The present invention relates to an optical device and more particularly but not exclusively to a device to be used in a bi¬ directional fibre communication system.

In bi-directional fibre communication systems a convenient way of separating incident and returned signals is to ensure the signals are in respective orthogonal polarisation states. An orthoganal polarisation relationship ensures minimum interaction between signals.

Previously, provision of signals in an orthoganal polarisation relationship has involved reciprical rotators or retarders such as a -/4 plate. A A/4 plate acts to retard light rays in respective ordinary and extra-ordinary planes, the degree of retardation in respective ordinary and extra-ordinary planes differing by a quarter period. A reciprical rotator provides a positive rotation for one traversal direction and a reciprical negative rotation for the reverse traversal with a consequent nett nil effect. This presents problems when rotation is required in single fibre communication systems ,as usually ""double pass routing" of signals thorough the reciprical rotator is necessary. Furthermore, to achieve circularly polarised light the A/4 plate must be set at 45° to an incident light beam.

An objective of the present invention is to provide a device which can substantially achieve an orthogonal polarisation transition in an incident light beam without " double pass routing".

According to the present invention there is provided an optical device for polarised light beams comprising a non-reciprical rotator

element and a reflective surface, the reflective surface being arranged relative to the rotator element for reflecting light beams incident from the rotator element back to the rotator element whereby _* in operation, light beams incident upon the rotator element traverse through the rotator element at least twice with consequential axial rotation of the polarised light beam plane on each traversal 1 being accumulative and additive to resultant chirality inversion produced by the mirror.

Preferably, the rotator element is a Faraday Rotator which may include a YIG crystal and inductive means.

The rotator element may have a rotation of 45°.

The device may be adapted to form a Fabry-Perot resonant cavity.

Also disclosed is a bidirectional fibre communication system comprising a light source of polarised light signals, a detector, a non- reciprical rotator element and a reflective surface, light signals from the light source being, in operation incident upon both the detector and the rotator element, light sign?ls incident upon the rotator element being rotated in the rotator element in the plane of the beam both on traversal to the reflective surface and reflection back through the rotator element to become incident upon the detector whereby light signals received at the detector directly from the light source can be distinguished from those light signals received via the rotator element by their respective polarisation states.

A method of rotation of polarised light signals comprising: - (i) directing the polarised light signals to a non-reciprical rotator element;

(ii) causing the polarised light signals to traverse the rotator element to become incident upon a reflective surface; (iii) the reflective surface reflecting the polarised light signals back through the rotator element whereby the polarised light signals are rotated in the plane of the signals by the rotator element said rotations being accumulative.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which:-

Figure 1 is a schematic diagram of an embodiment of the present invention; and,

Figure 2 illustrates orthogonal polarisation of linear, circular and elliptical light beams at positions A. B. C and D in figure 1.

A conventional mirror reflects light incident upon it in such a way that superposes the onward light field that would have existed without the mirror backwards towards the source. A more complex device known as a 'phase conjugate reflector' can be constructed that returns a light field such that each source point has its light returned to it. This function requires a non-linear transform (i.e. active adaptive component) and may be initialised to remove 'turbulence' in a light beam. Most of these devices work for single input/output polarisations but general devices have been reported.

With single mode fibre spatial reconstruction is not required as only a single source/receiver point exists. It can be shown that the conjugate reflector function (i.e. returning always a polarisation identical to the input) is still non-linear and thus active. However,

the 'orthoconjugate' function that returns always the orthogonal polarisation can be made from passive parts.

To achieve the orthogonal state of a light beam requires its polarisation to be rotated by 90° (or π/2) and its chirality to be reversed (i.e. left to right and vice versa).

Consider Figure 1, chirality reversal occurs when light 1 is reflected from a plane mirror 3, converting circularly polarised light into its orthogonal state but leaving linear states unchanged. As stated previously to produce an orthogonal linear state a 90° rotation is required, in this embodiment a 45° Faraday rotator 5 before mirror 3 is used. The rotator 5 effects a 45° rotation on both forward and return passses across the YIG crystal 7 consequently as such rotations are non-reciprocal and additive, the rotator 5 gives substantially the 90° rotation required. An arrangement such as that shown in Figure 1 will therefore retro-reflect the orthogonal polarisation state of any incident polarised light as shown in Figure 2 for linear, circular and elliptical polarisation states.

A typical device would be a 2.65 mm length of YIG crystal 7 as the Faraday rotator 5 and an aluminised mirror 3. Incident linearly polarised light at the YIG crystals 7 design wavelength of 1.55 was found to be returned in the orthogonal state with an extinction of better than 20-dB (measurement limited). A variety of reciprocal birefringent media and a 2 metre length of single mode fibre have been inserted between the incident polarised light 1 and the rotator 7, and the orthogonality of the returned light is still maintained to better than 20-dB. The rotation produced by the YIG crystal 7 had a wavelength variation of 5.14°/nm therefore an isolation of 20-dB

should be possible over a wavelength range of lOOnm. Other Faraday rotators 5 are, however, available which have a smaller wavelength dependence and could be used to produce broadband orthoconjugate reflectors 5.

A potential application of such a device would be in fibre bidirectional systems and measurements. The device can be used to separate the upstream and downstream signals into orthogonal states, but by varying the Faraday rotator polarisation modulation of the returned signal can be achieved. A broadband orthoconjugate reflector would cancel, on the returned signal, any spectral variations in polarisation caused by the fibre and hence would effectively restore any depolarisation effects. Localised non-reciprocities such as might be produced by external magnetic fields, would • disrupt the orthogonality of the returned state and hence a transducer or detector instrument may be constructed.

A Fabry-Perot cavity can also be formed by use of one or two such devices as reflectors having simple, regularly spaced modes even with random birefringment elements in the cavity which would otherwise disrupt them (because light would return with altered polarisation from a round trip). This has fibre resonator applications including in fibre lasers.

An alternative use may be in a non linear fibre logic system where a double pass system gives, at low power, a simple output because of reciprocity but more complex behaviour at higher powers. The Faraday rotator and the mirror have both been made from fibre components, so on all-fibre orthoconjugate reflector is possible.