GB2190744A

GB2190744A - Magnetic field sensors

Info

Publication number: GB2190744A
Application number: GB08711926A
Authority: GB
Inventors: Jingren Qian; Luksun Li; David Neil Payne
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-05-20
Filing date: 1987-05-20
Publication date: 1987-11-25
Also published as: WO1987007387A1; GB8612190D0; GB8711926D0; EP0267256A1; JPH01500615A

Abstract

Apparatus for the sensing of magnetic fields (3) by means of the Faraday effect incorporating an optical fibre light guide arranged in a coil (1) which has a diminished birefringence produced by spinning during the drawing process.

Description

SPECIFICATION Optical fibre apparatus and method This invention relates to optical fibres and, in particular to apparatus incorporating such fibres for the sensing of magnetic fields.

Electric-current monitors are widely employed in the electricity-generating industries.

Conventionally these measurements are made using large high-voltage current transformers which require extensive insulation and are therefore expensive and bulky. Moreover, the bandwidth of the measurement system is low, making the monitoring of fast transients (e.g.

lightning strikes) difficult. There is therefore a requirement for a cheaper and simpler alternative. Such an alternative is provided by the optical-fibre current monitor. This requires little or no insulation because the optical fibre is a dielectric and therefore does not conduct electricity. The sensor arm is the fibre, which, being light and flexible, is simple and convenient to employ. Finally, the monitor has a large bandwidth which facilitates the monitoring of very fast transients on the grid. Thus optical-fibre current monitors or transformers have considerable advantages in measuring current on high-voltage lines and in protective applications.

According to the present invention there is provided apparatus for the sensing of magnetic fields by means of the Faraday effect incorporating an optical fibre light guide arranged in a coil wherein said coil comprises an optical fibre having diminished birefringence produced by spinning said fibre during its drawing process.

The invention will now be particularly descxibed with reference to the accompanying drawings in which: Figure 1 shows an optical fibre current monitor Figure 2 shows two bifilar fibres wound on a former Figure 3 illustrates a cross-sectional representation of one type of fibre; and Figure 4 is a corresponding view of another type of fibre.

An optical-fibre current monitor or transformer (Fig. 1) is based on the Faraday effect, i.e. the rotation of linearly polarised light which occurs when a magnetic field is aligned with the direction of light propagation. A fibre coil 1 is wound around a conductor 2 and when an electric current flows the associated magnetic field 3 encircling the conductor interacts with the light propagating through the fibre. The Faraday effect results in a rotation of the output state of polarisation, as shown in Figure 1. When linearly-polarised light polarised in the direction of the arrow A is injected into the fibre from a laser 4 the output polarisation is rotated by an amount proportional to the line integral of the magnetic field contained within the single loop of fibre (i.e. to the current flowing).The amount of rotation depends only on the net electric current contained within the loop and is unaltered by the shape of the loop. If greater sensitivity to electric current is required, this can be provided by winding the fibre N times around the wire, whereupon the rotation is magnified by N. Te changed direction of polarisation of the light (indicated by the arrow B) is detected by a polarisation detector 5.

Unfortunately, the above description of the fibre current-monitor refers to the ideal case.

Practical single-mode optical fibres are linearly birefringent as a result of inadvertent ellipticity or asymmetric stress. This means that the fibre supports two orthogonally-polarised modes whose propagation constant spacing is equal to the birefringence Afi. Consequently, the state of polarisation of light evolves periodically along the fibre with a beat length Lp = 2/Afi, typically around 1 metre. The consequence of this is that the linear birefringence quenches the small Faraday rotation, thus making the optical-fibre current monitor inoperative in lengths longer than Lp/2. Furthermore, packaging and winding the fibre into coils creates further linear birefringence, making the interaction length even shorter. Worse still, the linear birefringence is indeterminate and varies with time.

It is important to design optical fibres which are suited to current transformer applications.

Since the linear birefringence in a real fibre quenches the small Faraday rotation, a first approach is to manufacture a fibre with diminished linear birefringence.

Spinning a conventional single-mode fibre with moderate linear birefringence has been shown to create a very low birefringence fibre. However, although the fibre contains no internal birefringence, it remains as sensitive as a normal fibre to external effects, such as bending or applied stress. Thus when the lowbirefringence spun fibre is packaged into a cable or coil, the birefringence could reappear and, unless extreme care is taken, would dominate the current measurement.

Helical-core circularly-birefringent fibres have recently been reported and shown to exhibit high circular birefringence. In this case both the external and internal linear-birefringence are swamped by the high circular birefringence and the fibre is suitable for monitoring current.

However, in practice, since the diameter of the fibre must be large enough to contain the helical core, the fibre is stiff and it is therefore most suited to applications requiring large coils. Moreover, launching into the fibre and splicing with other fibres requires care.

We have devised a method of overcoming these drawbacks, as will be apparent from the following description of a specific embodiment of the invention.

The starting fibre is a highly linearly-birefrin gent fibre which is spun during the fibre drawing process. The spin rate is chosen such that the linear birefringence Afi is not small compared with the twist rate r, as is the case when manufacturing a low birefringence fibre.

In the present case the twist/birefringence ratio 24awl is of order 1 to 2 and the linear birefringence is only partially cancelled by the twisting. Moreover, since the fibre is in a viscous state at the high fibre-forming temperature and cannot support significant shear stress, no torsional circular birefringence will be present.

The principle of operation of spun highlybirefringent fibres is as follows. The rotation of the principal axes of the linear birefringence of, for example, a Bow-Tie fibre, is equivalent to the existence of circular birefringence in rotating coordinates or the Poincaré sphere.

Consequently, the outcome is to create elliptical birefringence in the fibre, the magnitude of which depends on the spin ratio 2T/Ass. According to a coupled-mode analysis of spun fibres, when linear birefringence Afi is high and Afi (1-2)r, the fibre exhibits highly-elliptical birefringence.

The elliptical birefringence can be controlled by the spin ratio 2r/Afi. When 2z/Ass is nearly equal to or greater than 2 the fibre behaves similarly to a circularly-birefringent fibre and consequently has high sensitivity to the Faraday effect. Increasing the spin ratio still further results in a fibre with slightly increased current sensitivity, but the residual elliptical birefringence decreases rapidly. This is because for large spins the fibre approaches the limit of large spin and low birefringence disclosed previously. Thus a spin ratio of 2 gives a suitable compromise between high current sensitivity and sufficient residual birefringence to resist external perturbing effects.Since both the local (linear) birefringence and resultant (elliptical) birefringence of the fibre are high, additional linear birefringence caused by external perturbations, either microscopic (such as microbending) or macroscopic (such as applied stress or winding) are swamped by the high level of internal birefringence. We can therefore use the fibre in current transformers without paying too much attention to packaging, which is certainly not the case for low-birefringence fibres. Further advantages of this fibre are the relative ease of fabrication using existing technology and the simplicity of launching and splicing.

Since the fibre is elliptically birefringent, the state of polarisation of the output light is in general elliptical, even though the input light is usually chosen to be linearly polarised. The Faraday effect in the fibre caused by the flow of electric current results in the rotation of the polarisation plane of the elliptically-polarised light at the output and can be detected by a polarisation analyser. For small angles the rotation is closely proportional to the current.

Moreover, the sensitivity of the rotation to current flow is similar to that in a perfect isotropic fibre for values of twist ratio 2T/Ass). It is a further requirement that the total number of twists in the coil should be greater than 0.736 x 10-5 IN, where I is the current in amperes and N is the number of turns in the coil.

A disadvantage of the new fibre is that the elliptical birefringence present is temperature sensitive, owing to the change in Afi of the starting linearly-birefringent fibre with temperature. The effect is to produce an output state of polarisation which varies with temperature and this leads to difficulties in measurement of the Faraday rotation. One way of overcoming this is to electronically track the output polarisation state. However, in general it is good practice to temperature compensate the fibre coils in order to simplify output polarisation tracking. Temperature compensation may be achieved in a number of ways.

The compensation can be realised by winding two jointed fibres which have the same length and similar temperature behaviour. but which have opposite twist directions. The orientation of the principle axes of the two fibres are arranged to be orthogonal at the joint, i.e.

the fast and slow birefringent axes are interchanged. One fibre has a right-hand twist and the other has a left-hand twist.

Referring to Figure 2, the two fibres 21,22 are bifilar wound (i.e. side by side) on a former 23. The start and the finish of one fibre are marked 24 and 25 respectively, while 26 and 27 represent the other fibre. The ends 26 and 25 are then spliced to each other at joint 28 and if the light is launched into end 24, it will travel via 25, 26 and output at 27. The effect of temperature is now opposite in the two fibre lengths and, since the two fibres are in intimate thermal contact, good compensation can be achieved.

Typically, highly-birefringent fibres have a decrease in birefringence Afi with increasing temperature of order 0.15% per "C. Moreover, the birefringence Afi is inversely proportional to wavelength A. Thus a change in operating wavelength of similar order can be used to compensate the variation in output polarisation state.

A highly-birefringent fibre can be designed to have a very small variation in birefringence with temperature. Birefringence in fibres can be obtained either by thermal stress created by incorporating sectors of high-expansion coefficient glass within the cladding (as in Bow-Tie fibres), or by core ellipticity. Whereas the former is obviously strongly temperaturedependent, the latter is a purely geometric effect and is therefore temperature-insensitive.

Unfortunately, in order to create large birefringence by the geometric effect it is necessary to have a large index-difference between elliptical core and cladding. In general, this leads to a large core/cladding expansion coefficient mismatch and a consequent temperature-dependent stress-birefringence component of around 30% of the geometrical birefringence.

Although this is an improvement over a purely stress-birefringent fibre, it is an advantage to eliminate stress-birefringence altogether.

One method is to ensure that the expansion coefficient of the core matches that of the cladding. A further method of achieving zero stress-birefringence in a geometrically-birefringent fibre is shown in Fig. 3, where a1, a2 and a3 are the expansion coefficients of the substrate (i.e. outer cladding 31, the inner cladding 32 and elliptical core 33 respectively. The core (ellipticity typically 2:1) is embedded in a circular inner cladding of expansion coefficient a2 = a3 so as to create a circular central region of uniform expansion coefficient. Thermal stress will then be isotropic and will contribute negligibly to the total birefringence. In this way the temperature coefficient of the (geometric plus stress) total birefringence will be small.

Another method which balances the thermal stress caused by the high expansion coefficient of the elliptical core is shown in Figure 4.

Here thermal stress which opposes that of the core 41 is created by regions 42 of high expansion coefficient in the cladding 43. An exact balance can be achieved by appropriate design of the stress-creating regions and their expansion coefficients. A number of the wellknown stress-creating sector configurations can be used (e.g. circular regions, Bow-Tie regions, elliptical-clad etc.) The orientation of the sectors relative to the core ellipse shown in Fig. 4 assumes that the expansion coefficients a2 > key1. If a, > a2 (e.g. TiO2 doping) the sectors should be disposed orthogonally to the positions shown.

When a Bow-Tie fibre is stretched it experiences an increase in linear birefringence caused by the difference in Poisson's ratio between the Bow-Tie sectors and the silica cladding. An increase in temperature, on the other hand, reduces the birefringence. This suggests a compensation scheme whereby the thermal decrease in birefringence is counterbalanced by an axial extension of the fibre.

Axial extension of the fibre can be thermally created by using materials of high expansioncoefficient in contact with the fibre, for example metal or plastics. Computations show that if the fibre is wound on an aluminium coil-former the aluminium thermal expansion will stretch the fibre axially a sufficient amount for the strain-related increase in birefringence to balance the natural thermal decrease in birefringence. Alternatively, a plastic over-jacket on the fibre can be similarly used to create thermal extension of the fibre. Experiments show this to be of the right order to compensate thermal variations in birefringence.

As an example of the invention, an optical fibre current transformer has been designed and constructed.

A standard highly linearly-birefringent fibre (Bow-Tie) preform made by the MCVD method was placed inside a glass sleeving tube which had an inner and outer diameter of 11 mum and 14mm respectively, The diameter of the preform was 10.5mm. Normal unspun highly linear-birefringent fibre was firstly drawn from this preform and the linear birefringence was measured to be B -- 2.1 x 10-4, (Afi = 2.09 x 103 rads/m at 633nm, corresponding to a 3mm beat length). Based on this measurement, the spinning rate was designed according to the criterion 21/Ass = 2, i.e. 330 turns/metre (3mm pitch). The preform was rotated during the draw at an appropriate rate using a DC motor so as to give a spun fibre.

The speed of the motor was controlled such that the spun fibre had 330 turns per metre.

The fibre was coated during the draw with U.V. curable acrylate, as is conventional. The diameter of the resultant spun highly birefringent fibre was 100m (without coating) and was single mode at a wavelength of 633nm.

A fibre current transformer was constructed by winding 165 turns of this fibre on a former which had a diameter of 33mm. A current carrying electrical conductor was inserted through the former and linearly-polarised light injected into the coil. The output polarisation state was analysed using a polariser and a photodiode. Currents up to 400A could be measured and the observed Faraday rotation agreed well with theoretical predictions. However, the device was found to be temperature sensitive. In a control experiment, a similar coil was wound using conventional spun lowbirefringence fibre and it was found that no significant Faraday rotation occurred, no doubt as a result of the severe bend and pressureinduced birefringence present.

A further experiment to demonstrate a method of temperature compensation of the fibre was conducted. Following the same procedure as mentioned in the previous example, two 20-metre long fibres were drawn. The spin rates were the same, but the direction of the spin was opposite for the two lengths.

The two fibres were bifilar wound side by side on a former which had a diameter of 55mm and jointed using UV-cured adhesive.

The optical axes of the linear birefringence of the two fibres were rotated by 90" at the joint by observing the orientation of the Bow Tie sectors and ensuring their orthogonality.

Current measurements were carried out in a similar manner to those described above and it. was found that fibre response to current was unchanged. However, the effect of changing temperature was considerable reduced.

Claims

1. Apparatus for the sensing of magnetic fields by means of the Faraday effect incorpo rating an optical fibre light guide arranged in a coil wherein said coil comprises an optical fibre having diminished birefringence produced by spinning said fibre during its drawing process.

2. Apparatus for the sensing of magnetic fields by means of the Faraday effect as claimed in claim 1 wherein said fibre is produced from a fibre having a high initial birefringence.

3. Apparatus for the sensing of magnetic fields by means of the Faraday effect as claimed in claim 2 wherein the linear birefringence is not small in comparison with the twist rate of the fibre.

4. Apparatus for the sensing of magnetic fields by means of the Faraday effect as claimed in claim 3 wherein the twist/birefringence ratio is of the order of 1 to 2.

5. Apparatus for the sensing of magnetic fields by means of the Faraday effect as claimed in claim 3 wherein the twist/birefringence ratio is substantially equal to 2.

6. Apparatus for the sensing of magnetic fields by means of the Faraday effect as claimed in any one of the preceding claims incorporating means for compensating for variations in ambient temperature.

7. Apparatus for the sensing of magnetic fields by means of the Faraday effect as claimed in claim 6 wherein said means for compensating for variations in ambient temperature comprises means for adjusting the operating wavelength of light transmitted through the optical fibre light guide.

8. Apparatus for the sensing of magnetic fields by means of the Faraday effect as claimed in claim 6 wherein said means for compensating for variations in ambient temperature comprises a coil having a pair of bifilar fibres of substantially similar length and temperature sensitivity but opposite twist directions.

9. Apparatus for the sensing of magnetic fields by means of the Faraday effect substantially as herein described with reference to and as shown in the accompanying drawings.