CLOSED-LOOP INTERFEROMETRIC SENSOR USING LOOP GAIN FOR DETERMINING INTERFERENCE CONTRAST

Abstract

In order to measure the contrast of interference in an interference-based, closed-loop, phase-modulating optical sensor device, the gain of the feedback loop in a feedback controller (12) is evaluated. This gain is found to be a measure for the contrast. The contrast evaluated in this way can e.g. be used for period-disambiguation when determining the measurand of the sensor device. The sensor device can e.g. be a high-voltage sensor or a current sensor.

Claims

1. A method for measuring an interference contrast (A) in an interference-based, closed-loop, phase-modulating optical sensor device, said method comprising the steps of generating a first and a second optical wave by means of a light source, sending at least said first wave through a sensing element, wherein at least one refractive index of said sensing element depends on a measure and, sending said first and/or said second wave through a phase modulator for adding a phase shift modulation m between said first and second wave, wherein said phase shift modulation m is controlled by a control signal f, bringing said first and said second waves to interference and determining a measured interference signal (I) in a light sensor, using a modulation amplitude of said measured signal (I) for determining an error signal I and feeding said error signal I to a feedback loop for controlling said phase shift modulation m, wherein said feedback loop controls said control signal f with feedback loop gain G in order to keep said error signal I zero, adjusting said feedback loop gain G to a value $G = \frac{_{f}}{.Math. .Math. I} .Math. |_{.Math. .Math. I = 0},$ using said gain G for determining the interference contrast (A) of said waves.

2. The method of claim 1, further comprising the step of using said gain G for calculating said measurand.

3. The method of claim 2, comprising the step of using said gain G for a disambiguation of a quasi-periodicity of said measured signal (I) as a function of said measurand.

4. The method of claim 1, wherein said measurand is an electrical voltage, said sensing element is located in an electrical field generated by said electrical voltage, and said at least one refractive index depends on said electrical field.

5. The method of claim 1, wherein said measurand is an electrical current, said sensing element is located in a magnetic field generated by said electrical current, and said at least one refractive index depends on said magnetic field.

6. The method of claim 1, comprising the step of intermittently measuring a response of the measured signal (I) to a variation of said control signal f.

7. The method of claim 1, wherein said gain G is used to correct a systematic error of said sensor device.

8. An interference-based closed-loop, phase-modulating optical sensor device comprising a control unit a light source for generating a first and a second optical wave, a sensing element having at least one refractive index depending on a measurand, wherein said light source is positioned to send at least said first wave through said sensing element, a phase modulator configured to add a phase shift modulation m between said first and second wave, wherein said phase shift modulation m is controlled by a control signal f, a light sensor configured to determine a measured signal (I) from an interference of said first and said second wave after passage through said sensing element and said phase modulator, a feedback controller configured to use the modulated amplitude of said measured signal (I) for determining an error signal I and to generate said control signal f, wherein said feedback controller has an optimum gain G $G = \frac{_{f}}{.Math. .Math. I} .Math. |_{.Math. .Math. I = 0},$ said control unit is configured to use said gain G for determining an interference contrast (A) of said waves.

9. (canceled)

10. The method of claim 2, wherein said measurand is an electrical voltage, said sensing element is located in an electrical field generated by said electrical voltage, and said at least one refractive index depends on said electrical field.

11. The method of claim 3, wherein said measurand is an electrical voltage, said sensing element is located in an electrical field generated by said electrical voltage, and said at least one refractive index depends on said electrical field.

12. The method of claim 4, wherein said measurand is an electrical voltage, said sensing element is located in an electrical field generated by said electrical voltage, and said at least one refractive index depends on said electrical field.

13. The method of claim 2, wherein said measurand is an electrical current, said sensing element is located in a magnetic field generated by said electrical current, and said at least one refractive index depends on said magnetic field.

14. The method of claim 3, wherein said measurand is an electrical current, said sensing element is located in a magnetic field generated by said electrical current, and said at least one refractive index depends on said magnetic field.

15. The method of claim 2, comprising the step of intermittently measuring a response of the measured signal (I) to a variation of said control signal f.

16. The method of claim 3, comprising the step of intermittently measuring a response of the measured signal (I) to a variation of said control signal f.

17. The method of claim 4, comprising the step of intermittently measuring a response of the measured signal (I) to a variation of said control signal f.

18. The method of claim 5, comprising the step of intermittently measuring a response of the measured signal (I) to a variation of said control signal f.

19. The method of claim 7, wherein said systematic error of said sensor device is due to drift and/or component misalignment.

20. The method of claim 2, wherein said gain G is used to correct a systematic error of said sensor device.

21. The method of claim 3, wherein said gain G is used to correct a systematic error of said sensor device.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0045] The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. This description makes reference to the annexed drawings, wherein:

[0046] FIG. 1 shows a block diagram of a reflective optical voltage sensor with closed-loop MPD;

[0047] FIG. 2a shows the interference contrast A and unmodulated optical power I as functions of the applied voltage and the corresponding phase shift;

[0048] FIG. 2b shows the interference contrast A and expected loop gain G; the calculation for FIGS. 2a, 2b is performed for a reflective BGO voltage sensor, with a 40 nm FWHM Gaussian spectrum centered at 1310 nm, and a group delay offset .sub.0=60 fs, assuming I.sub.0=1 for loop gain;

[0049] FIG. 3 shows, in part (a), the polarimetric response curve and, in part (b), the square-wave modulation waveforms of phase shift .sub.m-.sub.f (dotted) and polarimetric output I (dashed) at the set point; for the plots, we assume .sub.+=0.1 and .sub.=0.05; the loop-gain calculation sequence occupies modulation cycles 0 to 3.

DETAILED DESCRIPTION

[0050] The design of a reflective optical voltage sensor using the closed-loop MPD phase shift detection scheme is shown in FIG. 1.

[0051] The device comprises an MPD optoelectronics module 1, which contains a low-coherence light source 2. The light from light source 2 is fed through a polarizer 3, a phase modulator 4 and into both polarization directions of a polarization maintaining (PM) fiber 5.

[0052] A collimator 6 sends these waves from PM fiber 5 through a 45 Faraday rotator 7 and into a first end of Pockels effect crystal 8, which is being exposed to the electrical field generated by the voltage to be measured. At the second end of the crystal, the waves are reflected by reflector 9 and sent back through the components 3-8.

[0053] A beam splitter 10 is arranged between light source 2 and polarizer 3 and sends at least part of the returning light into a light sensor 11.

[0054] The device of FIG. 1 further comprises a feedback controller 12 for controlling phase modulator 4 as well as a control unit 13 for controlling the operations of the device. These two components, whose functions are described in more detail below, can be implemented as a single hardware unit (such as a microcontroller) or as separate hardware units.

[0055] Faraday rotator 6 rotates both linear polarizations from PM fiber 5 by 45 before they propagate along the electro-optic axes (principal refractive index axes) of sensing crystal 8. The reflected waves pass through Faraday rotator 6 again, further rotating the polarizations by 45 in the same direction, thereby making a combined 90 rotation from the input polarizations, which is equivalent to a swap between the two orthogonal linear polarizations.

[0056] Phase modulator 4 is operating at a frequency adapted to the round-trip time of the light waves traveling from modulator 4 to reflector 9 and back, such that the relative phase shifts induced by the phase modulator in the two passes of the waves through it have opposite signs and the phase modulations in the two passes are consequently added due to the swap in light polarizations.

[0057] The coherence length of light source 2 is advantageously between 5.Math..sub.0 and 100.Math..sub.0, with .sub.0 being the center wavelength of the light source, in order to obtain a good variation of interference contrast A when changing the phase between the two polarizations by a few multiples of 2.

[0058] With such a low-coherence light source 1 and a properly selected group delay offset .sub.0 (e.g. by means of a birefringent element), the voltage measurement range can be set such that the interference contrast varies strongly and monotonically with the applied voltage (see FIG. 2a), thereby enabling the unambiguous determination of the voltage in a wide measurement range (>500 kV).

[0059] In the MPD system, a phase modulation .sub.m(t) is added in phase modulator 4 onto a phase shift of interest (.sub.0), i.e. here the phase shift caused by the measurand, and the polarimetric optical response is measured in the returning waves after they have passed polarizer 3. This response is the intensity I of the interfering waves

[00003] $I (t) = \frac{I_{0}}{2} .Math. {1 + A .Math. .Math. \cos [_{0} +_{m} (t)]},$

where I.sub.0 is the total light power, and A is the interference contrast.

[0060] In a closed-loop square-wave modulation system, the modulation .sub.m(t) is advantageously a square wave alternating between two levels .sub.=/2+.sub.f, where .sub.f is a dynamically controlled feedback phase [2]. .sub.f can be considered to be the control signal that feedback controller 12 uses for controlling phase modulator 4.

[0061] The corresponding measured polarimetric response at the two modulation levels are

[00004] $I_{} = \frac{I_{0}}{2} [1 A .Math. .Math. \sin (_{0} +_{f})]$

[0062] The feedback-loop error signal is the difference between the two polarimetric response levels

I=I.sub.I.sub.+=I.sub.0 sin(.sub.0+.sub.f)

[0063] The system is operated at a point that the error signal I is maintained at 0, which corresponds to .sub.0+.sub.f=0. Therefore, at the operating point, the phase shift offset to be measured .sub.0 is simply the opposite of the dynamically controlled feedback phase shift .sub.f. The expected loop gain in the designed voltage range is plotted in FIG. 2b.

[0064] The optimal loop gain of the feedback loop in feedback controller 12 is found to be the derivative of the feedback phase shift with respect to the error signal at the set point, i.e.

[00005] $G = \frac{_{f}}{.Math. .Math. I} .Math. |_{.Math. .Math. I = 0} = \frac{1}{{AI}_{0}}$

[0065] As an important control parameter for the control loop, the optimal loop gain G is determined by control unit 13 e.g. in the following way. Intermittently, in particular periodically (but possibly also in non-periodic manner), the response of the measured signal, in particular the response of the error signal I, to a variation of the control signal .sub.f applied to the phase modulator 12 is measured.

[0066] For example, a phase modulation sequence [.sub.++, .sub.+, .sub.+, .sub.] is invoked, whereby small phase deviations .sub. (typically .sub.+=.sub.) are added to or subtracted from the normal phase modulation .sub. (see FIG. 3):

[00006] $_{++} =_{+} + {.Math.}_{+} = \frac{}{2} +_{f .Math. .Math. 0} + {.Math.}_{+}$ $_{- +} =_{-} + {.Math.}_{+} = - \frac{}{2} +_{f .Math. .Math. 0} + {.Math.}_{+}$ $_{+ -} =_{+} - {.Math.}_{-} = \frac{}{2} +_{f .Math. .Math. 0} - {.Math.}_{-}$ $_{--} =_{-} - {.Math.}_{-} = - \frac{}{2} +_{f .Math. .Math. 0} - {.Math.}_{-}$

[0067] Here, .sub.f0 corresponds to the control signal .sub.f that presently sets the error signal I to zero.

[0068] Correspondingly, the polarimetric responses at the four modulation levels are

[00007] $I_{++} = \frac{I_{0}}{2} .Math. (1 - A .Math. .Math. \sin .Math. .Math. {.Math.}_{+}) \frac{I_{0}}{2} .Math. (1 - A .Math. .Math._{+})$ $I_{- +} = \frac{I_{0}}{2} .Math. (1 + A .Math. .Math. \sin .Math. .Math. {.Math.}_{+}) \frac{I_{0}}{2} .Math. (1 + A .Math. .Math._{+})$ $I_{- +} = \frac{I_{0}}{2} .Math. (1 + A .Math. .Math. \sin .Math. .Math. {.Math.}_{-}) \frac{I_{0}}{2} .Math. (1 + A .Math. .Math._{-})$ $I_{--} = \frac{I_{0}}{2} .Math. (1 - A .Math. .Math. \sin .Math. .Math. {.Math.}_{-}) \frac{I_{0}}{2} .Math. (1 - A .Math. .Math. {.Math.}_{-})$

[0069] From these, we can calculate two error signals corresponding to .sub. as

I.sub.+=I.sub.+I.sub.++=AI.sub.0.sub.+

I.sub.=I.sub.I.sub.+=AI.sub.0.sub.

[0070] The optimal loop gain can then be calculated as

[00008] $G = \frac{{.Math.}_{+} + {.Math.}_{-}}{.Math. .Math. I_{+} - .Math. .Math. I_{-}} = \frac{1}{{AI}_{0}}$

[0071] The calculated optimal loop gain G is then used for setting the amplitude of the feedback phase shift.

[0072] From the results above, one sees that the loop gain value G measured in the MPD system is related to the interference contrast A and the total light power I.sub.0. Conversely, one can calculate the interference contrast A as

[00009] $A = \frac{1}{{GI}_{0}}$

[0073] In the modulation scheme described above, the total light power I.sub.0 is just twice the optical response level I.sub. at the operating point .sub.0+.sub.f=0. Therefore, practically it can be measured as the DC offset of the optical response I.sub., while the AC oscillation amplitude I is used as the error signal to control the feedback loop.

[0074] It should however be noted, that in some applications, such as period disambiguation of the phase shift (e.g. for optical DC current sensing), it is not necessary to determine the absolute value of the interference contrast A, but it would be sufficient to follow its relative change. In such cases, one may record the relative variations of the loop gain G and the total optical power I.sub.0, without determining their absolute scales. For example, one may set the values of the loop gain and the total optical power measured at a certain moment (e.g. at the onset of operation) to a constant, and evaluate all subsequent measurements accordingly. If the optical power is maintained at a constant level, its measurement may not be needed at all.

[0075] With the interference contrast A measured, one can then determine, in which period the phase shift is residing, and convert the measured phase shift principal value to a corresponding unique full-range value, following the procedure that is described in [1], the disclosure of which is herewith enclosed in its entirety. For an optical voltage sensor, an unambiguous voltage value can then be calculated from the full-range phase shift.

[0076] In a FOCS sensor, the interference contrast calculated from the loop gain can likewise be used to extend the measurement range, which is, without history tracking, limited to 2 phase shift in current products.

[0077] Furthermore in some FOCS sensors, the current measurement scale factor is influenced by the polarization extinction ratio of the fiber link between the sensor head and the optoelectronics, which in turn directly affects the interference contrast. Therefore, the measurement of the interference contrast can be used to monitor the variation of the polarization extinction ratio, and to compensate the corresponding variation of the current measurement scale factor. Hence, also in this case, the interference contrast can be used to determine the measurand in a more precise manner.

[0078] In general, the determined gain G can be used to correct systematic errors, in particular systematic errors due to drift and/or component misalignment, in the sensor device, in particular if the systematic errors affect the interference contrast.

[0079] Notes:

[0080] Electro-optic crystal 5 can be replaced by any other electro-optic element that exhibits a birefringence depending on the applied electrical field.

[0081] In another application for measuring the current in a conductor, the sensing element can e.g. be a fiber wound around the conductor. In that case, a quarter-wave retarder is used instead of Faraday rotator 7.

[0082] Advantageously, the optical sensor device has a reciprocal configuration, e.g. in the form of a Sagnac interferometer or in a reflective form, where the phase shifts, in particular from PM fiber 5 and/or sensing element 8, are canceled. In that case, the phase modulator 4 is advantageously operated, as mentioned, to apply opposing phase shifts between the two waves at a frequency corresponding to a round trip from the modulator 4 to reflector 9 and back.

[0083] When the device is a voltage sensor, the techniques shown herein allow the compensation of systematic errors for an optical DC voltage sensor with measurement range >500 kV to achieve an accuracy <0.2%.

[0084] The techniques are ideal for applications in HVDC air-insulated systems, HVDC cables, and DC gas-insulated switching (GIS) systems. Such GIS may be filled with dielectric gas based on SF6 or alternative gases, such as fluoroketones or fluoronitriles, preferably in mixtures with a background gas, such as e.g. selected from: nitrogen, carbon dioxide and oxygen.

[0085] They allow to remove the periodwise measurement ambiguity in a wide range (>16 phase periods, or equivalently >500 kV).

[0086] They provide a means for the measurement of interference contrast in a closed-loop modulation phase detection system for ambiguity removal.

[0087] And they make it also possible to enhance the accuracy of fiber-optic current sensors (monitor and control of polarization extinction ratio, PER) by correction of variations in scale factor during operation

[0088] While presently preferred embodiments of the invention are shown and described, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

REFERENCES CITED

[0089] [1] WO 2015/124676 [0090] [2] H. Lefvre, The Fiber-Optic Gyroscope: Artech House, 1993. [0091] [3] K. Bohnert, P. Gabus, J. Nehring, and H. Brndle, Temperature and vibration insensitive fiber-optic current sensor, Journal of Lightwave Technology, vol. 20, pp. 267-276, 2002. [0092] [4] K. Bohnert, P. Gabus, J. Nehring, H. Brndle, and M. G. Brunzel, Fiber-Optic Current Sensor for Electro-winning of Metals, J. Lightwave Technol., vol. 25, pp. 3602-3609, 2007. [0093] [5] WO 2015/124678 [0094] [6] U.S. Pat. No. 7,911,196

LIST OF REFERENCE NUMBERS

[0095] 1: optoelectronics module [0096] 2: light source [0097] 3: polarizer [0098] 4: phase modulator [0099] 5: fiber [0100] 6: collimator [0101] 7: Faraday rotator [0102] 8: Pockels effect crystal 8 [0103] 9: reflector [0104] 10: beam splitter [0105] 11: light sensor [0106] 12: feedback controller [0107] 13: control unit

CLOSED-LOOP INTERFEROMETRIC SENSOR USING LOOP GAIN FOR DETERMINING INTERFERENCE CONTRAST

Inventors

Cpc classification

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G01J2009/0249

PHYSICS

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G01B9/02067

PHYSICS

Classification Explorer

G01B9/0201

PHYSICS

Classification Explorer

G01D5/266

PHYSICS

Classification Explorer

G01J2009/0253

PHYSICS

International classification

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G01B9/02

PHYSICS

Abstract

Claims

Description