GB2211956A - Optical switch - Google Patents

Abstract

An optical switch comprises a pair of single mode optical waveguides coupled together at two spaced locations to form couplers to mix equally light from each waveguide, the arms of the waveguides between the couplers being of substantially equal length, so as to form a Mach Zehnder interferometer 4; a polarization-dependent phase shifter 5 associated with one of the arms; and a polarization-independent phase shifter associated with the other arm. The phase shifter 5 comprises a piezo-electric devices which in use compress the optical fibre in one direction only. <IMAGE>

Description

Optical Switch This invention relates to optical switches of the type which comprise a pair of single mode optical waveguides, preferably fibres, which are coupled at two spaced locations, known as couplers, so as to mix equally light from each waveguide such that the two arms between the couplers are of substantially equal length, forming a Mach Zehnder interferometer. In particular the invention relates to a polarisation splitter/switch.

Generally, all optical fibre polarisation splitters are formed by fusing and tapering two single mode fibres side by side.

The tapering process is continued through the duration of many more power transfer cycles between the coupler arms than for a standard 3 dB coupler, until a further modulation is superimposed onto these power oscillations. Usually, polarisation splitters are formed at the first minimum of this superimposed modulation. As a result of this lengthened tapering, the performance of the splitters formed is in general highly dependent on the wavelength of the light to be analysed.

It is an object of this invention to provide an optical polarisation splitter which is switchable and can also be used over a wider range of wavelengths than conventional polarisation splitters.

According to the invention, an optical switch comprises a Mach Zehnder interferometer, as hereinbefore described, wherein a phase shifter is mounted on each of the interferometer arms, one of said shifters being polarisation-dependent, the other being pol ari sati on-Independent.

There is no longer any need for a lengthy interaction region, and each coupler is preferably a standard 3 dB coupler, so that wavelength dependence is substantially reduced.

In this way, it is possible to vary the optical power of light of either linear polarisation emerging from the switch.

Conveniently, where the waveguides are optical fibres, transducers in the form of piezo-electric squeezers are used to compress the optical fibre forming a polarisation-dependent phase shifter in which the relative phase difference between orthogonal polarisations may be electrically varied.

Each transducer acts on one light path so that the phase relationship between light of one polarisation in one lightpath may be altered relative to light of the same polarisation in the other light path. Light of the orthogonal polarisation is not affected.

To reduce the wavelength dependence of the coupling ratio still further, where the waveguides are fibres, one fibre at each coupler is pretapered to alter the propagation constant of the light guided by the fibre in the pretapered region.

The invention will now be described by way of example only, with reference to the accompanying drawings of which Figure 1 is a perspective view of a polarisation-dependent phase shifter for an optical fibre; and Figure 2 is a plan view of a fibre-optic Mach Zehnder interferometer incorporating the phase shifter of Figure 1.

As shown in Figure 1 a change in the phase of light in the optical fibre 1 is induced by compressing the fibre using piezo-electric squeezers 2 mounted on a U-shaped block 3. A bias force adjustment screw 7 is used to adjust the fibre compression.

The phase difference between light polarized in the x direction and light polarised in the y direction is induced in light travelling along the z-axis if the force exerted by the squeezers 2 is along the x-axis. A pair of insulating clamps 8 with inset electrodes is mounted on the block 3.

The interferometer 4 is formed in the normal way by forming two standard single mode optical fibre 3 dB couplers in series on a pair of single mode fibres using the fused biconical technique.

Each of the two arms of the interferometer 4 has a phase shifter 5, 6 mounted thereon, as shown in Figure 2. One of the phase shifters is a standard non-birefringent phase shifter 6, but the other is constituted by a piezo-electric squeezer 5 which induces a phase shift in light of one polarisation relative to light of the orthogonal polarisation.

Thus a switchable polarisation splitter is formed, which may be used to maintain either of two constant specified states of linear polarisation at both outputs.

An active device which may offer better long term stability of operation may be achieved by monitoring the state of polarisation at one output and using a feed back control system to optimise the phase shifts applied by the two transducers. This may be realised by dithering the control signals to the two transducers and detecting improvement in the polarisation match at the output.

It is envisaged that an integrated optic implementation of the invention would also be feasible, in which the phase shifters take one of the forms conventional in integrated optics.

Claims (5)

1. An optical switch comprising : a pair of single mode optical waveguides coupled together at two spaced locations to form couplers to mix equally light from each waveguide, the arms of the waveguides between the couplers being of substantially equal length, so as to form a Mach Zehnder interferometer; a polarization-dependent phase shifter associated with one of the arms; and a polarization-independent phase shifter associated with the other arm.

2. A switch according to Claim 1, in which each coupler is a standard 3 dB coupler.

3. A switch according to Claim 1 or 2, in which each waveguide is an optical fibre, the fibres being fused at the couplers.

4. A switch according to Claim 3, in which one of the fibres is pretapered at each coupler.

5. A switch substantially as described herein with reference to the accompanying drawings.