WO1994000768A1

WO1994000768A1 - Optical current sensor

Info

Publication number: WO1994000768A1
Application number: PCT/GB1993/001362
Authority: WO
Inventors: David Alfred Jackson; Ya Nong Ning; Andrew Paul Steer
Original assignee: Sifam Limited
Priority date: 1992-06-29
Filing date: 1993-06-29
Publication date: 1994-01-06
Also published as: GB9213736D0; AU4506793A

Abstract

An optical current sensor comprises a block (5) of transparent material capable of creating the Faraday effect, a light source (1), optical fibre (2), lens (3) and linear polarizer (4) for generating and directing a linearly polarized light beam into the block (5) for transmission therethrough in a magnetic field generated by an electrical current through a conductor (12) extending through an aperture (11) in the block (5), and a polarization analyser (13), second lens (15), optical fibre (14) and light intensity measuring device (16) for detecting and analysing a change in the polarization of the light beam emerging from the block (5). The block (5) is shaped so that the light beam is transmitted therethrough along a substantially triangular path with total internal reflection of the beam at the critical angle of the transparent material occurring at two separate faces (8, 9) of the block (5).

Description

OPTICAL CURRENT SENSOR

This invention relates to optical current sensors and in particular to such sensors which are capable of measuring an electrical current by use of a rotation of the polarization plane of a polarized light beam in a magnetic field, generally known as the Faraday effect.

In known optical current sensors of this type, a light beam is transmitted through a lens and a polarizer to produce a narrow beam of linearly or plane polarized light which is directed into a block of glass. As the beam travels through the glass, its plane of polarization is rotated by an amount which depends on the magnetic field present by virtue of the Faraday effect. Ideally, if the light beam is reflected within the glass block so that it encircles an electrical conductor carrying the current to be measured, then the Faraday rotation angle is proportion¬ al to the current. The beam emerging from the glass block is analysed by a second polarizer which converts the rota¬ tion into a change in light intensity, which is detected to generate a measurement of the current.

However, the optimal, linear polarization state of the light beam can be disturbed at the reflection of the beam within the glass block, which can lead to a loss of sensitivity of the sensor and possibly to a non-uniform distribution of the sensitivities along the light beam paths before, between and after the reflection. If this occurs, the sensor becomes susceptible to current effects from external conductors, as well as from the conductor carrying the current to be measured, so that an accurate measurement cannot be obtained.

It is therefore an object of the present invention to provide an optical current sensor of substantially improved sensitivity which is capable of maintaining the linear polarization state of the light beam after reflec¬ tion irrespective of the input azimuth or angle of polari- zation of the beam.

Accordingly, the present invention consists in an optical current sensor comprising a block of transparent material capable of creating the Faraday effect, means for generating a linearly polarized light beam and for direct¬ ing said beam into the block for transmission through the block in a magnetic field generated by an electrical cur¬ rent to be measured, and means for detecting and analysing a change in the polarization of the light beam emerging from said block, said change being indicative of the meas¬ urement of said electrical current, characterised in that said block is shaped so that the light beam is transmitted therethrough along a substantially triangular path with total internal reflection of the beam at the critical angle of the transparent material occurring at two separate faces of the block.

In a preferred embodiment, the sensor is arranged such that the polarized light beam enters the block through a first further face thereof and emerges from the block through a second further face thereof, said block being shaped so that the light beam is incident on each of said further faces substantially along the normal to said face, thereby minimising reflection at the further faces and reducing polarization errors.

An aperture is preferably formed in the block, through which current conducting means carrying the elec¬ trical current to be measured are intended to pass, the aperture being located within the loop formed by the trian¬ gular path of the beam.

Advantageously, the block may comprise two separa¬ ble portions with the aperture formed between opposing faces of the two portions, thereby enabling the block to be mounted around the current conducting means . The two portions may be hinged together along one side so that they are angularly movable relative to each other, or they may be arranged to be slidably movable relative to each other. Preferably, the two portions are shaped such that the light beam passes through their opposing faces substantially along a normal common to both of said faces, so as to minimize introduction of polarization effects.

The block is preferably made substantially symmet¬ rical in shape. In the embodiment in which the block is formed into two portions, the joining line of the portions may extend along the axis of symmetry of the block, so that both portions are substantially identical in shape and size.

In a specific example, the means for generating the polarized light beam includes a light source for emitting a light beam, a first lens, and a linear polarizer, the first lens being arranged to receive the light beam and to focus the beam on the linear polarizer to polarize the beam. The means for detecting and analysing a change in the polarization of the light beam may comprise a polarization analyser for converting the change in polarization into a change in light intensity of the beam, and a second lens for focussing the light beam of changed intensity onto a light intensity measurement device, the output of said device being indicative of the electrical current to be measured. Preferably, optical fibre means are provided to transmit the light beam from the light source to the first lens and/or from the second lens to the light intensity measurement device. Additionally, the light beam emitted from the light source may be arranged to pass through a depolarizer before being directed to the first lens.

The invention will now be further described by way of example with reference to the accompanying drawings, in which:-

Figure 1 shows schematically an optical current sensor in accordance with one embodiment of the invention showing a plan view of a glass block used therein,

Figure 2 shows a side view of the glass block shown in Figure 1, and Figures 3 and 4 show plan views of a glass block in accordance with an alternative embodiment, with the block in "closed" and "open" positions respectively.

Referring firstly to Figures 1 and 2, an optical current sensor comprises a light source 1, such as a laser, which emits a narrow light beam along an optical fibre 2 to a first lens 3. The lens 3 focuses the beam on a linear polarizer 4 which linearly polarizes the beam and directs it into a block 5 of transparent material, preferably SF6 glass, through a face 6 of the block. The polarized light beam then follows a triangular path 7 through the block 5 with two internal reflections of the beam occurring at faces 8 and 9 respectively and the reflected beam emerging from the block 5 through face 10 of the block.

The block 5 is formed with an aperture 11 which is located within the triangular path 7 of the light beam and through which an electrical current conductor 12 is passed. The conductor 12 carries the electrical current to be measured by the sensor and generates a magnetic field within the block 5 which causes a rotation in the plane of polarization of the light beam as it passes through the block. By virtue of the Faraday effect, the angle of rotation is substantially proportional to the current to be measured.

The light beam emerging from the block through the block face 10 is received by a polarization analyser 13 which converts the Faraday rotation into a change in light intensity. The light beam of changed intensity is then focussed through an optical fibre 14 via a lens 15 to a light intensity measuring device 16 including a photodiode 17 which detects the light intensity of the beam emerging from the optical fibre 14. The device 16 analyses the detected intensity and produces an output 18 indicative of the required current measurement.

Upon reflection of the light beam at faces 8 and 9 of the block, the in-phase orthogonal components of the linearly polarized beam can become out of phase and thus create elliptically or circularly polarized light which reduces the sensitivity of the sensor to the current to be measured. In order to minimize this effect, in the present invention, the light beam is totally internally reflected at the faces 8,9 at the critical angle of the material of the block 5. At this critical angle, there is no phase change between the orthogonal components, so that the components remain in phase and the linear polarization of the light is preserved. Furthermore, at the critical angle, both orthogonal components are reflected with equal intensity so that the angle of polarization is not affected by the reflection.

Additionally, it has been found that a perfectly collimated beam cannot, in practice, be produced in the glass block. As a consequence the beam cross section at the exit lens (15 in figure 1) increases with path length and, since the beam cross section is greater than that of the lens, a proportion of the light is lost. Also, some light is lost through absorption in the glass block. Thus, minimum path length is desirable because the reduction in sensitivity due to losses outweighs the increase in sensi¬ tivity due to greater Faraday effect rotation in a longer beam path.

Minimum beam path length is achieved in the present invention by the use of the triangular path beam which produces the shortest single loop with minimum number of reflections.

Another important feature of the preferred embodi¬ ment of the present invention is that the faces 6 and 10 of the block 5 through which the light beam passes are angled so that the light beam enters and exits the block along the normals to these faces, thereby minimizing reflection of the beam and avoiding undesirable polarization effects on entry and exit of the light beam.

It can also be seen that by use of a triangular beam path the block can be made in a substantially symmet¬ rical shape which renders the block relatively simple to manufacture. Additionally, as shown in Figure 2, the block has a relatively small, but substantially uniform, thick¬ ness.

Figures 3 and 4 show an alternative embodiment of the block 5, which can be used in place of the block shown in Figure 1. In this embodiment, the block is divided into two portions 19,20 of substantially identical shape and size which are joined together by a hinge 21 so that they can be moved angularly relative to each other between the "closed" and "open" positions shown. The aperture 11 is formed between opposing faces 22,23 of the two portions, which enables the block to be clamped around the conductor 11 rather than having to thread the block over the conduc¬ tor as with the embodiment shown in Figure 1. The block in Figures 3 and 4 is also generally rectangular in shape instead of the generally triangular shape of the first embodiment. However, as in the first embodiment, the generally rectangular block still has angled faces 6 and 10 through which the light beam respectively enters and exits the block along the respective normal to the face so as to minimize reflection at these faces, and the faces 8,9 are angled such that the beam is totally internally reflected at these faces at the critical angle of the material of the block. The light beam also follows a substantially trian¬ gular path through the generally rectangular block with the path crossing the opposing faces 22,23 of the block along a normal common to both faces, as shown at 24, so as again to minimize reflection and thus undesirable polarization effects at this point.

Whilst particular embodiments of the present inven¬ tion have been described, it will be envisaged that other modifications may be made without departing from the scope of the invention. For example, instead of the two block portions shown in Figure 4 being hinged together, the portions may be arranged to slide relative to each other or they may be mounted for movement relative to each other in any other suitable arrangement. Additionally, although the blocks are shown as being generally triangular or rectangu¬ lar in shape, they may have any other suitable shape in¬ cluding appropriately angled reflection faces 8,9 and preferably also entry/exit faces 6,10. In another embodi¬ ment, a fibre depolarizer, shown in broken lines at 25 in Figure 1, may be connected between the light source 1 and the input optical fibre 2. When a laser light source is used, the light beam emitted therefrom is polarized and if there is a perturbation of the input fibre 2, caused for example by vibration, the polarization state at the polar¬ izer 4 may fluctuate, thereby causing a fluctuation in light intensity which may give a spurious output signal. This problem can therefore be solved by using the fibre depolarizer which unpolarizes the light emitted by the laser light source.

Claims

CIAIMS

1. An optical current sensor comprising a block (5) of transparent material capable of creating the Faraday ef¬ fect, means (1 to 4) for generating a linearly polarized light beam and for directing said beam into the block (5) for transmission through the block (5) in a magnetic field generated by an electrical current to be measured, and means (13 to 18) for detecting and analysing a change in the polarization of the light beam emerging from said block (5), said change being indicative of the measurement of said electrical current, characterised in that said block (5) is shaped so that the light beam is transmitted there¬ through along a substantially triangular path with total internal reflection of the beam at the critical angle of the transparent material occurring at two separate faces (8, 9) of the block (5) .

2. A sensor as claimed in claim 1, wherein it is arranged such that the polarized light beam enters the block (5) through a first further face (6) thereof and emerges from the block (5) through a second further face (10) thereof, said block (5) being shaped so that the light beam is incident on each of said further faces (6, 10) substantially along the normal to said face (6, 10), there¬ by minimising reflection at the further faces (6, 10) and reducing polarization errors.

3. A sensor as claimed in claim 1 or 2, wherein an aperture (11) is preferably formed in the block (5), through which current conducting means (12) carrying the electrical current to be measured are intended to pass, the aperture (11) being located within the loop formed by the triangular path of the beam.

4. A sensor as claimed in claim 3, wherein the block (5) comprises two separable portions (19, 20) with the aperture (11) formed between opposing faces (22, 23) of the two portions (19, 20), thereby enabling the block (5) to be mounted around the current conducting means (12).

5. A sensor as claimed in claim 4, wherein the two portions (19, 20) are hinged together along one side so that they are angularly movable relative to each other.

6. A sensor as claimed in claim 4, wherein the two portions (19, 20) are arranged to be slidably movable relative to each other.

7. A sensor as claimed in claim 4, 5 or 6, wherein the two portions (19, 20) are shaped such that the light beam passes through their opposing faces (22, 23) substantially along a normal common to both of said faces (22, 23), so as to minimize introduction of polarization effects.

8. A sensor as claimed in any preceding claim, wherein the block (5) is made substantially symmetrical in shape.

9. A sensor as claimed in any one of claims 4 to 7 in combination with claim 8, wherein the joining line of the portions (19, 20) extends along the axis of symmetry of the block (5), so that both portions (19, 20) are substantially identical in shape and size.

10. A sensor as claimed in any preceding claim, wherein the means (1 to 4) for generating the polarized light beam includes a light source (1) for emitting a light beam, a first lens (3), and a linear polarizer (4), the first lens (3) being arranged to receive the light beam and to focus the beam on the linear polarizer (4) to polarize the beam. -lO-

ll. A sensor as claimed in any preceding claim, wherein the means (13 to 18) for detecting and analysing a change in the polarization of the light beam comprise a polariza¬ tion analyser ( 13) for converting the change in polariza¬ tion into a change in light intensity of the beam, and a second lens (15) for focusing the light beam of changed intensity onto a light intensity measurement device (16), the output of said device (16) being indicative of the electrical current to be measured.

12. A sensor as claimed in claim 10 or 11, wherein the light beam is transmitted from the light source (1) to the first lens (3) and/or from the second lens (15) to the light intensity measurement device (16) by optical fibre means (2, 14) .

13. A sensor as claimed in claim 10, wherein the light beam emitted from the light soure (1) is arranged to pass through a depolarizer (25) before being directed to the first lens (3) .