EP0293378A1 - Interferometric apparatus - Google Patents
Interferometric apparatusInfo
- Publication number
- EP0293378A1 EP0293378A1 EP19870901101 EP87901101A EP0293378A1 EP 0293378 A1 EP0293378 A1 EP 0293378A1 EP 19870901101 EP19870901101 EP 19870901101 EP 87901101 A EP87901101 A EP 87901101A EP 0293378 A1 EP0293378 A1 EP 0293378A1
- Authority
- EP
- European Patent Office
- Prior art keywords
- coil
- optical fibre
- signal
- loop
- beams
- Prior art date
- Legal status (The legal status 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 status listed.)
- Withdrawn
Links
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
Definitions
- the present invention relates to interferometric apparatus which is adapted to respond to a magnetic field and, more particularly, to such apparatus which is based on a Sagnac interferometer and which is designed for use as an optical fibre data ring or optical magnetometer for detecting magnetic field and measuring electrical current.
- Optical fibre links are commonly used as part of high speed data networks. In most applications, the fibre acts as a "point-to-point" link between a transmitter and a receiver. In other applications, the optical fibre may be used in conjunction with either conventional optical components, such as, beam splitters, or fibre optic directional couplers to form a data ring.
- V V( ⁇ ) is dispersive.
- a linearly polarised light beam with polarisation azimuth ⁇ can be considered as a combination of two orthogonal (left and right) circularly polarised beams with a phase difference of 2 ⁇ between them, which in the Jones calculus (see paper entitled “A new calculus for the treatment of optical systems" by R. C. Jones in J.Opt. Soc.Am, 31, 488 of 1941) is expressed as
- the Faraday rotation, ⁇ is basically the resultant differential phase retardation between the right and left circularly polarised components of the beam due to the magnetically induced circular birefringence of the medium. Since the magnitude of ⁇ is a linear function of both the path length and the magnetic induction, a measurement of the magnetic field strength, H, can be achieved by recording the observed azimuth rotation, ⁇ , of the linearly polarised beam. A general difficulty is experienced in the use of this technique in that the magnitude of the Verdet constant is relatively small for most optical materials. In particular, the magnetically induced circular birefringence may be obscured by other sources of birefringence. For example, in an early implementation of the technique (see paper entitled "Optical fibres for current measurement applications" by A.
- low birefringence fibre sensing elements are used.
- the low linear birefringence requirement is an essential one, since the magneto-optically induced azimuth rotation alternates in sign at half fibre beat length intervals, ⁇ /2, yielding a zero net value over one whole beat length or a multiple of it.
- the beat length, ⁇ is given by ⁇ / ⁇ n, where ⁇ is the vacuum wavelength of the light and ⁇ n is a measure of the linear birefringence of the fibre expressed by ( n f - n s ) where n f , n s are the refractive indices of the fast and slow polarisation eigen modes of the fibre respectively.
- a reciprocal path interferometric technique is used which is based on a Sagnac interferometer and in which a beam of light is amplitude divided and launched in counter-propagating paths into a single loop of single mode optical fibre, or a coil of such fibre forming the sensor.
- the optical intensity of the interferometer output is directly dependent on the Faraday rotation of the polarisation azimuths, after the two beams are recombined.
- non-magnetically induced circular birefringence optical activity
- the effects of linear birefringence are reduced, as more fully described below.
- the invention is much less dependent on environmental perturbations, such as, temperature and vibration, than hitherto known systems. This advantage is particularly valuable in the measurement of direct electric currents, where long term stability is of paramount importance.
- the invention enables the use of a low coherent light source. Moreover, an electronic synthetic heterodyne signal processing technique may be used and this obviates the requirement of opto-mechanical adjustment to set the instrumental operating point and has an effectively infinite dynamic range. With a heterodyne signal processing system, the measurand current is recovered from the phase modulation of an electrical or electronic carrier. Such processing is desirable because it provides a linear response function over an effectively infinite measurement range. In combination with the optical configuration utilised by the invention, this signal processing system does not rely on the adjustment of mechanical components (such as polarisers) to set the operating point of the system. The system may be based on the use of a piezo-electric modulating element which provides a dynamic phase bias in a manner analogous to that previously used with fibre optic gyroscopes.
- the Faraday effect is the magneto-optically induced circular birefringence in a medium.
- the transfer matrix of a finite element of an isotropic dielectric, possessing both linear and circular birefringence can be written as a product of two Jones matrices describing the linear and the circular birefringences of the medium where ⁇ and ⁇ express the degree of linear and circular birefringence respectively, and
- ⁇ 0 and ⁇ are affected by external fields such as temperature, strain, anisotropic pressure, etc, hindering a reliable quasi-steady measurement of ⁇ , and producing noise and operating point drifts.
- a ring configuration is used in which reciprocal effects such as birefringence of the fibre are reduced by common mode rejection.
- the fibre is considered as a linear array of N dielectric elements each possessing finite linear and circular birefringence whose individual transfer matrices are of the kind given in equation (3).
- the magnetic field is taken to be uniform along the sensing region.
- equation (9) refers to the preservation of symmetry in the Sagnac interferometer. This means that the effect of any external fields such as temperature changes or vibration is greatly reduced and takes a differential form expressed by
- ⁇ e is the overall error in the recorded interferometric phase due to the effect of the error field, M i , on the fibre element j , ⁇ M i j , and n j , l j . are the refractive index and the physical length of the jth element respectively.
- Figure 1 is a schematic diagram of a data ring and homodyne signal processing system according to the invention
- Figure 2 is a schematic diagram of a closed loop magnetic field or current detector and homodyne signal processing system according to the invention
- Figure 3 is a schematic diagram of a data ring or magnetic field or current detector and heterodyne signal processing system according to the invention
- Figure 4 is a schematic diagram illustrating another embodiment of the invention for magnetic field and current detection
- Figure 5 is a diagram illustrating the two electronically produced carriers utilised in the embodiment of Figure 4, after being squared up with 50% duty cycles (solid line: no current: dashed line: D.C. current applied),
- Figure 6 is a graph illustrating the results achieved with the apparatus of Figure 4 for direct current measurements using both a laser and an LED source
- Figure 7 is a graph illustrating the variation of Verdet constants with the square of light source frequency
- Figure 8 is a graph illustrating the results achieved with the apparatus of Figure 4 for alternating current measurements for both laser and LED light sources
- FIG 9 is a graph illustrating the environmental insensitivity of the apparatus of Figure 4.
- the data ring comprises a single loop 1 of optical fibre which is illuminated by a solid state light source 2 which may be either a solid state laser or a light emitting diode (L.E.D.).
- the light source 2 is connected to the optical fibre loop by a fibre optic directional coupler 3.
- the input light from the source is polarised and amplitude-divided into two beams at the directional coupler, which beams propagate in clockwise and anti-clockwise directions within the fibre loop.
- the latter is contained along most of its length in a protective sheath of, for example, flexible plastic or aluminium.
- After propagating through the fibre loop 1 these two beams mix coherently at the directional coupler 3 giving rise to an optical intensity which depends on the optical path difference between the clockwise and anti-clockwise beams.
- This optical interference signal is detected by a photodiode 4.
- a small electrical coil 5 is wound about the optical fibre of the loop 1 such that a current signal of frequency f s1 flowing through the coil produces a magnetic field with the field direction along the direction of propagation of the counter-propagating beams in the fibre loop.
- the polarisation azimuths of the counter-propagating optical beams within the fibre loop 1 are affected as they pass through the coil, by reason of the Faraday effect, such that the azimuth of each beam is rotated.
- the degree of rotation of each beam depends on the amplitude of the signal f s1 .
- these co-rotating azimuth modulated optical beams arrive at the directional coupler 3 they combine coherently and produce an optical interference signal which is amplitude modulated with a frequency spectrum containing harmonics of f s1 and including a major component at a frequency 2f s 1 .
- a plurality of electrical coils similar to 5, that is, coils 6, 7, etc may be used to induce signals into the loop 1 and, if their frequencies are different, that is, f s2 , f s3 etc then the electrical current of the photodiode output will contain signals at frequencies 2f s1 ,
- the data ring can be used to recover signals generated in electrical coils 5, 6 and 7.
- Analogue data can be recovered from the variation in the amplitude of the recovered signals at, for example, 2f s1 , 2f s2 , 2f s3 etc.
- Digital data can be impressed on the light beams by frequency switching the input current signals between f' s1 to f" s1 , f' s2 to f" s2 etc. The information contained in these signals may then be recovered uniquely by passing the output signal from the photodiode 4 through separate band pass filters 8 centered at 2f s1 , 2f s2 etc with bandwidths 2f' s1 to
- FIG. 2 illustrates an embodiment for detecting magnetic fields where the direction of the unknown magnetic field has a component along the direction of the optical fibre ring or loop 1.
- the magnetic field may be generated either by an electrical coil 10 wound about the fibre of the loop, as illustrated, or by a magnetic field in free space.
- input light from a solid state optical source 2 is polarised and amplitude-divided into two beams at the fibre optic directional coupler 3, and these two beams, propagate in clockwise and anti-clockwise directions about the fibre loop 1.
- the two beams are recovered and mixed coherently by the directional coupler 3 and the resulting interference signal is detected by the photodiode detector 4.
- the signal processing system includes a servo control arrangement comprising an electrical coil 11 wound about the fibre of the loop 1 , a difference amplifier having inputs connected to the outputs of the photodiode 4 and a second photodiode detector 13, which monitors the output of the light source 2, and having its output connected, via a current amplifier, to supply a feedback signal to the coil 11, as well as a signal to the next stage in the signal processing system.
- Servo control 12 includes the difference amplifier and current amplifier.
- the output signal at the detector 4 will contain signals at frequency 2F m related to A and at F m related to B.
- the amplitude of C is environmentally sensitive and represents a source of error in the measurement of A and B.
- the system illustrated in Figure 2 operates as follows.
- the output signal from the photodiode 4 is fed into one input of the difference amplifier, the output of which is electronically integrated and fed to coil 11 via the current amplifier.
- the other input to the difference amplifier is derived from the photodiode 13, the output signal of which is adjusted such that it corresponds to the 'quadrature' point in the interferometer's transfer function.
- the feedback signal supplied to the coil contains a DC component, which represents signal components B and C, and an alternating component at F m directly proportional to A.
- the constant of proportionality is obtained, for example, by applying a magnetic field of known magnitude to the system.
- This method of signal recovery can also be used if the interferometer is subject to more than one magnetic field and can therefore be used as an alternative method of signal recovery for the data ring.
- FIG. 3 An arrangement for recovering data in this manner is illustrated in Figure 3. It includes the basic ring configuration shown in the previous embodiments including a single optical fibre loop 1, a solid state light source 2, a directional coupler 3, a photodiode detector 4 and data coils 5, 6, 7.
- the electronic carrier is produced by incorporating a dynamic phase shifter into the optical fibre loop 1 close to the directional coupler 3.
- the dynamic phase shifter may, for example, be a Bragg cell or a piezo-electric element (around which part of the fibre loop 1 is wound).
- the latter is driven sinusoidally by an electronic oscillator 16 at a frequency f p , and the output of the photodiode 4 is fed through an electronic switch 17, which is. synchronously driven from the oscillator 16.
- the resulting signal is band pass filtered at 2f p to produce an electronic carrier at 2f p .
- This technique of phase modulation and carrier production is known to those skilled in the art of fibre optic gyroscopes.
- This signal centred about the base band, is then transposed onto the electronic carrier producing a phase modulated carrier of centre frequency 2f p by the combined action of the oscillator 16 and the electronic switch 17.
- the output carrier signal contains signals at f p ⁇ f s1 ,f p ⁇ f s2 and f p ⁇ f s3 with amplitudes proportional to A s1 , A s2 , A s3 etc.
- These signals can then be demodulated by conventional electronic methods, such as, a phase locked loop 18.
- A' piezo-electric phase modulator is particularly suited for data recovery where the bandwidth required for the system is less than 100kHz; for higher bandwidths the Bragg cell modulator is preferred.
- the heterodyne recovery system described with reference to Figure 3 may also be used as a detector of electric currents and magnetic fields.
- a typical application is where it is necessary to detect the amplitude A and frequency f m of mains current under both normal conditions and when there is the possibility of large surge currents due to overloads or surges in the supply.
- a single signal coil is used, for example, the coil 5.
- a signal related to the amplitude and frequency of the current may be recovered with a phase locked loop detector.
- a phase locked loop detector To enable measurements of the output signal associated with large surge currents an additional high speed digital phase tracker is incorporated into the system.
- the embodiment illustrated in Figure 4 comprises a coil 20 formed from single mode optical fibre 21 wound about a 30 cm diameter circular aluminium former with 45 turns.
- the fibre 21 is also twisted, for example, at a rate of approximately 120 rads/m in the process of winding onto the former to induce circular birefringence, thereby suppressing the quenching effect of linear birefringence in the fibre.
- An electrical coil 22, for example, a copper wire coil, is wound about the optical fibre coil to form a toroid with 580 turns.
- Polarised light from a solid state light source 23 is amplitude divided and launched into opposite ends of the coil 20 by a beam splitter 24 and lenses 25 so that light beams are propagated in opposite directions about the coil.
- the optical fibre 21 is wound about a piezo-electric phase shifter 28.
- a 2 cos 2 ⁇ [cos 2 ( ⁇ 2 ⁇ ) - ⁇ 2 sin 2 ( ⁇ +2 ⁇ ⁇ )]
- the interferometric irradiance function will exhibit an amplitude modulation of up to 2% for currents of the order of 1 A flowing through the toroid 22.
- the observed phase modulation, p is also affected by the scale factor and is smaller in magnitude as given in equation (17); the experimental results should, therefore, be corrected accordingly.
- the photodetector output is processed at 27 by band pass filtering synchronously gated segments of the signal to give two sine wave carriers at twice the modulation frequency, the phases of which vary by equal magnitudes but opposite signs in response to an applied field, H, to the ring (see Figure 2).
- H an applied field
- the effect of an applied magnetic field is to induce a phase retardation, 2 ⁇ , of the carrier, where ⁇ is the magnitude of the Faraday rotation of the azimuth of the polarisation state, determined by equation (15).
- a periodic electrical signal applied to the apparatus in the form of alternating current through the toroid 22, causes phase modulation of the carrier at the signal frequency ⁇ s ( 50 Hz, mains, in present experiments), which appears as sidebands about the
- Figure 6 illustrates the results achieved with the apparatus of Figure 4 for DC measurements obtained using a multi-longitudinal mode 5 mW HeNe laser and an LED source, at wavelengths of 633 nm and 780 nm respectively, using the scale factor obtained in the previous section after polarisation state considerations.
- the gradient of the straight line yields the Verdet constant at each wavelength.
- the upper limit on the maximum detectable signal is set by a phase change of 2 ⁇ radians, to avoid ambiguity due to the periodic nature of the interferometer transfer function, equivalent in the present tests to an applied current of approximately 100 A; the lower limit is determined by the signal processing scheme used.
- Phase sensitive signal processing schemes have been developed for use with fibre optic interferometric sensors capable of phase resolutions in the range of 1 m rad to 1 ⁇ rad.
- the minimum detectable current was ⁇ 0.1 mA.
- Figure 9 shows plots of the recorded interferometric phase and the temperature to which the fibre coil 20 as a whole was subjected in time.
- the instrument may be configured as a listen-only optical data bus.
- This technique has a number of important advantages over previously proposed optical data rings. Chief amongst these are the facts that, firstly the transmitting stations are simply magneto-fibre optic transducers, and are consequently non-intrusive, and may be connected without physically disrupting the waveguide; and secondly, that each transmitter draws no optical power from the ring. Therefore, the number of transducers is limited only by the available frequency bandwidth.
- the embodiment of Figure 4 may be used as a magnetometer for the measurement of direct and alternating electrical currents, and may be used with a heterodyne signal recovery scheme which exhibits excellent linearity over a wide dynamic range.
- the pseudo-reciprocity of the arrangement confers environmental insensitivity and allows cheap low coherence optical sources, to be used.
- the technique also has the potential of being applied to the production of an optical data ring with a large number of non-int.rusive transmitters, but without significant attenuation.
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- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Physics & Mathematics (AREA)
- Condensed Matter Physics & Semiconductors (AREA)
- General Physics & Mathematics (AREA)
- Gyroscopes (AREA)
Abstract
Un appareil à interférométrie, destiné à être utilisé comme anneau de données ou comme détecteur de champs magnétiques ou de courants électriques, comprend une boucle en fibre optique (1), une source lumineuse à semi-conducteurs (2), un coupleur directionnel de fibres optiques (3) et un détecteur à photodiode (4). La lumière d'entrée provenant de la source lumineuse (2) est polarisée et son amplitude est divisée, ce qui permet d'obtenir deux faisceaux se propageant dans des directions opposées autour de la boucle en fibre optique (1). Après leur propagation autour de la boucle, les deux faisceaux sont mélangés de façon cohérente au niveau du coupleur directionnel et le signal d'interférence optique qui en résulte est détecté par la photodiode (4). Une ou plusieurs bobines de données ou de signaux électriques (5, 6, 7) enroulées autour de la fibre optique de la boucle (1) produit/produisent des champs magnétiques dont les composantes sont orientées dans la direction de la fibre optique, l'effet de Faraday qui en résulte produisant la rotation des azimuts de polarisation des faisceaux se propageant en sens inverse dans la boucle. Lorsque ces faisceaux lumineux à modulation azimutale arrivent au niveau du coupleur directionnel (3), le signal d'interférence qui en résulte est détecté par la photodiode (4), laquelle produit un signal électrique traité à son tour afin de permettre la récupération des signaux produits par les bobines de données ou de signaux (5, 6, 7).An interferometry device, intended to be used as a data ring or as a detector of magnetic fields or electric currents, comprises a fiber optic loop (1), a semiconductor light source (2), a directional fiber coupler optics (3) and a photodiode detector (4). The input light from the light source (2) is polarized and its amplitude is divided, which makes it possible to obtain two beams propagating in opposite directions around the fiber optic loop (1). After propagation around the loop, the two beams are coherently mixed at the directional coupler and the resulting optical interference signal is detected by the photodiode (4). One or more coils of data or electrical signals (5, 6, 7) wound around the optical fiber of the loop (1) produces / produce magnetic fields whose components are oriented in the direction of the optical fiber, the Faraday effect which results producing the rotation of the polarization azimuths of the beams propagating in opposite direction in the loop. When these azimuthally modulated light beams arrive at the directional coupler (3), the resulting interference signal is detected by the photodiode (4), which produces an electrical signal processed in turn to allow signal recovery. produced by the data or signal coils (5, 6, 7).
Description
Claims
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GB8603375 | 1986-02-11 | ||
GB868603375A GB8603375D0 (en) | 1986-02-11 | 1986-02-11 | Fibre optic data ring & current detector |
Publications (1)
Publication Number | Publication Date |
---|---|
EP0293378A1 true EP0293378A1 (en) | 1988-12-07 |
Family
ID=10592883
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP19870901101 Withdrawn EP0293378A1 (en) | 1986-02-11 | 1987-02-11 | Interferometric apparatus |
Country Status (3)
Country | Link |
---|---|
EP (1) | EP0293378A1 (en) |
GB (1) | GB8603375D0 (en) |
WO (1) | WO1987004798A1 (en) |
Families Citing this family (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US5568049A (en) * | 1993-10-22 | 1996-10-22 | The United States Of America As Represented By The Secretary Of The Navy | Fiber optic faraday flux transformer sensor and system |
US5598489A (en) * | 1994-07-27 | 1997-01-28 | Litton Systems, Inc. | Depolarized fiber optic rotation sensor with low faraday effect drift |
DE19612993C2 (en) * | 1996-03-22 | 2003-12-18 | Forschungsverbund Berlin Ev | Method and device for detecting changes in the magnetic field |
US10495462B2 (en) * | 2017-05-30 | 2019-12-03 | California Institute Of Technology | Integrated optical gyroscope with noise cancellation |
Family Cites Families (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB2119536B (en) * | 1982-04-30 | 1986-01-08 | Arthur John Barlow | Fibre optic faraday rotation device and method |
GB2143634A (en) * | 1983-07-13 | 1985-02-13 | Standard Telephones Cables Ltd | Optical sensors |
-
1986
- 1986-02-11 GB GB868603375A patent/GB8603375D0/en active Pending
-
1987
- 1987-02-11 WO PCT/GB1987/000101 patent/WO1987004798A1/en not_active Application Discontinuation
- 1987-02-11 EP EP19870901101 patent/EP0293378A1/en not_active Withdrawn
Non-Patent Citations (1)
Title |
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See references of WO8704798A1 * |
Also Published As
Publication number | Publication date |
---|---|
WO1987004798A1 (en) | 1987-08-13 |
GB8603375D0 (en) | 1986-03-19 |
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Inventor name: JACKSON, DAVID, ALFRED Inventor name: AKHAVAN LEILABADY, PEDRAM Inventor name: JONES, JULIAN, DAVID, CLAYTON |