GB2345129A

GB2345129A - Optical Sensor Using Polarised Light

Info

Publication number: GB2345129A
Application number: GB9828190A
Authority: GB
Inventors: Mats Ekberg; Tord Bengtsson; Mats Leijon; Thomas Eriksson
Original assignee: Asea Brown Boveri AB
Current assignee: ABB AB
Priority date: 1998-12-21
Filing date: 1998-12-21
Publication date: 2000-06-28
Also published as: WO2000037953A1; AU1792000A; GB9828190D0

Abstract

A Faraday-effect magneto-optic sensor comprises a light source 2, a polariser 8, a polarisation-maintaining optic fibre 14 for conveying plane polarised light to a Faraday-effect transducer 9 which, in response to a magnetic field, directed along the direction of light propagation within the transducer, changes the plane of polarisation of the light by an amount proportional to the magnetic field. Light from the transducer 9 passes via a polarisation-maintaining optic fibre 16, via a second polariser 10 acting as an analyser to a photodetector 3 which supplies an output signal to signal processing circuitry 4. Alternatively, the light may pass via a polarising beam splitter to two photodetectors. Gradient index lenses may be mounted between the optic fibres 14,16 and the transducer 9. The optical sensor can be used to detect a partial electric discharge by sensing the associated magnetic field. In another embodiment either one or both of the polarisation-maintaining optical fibres 14,16 and the polarisers 8,10 respectively may be replaced by a polarising optical fibre.

Description

1 2345129 OPTICAL SENSORS The present invention relates to optical sensors

and, in particular, to optical sensors which measure the value of a parameter by detecting the rotation of plane- polarised light.

One application of the present invention is in the measurement of transient magnetic fields, such as are caused by so-called partial discharge activity in highvoltage apparatus. It is desirable to measure such partial discharge at an early stage in the process leading towards a dielectric breakdown in high-voltage apparatus. As is well known, any electric current has associated with it a magnetic field, and such a magnetic field can be detected and measured using an optical sensor based on magnetooptic effects, such as the Faraday effect. In such a sensor, a magnetic field causes the plane of polarisation of plane-polarised light to change by an angle which is in direct proportion to the strength of the magnetic field along the propagation direction.

A typical prior-art arrangement of such a magnetic field sensor is shown in Figure 1. A control module 1 comprises a light source 2, a photodetector 3 and a signal-processing circuit 4. Light from the light source 2 is conveyed via an efferent optic fibre 5 to a Faraday-effect sensor unit 6 which comprises in sequence a collimating lens 7, a polariser 8, a Faraday-effect transducer 9, a second polariser 10 acting as an analyser and a focussing lens 11 all mounted within a housing 12. The second lens 11 serves to reduce the beam diameter to a value suitable for transmission via an afferent optic fibre 13 to the photodetector 3 located within the control module 1.

Conventional optic fibres have the advantage of being able to transmit light over substantial distances without any significant absorption. However, when planepolarised light is transmitted along such optic fibres, the polarisation state of the light is not maintained.

Optic fibres are available which are structured so as to retain the polarised state of light being transmitted therethrough. In such polarisation-maintaining (PM) optic fibres, light is transmitted having two orthogonal planes of polarisation. Thus plane- 2 polarised light entering such a fibre, and which has a plane of polarisation intermediate these two permitted planes, is broken down into its two components in the two respective permitted planes, and the two components are transmitted along two separate channels in the optic fibre.

Furthermore, optic fibres are available which are structured so as to polarise light being transmitted therethrough. In such polarising (PZ) optic fibres, light is transmitted having only one plane of polarisation.

The inventors of the present invention have realised that it would be possible to apply such fibre-optic technology to the prior-art arrangement shown in Figure 1 so as to achieve a more compact optical sensor.

In accordance with a first aspect of the present invention, there is provided an optical sensor comprising means for generating a beam of plane-polarised light, a transducer located within said beam and responsive to a parameter being measured for causing a change in the plane of polarisation of said beam and means for detecting the change in plane of polarisation, wherein said detecting means comprises in sequence a polarisation-maintaining optic fibre, a polariser and a photodetector.

The detecting means preferably comprises a polarising beam-splitter, which directs the two components from the two respective channels to two respective photodetectors, the output signals of which are supplied to a comparator. The comparator supplies a signal to a signal-processing circuit in dependence on the difference between the two signals, the ratio of the two signals or the ratio of the difference between the two signals to the sum of the signals. With such a two-channel detection scheme, it is possible to detect the direction of the change of the plane of polarisation, which is not possible with single-channel detection.

In accordance with a second aspect of the present invention, there is provided an optical sensor comprising means for generating a beam of plane-polarised light, a transducer located within said beam and responsive to a parameter being measured for causing a change in the plane of polarisation of said beam and means for detecting the 3 change in plane of polarisation, wherein said detecting means comprises in sequence a polarising optic fibre and a photodetector.

The beam-generating means preferably comprises in sequence a light source, a polariser and a polarisation-maintaining optic fibre, but may alternatively comprise a light source and a polarising optic fibre. Such a PZ optic fibre both plane-polarises an incoming light beam and also retains the polarised state of the light beam. In this case, there is no need for a separate polariser to be associated with the efferent optic fibre.

Furthermore, it is desirable to orient the optic fibre of the beamgenerating means such that the emergent light has a plane of polarisation which intersects the two orthogonal planes of the PM fibre at substantially 45 degrees. Thus, with no change in the plane of polarisation in the transducer, equal intensities of light are transmitted along the two channels in the PM optic fibre. Altematively, especially when the sensor is used to detect dc current, the PZ optic fibre is oriented such that the emergent light has a plane of polarisation which intersects one of the two planes of the PM fibre at less than 45 degrees.

Thus, when the plane of polarisation is changed within the transducer, there is a reduction in the intensity of light transmitted along one of the two channels in the PM optic fibre and, at the same time, a corresponding increase in the intensity of light transmitted along the other channel. The difference signal generated by the comparator thus represents twice the change in intensity of light in each channel. In this way, the sensitivity of the system is enhanced.

In a preferred arrangement, light enters the transducer at a first surface and emerges from the transducer at a second surface opposite the first surface. A lens is preferably positioned between the or each optic fibre and the transducer so as to change the beam diameter between a small value appropriate for transmission along the optic fibre and a large value appropriate for transmission through the transducer.

The or each lens is advantageously a gradient index (GRIN) lens which is a miniature lens in the form of a cylindrical rod having a refractive index which varies 4 radially, such that the refractive index profile provides the same focussing action as a conventional lens having uniform refractive index but a radially varying shape. The resulting light beam which traverses the transducer is substantially perfectly collimated.

The GRIN lens is mounted to the end of its associated optic fibre by means of a substantially cylindrical housing which surrounds, and is in contact with, both the lens and the fibre end. The housing is preferably made from a dielectric material such that the sensor can be used in environments having high electric field strengths without the risk of electrical discharges.

PM optic fibres which are pre-aligned with, and mounted to GRIN lenses are commercially available, and the complex task of aligning the fibre with the focal point of the lens is thereby eliminated. Furthermore, any necessary angular adjustment of the PM optic fibre is effected by rotation of the associated GRIN lens, which causes a corresponding rotation of the attached fibre The transducer is advantageously remotely situated from the light source and/or the photodetector. In this way, it is possible for control circuitry associated with the light source and/or photodetector to be located remotely from the source of the magnetic field being measured, such as a high-voltage transmission line.

The transducer can be arranged to sense a magnetic field using the Faraday effect, as mentioned above.

In this case, the transducer advantageously comprises a rare-earth-doped iron garnet, such as YIG (yttrium iron garnet). Such a material exhibits a large change in the plane of polarisation of light passing therethrough for a given magnetic field strength and for a given length of transducer material in the direction of light transmission.

In order for the optical sensor to detect a partial discharge, it is advantageous that the frequency response of the transducer be substantially constant up to 3 GHz.

In a further aspect of the present invention, there is provided a method of measuring change in the plane of polarisation of a beam of planepolarised light comprising supplying the changed beam to a polarisation-maintaining optic fibre, a polariser, such as a polarising beam-splitter, and one or two photodetectors in sequence along an optical path and detecting the output of the photodetector.

The invention extends to a method of measuring a change in the plane of polarisation of a beam of plane-polarised light comprising supplying the changed beam to a polarising optic fibre and a photodetector in sequence along an optical path and detecting the output of the photodetector.

The invention also extends to a method of detecting electrical discharge by sensing the transient magnetic field associated with the discharge using an optical sensor of the above type, and to a method of detecting a partial electrical discharge using a magnetic field sensor, which preferably comprises a magneto-optic material such as rare-earth-doped garnet, to detect the magnetic field associated with said partial discharge.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings, wherein:

Figure 1 illustrates a prior-ail arrangement of a Faraday-effect sensor; Figure 2 illustrates a first embodiment of the present invention; Figure 3 illustrates a second embodiment of the present invention; Figure 4 illustrates an application of the Faraday-effect sensor of Figure 2 or Figure 3.

Referring to Figure 2, the Faraday-effect sensor of the fitst embodiment, as in the prior-art arrangement shown in Figure 1, comprises a light source 2, an efferent 6 optic fibre 14 which conveys light from the light source 2 to a sensor unit 15, an afferent optic fibre which conveys light from the sensor unit 15 to a photodetector 3 and a signal-processing circuit 4 which receives an output signal from the photodetector 3. The light source 2, photodetector 3 and signal-processing circuit 4 are mounted within a 5 control module 17 remote from the sensor unit 15.

In place of the optic fibres 5, 13 of Figure 1, PM optic fibres 14, 16 are provided which have the property of maintaining the polarisation state of light being transmitted therethrough. Thus, whereas in the arrangement shown in Figure 1, polarisation elements 8 and 10 are provided in the housing 12 of the sensor unit 15, these are positioned, in the arrangement of Figure 2, between the light source 2 and the efferent optic fibre 14 on the one hand and between the afferent optic fibre 16 and the photodetector 3 on the other hand.

In a variant of this embodiment, either or both of the polarisation elements 8,10 and the corresponding PM optic fibres 14,16 are replaced by polarising (PZ) optic fibres, resulting in an arrangement with fewer components.

In the second embodiment, as shown in Figure 3, the lenses 7,11 of the first embodiment are replaced by GRIN lenses 18,19, and these are mounted, together with the ends of the associated optic fibres 20,21, within respective substantially cylindrical housings 22,23 made of dielectric material.

Furthermore, the efferent PM fibre 14 of the first embodiment is replaced by a PZ optic fibre 20 such that there is no need for a polariser to be positioned between the light source 2 and the PZ optic fibre 20. The ends of the PZ and PM optic fibres 20,21, the GRIN lenses 18,19 and the cylindrical housings 22,23 are mounted, together with the Faraday-effect transducer 9, in a housing 24. The end of the PZ optic fibre 20 is oriented such that the plane of polarisation of the emergent beam intersects the two permitted planes of polarisation of the PM optic fibre 21 by 45 degrees. However, when the sensor is used to measure dc current, the end of the PZ optic fibre 20 is oriented such that the plane of polarisation of the emergent beam intersects one of the two permitted planes of polarisation of the PM optic 1Fibre by less than 45 degrees.

7 Light emerging from the PM optic fibre 21 is directed to a polarising beamsplitter 25 which directs the light transmitted along the two channels of the PM optic fibre 21 to two respective photodetectors 26,27, the output signals of which are supplied in turn to the two inputs of a comparator 28 which generates a signal proportional to (a) the difference between the two input signals, (b) the ratio of the two input signals or (c) the ratio of the difference between the two input signals to the sum of the two input signals. The output signal from the comparator 28 is supplied to a signal-processing circuit 29.

As with the first embodiment, the light source 2 and the light-detecting apparatus 25-29 are mounted together in a housing 30 remotely situated from the sensor unit 3 1.

In a variant of this embodiment, the efferent PZ optic fibre 20 is replaced with a corresponding polarising element and a PM optic fibre, as in the first embodiment.

In both embodiments, the transducer 9 is a Faraday-effect transducer made from a rare-earth-doped iron garnet, such as yttrium iron garnet, which has a frequency response substantially constant up to 3 GHz.

A particular application of the sensor is illustrated in Figure 4, wherein the sensor unit 15; 31 is shown positioned adjacent an electrical conductor 32 so as to detect the magnetic field 33 associated with current flowing along the electrical conductor 32.

It will be seen from this figure that the sensor unit 15; 31 is arranged with its longitudinal axis generally parallel to the lines of magnetic field, since the Faradayeffect sensor is arranged to sense the magnetic field along the direction of light propagation.

It will be appreciated that many variations and modifications may be made to the above-described preferred embodiments without departing from the scope of the present invention. For example, the respective single and double beam-sensing arrangements of the first and second embodiments could be interchanged.

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Claims

CLAIMS:

1. An optical sensor comprising means for generating a beam of planepolarised light, a transducer located within said beam and responsive to a parameter being measured for causing a change in the plane of polarisation of said beam and means for detecting the change in plane of polarisation, wherein said detecting means comprises in sequence a polarisation-maintaining optic fibre, a polariser and a photodetector.

2. An optical sensor as claimed in any Claim 1, wherein the polariser of said detecting means comprises a polarising beam-splitter and said detecting means comprises a further photodetector, the arrangement being such that the two respective components of the light transmitted by the optic fibre are directed by the beam splitter to the photodetector and the further photodetector respectively.

3. An optical sensor as claimed in Claim 2, wherein said detecting means further comprises a comparator connected to the output of the photodetector and the further photodetector.

4. An optical sensor comprising means for generating a beam of planepolarised light, a transducer located within said beam and responsive to a parameter being measured for causing a change in the plane of polarisation of said beam and means for detecting the change in plane of polarisation, wherein said detecting means comprises in sequence a polarising optic fibre and a photodetector.

5. An optical sensor as claimed in any preceding claim, wherein said beam generating means comprises in sequence a light source, a polariser and a polarisation-maintaining optic fibre.

6. An optical sensor as claimed in any one of Claims 1 to 4, wherein said beam generating means comprises a light source and a polarising optic flibre.

9

7. An optical sensor as claimed in Claim 5 or Claim 6, when dependent on Claim 1, wherein the ends of the respective optic fibres adjacent the transducer are oriented such that the plane of polarisation of the light emerging from the optic fibre of the beam-generating means intersects the two orthogonal planes defined by the polarisation-maintaining optic fibre at substantially 45 degrees.

8. An optic sensor as claimed in Claim 5 or Claim 6, when dependent on Claim 1, wherein the ends of the respective optic fibres adjacent the transducer are oriented such that the plane of polarisation of the light emerging from the optic fibre of the beam-generating means intersects one of the two orthogonal planes defined by the polarisation-maintaining optic fibre at an angle less than 45 degrees.

9. An optical sensor as claimed in any preceding claim, and arranged such that light enters the transducer at a first surface and emerges from the transducer at a second, opposite surface.

10. An optical sensor as claimed in any preceding claim, flirther comprising a lens disposed between the or each optic fibre and said transducer.

11. An optical sensor as claimed in Claim 10, wherein the or each lens comprises a gradient index lens.

12. An optical sensor as claimed in Claim 11, finther comprising means for retaining the or each gradient index lens in position relative to the end of its associated optic fibre.

13. An optical sensor as claimed in Claim 12, wherein said retaining means comprises a substantially cylindrical housing which surrounds, and is in direct contact with, both the optic fibre end and its associated gradient index lens.

14. An optical sensor as claimed in Claim 13, wherein said housing is made from a dielectric material.

15. An optical sensor as claimed in any preceding claim, wherein said transducer is remotely situated from said photodetector.

16. An optical sensor as claimed in any preceding claim, wherein said transducer is arranged to sense a magnetic field using the Faraday effect.

17. An optical sensor as claimed in Claim 16, wherein said transducer comprises a rare-earth-doped iron garnet.

18. An optical sensor as claimed in Claim 17, wherein said transducer comprises yttrium iron gamet.

19. An optical sensor as claimed in any preceding claim, wherein said transducer has a frequency response which is substantially constant up to 3 GHz.

20. A method of detecting electrical discharge by sensing the magnetic field associated with a partial discharge using an optical sensor as claimed in any preceding claim.

21. A method of measuring a change in the plane of polarisation of a beam of plane polarised light comprising supplying the changed beam to a polarisation maintaining optic fibre, a polariser and a photodetector in sequence along an optical path and detecting the output of the photodetector.

22. A method of measuring a change in the plane of polarisation of a beam of plane polarised light comprising supplying the changed beam to a polarisation maintaining optic fibre, a polarising beam-splitter and two photodetectors in sequence along an optical path and detecting the output of the photodetectors.

23. A method of measuring a change in the plane of polarisation of a beam of plane- polarised light comprising supplying the changed beam to a polarising optic 11 fibre and a photodetector in sequence along an optical path and detecting the output of the photodetector.

24. A method of detecting a partial electrical discharge comprising using a magnetic 5 field sensor to detect the magnetic field associated with a said partial discharge.

25. A method as claimed in Claim 24, wherein the magnetic field sensor comprises a magneto-optic material.

26. A method as claimed in claim 25, wherein said magneto-optic material is a rare earth-doped iron gamet.

27. An optical sensor substantially as hereinbefore described with reference to Figures 2 to 4 of the accompanying drawings.

28. A method of detecting electrical discharge substantially as hereinbefore described with reference to Figures 2 to 4 of the accompanying drawings.