GB2199713A

GB2199713A - Optical communication system

Info

Publication number: GB2199713A
Application number: GB8631047A
Authority: GB
Inventors: Isobel Jennifer Goddard
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1986-12-31
Filing date: 1986-12-31
Publication date: 1988-07-13
Also published as: GB8631047D0

Abstract

A difficulty in optical systems, especially where polarisation-dependent (PD) elements are used, is that the light to be dealt with is subject to fading. To minimise this, the input beam, e.g. from an optical fibre, is applied to a polarisation beam splitter (1), which resolves it into two orthogonally-related beams (2, 3). These are applied to PD elements (4, 5) and therefrom to two detectors (6, 7), or to one detector (6). In an alternative arrangement, only one PD element is used. Such an arrangement, in its "all fibre" version is usable at the input of a node of an optical fibre LAN. <IMAGE>

Description

OPTICAL COMMUNICATION SYSTEM This invention relates to optical communication systems1 and especially to the avoidance of polarisation fading in such systems.

In many optical systems, sub-systems or components, performance is affected by the state of polarisation (SOP) of an optical input. Where this applies, a technique is needed to limit the SOP of the optical input to a polarisation-dependent (PD) element to a polarisation which is constant in time and of the preferred type.

If the SOP of the incoming light is constant in time, but not of the preferred type, a simple polariser or retarder can be used to convert it to the preferred type. However, there are situations where the incoming SOP changes with time, e.g. the output of an optical fibre, and using a polariser would cause fading.

An object of the invention is to provide a method of minimising or even avoiding fading in such arrangements.

According to the invention, there is provided a method of minimising the effects of fading in an optical communication system, in which the light to be handled is resolved into two beams whose polarisations are orthogonal, in which the two beams thus produced are passed via polarisation-dependent means, and in which both of the beams are applied to detection means after their passage through the polarisation-dependent means.

An important application of such an arrangement is at the node of an optical fibre local area network, where the light arriving from the fibre network is demultiplexed into its constitutent channels each carried on a different wavelength, and either retained at the node or passed on along the bus.

Two embodiments of the invention will now be described with reference to Figs. 1 and 2 of the accompanying highly schematic drawing.

The basis of the method used in both embodiments is to resolve the light into two beams whose SOPs are orthogonal, pass each such beam through the PD element used, and detect in at least one of the two configurations. The PD elements may be of the multi-layer transmissive filter type which, when at an angle to the incident light has a different transmission characteristic for the two polarisations. One form of such an element is two stacks of reflective material with a Fabry-Perot etalon between them.

In the arrangement of Fig. 1, the light input, which may arrive via an optical fibre or from free space, is applied to a polarisation beam splitter 1, which produces output beams 2,3, whose SOPs are orthogonal.

Such a beam splitter can exploit the birefringent properties of certain materials. The outputs from the beam splitters, which may be via optical fibres, are applied to PD elements 4,5, through which they pass to detectors 6,7 respectively. Again the outputs from the PD elements may be via optical fibres.

The outputs from the detectors 6,7 are added electrically to provide the output of the arrangement.

However, in some cases one output may be used. Note that, as indicated by the broken line from the PD element 5 to the detector 6, both of the orthogonal beams may be applied to the same detector.

In the other arrangement, Fig. 2, the input passes, as from an optical fibre, via the beam splitter which gives the two orthogonal beams, as in Fig. 1.

However, in this case one beam has. its SOP rotated through 900 physically or with a half-wave plate, indicated at 8, to give the same SOP as the other beam.

The two beams are then passed through the same PD element 9, which may be an optical filter. The two beams pass through the element 9 and arrive at the detector at spatially-separated points, so that the beams do not interfere with each other. As indicated, the beams, which leave the PD element 9 separately, may be detected on different detectors.

The resolution of the incoming light into orthogonal SOPs is effected using a polarisation beam splitter, which may be in bulk optics, or made from optical fibres. The pass-band width of a multi-layer filter varies with SOP, being greater for light polarised parallel to the plane of incidence (P-polarisation) then for light polarised perpendicular to tht plane (S-polarisation). If the incoming light is polarised into S- and P- polarisations, and the latter rotated by 0 90 , both beams can pass through the filter, and both experience the narrow pass-band width. It is necessary to use both polarisations to ensure that the total effectively realised optical power is proportional to the incoming optical power.

As already indicated, one important application of an arrangement such as described above is as part of the receiving part of a node in an optical fibre local area network.

Claims

1. A method of minimising the effects of fading in an optical communication system, in which the light to be handled is resolved into two beams whose polarisations are orthogonal, in which the two beams thus produced are passed via polarisation-dependent means, and in which both of the beams are applied to detection means after their passage through the polarisation-dependent means.

2. An optical communications arrangement, which includes an input over which a light beam to be handled is received, a polarisation beam splitter to which said beam is applied and which derives therefrom two beams whose polarisations are orthogonal, polarisation-dependent means to which said beams are applied from the splitter, and detection means to which one or both of the beams are applied after the passage through the polarisation-dependent means.

3. An arrangement as claimed in claim 2, in which the polarisation-dependent means includes two polarisations-dependent elements, one per beam.

4. An arrangement as claimed in claim 2, in which the polarisation-dependent means includes a single polarisation-dependent element, the two beams passing therethrough arrive at the detection means at spatially-separated points.

5. An arrangement as claimed in claim 3 or 4, in which the detection means includes two detectors, one per beam.

6. An arrangement as claimed in claim 3 or 4, in which the detection means includes one detector to which both said beams are applied, spatially separated.

7. An optical communication arrangement, substantially as described with reference to Fig. 1 or Fig. 2 of the accompanying drawings.