WO2015124677A1 - Capteur interférométrique - Google Patents

Capteur interférométrique Download PDF

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Publication number
WO2015124677A1
WO2015124677A1 PCT/EP2015/053514 EP2015053514W WO2015124677A1 WO 2015124677 A1 WO2015124677 A1 WO 2015124677A1 EP 2015053514 W EP2015053514 W EP 2015053514W WO 2015124677 A1 WO2015124677 A1 WO 2015124677A1
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Prior art keywords
waves
sensor
wavelengths
measurand
phase shift
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PCT/EP2015/053514
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English (en)
Inventor
Xun GU
Sergio Vincenzo Marchese
Klaus Bohnert
Andreas Frank
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Abb Technology Ag
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Priority to DE112015000882.9T priority Critical patent/DE112015000882T5/de
Priority to CN201580009647.3A priority patent/CN106062506B/zh
Publication of WO2015124677A1 publication Critical patent/WO2015124677A1/fr

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
    • G01D5/266—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light by interferometric means
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
    • G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
    • G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
    • G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
    • G01D5/35306—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement
    • G01D5/35309—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement using multiple waves interferometer

Definitions

  • the invention relates to an interferometric sensor, wherein a change in the parameter to be measured is related to a relative phase shift between two waves, such as an electro-optic voltage sensor, particularly for DC electrical voltages, or a fiber-optic current sensor (FOCS) .
  • an electro-optic voltage sensor particularly for DC electrical voltages
  • a fiber-optic current sensor FOCS
  • an electro-optic DC voltage sensor consisting of a bismuth germanate (Bi 4 Ge 3 0i2, or BGO) crys ⁇ tal with its [001] crystal axis oriented along the optical path of the waves (see also reference [1] for further de- tails) has a corresponding n-voltage or an unambiguous measurement range of about 75 kV for light waves at 1310 nm.
  • a quarter-wave retarder (QWR) can be inserted to ob ⁇ tain an unambiguous measurement in the range [- ⁇ /2, ⁇ /2], i.e., centered about zero voltage.
  • two antiphase outputs are generated by means of two ana ⁇ lyzers at ⁇ 45° to the electro-optic axes, making it more robust against light power fluctuations (see ref. [1] for details) .
  • the sign ambiguity (between ⁇ and - ⁇ ) can be removed by combining two polarimetric signals with a (static) relative phase offset (preferably n/2, called quadrature signals) as shown for example in ref. [2]
  • the periodwise ambiguity (between ⁇ and ⁇ + 2nn) is an inherent problem for all interferometric measurements.
  • the measurement range can be extended by fringe counting, zero- counting or similar history-tracking techniques.
  • the measurement range can be extended by fringe counting, zero- counting or similar history-tracking techniques.
  • Electro-optic voltage sensors can also be built using the modulation phase detection (MPD) technique as described for example in [6] . It is generally implemented in a non-reciprocal phase modulation scheme and commonly used in fiber-optic gyroscopes and fiber-optic current sen ⁇ sors, see ref. [7, 8] . Reciprocal MPD sensors have excel- lent phase accuracy and DC stability.
  • the co-owned Patent US7911196 (cited herein as reference [9]) describes a volt ⁇ age sensor incorporating a voltage sensing element (or several such elements), a 45° Faraday rotator, and the MPD modulation and detection electronics.
  • Patent EP0864098B1 [11] describes a method combining two independent measurements of different sensitivities.
  • the first Faraday element has a low current sensitivity, but has a sinusoidal response curve whose period is longer than twice the target measur ⁇ ing range. Therefore, it can provide a low-resolution but unambiguous measurement.
  • the second Faraday element has a higher resolution but is periodwise ambiguous. Combining the two measurements, an unambiguous and high-resolution result can be obtained.
  • the measure ⁇ ment range extension factor (from the second ambiguous high-resolution measurement) in this approach is basically the periodicity ratio of the two measurements.
  • the choice of the sensing medium is limited, and it may be difficult to find a proper medium suitable for the low-sensitivity measurement.
  • Bi 4 Ge 3 0i 2 (BGO) and to a lesser extent Bii 2 Ge02o or Bi 4 Si30i2 are known as practical sensing crystals.
  • the crystals provide good voltage sensitivity, but none is suitable for the low- sensitivity measurement, as their n-voltages are much smaller than the 100s of kilovolts that would be required for unambiguous voltage measurement in HV applications.
  • an interferometric sensor with one or more wave generators generating at least a first set of two waves both centered at a first wavelength ( ⁇ ) and a second set of two waves both centered at a different second wavelength ( ⁇ 2) , and a sensing element whereby a measurand induces a first relative phase shift between the first set of waves and a second relative phase shift between the second set of waves, respectively, at least one detector measuring a first interference signal between the first set of waves and a second interference signal between the second set of waves, and further including a signal pro- cessing unit adapted to determine from the first and second interference signals two quantities representative of the principal values of the first and the second relative phase shifts unambiguously within a 2n range, respectively, and to derive a measurand value from their combination.
  • the senor is capable of performing individual phase shift measurements at two different wavelengths, both in a full 2n range, and using their combination to unambiguously determine the measurand.
  • the term "wave” here is meant in the general physical sense of the word, including all types of oscil ⁇ lations traveling in space and time.
  • the wave may have narrow or broad spectral content, may be long-lasting or be limited in duration, and may be generated by one source or be synthesized from multiple sources.
  • the nature of the wave may be mechanical (acoustic) , electro-magnetic (optical), or of any other type.
  • the invention is described using light waves as ex ⁇ amples.
  • the two interfering waves can be for example two orthogonal linear or circular polarization modes of a light wave .
  • the two sets of waves at X and %2 may be gen ⁇ erated by separate sources, or may be spectral portions of one set of waves, with each portion centered at a different wavelength.
  • the wavelength selectivity of the two inter- ference signals can be realized by a wavelength selection component (for example a spectral filter or a wavelength- division multiplexer (WDM) component) , two detectors of different spectral response, or other similar means.
  • a wavelength selection component for example a spectral filter or a wavelength- division multiplexer (WDM) component
  • WDM wavelength- division multiplexer
  • phase shift measurement which can unambig- uously determine a phase shift in a 2n range may be used for the implementation of this invention.
  • Two examples are given: the polarimetric method and the modulation phase detection method.
  • the polarimetric method various implementations are possible: one can have two quadrature channels and one additional channel measuring the total light power; alternatively the total light power can be measured by the sum of the two antiphase outputs of a polarizing beamsplitter; or the total light power can even be monitored or stabilized by detectors and electronics before the sensing medium.
  • quadrature detec ⁇ tion a 90° phase offset between the channels is desired but not imperative.
  • the sensing element can com- prise an electro-optic crystal, a crystalline electro-op ⁇ tic fiber, a poled fiber, or a fiber or bulk optic material attached to a piezo-electric element.
  • the sensing element can comprise an optical fiber or a bulk optic mate- rial.
  • the sensing element can comprise optical fibers or waveguides, including specialty low birefringent fibers, flint glass fibers, or spun highly-birefringent fibers, bulk magneto-optic materials, such as yttrium iron garnet crystals or fused silica glass blocks, or optical fibers, waveguides, or bulk optical ma ⁇ terials attached to a magneto-strictive element or combi ⁇ nations thereof.
  • optical fibers or waveguides including specialty low birefringent fibers, flint glass fibers, or spun highly-birefringent fibers, bulk magneto-optic materials, such as yttrium iron garnet crystals or fused silica glass blocks, or optical fibers, waveguides, or bulk optical ma ⁇ terials attached to a magneto-strictive element or combi ⁇ nations thereof.
  • the two center wavelengths [ ⁇ , ⁇ 2] are pref- erably chosen such that the trace of the first and second phase shift principal values [ ⁇ , cp2] in the measurement range evenly fills the 2-dimensional phase space (-n/2, n/2] x (-n/2, n/2] .
  • a particularly suitable guideline is that the difference in the inverse response periods of the sensor at the two wavelengths I Aq
  • I qi - q2 I is close to ⁇ * 2n / L, where L is the size of the measurement range of the sensor, and ⁇ is an integer.
  • Another aspect of the invention relates to a method of performing an interference measurement including the steps of:
  • FIG. 2 illustrates an example of the invention using the polarimetric quadrature signal detection scheme;
  • FIG. 3 illustrates an example of the invention using the modulation phase detection scheme
  • FIG. 6 shows an extended-range DC voltage sen ⁇ sor
  • FIG. 7 is another extended-range DC voltage sensor design
  • FIG. 8 shows a reflective-configuration version of the sensor shown in FIG. 6;
  • FIG. 9A is another extended-range DC voltage sensor design with two-wavelength period disambiguation; and
  • FIG. 9B is another extended-range DC voltage sensor design with two-wavelength period disambiguation.
  • the output of a polarimetric interference sen ⁇ sor is a sinusoidal function of a relative phase shift, which relates to the measurand x.
  • the outputs at the two different wavelengths are
  • Y2 (x) COS (c
  • 2) COS (q2 * x + 02)
  • ⁇ ⁇ and ⁇ 2 are the relative phase shifts
  • qi and q2 are the inverse response periods
  • ⁇ ⁇ and ⁇ 2 are the phase offsets of the sensor at wavelengths ⁇ and X2, re- spectively.
  • the outputs yi and y2 are normalized, such that their am ⁇ plitudes are set to 1, and their offsets to 0.
  • the measurand is the voltage to be measured, and the inverse response period where ⁇ is the wavelength, n is the refractive index, and r 4 i is the electro-optic coefficient.
  • ⁇ is the wavelength
  • n is the refractive index
  • r 4 i is the electro-optic coefficient.
  • the shape of a Lis ⁇ sajous figure is characteristic of the ratio q ⁇ /q 2 as well as the phase offset difference ⁇ 1 — ⁇ 2 . Therefore, it is widely used in engineering applications such as visualiza ⁇ tion of the relationship between harmonic signals.
  • a two-wavelength measurement maps a ID variable x to a point along the trace Y(p ) in the 2D yi-y 2 plane of FIG. 1A. If the ratio is rational, the period d of Y(p ) is increased Nj-fold from the single-wavelength measurement period 2n/qi ; if is irrational, the trace is not closed, meaning the two-wavelength measurement Y(p ) is ape ⁇ riodic. Therefore, one can use the two-wavelength method to significantly increase the unambiguous range of the po- larimetric measurement.
  • EP1179735 [14] recognized the existence of these ambiguity points, and proposed using three or more wavelengths as a solution.
  • a trace Y(x) [y ( . x), y 2 ( . x),y3(x)] in the 3D space, known as a Lissajous knot, generally does not make crossings upon itself as the measurand x pro ⁇ gresses (with the exception of some isolated degenerate cases). Therefore, no ambiguity generally exists.
  • ⁇ ever, operating light sources, detectors and other optical components at three or more wavelengths increases the com ⁇ plexity and cost, and reduces the reliability of the entire sensor system.
  • mapping a 3D (or higher-dimensional) measurement to a single measurand value also involves more complicated signal processing. Therefore, it is not a preferred approach to solve the ambiguity prob ⁇ lem.
  • the properties of the ambiguity points can be better studied by performing an arccos transformation on the Lissajous figure
  • FIG. IB A plot of Z in the z-z 2 plane as the measurand x progresses is shown in FIG. IB. Because zi and Z2 now are each segmented linear functions of x, the Z trace con ⁇ sists of a series of straight lines, and the measurand x is uniformly distributed along these lines.
  • An ambiguity point is created by the crossing between a positive-slope segment and a negative-slope segment. All the positive-slope segments are parallel to and evenly spaced between each other, as is true also for the nega ⁇ tive-slope segments. Consequently, the crossing points are also evenly spaced along the segments. Because the measurand x is uniformly distributed along the straight lines, this also means that the ambiguity points are quasi- uniformly distributed in the entire measurand range.
  • the number of the segments is given by the number of reflections.
  • the analysis presented is approximate in na- ture, but gives a reasonable estimate of the number of ambiguous measurand values.
  • the total number of ambiguous voltage values in this sensor is there ⁇ fore 100, meaning on average an ambiguous voltage value occurs every 4.5 kV.
  • the present invention completely eliminates ambiguity in a two-wavelength measurement in a given meas ⁇ urement range which is larger than response periods at either of the two wavelengths. It can be described as follows. Suppose, by a suitable method (two such examples will be described in the next section) , the principal val- ues of the phase shifts are determined unambiguously within a 2n range, i.e.,
  • ⁇ 12 + ⁇ .
  • the first case of rational period ratio still ap ⁇ plies, meaning that the unambiguous measurement range is limited to L ⁇ d . Therefore, one should carefully choose the ratio q2/qi to ensure that ⁇ exhibits no periodic be ⁇ havior within the measurement range.
  • Any method that determines a phase shift unam ⁇ biguously in a 2n range (at each of the wavelengths) can be used for the implementation of this invention. Next, two examples of such methods are described.
  • the first example is the quadrature polarimet- ric method.
  • FIG. 2 shows schematically components of a typical sensor where a static optical phase shift bias is introduced between two detection channels.
  • the components as shown are a light source 20, an input polarizer 21, a sensing element 22 (which in use would be exposed to the measurand) , a first beamsplitter 23-1, a second beamsplit ⁇ ter 23-2, a quarter wave retarder (QWR) 24, a first output polarizer 25-1, a second output polarizer 25-2, and three optical power detectors 26-1, 26-2, 26-3.
  • the beam path of the wave is shown as dashed line (s) .
  • Three detectors are connected to the output beam path: the first detector 26-1 with no polarizer attached, the second detector 26-2 with the linear output polarizer 25-1, and the third de ⁇ tector 26-3 with a quarter-wave retarder 24 and the linear output polarizer 25-2.
  • the detectors are connected to a signal processing unit 31 performing at least some of the processing described below.
  • the optical powers measured at the detectors are, up to some proportionality constants, respectively,
  • phase shift ⁇ a number of signal processing recipes exist.
  • a vector or complex variable Y can be calculated in the fol ⁇ lowing way
  • the detection scheme as represented by FIG. 2 also works without an exact 90° phase offset (although preferred) and only requires having two polarimetric channels that have a certain known relative phase difference other than 0° or 180°. Indeed, if channel 3 has an additional phase offset ⁇ , a complex variable can be calculated as
  • the QWR element can be replaced by any phase retarder other than full and half-wave retarders .
  • Io measure Io could be having detectors measuring both antiphase outputs from a polariz ⁇ ing beamsplitter (replacing polarizer 25-1 or 25-2) in at least one of the two polarimetric detector channels as represented by detectors 26-2 and 26-3. The sum of the two anti-phase outputs would then yield the total light power Io. It should further be noted that the Io measure ⁇ ment is not required in cases where the total light power is known and/or kept constant, or can be readily monitored or controlled through electronic means or other detectors located before the sensing optical subsystem.
  • the second example of a phase shift measurement method in a full 2n range is the modulation phase detection (MPD) technique, which is often implemented in a "non- reciprocal phase modulation" scheme and commonly used in fiber-optic gyroscopes and current sensors.
  • MPD modulation phase detection
  • FIG. 3 a transmissive open-loop MPD setup is shown in FIG. 3.
  • the basic compo ⁇ nents as shown are a light source 20, an input polar ⁇ izer 21, a sensing element 22 (which in use would be exposed to the measurand) , an output polarizer 25, and a detector 26.
  • the beam path of the wave is shown as dashed line(s) .
  • a birefringent phase modulator 30 is also included in the beam path.
  • the phase modulator 30 and the detector 26 are coupled through a signal processing unit 31 for performing the signal analysis as described in the following.
  • the birefringent phase modulator 30 is con ⁇ nected in series with the sensing medium 22, so that an additional phase shift modulation P(t) can be added to the phase shift to be measured.
  • the detector 26 measures the modulated optical power after the linear polarizer 25.
  • a sinus ⁇ oidal modulation ⁇ sinQt is imposed by the phase mod ⁇ ulator.
  • the detector signal of eq. [4] can be written in a Fourier expansion as a series of harmonics at different orders k of the modulation frequency ⁇ , i.e.,
  • the signs of the harmonic components can be ascertained by comparing the phases of the harmonic components with that of the excitation waveform.
  • a vector or complex numbers can be formed from the above representation which allows to derive the phase shift principal value from the detected signal.
  • phase shift principal value cp can be cal ⁇ culated as the argument of Y, which is again defined in the range (-n, n] .
  • the phase modulation amplitude P can be arbitrarily small.
  • the modulation amplitude P can also be calculated from the measured harmonic amplitudes, e.g., for the purpose of stabilizing the amplitude.
  • a sinusoidal modulation other waveforms, in particular square wave modulation, may be used. Further details of the open-loop MPD signal processing can be found in the references [6, 16] . There are several different approaches with re ⁇ gard to the signal processing procedure as performed by unit 31 of converting the measured two-wavelength outputs to an unambiguous measurand value.
  • phase shift principal values [ ⁇ 1, ⁇ 2] a t the two wave ⁇ lengths
  • two lists of possible full values of the phase shifts i + ⁇ ⁇ ⁇ , ⁇ 2 + 2 ⁇ ⁇ ] (m and n are integers) in the measurement range.
  • two lists of possible measurand values are calculated from these phase shifts
  • the two lists can be compared to identify a pair of [ ⁇ , ⁇ 2 ''] with the smallest difference x ⁇ — x ⁇
  • the output can be set as the average of both x— x m + x n /2 ⁇
  • a second more sophisticated method is the fol- lowing:
  • a mn /j ql + q 2 is the signed per- pendicular distance from the origin to the segment with the index pair [m, n] , with its sign indicating on which side of the origin the segment lies. Therefore, in the entire unambiguous measurement range, each index pair [m, n] corresponds to a unique A mn and vice versa.
  • This mapping can be pre-calculated and saved in a ID tabular form.
  • An example of the one-to-one correspondence between A mn and [m, n] is shown in FIG.
  • phase shift principal values [cpi, cp2] From the measured phase shift principal values [cpi, cp2], one can calculate A mn using eq. [6] and look up the corresponding indices [m, n] from the pre- calculated table. Finally, the full values of the phase shift and the corresponding measurand value can be calcu ⁇ lated .
  • the first method involves a search in a 2D space consisting of two dynamic lists, whereas the second method involves only a ID lookup in a static list. There ⁇ fore, in terms of computational complexity, the second method is the preferred method.
  • the sensor design can start with one given wavelength ⁇ and the corresponding qi .
  • ⁇ 2n i / L. Therefore, the meas- urement at ⁇ results in i line segment end points on the left (or right) boundary of the 2D phase space (-n/2, n/2] x (-n/2, n/2] .
  • the end points on the vertical phase-space boundary should be evenly distributed, i.e., the separation between two adjacent end points should be 2n / i .
  • This can be achieved by making the measurement at ⁇ 2 fill a 2n range, i.e., q2 2 ⁇ / L (the dotted line in FIG. 5) .
  • the condition is satisfied for all q2 2 ⁇ 2 / L as long as i and 2 are coprime, i.e., their greatest common divisor is 1. This leads to the following crite ⁇ rion :
  • Eq. [7] provides a list of candidate wave ⁇ lengths defined by an integer ⁇ (or equivalently N2) , that satisfy the uniform-filling condition. A selection of sensor wavelengths can be then made taking into account other considerations.
  • the present invention of two-wavelength disambiguation can be readily applied to the design of extended- range electro-optic DC voltage sensors. Next, some exam ⁇ ples are presented.
  • the outputs of two light sources of different center wavelengths are combined.
  • Low-coherence light sources e.g., SLED source, 40 nm FWHM bandwidth
  • An electro-optic crystal is used as the sensing medium to convert the voltage to be measured to a phase shift between the orthogonal polarization modes in the crystal.
  • An electro-optic crystal without natural bi ⁇ refringence such as Bi 4 Ge 3 0i2, BGO
  • An elec ⁇ tro-optic waveguide or fiber can also be used as the volt ⁇ age sensing medium.
  • the end faces of the BGO crystal (cut perpendicular to the [001] direction) are electrically con ⁇ nected to the electrodes that provide the voltage drop.
  • the full voltage (not a fraction thereof) is applied across the sensing crystal.
  • the voltage is applied in the longitudinal direction of the crystal, thus the path integral of the electric field in this direction (i.e. the applied voltage) is measured. Therefore, the voltage meas ⁇ urement is independent of the internal charge redistribu ⁇ tion in the crystal.
  • FIG. 6 shows a design of an extended-range DC voltage sensor in a transmissive config- uration.
  • the basic components are those already described in FIG. 2. However the sensor is expanded to accommodate operations at two different wavelengths X and %2 ⁇
  • the components relating to the second wavelength carry an apos ⁇ trophe.
  • light source 20 generates light centered at wavelength X
  • light source 20' generates light centered at wavelength %2 ⁇
  • the signals at the two wavelengths are combined and separated by WDM filters 60.
  • three detector channels 26 and 26' yield the total light power and two quadrature polarimetric signals, re ⁇ spectively.
  • the phase shifts at each wavelengths are eval- uated as described above (see eqs .
  • the axes of the polarizers 21 and 25-1 and 25-2 are aligned at ⁇ 45°, and the QWR axes are aligned parallel to the electro-optic axes of a BGO crystal.
  • the beamsplitters 23-1 and 23-2 should be aligned with their axes at 45° relative to the BGO crystal axes, in order to equalize any possible phase shift the two polarization waves may experience from the beamsplitter.
  • Any residual system phase shifts for example from the beamsplitter or from the residual natural birefringence of the BGO crystal, can be characterized and taken out by calibration.
  • the residual birefringence of BGO can also be reduced by combining two BGO crystals in series, with antiparallel [001] axes and the x/y axes ro ⁇ tated 90° against each other. In this arrangement, the electro-optic phase shifts add up, while the intrinsic bi ⁇ refringence cancels, leading to a better zero-point sta ⁇ bility.
  • FIG. 7 shows an alternative polarimetric de ⁇ tection scheme, where only one birefringence-free beamsplitter 23-2 is used, and where the two detector pairs 26-1, 26-1' and 26-2, 26-2' are each connected to one of the two outputs of a polarizing beamsplitter 65 at each wavelength.
  • the total optical power is given by the sum of powers at the detectors 26-1 and 26-1' and 26-2 and 26- 2', while detectors 26-1 and 26-1' (or 26-2 and 26-2') and detectors 26-3 and 26-3' constitute two pairs (one for each wavelength) of quadrature polarimetric channels.
  • the polarizers and waveplates should have a working bandwidth broad enough to cover both wavelengths, which are combined before and separated after the common polarimetric sensing device 22.
  • the optical components in the sensor including the polar ⁇ izers, WDMs and the QWR may be bulk-optic components, or their fiber-optic counterparts.
  • the detectors may be di ⁇ rectly attached to the sensor, or alternatively they may be connected to the sensor via single-mode or multimode optical fibers.
  • the light source end of the sensing crystal are connected to the ground potential, and the detector end of the crystal is connected to the high- voltage potential.
  • the detector end of the crystal is connected to the high- voltage potential.
  • the polarimetric voltage sensors can also be designed in a reflective configuration.
  • An example is shown in Fig. 8, where a reflecting optic 70 is placed at one end of the crystal, and all the other optics are located at the other end.
  • Other elements are identical or similar to those described already when referring to FIG. 6 above.
  • the reflecting optic 70 may be a flat/curved mirror, a roof mirror, a corner-cube retroreflector, or simply a reflective thin film coating deposited on the end face of the crystal.
  • the reflection at the reflecting optic should ideally preserve the polarization state of the light without rotation or polarization-dependent phase shift. Any residual phase shift from reflection can also be characterized and taken out by calibration.
  • the light source and detector end of the crystal is con- nected to the ground potential, and the reflector end to the high voltage potential, in order to keep the light source and the detectors at the ground potential.
  • the sensor of FIG. 8 is a reflective-configuration design corresponding to the sensor shown in FIG. 6, analogously, the sensor shown in Fig. 7 can also be adapted to a reflective configuration (not depicted) .
  • the modulation phase detection scheme is another approach to simultaneously measure the phase shift, as described above.
  • a MPD-based voltage sensor in the transmissive configuration similar to what is de ⁇ picted in FIG. 3, has disadvantages in real-world applica ⁇ tions, because the phase modulator and the connecting po ⁇ larization-maintaining fibers are generally very sensitive to temperature and/or stress variations. Therefore, the output of a transmissive MPD-based voltage sensor is easily affected by environmental disturbances, and hence not re ⁇ liable for field applications.
  • the key to a robust MPD sensor is a reciprocal optical design, whereby the two interfering waves either counter- propagate in a loop (in the case of a fiber-optic gyro ⁇ scope) or retrace the optical path upon reflection with swapped orthogonal polarizations (in the case of a reflec ⁇ tive fiber-optic current sensor) .
  • the intrinsic phase shifts of the phase modulator and inter ⁇ connecting polarization maintaining (PM) fibers automatically cancel along the reciprocal path (hence no tempera ⁇ ture dependence thereof) , while the phase modulation and the measurand-induced phase shift double.
  • Non-reciprocal phase modulation This is usually referred to as "non-reciprocal phase modulation.”
  • the co-owned Patent US7911196 [9] describes a non-reciprocal phase modulation voltage sensor incorporat ⁇ ing a voltage sensing element (or several such elements) , a 45° Faraday rotator, MPD modulation and detection elec- tronics.
  • the sensor in this patent is only capable of unambiguously measuring a DC electro-optic phase shift of
  • the sensors described in US7911196 can be modified to unambiguously measure a DC voltage in an extended range.
  • FIG. 9A A re ⁇ flective version of a MPD based sensor is shown in FIG. 9A using elements already described when referring to the figures above.
  • low-coherence light sources 20, 20' and photodetectors 26, 26' are connected via a 1x2 fiber ⁇ optic coupler to the sensing element 22 with WDM elements 60 providing the combination and separation of the wave- lengths X and %2 ⁇
  • the light first passes through a linear polarizer 21, enters into a PM fiber 80 and is coupled into both axes of a fiber-optic birefringent phase modulator 30 through a 45° splice 81.
  • the beam is then passed through a collimator 82, passes through a 45° Faraday rotator 83, and enters the sensing element 22 (BGO crystal with beam propagating along the [001] axis), whose ends are electri ⁇ cally connected to the high voltage and ground potentials, respectively.
  • the light is reflected back into the sensing medium, the birefringent crystal, the Faraday rotator and the PM fiber 80 in sequence, by a reflecting optic 70 at the far end of the sensing medium 22.
  • the reflecting optic 70 may be a flat/curved mirror, a roof mirror, a corner- cube retroreflector, or simply a reflective thin film coating deposited on the end face of the crystal.
  • the reflection at the retroreflector should pre ⁇ serve the polarization state of the light without rotation or polarization-dependent phase shift. Due to the two-time 45° Faraday rotation in the reciprocal path, the interfering orthogonally polarized waves swap their polarizations as they reenter the PM fiber 80 and the birefringent phase modulator 30, thereby can- celing the associated intrinsic phase shifts and eliminat ⁇ ing the temperature dependence thereof.
  • the 45° splice 81 and the polarizer 21 on the return path allows the coherent mixing of the polarization waves, and the photodetectors 26, 26' measure the resulting light power for the two dif- ferent wavelengths.
  • a signal processing and control unit 31 con ⁇ trols the phase modulation waveform and measures the light powers at the photodetectors at the two wavelengths. It calculates the phase shift principal values, for example according to the procedure described in eqs . [4'] and [5], individually at each of the wavelengths. Finally, the two phase shift principal values are again combined using one of the methods described above to yield an unambiguous measurement of the voltage V.
  • Any residual system phase shifts can be characterized and taken out by calibration.
  • FIG. 9B A loop configuration where the reflecting optic 70 of FIG. 9A is replaced with a Y-type phase modulator 30 and another set of collimator 82' and Faraday rotator 83', is shown in Fig. 9B . Due to the Faraday rotators 83 and 83' , the waves are polarized along orthogonal electro-optic axes of the sensing medium 22 in the two counterpropagating directions.
  • An advantage over the reflective configuration of FIG. 9A is that there are more degrees of freedom in the alignment of the optical components, which may simplify the light coupling from electro-optic crystal back into the PM fiber 80, a potential problem when using bulk opti- cal sensing elements.
  • a possible drawback is the larger number of components. As can be seen from the above embodiments and also from the figures, the first set of waves and said second set of waves all pass sensing element 22.
  • an interferometer in the RF or other electro-magnetic radiation bands can also benefit from the same unambiguous extension of the measurement range.
  • the invention is also not limited to electro-magnetic waves. Any interferometer involving waves, be it acoustic, density, or other types of waves, can also benefit in the same way .

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Instruments For Measurement Of Length By Optical Means (AREA)
  • Length Measuring Devices By Optical Means (AREA)

Abstract

La présente invention concerne un capteur interférométrique comprenant un ou plusieurs générateurs d'onde (20, 20') générant au moins un premier ensemble de deux ondes centrées toutes deux sur une première longueur d'onde et un second ensemble de deux ondes centrées toutes deux sur une seconde longueur d'onde différente, un élément de détection (22) qui permet à une grandeur à mesurer d'induire un premier déphasage relatif dans le premier ensemble d'onde et un second déphasage relatif dans le second ensemble d'ondes, respectivement, au moins un détecteur (26, 26') mesurant un premier signal d'interférence dans le premier ensemble d'ondes et un second signal d'interférence dans le second ensemble d'ondes, et comprenant en outre une unité de traitement de signal (31) conçue pour déterminer de manière non ambiguë, à partir des premier et second signaux d'interférence, deux quantités représentatives des valeurs principales des premier et second déphasages relatifs au sein d'une plage 2π, respectivement, et pour dériver une valeur de grandeur à mesurer à partir de leur combinaison.
PCT/EP2015/053514 2014-02-21 2015-02-19 Capteur interférométrique WO2015124677A1 (fr)

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CN112097808A (zh) * 2020-08-18 2020-12-18 中国科学院空天信息创新研究院 一种基于相位生成载波调制的f-p干涉光纤传感系统

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EP3729106A1 (fr) * 2017-12-22 2020-10-28 ABB Power Grids Switzerland AG Détection optique de polarisation à précision améliorée dans le régime de signal élevé
CN110531103B (zh) * 2019-09-30 2021-08-10 浙江海洋大学 一种基于利萨如图形的光速测量方法与装置

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EP3598149A1 (fr) * 2018-07-19 2020-01-22 Lumiker Aplicaciones Tecnologicas S.L. Procédé pour mesurer le courant circulant à travers au moins un conducteur avec un équipement de mesure à fibre optique et équipement de mesure
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CN112097808A (zh) * 2020-08-18 2020-12-18 中国科学院空天信息创新研究院 一种基于相位生成载波调制的f-p干涉光纤传感系统

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