GB2046470A - Optical interferometers - Google Patents

Description

SPECIFICATION Optical interferometers The invention relates to optical interferometers of the type comprising an optical single mode waveguide which is wound to form a coil.

Ring Interferometers of this type, often described as ring-interferometers, are based on the Sagnac effect, which produces non-reciprocal transit time differences in the coil when the arrangement rotates relative to a reference system, which differences can be established by interferometer analysis. Atypical application of a ring interferometer of this kind is in the field of navigation.

It has been observed that the bi-refringent properties of an optical single mode waveguide can lead to difficulties. In order to eliminate these difficulties, it has been proposed that polarisation filters whose polarisation planes are parallel to one another be employed between the ends of the single mode light waveguide and receiving photo-detectors, as is described for example in our co-pending United Kingdom Patent Applications Nos. 79.03274 (Specifi- cation No. 2,014,329A); No.79.03275 (Specification No. 2,013,929A); and No.79.11687 (Specification No.

2,01 8,456A).

One object of the present invention is to provide a ring interferometer of the type referred to in the introduction which does not require polarisation filters of this kind, but in which ensures that the birefringence properties of optical waveguides do not lead to difficulties.

The invention consists in an optical interferometer comprising a single mode light waveguide which is wound to form a coil, said single mode light waveguide consisting of one or more waveguides each capable of conducting only one polarisation state, and/or utilising optical directional couplers which can couple only one polarisation state to couple associated waveguides.

This proposal, which appears surprisingly simple, has been found to promptly and readily eliminate the difficulties which can be caused due to birefringence.

An optical waveguide which is only capable of conducting one polarisation state can consist of a fibre or a strip conductor. Optical waveguides of this kind, which are capable of assuming only one polarisation state, are disclosed in an article published in Appl. Phys. Lett. (1978) on pages 814-816, and in an article published in Appl. Phys.

Lett. 33(1978) on pages 699-701, which maintain a linear polarisation.

The proposed arrangement of the present invention can be applied to all the above-mentioned ring interferometers, irrespective of their construction, and may be utilised in still other ring interferometers to advantage.

In embodiments in which the light is fed to the coil or discharged therefrom in free beam paths, it is essential that the light waveguide of the coil should only conduct one polarisation state. This is the case for example in our co-pending Patent Application No.79.30275 (Specification No. 2,013,929A); and in Application No. 79.11687 (Specification No.

2,018,456A), see Figures 1 to 3 therein.

The invention will now be described in detail as applied to the known types described in the copending Applications listed above.

If the light in an optical circuit is fed to the coil of an interferometer, and/or discharged therefrom via an additional Iightwaveguide, it is expedient that these additionally provided light waveguides should be able to conduct only one polarisation state. An optical interferometer to which this comment applies is described and illustrated for example, in the exemplary embodiment shown in Figure 1 of our co-pending Patent Application No. 79.03274 (Specification No.2,01 4,329A) or in Figure 4 of the two other Patent Applications listed above.

This circuit also contains optical directional couplers. It is expedient if these optical directional couplers are also only able to couple one polarisation state.

The three alternative possibilities can be employed individually, or in any arbitrary combination.

It will be obvious that the or each lightwaveguide which can conduct only one polarisation state, or the or each directional coupler which can couple only one polarisation state, should be so used in the construction of the ring interferometer that they achieve the same effect as that achieved by polarisation filters in the constructions described in the aforementioned Patent Applications.In the case of our Patent Application No. 79.03274 (Specification No. 2,014,329) this effect is safely achieved if both of the existing directional couplers can couple only one and the same polarisation state, and/or if ail the light waveguides of the optical circuit which are provided in addition to the coil, and from which the directional couplers are also constructed, can conduct only one and the same polarisation state. The waveguide of the coil itself can also consist of such an optical waveguide.If the waveguides of the optical circuit are conventional waveguides and their structure is provided for example with small curvatures and a short length, so that they do not involve any fundamentally disturbing effects, it is sufficient if only the light waveguide of the coil is a light waveguide which can conduct only one polarisation state.

1. An optical interferometer comprising a single mode light waveguide which is wound to form a coil, said single mode lightwaveguide consisting of one or more waveguides each capable of conducting only one polarisation state, and/or utilising optical directional couplers which can couple only one polarisation state to couple associated waveguides.

2. An optical interferometer as claimed in Claim 1, in which an additional optical single-mode waveguide capable of conducting only one polarisation state is provided, coupled to the existing waveguides of an optical interferometer in place of another type of polarisation filter.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Optical interferometers The invention relates to optical interferometers of the type comprising an optical single mode waveguide which is wound to form a coil. Ring Interferometers of this type, often described as ring-interferometers, are based on the Sagnac effect, which produces non-reciprocal transit time differences in the coil when the arrangement rotates relative to a reference system, which differences can be established by interferometer analysis. Atypical application of a ring interferometer of this kind is in the field of navigation. It has been observed that the bi-refringent properties of an optical single mode waveguide can lead to difficulties. In order to eliminate these difficulties, it has been proposed that polarisation filters whose polarisation planes are parallel to one another be employed between the ends of the single mode light waveguide and receiving photo-detectors, as is described for example in our co-pending United Kingdom Patent Applications Nos. 79.03274 (Specifi- cation No. 2,014,329A); No.79.03275 (Specification No. 2,013,929A); and No.79.11687 (Specification No. 2,01 8,456A). One object of the present invention is to provide a ring interferometer of the type referred to in the introduction which does not require polarisation filters of this kind, but in which ensures that the birefringence properties of optical waveguides do not lead to difficulties. The invention consists in an optical interferometer comprising a single mode light waveguide which is wound to form a coil, said single mode light waveguide consisting of one or more waveguides each capable of conducting only one polarisation state, and/or utilising optical directional couplers which can couple only one polarisation state to couple associated waveguides. This proposal, which appears surprisingly simple, has been found to promptly and readily eliminate the difficulties which can be caused due to birefringence. An optical waveguide which is only capable of conducting one polarisation state can consist of a fibre or a strip conductor. Optical waveguides of this kind, which are capable of assuming only one polarisation state, are disclosed in an article published in Appl. Phys. Lett. (1978) on pages 814-816, and in an article published in Appl. Phys. Lett. 33(1978) on pages 699-701, which maintain a linear polarisation. The proposed arrangement of the present invention can be applied to all the above-mentioned ring interferometers, irrespective of their construction, and may be utilised in still other ring interferometers to advantage. In embodiments in which the light is fed to the coil or discharged therefrom in free beam paths, it is essential that the light waveguide of the coil should only conduct one polarisation state. This is the case for example in our co-pending Patent Application No.79.30275 (Specification No. 2,013,929A); and in Application No. 79.11687 (Specification No. 2,018,456A), see Figures 1 to 3 therein. The invention will now be described in detail as applied to the known types described in the copending Applications listed above. If the light in an optical circuit is fed to the coil of an interferometer, and/or discharged therefrom via an additional Iightwaveguide, it is expedient that these additionally provided light waveguides should be able to conduct only one polarisation state. An optical interferometer to which this comment applies is described and illustrated for example, in the exemplary embodiment shown in Figure 1 of our co-pending Patent Application No. 79.03274 (Specification No.2,01 4,329A) or in Figure 4 of the two other Patent Applications listed above. This circuit also contains optical directional couplers. It is expedient if these optical directional couplers are also only able to couple one polarisation state. The three alternative possibilities can be employed individually, or in any arbitrary combination. It will be obvious that the or each lightwaveguide which can conduct only one polarisation state, or the or each directional coupler which can couple only one polarisation state, should be so used in the construction of the ring interferometer that they achieve the same effect as that achieved by polarisation filters in the constructions described in the aforementioned Patent Applications.In the case of our Patent Application No. 79.03274 (Specification No. 2,014,329) this effect is safely achieved if both of the existing directional couplers can couple only one and the same polarisation state, and/or if ail the light waveguides of the optical circuit which are provided in addition to the coil, and from which the directional couplers are also constructed, can conduct only one and the same polarisation state. The waveguide of the coil itself can also consist of such an optical waveguide.If the waveguides of the optical circuit are conventional waveguides and their structure is provided for example with small curvatures and a short length, so that they do not involve any fundamentally disturbing effects, it is sufficient if only the light waveguide of the coil is a light waveguide which can conduct only one polarisation state. CLAIMS

1. An optical interferometer comprising a single mode light waveguide which is wound to form a coil, said single mode lightwaveguide consisting of one or more waveguides each capable of conducting only one polarisation state, and/or utilising optical directional couplers which can couple only one polarisation state to couple associated waveguides.

2. An optical interferometer as claimed in Claim 1, in which an additional optical single-mode waveguide capable of conducting only one polarisation state is provided, coupled to the existing waveguides of an optical interferometer in place of another type of polarisation filter.

3. An optical interferometer as claimed in Claim 1, substantially as described hereinbefore.