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WO1996000905A1 - Capteurs optiques - Google Patents

Capteurs optiques Download PDF

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Publication number
WO1996000905A1
WO1996000905A1 PCT/GB1995/001513 GB9501513W WO9600905A1 WO 1996000905 A1 WO1996000905 A1 WO 1996000905A1 GB 9501513 W GB9501513 W GB 9501513W WO 9600905 A1 WO9600905 A1 WO 9600905A1
Authority
WO
WIPO (PCT)
Prior art keywords
optical sensor
light
sensing element
polarisation
polarised light
Prior art date
Application number
PCT/GB1995/001513
Other languages
English (en)
Inventor
Wamadeva Balachandran
Franjo Cecelja
Michael Berwick
Srboljub Radoslav Cvetkovic
Original Assignee
University Of Surrey
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by University Of Surrey filed Critical University Of Surrey
Priority to AU27995/95A priority Critical patent/AU2799595A/en
Publication of WO1996000905A1 publication Critical patent/WO1996000905A1/fr

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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
    • G01R29/00—Arrangements for measuring or indicating electric quantities not covered by groups G01R19/00 - G01R27/00
    • G01R29/08—Measuring electromagnetic field characteristics
    • G01R29/0864—Measuring electromagnetic field characteristics characterised by constructional or functional features
    • G01R29/0878—Sensors; antennas; probes; detectors
    • G01R29/0885—Sensors; antennas; probes; detectors using optical probes, e.g. electro-optical, luminescent, glow discharge, or optical interferometers

Definitions

  • the invention relates to optical sensors, and particularly to optical sensors including electro-optic sensing elements (sensitive to electric field) and magneto-optic sensing elements (sensitive to magnetic field) .
  • the invention relates particularly, though not exclusively, to such optical sensors that are suitable for the measurement of the electric and/or magnetic field components of weak electromagnetic fields, especially in the near field.
  • the present invention provides an optical sensor comprising either an electro-optic sensing element in the form of a crystal of cadmium telluride (CdTe) with a cubic crystalline structure or a magneto-optic sensing element in the form of a crystal of manganese-doped cadmium telluride (Cd, Mn Te) with a cubic crystalline structure.
  • an electro-optic sensing element in the form of a crystal of cadmium telluride (CdTe) with a cubic crystalline structure
  • a magneto-optic sensing element in the form of a crystal of manganese-doped cadmium telluride (Cd, Mn Te) with a cubic crystalline structure.
  • an optical sensor comprising, a light source, a sensing probe comprising a sensing element, polarising means for converting light from the light source to polarised light having a predetermined polarisation and analyser means, wherein said polarised light passes through the sensing element, the sensing element comprises either an electro-optic sensing element in the form of a crystal of cadmium telluride (CdTe) with a cubic crystalline structure which is effective to modify said predetermined polarisation according to an applied electric field or a magneto-optic sensing element in the form of a crystal of manganese-doped cadmium telluride (Cd, ⁇ —XMnXTe) with a cubic crystalline structure which is effective to modify said predetermined polarisation according to an applied magnetic field and said analyser means produces an output in response to the polarisation of the polarised light after the polarised light has passed through the sensing element, the sensing probe having an elongated configuration
  • an optical sensor comprising an electro-optic sensing element in the form of a crystal of cadmium telluride (CdTe) with a cubic crystalline structure, means for directing a beam of polarised light through said crystal of cadmium telluride (CdTe) in a direction perpendicular to the (110) crystallographic plane, the polarised light having a predetermined polarisation and the sensing element being effective to modify the predetermined polarisation according to an applied electric field, and means responsive to the polarisation of the polarised light after the polarised light has passed through the sensing element to produce an output representative of the applied electric field.
  • an electro-optic sensing element in the form of a crystal of cadmium telluride (CdTe) with a cubic crystalline structure
  • an optical sensor comprising, a magneto-optic sensing element in the form of a crystal of manganese-doped cadmium telluride (Cd, ⁇ *-&MnXTe) with a cubic crystalline structure, means for directing a beam of polarised light through said crystal, wherein the beam passes through the crystal more than once, the polarised light has a predetermined polarisation, and the sensing element modifies said predetermined polarisation according to an applied magnetic field, and means responsive to the polarisation of the polarised light after the beam has passed through the sensing element to produce an output representative of the applied magnetic field.
  • a magneto-optic sensing element in the form of a crystal of manganese-doped cadmium telluride (Cd, ⁇ *-&MnXTe) with a cubic crystalline structure
  • means for directing a beam of polarised light through said crystal wherein the beam passes through the crystal more than once, the polarised light has a predetermined polarisation
  • Figure 1 shows a diagrammatic view of an optical sensor according to the invention
  • Figure 2a shows a longitudinal sectional view through an electro-optic sensing probe in the optical sensor of Figure 1
  • Figure 2b shows a longitudinal sectional view through a magneto-optic sensing probe in the optical sensor of Figure 1,
  • FIG. 3 is a block diagram showing a signal processing circuit in the optical sensor of Figure 1, and
  • Figure 4 is a longitudinal sectional view through a collimator used in the electro-optic and magneto-optic sensing probes shown in Figures 2a and 2b.
  • the optical sensor comprises a sensing probe 10, a light source 20 in the form of a laser diode, a photodetection unit 30, a fibre-optic link 40 for directing light from the light source 20 to the sensing probe 10 and from the sensing probe 10 to the photodetection unit 30, and a signal processing unit 50 for processing an output from the photodetection unit 30.
  • the sensing probe 10 may contain either an electro-optic sensing element (sensitive to electric field) or a magneto-optic sensing element (sensitive to magnetic field) and, in accordance with the present invention, the electro-optic sensing element comprises a crystal of cadmium telluride (CdTe) with a cubic crystalline structure, whereas the magneto-optic sensing element comprises a crystal of manganese-doped cadmium telluride (Cd,” ⁇ XMnXTe) with a cubic crystalline structure.
  • the electro-optic sensing element comprises a crystal of cadmium telluride (CdTe) with a cubic crystalline structure
  • the magneto-optic sensing element comprises a crystal of manganese-doped cadmium telluride (Cd,” ⁇ XMnXTe) with a cubic crystalline structure.
  • Crystalline cadmium telluride (the electro-optic sensing element) exhibits a linear electro-optic effect known as the Pockel's effect in which linear birefringence is induced in the crystalline material , in proportion to the strength of an applied electric field.
  • This effect gives rise to a change in the ellipticity of polarised light propagating through the crystal, which can be expressed as a phase retardation ⁇ between two mutually orthogonal linearly polarised components of the light. More specifically, the phase retardation ⁇ is related to the component of electric field E perpendicular to the (110) crystallographic plane and, at the same time, perpendicular to the direction of propagation of the polarised light, by the equation
  • n and r.. are refractive index and electro-optic coefficient for the crystalline material respectively, ⁇ is the wavelength of the propagating light and 1 is the distance traversed by the polarised light through the crystal.
  • the phase retardation hence the ellipticity of the polarised light is caused to oscillate at the frequency of the field.
  • Crystalline manganese-doped cadmium telluride exhibits a linear magneto-optic effect known as the Faraday effect in which the polarisation plane of linearly polarised light is caused to rotate through an angle ⁇ in proportion to the strength of an applied magnetic field.
  • the rotation angle ⁇ is caused to oscillate at the frequency of the field.
  • crystalline cadmium telluride and crystalline manganese-doped cadmium telluride respectively exhibit high sensitivity to applied electric and magnetic fields, especially at high frequencies of up to 2GHz and yet, compared with known electro-optic and magneto-optic materials are relatively insensitive to environmental changes.
  • the present materials find particular, though not exclusive application in the measurement of weak electric and magnetic fields against a background of environmental noise, as may arise, for example, in the near field of some r.f. transmitting antennas, such as the antenna of a mobile telephone.
  • the optical sensors described with reference to the accompanying drawings are designed to exploit the high sensitivity of the electro-optic and magneto-optic materials used.
  • the electrical and electronic components of the sensor namely the light source 20, the photodetection unit 30 and the signal processing unit 50 are shielded to reduce electromagnetic interference (EMI), and the sensing probe 10 is remotely coupled to the light source 20 and photodetection unit 30 by means of the fibre-optic link 40.
  • the sensing probe 10 contains only dielectric materials and is electrically passive. These measures are effective to reduce perturbation of the measured field within the electro-optic or magneto-optic sensing element of the sensing probe.
  • Figure 2a of the accompanying drawings shows a longitudinal, sectional view through an electro-optic sensing probe which is sensitive to electric field and has an electro-optic sensing element 11 in the form of a crystal of cadmium telluride (CdTe) with a cubic crystalline structure
  • Figure 2b shows a longitudinal, sectional view through a magneto-optic sensing probe which is sensitive to magnetic field and has a magneto-optic sensing element 12 in the form of a crystal of manganese-doped cadmium telluride
  • sensing probes shown in Figures 2a and 2b have a generally elongate con iguration, with the respective sensing element 11,12 in each probe being mounted at one extreme end, in remote, spaced relationship to other optical components in the probe.
  • This configuration reduces any perturbation of the measured field to which the optical components might otherwise give rise were they to be positioned in close proximity to the associated sensing element.
  • Beam B- passes through a polariser 14, is reflected through 180 by a complementary reflector 15 and then passes through the electro-optic sensing element 11 towards a polarisation state analyser 16.
  • Polariser 14 comprises a polarising element 141 in the form of a Glan Thompson prism polariser which converts light received from source 20 to linearly polarised light having a defined polarisation plane, and a retardation element 142 in the form of a quarter-wave plate which is set to convert the linearly polarised light from the polarising element 141 to circularly polarised light.
  • a polarising element 141 in the form of a Glan Thompson prism polariser which converts light received from source 20 to linearly polarised light having a defined polarisation plane
  • a retardation element 142 in the form of a quarter-wave plate which is set to convert the linearly polarised light from the polarising element 141 to circularly polarised light.
  • an applied electric field causes a change in the polarisation state of light passing through the electro-optic sensing element 11.
  • an applied electric field converts the circularly polarised light in beam B- to elliptically polarised light having an ellipticity related to the strength of the applied field.
  • the ellipticity of polarised light in beam B after the beam has passed through the electro-optic sensing element 11 gives a measure of an applied electric field to which the sensing element is exposed.
  • polarisation state is measured indirectly using the polarimetric analyser 16 which produces a measurable modulation of light intensity in beam B, according to the polarisation state of the light.
  • the polarimetric analyser 16 comprises a beam splitter 161, a right-angle prism 162 and two polarisers 163,164 having mutually orthogonal polarisation planes.
  • the beam splitter 161 divides beam B into two beams B ,B of equal intensity and, in conjunction with right-angle prism 162 directs each beam B 2' B 3 through a respective polariser 163,164.
  • the polarisation state analyser 16 resolves elliptically polarised light in beam B- into two, mutually orthogonal, linearly polarised components whose relative intensities are related, in a known manner, to the required ellipticity representative of an applied electric field that is to be measured.
  • each polariser 163,164 is supplied to a respective photodetector 31,32 in unit 30 via a respective collimator 17,18 and a respective multi-mode fibre 42,43 in fibre-optic unit 40, and each photodetector 31,32 produces a detection signal representative of the detected light intensity.
  • each channel C. ,C_ in unit 50 comprises an amplification stage 51,51' and a bandpass filter 52,52' having a centre frequency tuned to select a desired frequency range of applied electric field.
  • the passband of filters 52,52' is centred on the applied field frequency (e.g. 900 or 1800MHz) .
  • the amplified and filtered signals in each channel are combined in a differential amplifier 53 to produce an output 0/P(l) representative of the amplitude and frequency of the applied electric field to which the electro-optic sensing element 11 is exposed.
  • the signals in channels C,,C_ could be combined in a variety of different ways to derive the required output signal.
  • the sensitivity of the electro-optic sensing element 11 to an applied electric field depends, inter alia, on the orientation of the cadmium telluride crystal and, for optimum sensitivity, it is found that beam B- should be aligned in a direction perpendicular to the (110) crystallographic plane. This orientation is found to offer the optimum sensor characteristics i.e. the maximum electro-optic effect. Furthermore, in this preferred orientation, the crystal has fixed, induced eigenaxes and is predominently sensitive to electric field in a single direction only; namely, the direction orthogonal to the (110) plane.
  • the optical components in the sensing probe should preferably be aligned to an accuracy of better than 7mrad.
  • collimators 13,17,18 are of particular importance and these components have been specially designed with a view to improved sensitivity.
  • each collimator comprises a ball lens 60 which is accurately centred on the longitudinal axis of an associated optical fibre 61.
  • the lens 60 and the optical fibre 61 are mounted in respective, closely-fitting cylindrical holders 62 and 63.
  • the axial position of the fibre within the holder 63 can be readily adjusted to optimise the spacing between the lens 60 and tip of the fibre 61.
  • lens 60 forms light from fibre 61 into an outgoing beam B. having substantially the same diameter as that of the lens (typically 2mm), whereas in the case of collimators 17,18, lens 60 focuses light in an incoming beam B _,B_ onto the end face of the optical fibre.
  • the size of the electro-optic effect produced in a crystal of cadmium telluride and also the extent of optical absorption in this material depends upon the wavelength of light used.
  • the preferred wavelength for cadmium telluride is 1300nm.
  • surfaces s i' S p are polished, preferably to a quality better than ⁇ /8 (where ⁇ is the wavelength of light) and are provided with anti-reflective coatings AR which, in this example, would have a centre wavelength of 1300nm (the wavelength of light in beam B-).
  • Beam B ' passes in turn through a polariser 14' and the magneto-optic sensing element 12 and is then reflected by 180 complementary reflection 15* back through the sensing element 12 towards a polarimetric analyser 16 ' .
  • Polariser 14* comprises a Glan Thompson prism polariser 141' and a half-wave plate 142' which, in combination, convert light from source 20 into linearly polarised light having a preset polarisation azimuth.
  • magneto-optic sensing element 12 The effect of magneto-optic sensing element 12 is to rotate this preset polarisation plane through an angle ⁇ which is proportional to an applied magnetic field in the propagation direction of beam B ' .
  • the rotation angle ⁇ is also proportional to the distance traversed by the beam through the magneto-optic sensing element 12, and so the effect of reflecting the beam back through the sensing element is to double the rotation angle, giving improved sensitivity.
  • a multi-path arrangement in which beam B- ' transverses the sensing element 12 more than twice would give even greater sensitivity.
  • the polarimetric analyser 16' comprises a beam splitter 161', a right-angle prism 162', and two polarisers 163', 164' (e.g. Glan Thompson prism polarisers) having mutually orthogonal polarisation planes, respectively set at +45° and -45 relative to the preset polarisation plane of polariser 14'.
  • analyser 16' resolves the linearly polarised light in beam B, ' into two mutually orthogonal, plane polarised components whose relative intensities are related, in known manner, to the rotation angle ⁇ which represent the applied magnetic field that is to be measured.
  • each polariser 163 ',164' is supplied to a respective photodetector 31,32 in unit 30 via a respective collimator 17', 18' and a respective multi-mode fibre 42',43" in unit 40, and each photodetector 31,32 produces a detection signal representative of the detected light intensity.
  • the detection signals from photodetectors 31,32 are processed, as before, in respective channels c i' C 2 of the signal processing unit 50 and are combined in differential amplifier 53 to produce an output 0/P(2) representative of the measured rotation angle ⁇ , in turn proportional to the applied magnetic field.
  • the sensitivity of the magneto-optic sensing element 12 depends upon the proportion of manganese in the manganese-doped cadmium telluride crystal and, as in the case of the electro-optic sensing element 12, upon the wavelength of light used.
  • the proportion of manganese in the crystal affects the frequency response characteristics of the crystal (the higher the proportion of manganese the greater the frequency range) and also the absorption and temperature response characteristics of the material.
  • the sensitivity of the magneto-optic sensing element 12 does not depend critically upon orientation; however, in order to reduce such unwanted effects as variation of polarisation state due to intrinsic linear birefringence and scattering and or internal reflection of light, it is preferred to align the incident light beam B- * with the optic axis of the crystal, this being particularly beneficial in the case of a multipath configuration such as that described with reference to Figure 2b.
  • the sensing element is cylindrical such that circular symmetry minimises the effect of induced linear birefringence.
  • the sensitivity degradation is minimised.
  • the anti-reflective coating would have a centre wavelenth of 650 nm.
  • the polarisers used in the electro-optic and magneto-optic sensing arrangements should have an extinction ratio of better than 1:10 4
  • the quarter-wave plate 142 (used in the electro-optic sensing arrangement) and the half-wave plate 142' (used in the magneto-optic sensing arrangement) should have a phase retardation accuracy better than lmrad.
  • the single mode fibre 41,41' used to conduct light from the laser-diode to the sensing probe should be highly, linearly birefringent - a so-called Hi-Bi optical fibre.
  • a Hi-Bi optical fibre When correctly orientated, a Hi-Bi optical fibre will preserve the linear polarisation state of light from the laser-diode, and is thereby able to maintain a constant optical power to the sensing probe, even when the fibre is subjected to environmental changes (e.g. bending and temperature changes) which tend to have an adverse effect on the transmission characteristics of multi-mode or low-birefringence single mode fibres.
  • the level of output noise from the laser-diode should, of course, be as low as possible and, for optimum sensitivity it is desirable that the level of output noise should be less than 20pW(Hz) "• - . Since a high proportion of the output noise is derived from the electrical power supply to the laser, the supply noise should preferably not exceed l ⁇ A for a maximum laser-diode output power of 20mW.
  • the laser diode has a stabilised power output and, in this embodiment, a stabilising control signal C_ is generated in a feedback circuit shown in Figure 3. Control signal C_ is derived from the signal processing unit 50.
  • a respective low-pass filter 54,55 extracts a low frequency component ( ⁇ lMHz) from each channel C-,C_ in unit 50 and an averaging circuit 56 averages the extracted, low frequency components to produce the stabilising control signal C_, which is then supplied to the laser-diode drive circuit 70.
  • Electro-optic and magneto-optic sensors described with reference to the drawings have high sensitivity to applied electric and magnetic fields over a frequency range up to about 2GHz. It has been determined that when suitably optimised, as described herein, the electro-optic sensor will have a resolution (i.e. minimum measurable electric field) of about 2Vm ⁇ , and the magneto-optic sensor will have a resolution (i.e. minimum measurable magnetic flux density) of about 0.25 ⁇ T, and both sensors will have a dynamic range greater than 60dB, a linearity (in the respective ranges 2Vm to lOOOVm and 0.25 ⁇ T to lmT) of better than 1% and a spatial resolution of approximately 1cm.
  • the electro-optic sensor will have a resolution (i.e. minimum measurable electric field) of about 2Vm ⁇
  • the magneto-optic sensor will have a resolution (i.e. minimum measurable magnetic flux density) of about 0.25 ⁇ T
  • both sensors will have a dynamic range greater than 60
  • sensors according to the present invention are passive devices and are therefore inherently safe and have low power consumption.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Electromagnetism (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • Measuring Magnetic Variables (AREA)

Abstract

Capteur optique comprenant une sonde (10) de détection, une source (20) lumineuse à diode laser, un photodétecteur (30), une liaison (40) par fibre optique destinée à diriger la lumière provenant de la source (20) lumineuse vers la sonde (10) de détection, et celle provenant de cette sonde (10) vers le photodétecteur (30), ainsi qu'une unité (50) de traitement de signaux aux fins de traitement d'un signal de sortie provenant du photodétecteur (30). La sonde (10) de détection contient soit un élément (11) de détection électro-optique sous la forme d'un cristal de tellurure de cadmium (CdTe) possédant une structure cristalline cubique, soit un élément (12) de détection magnéto-optique sous la forme d'un cristal de tellurure de cadmium dopé au manganèse (Cdl-xMnxTe) et possédant une structure cristalline cubique.
PCT/GB1995/001513 1994-06-30 1995-06-28 Capteurs optiques WO1996000905A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU27995/95A AU2799595A (en) 1994-06-30 1995-06-28 Optical sensors

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB9413179.4 1994-06-30
GB9413179A GB9413179D0 (en) 1994-06-30 1994-06-30 Optical sensors

Publications (1)

Publication Number Publication Date
WO1996000905A1 true WO1996000905A1 (fr) 1996-01-11

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Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/GB1995/001513 WO1996000905A1 (fr) 1994-06-30 1995-06-28 Capteurs optiques

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AU (1) AU2799595A (fr)
GB (1) GB9413179D0 (fr)
WO (1) WO1996000905A1 (fr)

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2342161A (en) * 1998-09-30 2000-04-05 Ando Electric Electro optic probe
EP0992800A1 (fr) * 1998-10-09 2000-04-12 Nippon Telegraph and Telephone Corporation Appareil et procédé pour mesurer la fréquence de résonance d'un circuit électrique
RU2170439C1 (ru) * 1999-12-28 2001-07-10 Московский государственный университет леса Микрорезонаторный волоконно-оптический датчик электрического тока
US6348787B1 (en) 1998-09-30 2002-02-19 Ando Electric Co., Ltd. Electrooptic probe

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS58196463A (ja) * 1982-05-12 1983-11-15 Hitachi Ltd 光電界測定装置
EP0454860A1 (fr) * 1989-11-13 1991-11-06 Dai Nippon Printing Co., Ltd. Capteur de potentiel utilisant un cristal electro-optique et procede de mesure de potentiel

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS58196463A (ja) * 1982-05-12 1983-11-15 Hitachi Ltd 光電界測定装置
EP0454860A1 (fr) * 1989-11-13 1991-11-06 Dai Nippon Printing Co., Ltd. Capteur de potentiel utilisant un cristal electro-optique et procede de mesure de potentiel

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
PATENT ABSTRACTS OF JAPAN vol. 8, no. 45 (P - 257) 28 February 1984 (1984-02-28) *

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2342161A (en) * 1998-09-30 2000-04-05 Ando Electric Electro optic probe
GB2342161B (en) * 1998-09-30 2000-12-20 Ando Electric Electro-optic probe
US6348787B1 (en) 1998-09-30 2002-02-19 Ando Electric Co., Ltd. Electrooptic probe
US6507014B2 (en) * 1998-09-30 2003-01-14 Ando Electric Co. Ltd. Electro-optic probe
DE19946664C2 (de) * 1998-09-30 2003-01-30 Ando Electric Elektrooptische Sonde
EP0992800A1 (fr) * 1998-10-09 2000-04-12 Nippon Telegraph and Telephone Corporation Appareil et procédé pour mesurer la fréquence de résonance d'un circuit électrique
US6288530B1 (en) 1998-10-09 2001-09-11 Nippon And Telegraph And Telephone Corp. Apparatus and method for measuring resonance frequency of electric circuit
RU2170439C1 (ru) * 1999-12-28 2001-07-10 Московский государственный университет леса Микрорезонаторный волоконно-оптический датчик электрического тока

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Publication number Publication date
GB9413179D0 (en) 1994-09-28
AU2799595A (en) 1996-01-25

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