The present invention will be described herein with reference to an optical polarisation mode interferometer for use in an electric field measuring device, however, it will be appreciated that the present invention does have broader applications, including e. g. to polarisation mode interferometers for use in sensors.

Background of the invention To optimise the stability of an optical interferometric sensor, it is desirable to have both the sensing arm and the reference arm subject to the same environmental perturbations.

Using the two polarisation modes of a birefringent optical fibre can allow the design of a single fibre interferometer, thus insuring that both paths, i. e. the light signals travelling in the respective polarisation modes along the poled optical fibre, are subjected to substantially identical environmental perturbations. Poled optical fibre, used in electric field sensors, is normally inherently birefringent, making the single fibre interferometer an ideal choice for this type of sensor.

However, it has been recognised by the applicant that in such poled optical fibre interferometers, the two polarisation modes will propagate along the poled optical fibre with different velocities due to the birefringence of the poled fibre. This results in different optical path lengths for the respective optical signals in the two polarisation modes. The optical path length difference will be temperature dependent, since the birefringence is temperature dependent, which introduces a drift in the interferometer. Such a drift is generally not desirable, in particular where DC measurements are to be carried out by the poled optical fibre interferometer, e. g. in a poled optical fibre DC voltage sensor.

Narrow band light sources are typically used in such prior art poled optical fibre sensors, so that the coherence length of the source is greater than the path length difference of the interferometer. However, narrow band light sources generally result in poorer noise performance of the sensor and are difficult to depolarise. It will be apparent from the above that it would be desirable to provide a poled optical fibre interferometer which can avoid at least some of the disadvantages associated with the difference in velocity of the respective polarisation modes.

Summary of the invention In accordance with a first aspect of the present invention there is provided an optical polarisation mode interferometer comprising a poled optical fibre, and polarisation manipulation means located substantially mid-length of the poled optical fibre for, in use, swapping the polarisation states of the polarisation modes of an optical signal propagating in the poled optical fibre, and wherein the poling direction in the poled optical fibre is reversed substantially mid-length of the poled optical fibre.

The poled optical fibre may comprise a transversely poled optical fibre. The transversely poled optical fibre may be disposed in a helix. The helix preferably has a constant pitch angle. In one embodiment, the transversly poled optical fibre is disposed in an interwound fashion, wherein a loop is formed substantially mid-length of the transversely poled optical fibre, whereby the interwound lengths of the transversely poled optical fibre extend from the loop.

In an alternative embodiment, the poled optical fibre may comprise a longitudinally poled optical fibre. The longitudinally poled optical fibre may be disposed in a substantially straight line. The longitudinally poled optical fibre may be disposed in a manner such that a loop is formed substantially mid-length of the longitudinally poled optical fibre, whereby lengths of the longitudinally poled optical fibre co-extend from the loop in substantially straight lines.

In yet another embodiment, the poled optical fibre may comprise a helical core poled optical fibre. The helical core poled optical fibre is preferably poled longitudinally. The helical core longitudinally poled optical fibre may be disposed in substantially a straight line. The helical core longitudinally poled optical fibre may be disposed in a manner such that a loop is formed substantially mid-length of the helical core longitudinally poled optical fibre, whereby lengths of the helical core longitudinally poled optical fibre co-extend from the loop in substantially straight lines.

The poled optical fibre may be provided in the form of two poled optical fibre segments having substantially the same length.

The polarisation manipulation means may comprise a splice formed between the two poled optical fibre segments, wherein the two poled optical fibre segments are arranged in a manner such that respective birefringent axes of the poled optical fibre segments are at a substantially 90° angle relative to each other at the splice, and wherein the poling directions in the respective poled optical fibre segments are reversed with respect to each other.

The means for swapping the polarisation modes may comprise an active device.

The active device may be located, in use, between the two poled optical fibre segments.

The active device may comprise a Faraday-effect device or a Kerr-effect device.

The interferometer may further comprise an elongated body of an electrically insulating material supporting the poled optical fibre. The material of the elongated body may comprise a fibre reinforced polymer material.

The interferometer may further comprise layers and outer sheds of a polymer material enclosing the poled optical fibre and the elongated body.

The interferometer may further comprise a light emitting means for launching the optical signal into the poled optical fibre.

The interferometer may further comprise a detecting means for receiving the optical signal transmitted through the poled optical fibre and for generating an output signal corresponding to a phase difference between the polarisation modes of the received optical signal.

Preferably, the interferometer further comprises a reflecting means disposed at one end of the poled optical fibre, whereby, in use, the optical signal propagates twice through the poled optical fibre.

In accordance with a second aspect of the present invention there is provided a method of conducting optical polarisation mode interferometry, the method comprising the step of swapping polarisation states of the polarisation modes of an optical signal propagating in a poled optical fibre at substantially mid-length of the poled optical fibre, wherein the poling direction in the poled optical fibre is reversed at the substantially mid-length of the poled optical fibre.

Brief description of the drawings Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying figure.

Figure 1 is a schematic drawing of an electric field measurement device embodying the present invention.

Detailed description of the embodiments The preferred embodiment described provides an optical polarisation mode interferometer device in which drift as a result of different propagation velocities of the respective polarisation modes is eliminated.

In Figure 1, an electric field voltage measuring device 10 embodying the present invention is arranged for measuring an electric field 12. The device 10 comprises a central rod 24 of a fibre reinforced polymer material, such as glass fibre reinforced epoxy.

A poled optical fibre 22 is wound around the rod 24 and forms a helix that has a constant pitch angle.

At the lower end of the rod 24 the poled optical fibre 22 is connected to a polarising beam splitter 14. Two optical fibre connections 16,18 are connected to the split bidirectional ports of the polarising beam splitter 14. One of the fibre connections 16 is connected directly to a first lead of a so-called 3x3 fibre optic coupler 26. A polarisation rotation unit 20 is provided in-line with the other optical fibre connection 18, which is connected to a second lead of the 3x3 fibre optic coupler 26. A third lead of the 3x3 fibre optic coupler 26 is unused.

The"outward"facing leads 30,32, 34 of the 3 x 3 coupler 26 are connected to an interrogation device 36 for interrogation by known techniques to obtain the relative phase shift between the respective polarisation modes undergone during propagation through the poled optical fibre 22 for measuring the electric field 12.

Mid-length of the poled optical fibre 22 at an upper loop 23, two counter poled optical fibre segments 22a, 22b of substantially identical lengths are spliced together, fonning the poled optical fibre 22.

In the embodiment shown in Figure 1, the poled optical fibre 22 is transversely poled, with the poling directions 19,21 being reversed between the poled optical fibre segment 22a and 22b, as illustrated in inlets 25,27 respectively.

The operation of the device 10 will now be briefly described.

A linearly polarised optical signal is launched into the poled optical fibre 22 from the interrogation device 36 via the 3 x 3 coupler 26. Immediately after the 3x3 coupler 26, the optical signals propagating in the optical fibre connections 16,18 respectively are of identical linear polarisation, denoted H for clarity. The optical signal propagating along the optical fibre connection 18 is subjected to the polarisation rotation unit 20, as a result of which the polarisation of that optical signal is rotated by 90°. Accordingly, at the polarising beam splitter 14, an H linearly polarised optical signal enters via the optical fibre connection 16, whereas a V linearly polarised optical signal enters via the optical connection 18. As a result, an optical signal containing both H and V linearly polarised modes is launched into the poled optical fibre segment 22a.

Within the poled optical fibre segment 22a, the electric field 12 will induce different refractive index changes in directions parallel and perpendicular to the poling direction. While propagating along the poled optical fibre segment 22a in the presence of the electric field 12, the two polarisation modes undergo a relative phase shift with respect to each other.

At the upper loop 23, the polarisation states of the modes are"swapped", i. e. each mode is now propagating as the other mode as a result of the 90° relative orientation of the birefringent axes between the fibre segments 22a and 22b respectively. However, the relative phase shift between the modes is not changed at the splice.

The now swapped polarisation mode signals will continue to undergo a relative phase shift in the presence of the electric field 12 while propagating along the poled optical fibre segment 22b. Importantly, since the poling direction is reversed between the poled optical fibre segments 22a, 22b (compare poling directions 19 and 21 in inlets 25,27 respectively), the relative phase difference, due to the electric field, will continue to increase.

The optical signal propagates along the poled optical fibre segment 22b, which terminates in a mirror 38. Upon reflection of the optical signal at the mirror 38, it will propagate back firstly through the poled optical fibre segment 22b and then through the optical fibre segment 22a and to the polarising beam splitter 14.

It will be appreciated by a person skilled in the art that the helically interwinding of the counter-poled optical fibre segments 22a, 22b in the example embodiment provides for matching of thermal conditions, which could otherwise introduce artefacts in the overall measurement. However, where such matching is not critical, this may not be required, i. e. the poled optical fibre may be disposed in a single winding manner.

As a result of the swapping of the polarisation modes between the poled optical fibre segments 22a and 22b, the differences in propagation velocity due to the birefringence of the poled optical fibre 22 are effectively eliminated, since each polarisation mode signal will travel at the different velocities for half the length of the entire path through the poled optical fibre 22.

Accordingly, the drift experienced in prior art polarisation mode interferometers can be suppressed.

After the polarising beam splitter 18, the two different polarisation modes, denoted H and V, enter the optical fibre connections 16 and 18 respectively. Again, the polarisation of the signal propagating along the optical fibre connection 18 will be rotated in the polarisation rotation unit 20, and thus light signals of identical linear polarisation, denoted H, will enter the 3x3 optical coupler 26 from the respective optical fibre connections 16,18. In the interrogation device 36, known processing techniques are utilised to determine the phase shift experienced, for measuring the electric field 12.

Furthermore, it is noted that the present invention can also facilitate the use of broadband light sources for launching into the poled optical fibre 22, since the optical path lengths are substantially identical. This removes the requirement for a long coherence length (narrow band) source. It will be appreciated by the person skilled in the art that numerous modifications and/or variations may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described.

The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

In the claims that follow and in the summary of the invention, except where the context requires otherwise due to express language or necessary implication the word"comprising"is used in the sense of"including", i. e. the features specified may be associated with further features in various embodiments of the invention.