Optical waveguides – With optical coupler – Particular coupling function
Patent
1992-02-28
1995-09-05
Bovernick, Rodney B.
Optical waveguides
With optical coupler
Particular coupling function
385 47, G02B 626, G02B 642
Patent
active
054486637
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an optical coupler for a communication network.
A multipoint communication network, such as a local exchange for telephony, has conventionally used a copper wire central exchange-based switching network and electronic or mechanical switching. Following the development of optical fibres, optical networks have been developed. The advantage of an optical fibre-based network is that it can support considerably more simultaneous communications and/or communications of a considerably greater bandwidth over the same optical fibre.
Current passive optical networks (PONs) are based on conventional centralised switching techniques, using electronic switching devices. Their reliance on electronic switching in an otherwise optical fibre-based network band limits the paths in the network to the capacity of the switches, creating an `electronic bottleneck`. Typically, current generations of electronic switches can support 64 kbit/s throughput. This is sufficient for a small number of simultaneous voice channels but it is inadequate for two-way switching of video as used, for example, in video conferencing. It is also inadequate for supporting broad band broadcast communications, such as high definition television.
Distributed switching is known in which the paths in a network are connected to nodal connectors. The network is broadcast based, each customer being able to receive the output of the other customer terminals.
However, these are currently limited to electronic networks because of the potential losses associated with the optical connectors which would be required at the network nodes.
U.S. Pat. No. 4,787,693 discloses a passive star optical coupler for a broadcast type local area network (LAN) in which a message transmitted by one user can be received by all others. The basic coupler is a reflective star arrangement in which one port of a transmissive star is terminated in a reflective surface in order to produce a coupler which is transparent to all terminals connected to the remaining ports.
There are, of course, various ways in which a reflective star coupler can be constructed which will be apparent to the skilled person. Another simple reflective coupler arrangement for a serial data bus is disclosed in U.S. Pat. No. 4,457,581.
However, as mentioned above, the main problem to be overcome is that of the losses in the coupler which can lead to a degradation in the signal passing through it. It is this which limits the applicability of optical fibre couplers to network applications in which a considerable number of couplers are required.
According to the present invention there is provided an optical coupler comprising a plurality of light transmissive elements communicatively coupled at a coupling point whereby in use light transmitted in each of the elements is coupled into each other of the elements, one of the elements being provided with light retransmissive means for reapplying light leaving the coupler in that element back to the coupling point, characterised in that the said one element is also provided with signal conditioning means in the path of the light transmitted along the one element.
Thus, the coupler is transparent to signals in the various elements, but the losses associated with the coupler can now be counteracted by the signal conditioning element. Alternatively, the signal conditioning element may be used to render the coupler transparent only at predetermined wavelengths or ranges of wavelengths.
In one form, the signal conditioning means are an amplifier. The signal conditioning means may also be a filter or a filter/amplifier combination.
When a filter is used it may be frequency domain or time domain selective.
The invention also extends to an optical switching network, for example, a passive optical network incorporating couplers according to the invention. The network may be a distributed switching network in which at least some of the nodes are constituted by the couplers.
According to another aspect of the invention there is provided an optical signal co
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Faulkner David W.
Smith Philip J.
Bovernick Rodney B.
British Telecommunications public limited company
Wise Robert E.
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