Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/02—Measuring direction or magnitude of magnetic fields or magnetic flux
  • G01R33/032—Measuring direction or magnitude of magnetic fields or magnetic flux using magneto-optic devices, e.g. Faraday or Cotton-Mouton effect
  • G01R33/0322—Measuring direction or magnitude of magnetic fields or magnetic flux using magneto-optic devices, e.g. Faraday or Cotton-Mouton effect using the Faraday or Voigt effect
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R15/00—Details of measuring arrangements of the types provided for in groups G01R17/00 - G01R29/00, G01R33/00 - G01R33/26 or G01R35/00
  • G01R15/14—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks
  • G01R15/24—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using light-modulating devices
  • G01R15/245—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using light-modulating devices using magneto-optical modulators, e.g. based on the Faraday or Cotton-Mouton effect
  • G01R15/246—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using light-modulating devices using magneto-optical modulators, e.g. based on the Faraday or Cotton-Mouton effect based on the Faraday, i.e. linear magneto-optic, effect
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/09—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on magneto-optical elements, e.g. exhibiting Faraday effect
  • G02F1/095—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on magneto-optical elements, e.g. exhibiting Faraday effect in an optical waveguide structure

Definitions

• 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 li.ttle 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.
• the monitor has a large bandwidth which facilitates the monitoring of very fast transients on the grid.
• optical-fibre current monitors or transformers have considerable advantages in measuring current on high-voltage lines and in protective applications .
• 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.
• 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
• 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.
• the starting fibre is a highly linearly-birefringent fibre which is spun during the fibre drawing process.
• the spin rate is chosen such that the linear birefringence ⁇ is not small compared with the twist rate ⁇ , as is the case when manufacturing a low birefringence fibre.
• the twist/birefringence ratio 2 ⁇ / ⁇ is of order 1 to 2 and the linear birefringence is only partially cancelled by the twisting.
• 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 elliptical birefringence can be controlled by the spin ratio 2 ⁇ / ⁇ .
• 2 ⁇ / ⁇ (5 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.
• a spin ratio of 2 gives a suitable compromise between high current sensitivity and sufficient residual birefringence to resist external perturbing effects.
• 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 elliptlcally-polarised light at the output and can be detected by a polarisation analyser. For small angles the rotation is closely proportional to the current.
• the sensitivity of the rotation to current flow is similar to that in a perfect isotropic fibre for values of twist ratio 2 ⁇ / ⁇ ).
• 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 1s 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 ⁇ of the starting linearly-birefrlngent 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.
• 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.
• 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.
• highly-birefringent fibres have a decrease in birefringence ⁇ with increasing temperature of order 0.15% per °C.
• the birefringence ⁇ is inversely proportional to wavelength A.
• 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 temperature-dependent, 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.
• ⁇ 1 , ⁇ 2 and ⁇ 3 are the expansion coefficients of the substrate (i.e. outer cladding 31, the inner cladding 32 and elliptical core 33 respectively.
• Axial extension of the fibre can be thermally created by using materials of high expansion-coefficient 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.
• 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 11mm and 14mm respectively.
• the diameter of the preform was 10.5mm.
• 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 lOO ⁇ m (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.
• a similar coil was wound using conventional spun low-birefringence fibre and it was found that no significant Faraday rotation occurred, no doubt as a result of the severe bend and pressure-induced birefringence present.

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• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• General Physics & Mathematics (AREA)
• Nonlinear Science (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• Optics & Photonics (AREA)
• Measuring Instrument Details And Bridges, And Automatic Balancing Devices (AREA)

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• 1986
  • 1986-05-20 GB GB868612190A patent/GB8612190D0/en active Pending
• 1987
  • 1987-05-20 EP EP19870903362 patent/EP0267256A1/en not_active Withdrawn
  • 1987-05-20 JP JP50313487A patent/JPH01500615A/ja active Pending
  • 1987-05-20 GB GB08711926A patent/GB2190744A/en not_active Withdrawn
  • 1987-05-20 WO PCT/GB1987/000345 patent/WO1987007387A1/en not_active Application Discontinuation

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