GB2203259A

GB2203259A - Polarisation rotator associated with reflective surface

Info

Publication number: GB2203259A
Application number: GB08704528A
Authority: GB
Inventors: William James Stewart; Colin Edge
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1987-02-26
Filing date: 1987-02-26
Publication date: 1988-10-12
Also published as: JPH01502780A; GB8704528D0; WO1988006745A1; EP0305430A1

Abstract

An optical device wherein rotation of a polarised beam of light can be achieved with passive optical elements. The device comprises a non-reciprocal rotator element before a reflective surface. The light passes in to the rotator and is reflected back again by the reflective surface the beam thus being rotated on each passage to give an accumulative rotation. An assembly including a further partially reflective surface can be constructed which approximates a Fabry-Perot cavity.

Description

An Optical Device 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 objective of the present invention is to provide a device which can substantially achieve an orthogonal polarisation transition in an incident light beam.

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 accomulative.

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 450.

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 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 signals 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 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 reflector1 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 sourcelreceiver 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 polerisation to be rotated by 900 (or it!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 900 rotation is required, in this embodiment a 450 Faraday rotator 5 before mirror 3 is used. The rotator 5 effects a 450 rotation on both forstard and return passses across the YIG crystal 7 consequently as such rotations are non-reciprocal and additive, the rotator 5 gives substantially the 900 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 m, 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 were inserted between the incident polarised light 1 and the rotator 7, and the orthogonality of the returned light was still maintained to better than 20-dB.The rotation produced by the YIG crystal 7 had a wavelength variation of - 5.140/nm therefore an isolation of 20-dB should be possible over a wavelength range of 100nm. 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 systems where a double pass system gives at low power a simple output because of reciporcity but more complex behaviour at higher powers.

The Faraday rotator and the mirror habe both been made from fibre components, so on all-fibre orthoconjugate reflector is possible.

Claims

1. An optical device for polarised light beams comprising a 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 being accumulative and additive to resutland chirality inversion produced by the mirror.

2. An optical device as claimed in claim 1 wherein a partially reflecting surface is located at the opposite end of the rotator element to the reflective surface.

3. An optical device as claimed in claim 1 wherein the rotator element is a Faraday Rotator.

4. An optical device as claimed in any preceding claim wherein an inductive element is perpherial with the Faraday Rotator.

5. An optical device as claimed in claim 3 wherein the Faraday Rotator includes a nG crystal.

6. An optical device as claimed in claim 5 wherein the YIG crystal in the Faraday Rotator has a length substantially of 2.65 mm.

7. An optical device as claimed in claim 4 wherein the inductive element provides a constant magnetic field to the rotator element whereby variation in rotation of the plane of the polarised light beam are thus indicative of variation in external magnetic fields.

8. An optical device as claimed in any preceding claim wherein the rotator element has a rotation of 45".

9. An optical device substantially as hereinbefore described with reference to figure 1.

10. An optical device comprising a rotator element for rotation of polarised light about its axis with first and second ends, said first end being reflective and the second end being partially reflective with the polarised light passing through the partially reflective end before becoming inadent upon the reflective end such that the device constitutes substantially a Fabry-Perot cavity.

11. An optical device for detection of external magnetic field variations wherein operation a rotator element under the influence of an imposed constant magnetic field rotates a polarised light beam in its plane and the degree of rotation being altered by external magnetic field variation.

12. A bidirectional fibre communication system comprising a light source of polarised light signals, a detector, a rotator element and a reflective surface, light signals from the light source in operation being incident upon both the detector and the rotator clement, light signals 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.

13. A bidirectional fibre communications system as claimed in claim 12 wherein modulation of the light signals is achieved by modulating the magnetic field applied to the Faraday rotator.

14. A bidirectional fibre communications system as claimed in claim 12 wherein modulation of the light signals is achieved by an electro-optic modulator between the Faraday rotator and the reflector.

15. A method of rotation of polarised light signals comprising: (i) directing the polarised light signals to a 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.

16. A method of rotation of polarised light signals as claimed in claim 14 wherein the rotator element is a Faraday Rotator.

17. A method of rotation of polarised light signals as claimed in claim 14 wherein the Faraday Rotator includes a YIG crystal.

18. A method of rotation of polarised light signals substantially as hereinbefore described.

19. An optical logic device using a non-linear birefringent element, such as a fibre, followed by a rotator and frflector such as claimed in claim 13 wherein operation light is returned in its orthogonal state at low magnetic field strengths but at a different state at higher magnetic field strengths.