GB2243680A

GB2243680A - Optical interferometer device

Info

Publication number: GB2243680A
Application number: GB9009920A
Authority: GB
Inventors: David Dominic Hall
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1990-05-02
Filing date: 1990-05-02
Publication date: 1991-11-06
Anticipated expiration: 2010-05-02
Also published as: GB2243680B; GB9009920D0

Abstract

An optical interferometer device comprises a waveguide 12 which divides to form a pair of arms 18, 20, electrodes 22, 24, 26 being provided such that a voltage to be measured can be applied across one arm and a reference voltage applied across the other arm. Alternatively, the electrodes are arranged such that the measured voltage and the reference voltage are both applied to each arm of the waveguide. The device can form part of a voltage sensor. <IMAGE>

Description

Optical Interferometer Device The present application relates to an optical interferometer device, typically acting as a voltage sensor.

Optical interferometers generally comprise a optical pathway which is divided into two similar paths at some point along its length and subsequently rejoined. By changing the optical behaviour of one or both of the paths, the light transmitted along one path can be caused to interfere with the light transmitted along the other path at the point where the two paths rejoin. The output can be monitored and an evaluation of the change to which the interferometer has been subjected can be made. In electro-optical interferometers, the changes to the optical paths are effected by applying a voltage across pairs of electrodes adjacent to the optical paths, or waveguides; hence the device acts essentially as a voltage sensor. However, such devices can suffer problems of drift when a d.c. voltage is applied to the electrodes, causing inaccuracy in measurement.

The present invention arose in an attempt to overcome the problems of drift in such devices.

In accordance with the first aspect of the present invention, there is provided an optical interferometer device comprising a waveguide which divides to form a pair of arms, electrodes being provided such that a voltage to be measured and a reference voltage are both applied to each arm of the waveguide.

In an alternative aspect of the invention, the electrodes are arranged such that a voltage to be measured can be applied across one arm and a reference voltage applied across the other arm.

Preferably, the reference voltage is selected to give a constant output from the device, irrespective of the voltage to be measured, in which case the reference voltage is chosen to create an equal but opposite effect in the interferometer to the voltage being measured. The reference voltage is typically variable and can be varied under the influence of a feedback loop in which the output from the device is used to determine the variation in the reference voltage required in order to obtain the required constant output. In one embodiment, the desired output is a minimum output of the device, since this allows the optimum operation of a feedback loop.

The invention also provides a sensor incorporating a device as defined above.

The sensor is preferably passive, i.e. containing no permanent power source and the reference voltage can be provided by a photo-electric cell or the like. The photo-electric cell can be activated by light from a separate optical fibre from a remote control and monitoring point.

This light can be adjusted to allow the voltage generated by the photo-electric cell to equal the desired reference voltage. This voltage will be known within the control unit through calibration and stabilisation loops.

In general, and for the case of generating the reference voltage by means of a photo-electric cell, the reference voltage will be uni-polar and this leads to two distinct operating methods. In the first method a uni-polar voltage is to be measured. The interferometer will be permanently set to operate at the minimum of its optical output (that is a fixed 1800 optical phase shift between the two optical paths). The effects of the reference voltage and of the voltage to be measured can be set to balance exactly, thus avoiding the possibility of voltage-induced drift completely.

In the second method a bi-polar voltage is to be measured. The interferometer will be permanently set as if to operate away from a minimum of its optical output, but will still actually be operated at the nearest minimum of its optical output, thus a finite value of reference voltage will have to applied even when the voltage being measured is zero.

This will allow bi-polar measurements.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figures 1 and 2 show diagrammatic representations of interferometer devices according to the present invention; and Figure 3 show a schematic representation of a device incorporating an interferometer as shown in Figure 1 or 2.

Referring now to Figure 1, the interferometer shown therein comprises an integrated optic chip 10 having a waveguide 12 defined thereon. The waveguide 12 has an input 14 typically fed by a laser and an output 16 which feeds a signal to a photo-diode or the like. Intermediate the input 14 and the output 16, the waveguide 12 is divided in to two arms 18, 20. Three electrodes 22, 24, 26 are deposited on the chip 10 adjacent the arms 18, 20, a first electrode 22 being provided between the arms 18, 20 and further electrodes 24, 26 being provided on the opposite side of a respective arm from said first electrode 22. Two sets of electrical connectors are provided for the electrodes, one set of connectors serving to provide an unknown voltage Vmeas and another for the application of a reference voltage Vref.The first electrode 22 is common to both sets of connectors and in use, it is arranged that Vref should have opposite polarity to Vmeas.

In the alternative embodiment shown in Figure 2, the chip 10 and waveguide 12 are as described above. However, in this case the arrangement of electrodes is quite different there being two sets of two electrodes 30, 32 and 34, 36.

One electrode 30, 34 of each set is disposed between the arms 18, 20 and the other electrode 32, 36 embraces both arms 18, 20. In this case Vmeas is applied to the electrodes 30, 32 and Vref is applied to the electrodes 34, 36. The electrodes 30, 32 apply the same voltage to both arms 18, 20 and the Vref voltage is applied to both arms 18, 20 with opposite polarity to Vmeas. Hence the same resultant voltage is applied to each arm.

In use, a light signal is passed along the waveguide 12 in to the two arms 18, 20. The unknown voltage Vmeas causes a phase change in the signal in one arm 18 (Figure 1) or both arms 18, 20 (Figure 2). The reference voltage is applied with opposite polarity with the object of obtaining a predetermined optical amplitude at the output 16 when the two signals are recombined. In the case where the voltage to be measured is uni-polar the net phase shift (relative to as-made value) required is oO and Vref has equal magnitude to Vmeas but opposite polarity. If the voltage to be measured is bi-polar the net phase shift (relative to as-made value) required might be say 900 rather than 0 in which case Vref will not have the same magnitude as Vmeas.

Referring now to Figure 3, the device shown therein comprises a controller including a laser, a photo-cell source and a photo-diode, each being connected via an optical fibre in a cable to a sensor including an integrated optical (I.O.) chip and photo-cell. The photo-cell is connected directly to the source and the laser provides an input for the I.O. chip and the output is fed to the photo-diode. The output of the photo-cell provides the Vref signal to the I.O. chip and the Vmeas signal is provided from an external source. By feeding the output from the I.O. chip to the photo-diode and hence, via some electronics, to the source, a feedback loop is established for setting Vref so as to produce the desired output from the chip and so measure Vmeas as this can be displayed at the control unit or further used in other devices.

Multiple sensors can be driven from one control unit, and if necessary the optical signals can be multiplexed onto a few fibres or even just one fibre.

Claims

1. An optical interferometer device comprising a waveguide which divides to form a pair of arms, electrodes being provided such that a voltage to be measured can be applied across one arm and a reference voltage applied across the other arm.

2. An optical interferometer device comprising a waveguide which divides to form a pair of arms, electrodes being provided such that a voltage to be measured and a reference voltage can be applied across each arm of the waveguide.

3. A device as claimed in claim 1 or 2, wherein the reference voltage is arranged to have an opposite polarity to the voltage to be measured.

4. A device as claimed in any preceding claim, wherein the reference voltage is determined by a feedback loop utilising the output from the device.

5. A device as claimed in any preceding claim, wherein the output from the device is arranged to be constant, irrespective of the voltage being measured.

6. A device as claimed in claim 5, wherein the output is a minimum, the reference voltage having the same magnitude and opposite polarity to the voltage being measured.

7. A device as claimed in claims 1, 5 and 6, wherein the phase difference in signals in the arms of the waveguide is maintained at 1800 at all times.

8. An optical interferometer device which is substantially as herein described in relation to Figures 1 and 2 of the accompanying drawings.

9. A sensor incorporating a device as claimed in any preceding claim.

10. A sensor which is substantially as herein described in relation to Figure 3 of the accompanying drawing.