FR2475230A1 - Optical method for current measurement - uses magneto=optical interaction between field around wire and laser light passing through coiled optical fibre - Google Patents

Abstract

The system uses a laser (3) producing a beam of light (21) which is linearly polarised. The light passes through a half-wave plate (4) before entering an optical cube (5). The cube splits the beam into two parts (22,23). One part of the beam is sent via further optical components (6,8) to one end of an optical fibre (2). The fibre is coiled around a former (1a) on the current carrying wire. The light passes through a fibre around the coil and emerges at the other end (2b) to be reintroduced into the optical system. Two signals are derived and compared by a differential amplifier (17) to determine the differential modulation between them. As a result of the magneto-optical effect between the current and the beam this provides a signal indicating the magnitude of the current.

Description

!

The invention relates to a device for measuring intensity

  tee of the electric current in a conductor by magneto optic effect, oh a polarized light beam coming from a coherent source travels in a material optical medium subjected to the magnetic field created by the current, the current measurement resulting from the measurement of beam variation under

  the effect of the cross magnetic field.

By material optical medium is meant a transparent medium having a refractive index different from the vacuum index, so that the speed of propagation in the material optical medium is less than the speed of the

light in the void.

  Faraday's work has shown that in a

  normally isotropic material optic medium, a magnetic field

t. a refractive anisotropy appeared

vant the direction of this field, which results in rotations of the plane of polarization of a polarized beam propagating parali.lement in the field. A circularly polarized beam

  pro-age at different speeds depending on whether its polarization

  tion es4 dextroyre or levogyre in relation to the orientation

  of the magnetism chamcp. The angle which the plane of polarization rotates

  tion is the product of the integral of the beam

  ceau following the magnetic field by a specific constant of the environment called "verdant constant". In fact the induced birefringence derpen.d of the magnetic induct-onr in the medium, of

  so that Verdet's constant is a function of permeability -

  magnetic lit4 in the middle, and therefore weak for mid-

diariagn4tiaiues and strong places for ferromagneti-

  aues. On the other hand, for ferromagnetic media, the cons-

  aunt de Verdet varies with temperature and saturation.

  It will be noted that the birefringence induced by a magnetic field is sensitive to the direction of propagation of an electromagnetic wave, while the birefringence induced by

  an electric field or a mechanical stress are insensi-

I have the sense of propagation. In this sense the birefringence in-

  duite by a magnetic field can be called birefringence with polar orientation, by allusion to the North-South orientation of the

mngngtiaue field.

  La mes.re d eorur-tn + tra-versart ur c ^ dcte.r 'ecto.-ue by rmesure of the effects of blréfringeree Lind, Àtîe rar le C: kmp mag.ntioue created by the pasa-e of 2Gcrar.t darln the cond _ eur

in an optic medium material l diaagn'tiue nr {ente r-

obvious interest when there are currents _es 4l - s d - conductors carried 'un- na-te tensortn by r - 3por - earth

due to prop.r tts - soIantes in-rins'.ues d_ = eu ---

tioue attached to the possibility of interposing between e - 'optical optics and the conductor of the msalar'tes c.rveab eF

  and the absence of s-turatin from the magnetic medium. In or-

  being the material optic medium does not draw energy from.

  current flowing through the conductor, this is not the case for conventional devices of current intensity such as coupled transformer. ammeters. Finally mag.'ito - opti ues devices will also be sensitive to direct currents o_'à dje constant conients,

  It was gold opo-, to measure the current in a conduc-

tor, to spiral an optical fiber around the conductors so that the optical fiber is so united, a magnetic field that

  uniform according to its! on eur, and to determine! - ng'e de rc-

  tation of the plan of: the spread of a coherent beam, penetrating

  trant in the fiber by one end face and omering by the other end face. However, this arrangement is not very precise and not very sensitive because, in-e p: rt, the fib. re maybe

the accidental birefringenc-s seat of the constrained

  your thernics or mncan: ales (flexions or vibrat o; s), or has electr-read fields, and on the other hand, the range of angles of rotation 7o nlan of dalariFLon does not exceed, except rarticuières provisions including use

  of a reference beam in spatial coherence with the do

ceau de mLure, this yes i ... 'ioue path lengths of the same order of magnitude r:,. r' .es acux beams between a separation wedge and a point -omparaeleon, s.- n that the noise affecting the rotation of rotation is important on the 3 reference beam; --i limited: you seriously the length of usable fiber and p:% repeating the sense.

  The invention has for -, jet a resure device

  rant in a conductor ': r - tîcc cap: .ganrtiu- resul-

  both on an optic fiber spiraled around the conductor and

traversed by a polarized light beam, where the birefrin-

accidental injuries are compensated.

  The invention also relates to a device for measuring

  of current of this kind where the optical fiber can be

great length and high sensitivity.

For these effects, the invention proposes a device for measuring the intensity of electric current in a conductor by magneto-optical effect, where a polarized light beam from a

  coherent source travels in a material optical medium

placed in the magnetic field created by the current, the measurement of

  rant resulting from the measurement of the path variation of the beam under the effect of the magnetic field crossed, device o the material optical medium consists of an optical fiber with two end faces, spiraled around the conductor so that the magnetic field is longitudinally directed

  daes fiber, characterized in that it comprises an optical means

  that separator adapted to divide the polarized beam in two

  secondary beams with penetrating circular polarization

  pectively in the fiber by one of the two end faces

  tee and energenat by the other, and to gather the beams

emerging in collinearity so that they interfere, a mo-

detection yen receiving the interfering beams and deli-

  signaling a differential modulation signal.

As the two secondary beams traversing the same optical chewin in the fiber, but in opposite direction of each other, all the disturbances due to birefringences

accidental, which have the same effects on the beams

  condaries regardless of the direction of propagation, will induce the same path differences on the two beams, while

  that the path differences caused by the magnetic field

that will be of opposite directions for the two respective beams

and their effects will be added to the figure of inter-

ference. Furthermore, the magnetic field being directed in the direction of propagation of the beams, the induced birefringernce simply results in a speed variation of

  propagation in the fiber, and a phase shift of the emerging beams

  gents, a displacement of the interference patterns between these beams, to which the detection means will be sensitive

without complex provisions.

  Preferably, the beam coming from the coherent source being with linear polarization, two quarter-wave plates will be inserted in the respective paths of the secondary beams, with a preferred axis orientation at 450 of the plane of

  polarization. Thus, in optical fiber, .the beams se-

with circular polarization after crossing of the respective quarter-wave plate, and after emergence of the fiber and

crossing of the other quarter wave plate, will resume pola-

  parallel linear risations to interfere and fall on

the detector.

  As the detector is sensitive to the absolute value of the path difference, the device will be, without special precautions, sensitive to the absolute value of the current in the conductor, and will not give information on the direction of the current. Also preferably we will insert on the -traject of one of the secondary beams an optical modulator which will move

  by a chosen quantity the frequency of this beam. The pre-

sence of the optic modulator induces a continuously increasing phase shift in a determined direction, to which will be added or subtract the phase shift due to the magnetic field, the

people will become noticeable as well.

In preferred arrangement, part of the beam from the source will be diverted to come, in combination with the interfering beams, to act on a heterodyne balance detection device with two detectors at the input of a

differential amplifier. We then take advantage of the awareness-

  noise immunity inherent in this device

detection.

  By interposing a Faraday cell on a path of

secondary beam, the Faraday cell, which induces a re-

  late walk proportional to a control current, then compensates for the difference in walk induced in the optical fiber by the magnetic field. The control current, measured by a conventional device, will be an image of the current

  in the conductor for exact compensation of differences

these walking.

Preferably, the control current is saved using the

  of an electronic regulator attacked by the detector,

  in order to automatically obtain the exact compensation.

The characteristics and advantages of the invention stand out

  will derive from the description which follows, for

  example, with reference to the accompanying drawing.

  Depending on the embodiment chosen and shown,

a conductor 'el or'a high voltage conductor of

  electriu bucket, crossed by a current that we want to measure-

  rer, passes through a mandrel la, insulating cylindrical, on which is spiraled, coiled turns, an optical fiber 2, whose ends 2a and 2b, spaced from the mandrel la, are provided

  optics or input faces with parallel axes.

A gas laser 3, neon helium, emits a beam 21 pola-

  ri.é linearly, which crosses a half-wave plate 4 before peo, -re in an optical cube 5, with a plane section passing through two parallel ridges so that the beam falls to

  4gE of cutting plau. Such an optical cube, called a polar cube,

sor, rof Ignt the light with linear polarization parallel P0 to the asrtedils 1st section plane and let the light pass] larîartîo lin (area perpendicular to these edges. Like a half-wave plate rotates the plane of polarization of the wave ..merging with respect to the plane of polarization of a wave inci <ent linear polarization, by an angle twice that favored by the optical axis of this blade by

compared to the polarization plau of the incident wave, the orient-

  The blade 4 allows adjustment of the intensity ratio.

  umine-uses tEFs of beams 22 and 23 yes emerge from the cube, with linear Dolarization, respectively parallel

  and perpendicularly to the plane of the figure.

  The beam 22 falls on a semi-reflecting plate 6, oriented e 4 in the direction of the beam 22, and which divides the beam 22 into two secondary beams, reflected 24 and tran.mis 25. The secondary beam 24 crosses a quarter plate

wave 8, with a privileged axis at 45 from the direction of po-

  the beam 24, so that the beam 24 'emerges

  giant of blade 8 is circularly polarized before

fall on the input optics of the end 2a of the fiber 2.

  The secondary beam 27 crosses a quarter-wave plate c

oriented to give a 25 'beam to polarisator c.re.iai-

re yes is reflected at 90 by a reflecting plate 7, and

  after crossing a Faraday 10 cell falls on the opti-

as input from the end 2b of the fiber 2. The beams 24 'and 25' therefore travel in the optical fiber in opposite directions to one another, and emerge from this fiber respectively by the ends 2b and 2a , cross

  the waveforms 9 and 8 respectively, which restore

  feels a linear polarization of the beams and falls on the semi-reflecting plate 6. It will be noted that the geometrical paths of the beams 24.24 'and 25.25' between the two

  impacts on the semi-reflecting plate 6 are identical.

  The beam 26 emerging from lname 6, substantially in line with

  with beam 24, is the seat of an interfacing figure

  due to the superposition of the beams 24.24 'and 25225', and comes to fall on a semi-reflective plate 14, oriented

to 45.

  Furthermore, the beam 23, coming from the polarizing cube 5 by reflection, is returned to 90 by a reflecting plate 11 to pass through an acousto-optical modulator 12, or a half-wave plate 13; the beam 27 aui emerges from the blade 13 comes to fall, at right angles to the beam 26, on the blade

semi-reflective 14, oriented along the internal bisector

less than the angle of the beams 26 and 27. The beams 28 and 29 which emerge from the blade 14, consist respectively, for the beam 28, of the reflected fraction of the beam 26 and of the transmitted fraction of the beam 27, and the beam 29,

  of the fraction transmitted from the beam 26 and of the fraction fraction

  deflected of the beam 27. As the reflection introduces a delay of a quarter wavelength between the reflected beam and the transmitted beam, the beats between the beams 26 and 27 are in phase opposition in the beams 28 and 29 which fall respectively on photoelectric detectors 15

  and 16, coupled respectively to the in-erseuse and di-

  of a differential amplifier 17. The assembly, compre-

  nant the blade 14, the detectors 15 and 16 and the amplifier 17, is known as d: coherent detection with balance

  heterodyne, and has the advantage of doubling the detected signal

  useful, while compensating for spurious signals which are

  consistent with the incident beams.

The current yes crosses the conductor l, which is sensi-

  clearly rectilinear over a large length relative to the radius of the mandrel on either side thereof, creates a magnetic field with circular lines of force centered on the conductor. This magnetic field creates, in the optical fiber 2 a directed induction following the length of the spiral part, by inducing an orientation birefringence

polar (Faraday effect) so that the optical medium

riel formed by the fiber has a different index for circularly polarized waves depending on whether they travel

in the fiber in the direction of induction or in the opposite direction.

  The difference in index is proportional to the induction, therefore for a medium with constant permeability, to the magnetic field

and therefore at the intensity in the conductor l, with a coefficient

  reverse of the spiral radius of fiber 2.

  Furthermore, the difference in optical path between the two beams 24 'and 25', which travel in opposite directions in the fiber 2 is proportional to the length of the spiraling. We

note here that the ends of the fiber, beyond the spira-

  The beams are subjected to substantially transverse magnetic fields, the longitudinal components of which, except at

connection to the spiral compensate for an extreme

  mit4 to the other. In addition, accidental birefringences,

due to mechanical stresses ThermalF, or electric field

  cudgel do not have a polar orientation and induce

the same index variations for reverse propagations.

Also the differences in path of the two beams 24 'and 25' result solely from the effect of the magnetic field on the

spiral part of the fiber.

  If we call a n the difference in index caused by

  a magnetic field H, in a material which has a consistency

aunt of "Verdet" Ve, for a wavelength X we have: [Sn = - Ve H. (1) Ir Taking into account the relationships linking the magnetic field to the current I in a conductor, for a spiraling at N turns the path difference expressed in phase shift Af is: [2 IN ve] I. (, 2) In the beam 26, the superposition of two beams having undergone a path difference AI forms a figure of in-

terference phase shifted by / l with respect to the figure of interfere

  in the absence of current. Detection in summer balance-

rodyne by the device constitutes by the blade 14 the detectors

  tors 15 and 16 and the amplifier 17, of the combination of the

  bundle 27 forming a reference or, according to the usual terminology-

  the in detection, local oscillator and of the modulated beam 26,

brings up a modulation signal at the output of the amplifier

  ficitor 17. The electronic device 18 is adapted, so

  known lesson, in particular by counting, to compare the phase of

modulation at the original phase, in the absence of current.

However, the central frequency of the detected signal is zero and the detected signals corresponding to two phase shifts

Opposite signs are equal. The operation of such if-

generally encountered known difficulties, and on the other hand the noises generated by the detector are generally significant

at low frequencies.

  This is the reason why we inserted in the journey

of the beam 23 an acousto-optical modulator 12 capable of

  duire, by interaction of ultrasonic waves with a light beam an offset of the frequency of the light wave. The beam 27 leaving the modulator is thus at a frequency which differs from a known amount of the frequency of the beam

  22, so that the detection output signal is constituted

  killed by offset frequency signal, phase modulated

  by the phase shift induced in the fiber.

  Furthermore, the modulator 12 can be coupled to the device

  electronic sitive 18 so that a variation of the deca-

  frequency linkage induced by the acousto-optical modulator 12 is taken into account by the device 18 so as not to influence

on the phase shift measurement (synchronous detection).

It was previously reported that a Faraday cell 10 was interposed on the path of the secondary beam 25 '

24? 5230

before entering the endpoint view. ' 2b. The Lie Farada.y cell is an optical device o a transparent bar made up of a material with a "Verdet" _evé material, such as a compound of a rare earth metal, which would be subjected to a longitudinal magnetic field created by a current of com-

  mande, de sr-te ou'il i appear a b-r4frlngence to orienta-

polar tion, Gold will have eomprLs -ue a Clle de Feraday 10

is available to induce a different operating difference

tre le fai-scea-u 95'.avent sFn affects fiber 2 and makes it 24 'after its emergence from i: [rre, qu. comes in compensation for the difference in industrial flux in the fiber

2, the intensity and the current of the common current

fairly settled. This is achieved by attacking an amplifier

  tor 19 by another output from the device 1-, where Y appears a

general feedback function of the phahsn_, detected at output

  of the differential amplifier 17. This constitutes an asser-

v.emen ... the struct.ure classioue, which it is useless to develop

  onoperate foO; ioanement. The current delivered by the amplifier

tior 19 to shoulder 10 is mesFur p..ir r amnpèremeter classi-

  hearing, analog or digital, or by a device

/ meti; eur de Tllmesure. It is clear that the control current

  of cell O10 is a thin, large and if-

gn-ie, of the current which crosses concuctour 1, while the decoupling in tePnson is ensured. On an igneous energy transport line, the device of iViventicon assumes the same function or an intensity transformer. Otherwise,

  damage to the device of the invention does not intro

  due to no disturbances on the transmission line, while

  yes we know 'r- dangers What does an information transformer

  ternFity whose secondary is in open ciLrcuit.

Z1 results from the formula? that the sensitivity of the device

  t.iú increases with the number of turns5 of the spiraling of the fiber

optic 2 on the mandrel 1a. Der- Considerations on the absorption-

  t ion of the light, and the emissFion "tum.u ': in the optic fibers

  ouea allow to; d. define lengths? opt-.males of sp: ra-

  lage, or -atte gnent several m:] 'bind? of meter F. ri is c '-_ ar or' a ctua'clement the lengths of,: - usable raage

srSt liitêes para ies.iffi-. u, '- tehnJhr- 9 g-.e -e f abr "-

tAcr. aC YUr r.e -: - ^ -

  t ar ai ieur, a t r; -? iti3 n e e? from e-uren, -

  minimum tectabe ritionr of c.urlt t er relation r the minimum. Indeed, i, = eren-e ae cihemln opt + inulite in the fiber by the chamrp magrL4ti aue created by the coureat attLnt 1 length 6th coh4renee, the measurement of S-phasing is no longer significant. onue les.originep Jet a- of the two secondary beams ns eor-t- ras then '-', ré!

  Of course, the device described admits numerous values.

  laughing in structure without departing from the framework of the information

  vention. I1 is clear notammernt cua the modulator acousto-

  optioue 12 could be arranged on the path of the beam 22, and that the Farada cell, 10l could be placed between the blade 8 and the optical input 2a of the fiber without altering the operation. In addition, the Faraday cell 10, with the amplifier 19 and the measuring device can be omitted in applications which do not require generation.

of an image current.

  In addition, optical detection arrangements are known.

coherent which allow measurements of dgphasing between beams

ceaux = nterferents, with determi: ration of the direction of the dephaling,

  without involving a frequency shift er.tre done-

  measurement ranges and reference beam. It is clear that

we can then replace the whole acousto-modulator

optic 12, and of the h6terodyre balance detector 14, i5,16,7 by an assembly of the type yes gives -e so. the direction of the phase shift,

  if they provide sufficient immunity against noise

re1es with the laser beam.

l1

R E V E N D I CAT I 0 N S

  1. Device for measuring the intensity of electric current

aue in a conductor by magneto-optic effect, or a

this polarized light beam from a coherent source travels in a material optical medium subjected to the magnetic field

  cr, é by the current, the current measurement resulting from the me-

variation in beam path under the effect of the magnetic field crossed, device o the material optical medium

consists of an optical fiber with two end faces

  moth, spiraling around the conductor so that the field ma-

  is directed longitudinally in the fiber, character-

  terized in that it includes a suitable separating means

  tee to divide the polarized beam into two secondary beams

  res with circular polarization penetrating respectively into the fiber by one of the two end faces and emerging

  by the other, and to gather the emerging beams in coli

  4a-rity so that they interfere, a means of detection re-

  cevnlt the interfering beams and delivering a signal of

* differential modulation.

  2. Device according to claim 1, characterized in that read the beam from the coherent source being linearly polarized, the optic separator means comprises two blades

  waveforms inserted on the beam path respectively

  secondary ceaux with an axis orientation at 45 from the plane of

polarization.

3. Device according to claim 1 or claim-

  tion 2, caraectrized in that, on the path of the beam coming from the coherent source, there is a means for deriving a reference beam, the detection means comprising two

  sensors at the input of a differential amplifier, these

  tectors each receiving a component of the beam of ref4-

  rence and a composition of fa: seals: Inter-rents with oppo-

  phase sition between homologous composite lees received by

both of the detectors.

  4. Device according to claim 3, characterized in that it comprises on the path either of the reference beam,

  either from the beam coming from the source an opti modulator d-

  placing a chosen quantity of the beam frequency

slope.

  5. DipFositif according to claim 3 or l1a revendica-

  tion 4, cPracteris in that it csmporte, between separate means

  tor and an end face of the optical fiber, a common Firadiy commarnd-'e rar cell, coming from a variable source

  a measurement monitor, so as to compensate for the modu-

  lation induced in l3 fiber by the magnetic field.

  6. Device according to claim 5, characterized in

  what the detection means attacks an electro-regulator

  nique commanding said variable source so as to compensate

exactly the induced modulation.