Optical Fiber Multiport Apparatus for Detecting Phase Shifts

This invention relates to optical apparatus and, in particular, to optical fibre multiports for use in coherent detection.

Coherent detection is used in optical fibre gyroscopes to measure signal phase shift, in long haul optical fibre communication to increase detector sensitivity and in wavelength multiplexed schemes to provide wavelength selective detection. In each of these applications the polarisation state of the signal must be matched to that of the reference <or local oscillator) at the photodetector. However, in conventional monomode optical fibres the polarisation state of the signal can change continuously--in order to realign signal polarisation state one needs polarisation controllers which can be expensive, bulky and unreliable.

Optical fibre multiports can be used to detect the complete envelope of signal phase or in long haul communications to provide "phase diversity"--coherent detection of the signal which is independent of fluctuation is signal phase. Such multiports are difficult to make because they require several fibres to be fused with precise symmetry. A three fibre device has been described by Davis A. W. and Wright S. ("A phase sensitive homodyne optical receiver" IEE Colloquium Digest No 185/30 Paper 11), but while the properties required for a four-fibre device have been Know for some time it is only recently that such a device has been successfully made. Even now the necessary control of the extra fourth mode in the four fibre device is difficult.

It is possible to imagine that a "full diversity", i.e. coherent detection of a signal independent of either its phase or its polarisation state might be provided by arrays of multiports. For example, a polarisation splitter could be used to separate two orthogonally polarised components of a signal into a pair of phase diversity multiports. Alternatively such an array might be used for complete detection of each polarisation state of the signal phase and/or polarisation state. However, such arrays of couplers are bulky and expensive.

It is proposed here to make a single multiport which provides for coherent detection with full diversity. While the minimum number of outputs required in an array is six (three outputs to detect the phase of each polarisation state of the signal) this multiport need only have five outputs.

According to the present invention there is provided a fibre optic multiport having at least five outputs.

The invention will now be particlarly described with reference to the accompanying drawings in which:

FIG. 1 shows the cross-section of a preform for a symmetrical arrangement of five fibres, and

FIG. 2 illustrates the profile of a fused coupler produced from the preform of FIG. 1.

The optical properties required of a full diversity multiport with five outputs are complicated but can be defined for present purposes by imagining a structure which is the four dimensional equivalent of a tetrahedron. If the four dimensions comprise a pair of mutually orthogonal Argand diagrams (one for each polarisation state) then the position vectors of the five corners of the structure represent the required state of the reference at each output.

Given such symmetry it might be expected that a symmetrical arrangement of fibres (FIG. 1) would be most likely to make the five output device, particularly since previous three and four fused fibre multiports have had symmetrical fibre arrangements.

Neglecting polarisation state for the present, five correctly fused fibres will support five modes. These are the zero order mode, and horizontal and vertical pairs of the first and second order modes: ##EQU1## Light injected into a single input will excite equal proportions of each mode. The modes will then propagate at various rates down the fused section, but modes of the same order propagate at the same rate: e.sup.j.beta..sbsp.1h =e.sup.j.beta..sbsp.1V and e.sup.j.beta..sbsp.2h =e.sup.j.beta..sbsp.2V. It follows that there are only two variables of the modal phase: the phases of the first and second order modes relative to the zero order mode. This is rather fortunate because it means that the five fibre device will have no more variables than the four fibre device so should not be more difficult to control during fabrication.

The fraction of the input delivered to each output is determined by summing the phases and amplitudes for each mode at the end of the fused section. The optical properties specified for the device require that equal powers of any sole input be delivered to each output. This will happen if the fibres are fused and tapered until .beta..sub.1 -.beta..sub.0 =2.pi./5 and .beta..sub.2 -.beta..sub.0 =-2.pi./5.

It remains to set up appropriate phase changes between inputs and outputs. It fact it so happens that once a coupler has been made as described above, it will automatically have a suitable set of phase changes provided that it is injected with not one but two references.

The references must be of equal power but orthogonal polarisation state and are injected into adjacent inputs (FIG. 2). It is assumed that the optical properties of the coupler be equal for both polarisation states. This will be true by arguments of symmetry provided the constituent fibres have zero birefringence. and the polarisation states of both injected references are circular (clockwise and anticlockwise).

For operation of the device, the signal should be injected into one of the inputs adjacent to the pair into which the references are being injected. The detected powers p.sub.1 are then the sum of the contributions from each polarisation state, with:

If the coherently modulated components of the output phase are then squared and summed, the result can be shown to be independent of the signal phase or polarisation state: ##EQU2## It might be argued that if this device requires two reference inputs then an extra coupler will be required to split the reference into two and the assembly will effectively become an array. However, the two outputs can at least In principle be derived from any reference laser by coupling light out from both ends of the lasing cavity. An adjustment of polarisation may subsequently be required, but this would be permanent so no moving parts would be needed.

In order to facilitate fabrication of the device a sixth optical fibre of slightly narrower diameter than the other be inserted down the centre of the device. This preserves the symmetry of the preform as it is fused, and may assist coupling between pairs of fibres which are not adjacent.

A coherent detector which is insensitive to signal or polarisation state may be assembled using a fused optical fibre multiport constructed from five optical fibres. To make the multiport the fibres are preferably fused in a circularly symmetric arrangement, then tapered until power injected into any input is equally distributed to all outputs. To operate the coherent detector two references (or local oscillators) of equal power but orthogonal polarisation state are injected into adjacent inputs, and the signal injected into either input adjacent to those used for the references. The sum of the squares of the coherently modulated components of the five output powers is proportional to the product of the signal and the reference power.