electronic signals, such as voice signals, therefrom.

While, in some instances, the optical signals passing from the premises 12 to the combiner 32 will be as above described, that is to say separated in time so that no two arrive at the same instance, in other systems different arrangements may be employed, particularly being ones where there may be signals present at more than one of the links 20 at any one time. An example would be where each laser 36 at the customer premise is modulated at different frequencies in the electrical domain. That is, electrical frequency division multiplexing is employed.

In order that the combiner 32 may be effective to generate satisfactory electrical signals for application to the laser 34 both in the situation where signals may be present only on one link 20 at any one time, (optical time division multiplexing) and where signals may be present on more than one link 20 at once, it is necessary that the combiner to be specifically adapted to deal with the simultaneous presence of such input signals. A particular difficulty with, for example, passive optical networks, arises where optical signals are combined directly. In particular, one usual form of multiplexing for certain types of signal transmissions, such as analogue video transmissions, involves signals separate in the electrical domain modulating optical signals of very nearly the same frequency. In these cases, if the optical signals are directly combined together and then converted back into the electrical domain, optical interference noise will occur. Passive systems are seriously limited in these cases as to the number of input channels which can be handled. On the other hand, it is possible to at least substantially avoid this difficulty, in accordance with the present invention, by forming the combiner 32 such that the optical signals, arising on the different links 20 are combined by a specific mechanism next described.

Turning now to Figure 2, the combiner 32 shown therein includes a large surface photodetector 40 arranged adjacent ends 42a of respective optical fibres 42 which are arranged in a bundle at the regenerator 22 and which comprise respective ones of the links 20. The ends are arranged at end portions 42b of the fibres which are held in closely spaced side-by-side array, by fitting of a ferrule 44 therearound. The photodetector 40 is arranged to be sensitive to light falling on a major surface 40a thereof to generate an electrical signal corresponding thereto. In this case, the ends 42a of the fibres 42 are formed by cutting off of the end portions 42a in a single transverse plane which is parallel to and closely spaced to surface 40a of the photodetector. The spacing of the fibres from each other, and the spacing of the fibre ends from the surface 40a, are arranged to be such that light incident on the surface 40a from the ends 42a, after passing thereto from the fibres 42, falls upon discrete spots on the surface 40a, each corresponding to a respective one of the ends 42a. The fibres may be exactly adjacent each other in touching engagement, or they may be spaced apart somewhat to facilitate this discrete location of the areas on which light falls from die ends 42a. By arranging that light from the ends 42a falls in discrete spots on the surface 40a, overlap on the surface as between light beams from adjacent ends 42a may be eliminated or at least made very slight. By this, the electrical signal produced by the photodetector 40 will be largely free from optical interference noise which would arise if the optical signals were combined together optically, as would occur where more than one of the spots of light from the ends 42a, at which light is incident on the photodetector surface 40a, substantially overlaps. Thus, the electrical signal produced for application to the laser 34 is free, substantially, from this noise and the combined optical signal directed on the link 24 to the exchange 10 from that laser is similarly free from these effects, enabling effective demodulation to be carried out at the exchange 10 or wherever required. The photodetector is connected to a wide band linear amplifier. The electrical signals can be manipulated using filter circuits.

Figure 3 shows a modified form for the optical combiner. Here, the large surface photodetector 40 may be formed in the same way as described in relation to the embodiment of Figure 2, with its surface 40a adjacent ends 42a of the fibres 42, the fibre ends 42a being arranged in a single transverse plane. The arrangement in this case is such that the ferrule 44 has a hollow cylindrical portion 46 which loosely receives the optical fibres 42 which, as is usual, have plastics or other outer buffer coatings 42c. The portions 42b of the fibres are not however coated, the buffer coatings 42c being removed over these portions, the portions 42b being led in closely spaced relationship. They are so led from the portions 46 through an opening 48a in an annular end portion 48 of the ferrule 44 which is sized so as to maintain the fibres in closely adjacent position and in the annular array shown in Figure 4. A tapered portion 50 of the ferrule 44 is positioned between the cylindrical portion 46 and the portion 48 so to provide a tapered lead in from the larger inner diameter of the portion 46 to the smaller inner diameter of the portion 48. The whole assembly may be held together by epoxy resin which can be inserted, for example from the end of portion 46 opposite portion 48, to be positioned in the tapered portion 50 and to pass into the wall opening 48a to occupy the interstices between the side by side fibres 42. The assembly may be easily effected by first stripping the ends 42b of the fibres then positioning unset epoxy resin in the portion 50 of the ferrule and then passing the fibres into portions 46 of the ferrule, through portion 50 and therethrough opening 48a to project from the portion 48, after which, on setting of the epoxy resin, the end surface 44b of the ferrule, opposed to the surface 40a of the detector 40, is finished by a suitable grinding and polishing operation to cause the fibre ends 42a to be flush with the surface 44b. The epoxy resin is shown at 52 in Figures 3 and 4.

In the form of the invention illustrate in Figures 5, 6 and 7, the end portions 42b are reduced in diameter by causing them to be tapered towards the ends 42a, such as by a suitable etching process. Alternatively, the bundle of fibres is heated, with the ends 42b adjacent to each other, and longitudinal tension is applied so as to cause the fibres to partially fuse together at the sides of portions 42b which are adjacent and, at the same time, reducing the overall diameter of the bundle of fibres (and of the fibre end portions themselves). In such an arrangement, it is possible to so have the fibres partially fused together that they occupy, over the combined cross-section at the ends 42a thereof, a substantially reduced diameter compared with the diameter which would otherwise be occupied by the side by side bundle of fibres, while still providing sufficient optical integrity as concerns the signals applied on the individual fibres to the ends 42a. That is to say, by first effecting the fusing, it is possible to select a location for transverse cutting off of the ends 42 where the fused fibres exhibit substantially reduced cross- sectional areas but such that the fibres are still sufficiently discrete that leakage of light from one such adjacent fibre to the other can be kept to a level sufficiently small as to not interfere with operation.

In the described network of Figure 1, it is not necessary to use high powered laser sources at the customer premises 12, for outward signal transmission, since the regenerator 22 may be positioned locally relative to a group of the premises 12, whereby the signal paths from the premises to the

SUBSTITUTE SHEET regenerator are short, and signal attenuation in the links 20 will be practically negligible. Thus, the lasers 36 may be, for example, 850nm laser diodes. They may even be replaced by light emitting diodes. The laser light source 34 may be a low power laser diode or a high power 1300nm laser diode, depending upon the length and signal attenuation of the link 24. However, since in a practical network relatively few regenerators 22 would be needed, the higher costs associated with use of more powerful lasers in the regenerators 22 will not be so unattractive. Similarly, if more powerful lasers are used at the exchange 10, for source 26, the cost penalty so incurred will likewise be less critical, since there are considerably fewer of these than there are customer premises 12.

Claims

CLAIMS:

1. An opto-electronic combiner comprising a bundle of optical fibres for respective transmission therethrough of optical signals, said fibres terminating at ends which are at or adjacent a single photodetector effective in use to produce electrical signals in correspondence with the optical signals incident thereon, the fibre ends being so disposed relative to the photodetector that areas of the photodetector on which the optical signals from the respective fibre ends in use impinge are substantially discrete.

2. An opto-electronic combiner comprising a single photodetector for producing electrical signals in correspondence with optical signals incident thereon, and a plurality of optical fibres having ends arranged for respective transmission of discrete optical signals therethrough so as in use to be incident on the photodetector substantially without optical interference between the discrete optical signals.

3. An opto-electronic combiner according to claim 2 wherein the ends of the optical fibres are arranged such that respective optical signals emitted therefrom, when incident on said photodetector, are spatially displaced from one another.

4. An opto-electronic combiner according to any preceding claim in which end portions of the optical fibres, at which said ends are defined, are disposed in adjacent closely spaced positions.

5. An opto-electronic combiner according to any one of claims 1 to 3, in which end portions of the optical fibres, at which said ends are defined, are at least partially fused together.

6. An opto-electronic combiner according to claim 5 wherein the cladding of the fibre end portions are fused such that the discrete nature of the optical signals in use carried by the respective fibres is maintained.

SUBSTITUTE SHEET

7. An opto-electronic combiner according to claim 6 wherein the collective cross-sectional area of the optical fibres at the ends thereof is less than the sum of the individual fibre cross-sectional areas.

8. An opto-electronic combiner wherein the fibre end portions are secured within a ferrule by a resin, the ends of the fibres being formed flush with an end surface of the ferrule.

9. A method of forming an opto-electronic combiner comprising forming a bundle of optical fibres and securing end portions thereof in close adjacent relation, cutting said end portions such that the end surfaces of the optical fibres lie in a single transverse plane, and securing an optical sensing surface of a photodetector closely adjacent and substantially parallel to said transverse plane such that optical signals emitted from the ends of the fibres is incident on said sensing surface.

10. A method according to claim 9 wherein the fibre end portions are secured within a ferrule by a resin, the ends of the fibres being formed flush with an end surface of the ferrule.

11. A method according to claim 9 or 10 including a step of heating the fibre end portions while adjacent to each other and applying a longitudinal tension so as to cause the fibre end portions at least partially fuse together and so as to reduce the overall diameter of the bundle of fibres at the end portions thereof.

12. A method according to claim 11 wherein the end portions are cut in a transverse plane in which the fibres are partially fused but whilst substantially inhibiting cross-talk as between optical signals transmitted through individual fibres in the bundle.

13. An optical communications network having a first terminal coupled by optical communications links to a plurality of second terminals and wherein the optical links include a first optical fibre link for transfer of optical signal from the first terminal to an optical coupler and respective second optical fibre links coupled to the second terminals, the optical coupler being coupled to in use transfer signal from the first link to the second links, the links further including a plurality of third optical fibre links coupling from the second terminals, to an opto-electronic regenerator and a fourth optical fibre link, the opto-electronic regenerator being coupled to receive signal from the third links, to convert the optical signal so received to a corresponding electrical signal, the electrical signal representing the combined input optical signals, and to convert the electrical signal to a combined optical signal for transfer via the fourth link to the first terminal, wherein the opto-electronic regenerator includes an opto-electronic combiner comprising end portions of optical fibres forming said third links, a photodetector, and an optical source, the ends of the fibre portions being arranged so that optical signal therefrom are in use incident on the photodetector at respective substantially discrete areas thereon, the photodetector being arranged to generate said electrical signal pursuant to incidence of the optical signals thereon and the optical source being coupled to in use receive said electrical signal and generate therefrom said combined optical signal for transmission of the combined optical signal along said fourth link to said first terminal.

14. An optical communications network substantially as hereinbefore described with reference to the drawings.

15. An opto-electronic combiner substantially as hereinbefore described with reference to the drawings.