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WO1997023766A1 - Capteur a fibre optique se presentant sous l'aspect d'un interferometre de fabry-perot comportant un ou deux elements reflechissants conçus comme un reseau de bragg - Google Patents

Capteur a fibre optique se presentant sous l'aspect d'un interferometre de fabry-perot comportant un ou deux elements reflechissants conçus comme un reseau de bragg Download PDF

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Publication number
WO1997023766A1
WO1997023766A1 PCT/SE1996/001731 SE9601731W WO9723766A1 WO 1997023766 A1 WO1997023766 A1 WO 1997023766A1 SE 9601731 W SE9601731 W SE 9601731W WO 9723766 A1 WO9723766 A1 WO 9723766A1
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WIPO (PCT)
Prior art keywords
fibre
grating
strain
interferometer
function
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.)
Ceased
Application number
PCT/SE1996/001731
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English (en)
Inventor
Anders Henriksson
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Flygtekniska Forsoksanstalten
Original Assignee
Flygtekniska Forsoksanstalten
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Publication date
Application filed by Flygtekniska Forsoksanstalten filed Critical Flygtekniska Forsoksanstalten
Priority to AU12198/97A priority Critical patent/AU1219897A/en
Publication of WO1997023766A1 publication Critical patent/WO1997023766A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L1/00—Measuring force or stress, in general
    • G01L1/24—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet
    • G01L1/242—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet the material being an optical fibre
    • G01L1/246—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet the material being an optical fibre using integrated gratings, e.g. Bragg gratings
    • 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
    • G01D5/35312—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 using a Fabry Perot

Definitions

  • Fibre-optic sensor in the form of a Fabry-P ⁇ rot interferometer having one or both reflecting elements made as a Bragg grating
  • the present invention relates to a fibre-optic sensor in the form of a Fabry-Perot interferometer with one or both beam-reflecting elements designed as a Bragg grating.
  • Fibre-optic sensors have been gaining ground for many years and have frequently been recommended for their many good properties, such as smallness, sensitivity, great dynamic range, flexibility, non-interfering, immunity to electromagnetic inter ⁇ ference, corrosion immunity, signal range etc. In spite of all these good properties, the fibre-optic sensors have had difficulty in keeping up with conventional electric sensors. Above all this probably depends on the fact that their advantages have not been great enough in relation to drawbacks such as higher expenses and greater complexity.
  • a fibre-optic Bragg grating is a periodic modulation of the refractive index in the core of an optical fibre.
  • the grating pitch of the Bragg grating coincides with half the wavelength of the light, the reflection conditions of the first order are satisfied and part of the light will be reflected and still be conducted in the core of the fibre.
  • the reflectance and bandwidth of the grating are decided by the length and modulation depth of the grating. Longer gratings and greater modulation depths result in higher reflectance, while essentially the length of the grating decides the bandwidth thereof. This means that the gratings can be tailor-made to a great extent.
  • an "efficient reflective surface” is usually defined, which corresponds to the centre of the grating lines that have contributed to the reflection. This is not only a definition but also a physical consequence of the reflected light simultaneously satisfying the reflection condition.
  • a technique that has frequently been described to increase the bandwidth of the grating is to change the grating pitch over the length of the grating and then obtain a so-called chirped grating.
  • Light of a wider spectrum could then be reflected in the same grating, which would then of course obtain a greater bandwidth in the charac ⁇ teristic.
  • the different wavelengths of the light would not be reflected from the same area in the grating, which is utilised by Kersey and Davis, 1994, to measure strain by fibre-optic Michelson interferometers.
  • the Michelson interfero- meter is a construction where one measures the differences in optical path length between two arms which are independent of one another, one measuring arm and one reference arm.
  • the light is conducted in the two arms, is reflected so as then to interfere when meeting again.
  • a chirped Bragg grating in the interferometer it is used as a reflector in the measuring arm, and the used light is monochromatic and coherent.
  • the measuring arm and the grating are strained, not only the grating in its entirety is moved, but also the centre of the reflection inside the grating, which results in an additionally increased optical path difference.
  • the chirp in the grating functions as a mechanical signal amplification of the measurand.
  • Fig. 1 illustrates a Fabry-Perot interferometer having reflectors of chirped Bragg gratings BG. The position of the centre of the reflection within a chirped Bragg grating is called x r .
  • the Fabry-Perot interferometer is an interferometer where both the modulated light beam and the reference beam take the same way or the same fibre.
  • the beams are divided by a semitransparent mirror, where the reflected light constitutes the reference beam and the transmitted light is allowed to pass the measuring range L so as to be then reflected in a second mirror and back through the semitransparent mirror, whereupon the two beams can interfere.
  • What such a sensor senses is changes of the optical path length over the measuring range between the mirrors, such as displacement ⁇ of the mirrors in relation to each other, which for a sensor can be caused by, for instance, applied strain.
  • the two mirrors can of course consist of Bragg gratings.
  • the two mirrors would consist of chirped gratings, where the chirps have a varying or opposite inclination. Applied strain would then not only cause a displacement of the mirrors in relation to each other, but also displace the centre of the reflection in the gratings and thus obtain an increased deflection.
  • the present invention does not concern a signal-amplified Fabry-Perot interfero ⁇ meter per se, since it may be discussed if such an interferometer should be consid ⁇ ered obvious in view of prior art, but a sensor comprising such an interferometer, which is made invariant to some non-desired measurands by containing at least one grating having a specially designed chirp. This occurs by the invention having the design as defined in the independent claim. Suitable embodiments of the inven ⁇ tion are stated in the dependent claims.
  • Fig. 1 shows a Fabry-Perot interferometer with reflectors of chirped Bragg gratings
  • Fig. 2 shows the phase shift as a function of strain and temperature in an ordinary Fabry-Perot interferometer with straight Bragg gratings as reflectors
  • Fig. 3 shows an example of the chirp function ⁇ 0 (x) in the Bragg grating
  • Fig. 4 shows the phase shift as a function of strain and temperature for a temperature-compensated Fabry-Perot interferometer
  • Fig. 5 shows the phase shift as a function of strain and temperature for an embedded Fabry-Perot interferometer with straight gratings as reflec ⁇ tors
  • Fig. 6 shows the phase shift as a function of strain and temperature for an embedded Fabry-Perot interferometer with gratings having a tempe ⁇ rature-compensating chirp.
  • the signal obtained from an interferometer represents an optical phase difference.
  • this phase difference is determined by the difference in optical path length between two reflections. This optical path difference is then affected by several factors such as: 1) temperature
  • the principle accomplished according to the invention is controlling 4) by controlling the chirp of the Bragg grating such that the combined change of the optical path difference, caused by one arbitrary measurand of 1) or 2) and 3) and 4) will be zero under influence.
  • Temperature sensor invariant to longitudinal strain • Sensor for acoustic waves invariant to global strain
  • the theoretical part also describes temperature-invariant strain sensors for two cases, one in which the sensor is free, and one in which the sensor is embedded in a cross laminate of carbon fibre/epoxy composite.
  • strain sensors which are invariant to both longitudinal and transverse strain. All these descriptions include how the chirp should be designed for the desired effect.
  • the case involving the embedded sensor illustrates an example of how the form of the solution is adapted to amended marginal conditions.
  • the chirp algorithm is, however, expected to need adjustment in order to compensate for various deviations from the theoretical case.
  • the form of the chirp algorithm must be the one that is given in the solution.
  • the core of the fibre is exposed to an interference pattern of UV laser light such that the refractive index and density will vary periodically along the fibre for some fraction of a milii- metre up to thirty or forty millimetres.
  • the period is typically about 500 nm and the radius of the core is typically about 5 ⁇ m.
  • ⁇ Small variation
  • ⁇ Strain ⁇ xt Strain in x direction caused by thermal expansion
  • ⁇ zt Tension in z direction caused by thermal expansion
  • ⁇ Optical wavelength
  • all the other parameters and finally also the phase difference are set to be zero.
  • an expression containing the non-desired measurand and the shift of position of the centre of the reflection in the grating and the relation can easily be solved. This relation will be valid in the zero point for the remaining parameters, but since the relations are rela ⁇ tively linear, the errors will be small also for moderate variations.
  • ⁇ C x ⁇ - Xa
  • x, x r ( ⁇ g, AT) for each grating
  • ⁇ C can of course be obtained through an optional number of combinations, and the selected combination is determined by boundary conditions and technical restric ⁇ tions in the manufacture.
  • a lower chirp is to be preferred, which also results in x f being considerably greater than ⁇ C .
  • the chirp of the grating which yields ⁇ C X in the shift of position of a grating is solved below. The chirp in the function can then easily be divided by an optional scaling factor, whereupon the shift of position of the centre of the reflection will be multiplied by the same scaling factor.
  • the length of the physical period as well as
  • ⁇ A are set to be zero.
  • ⁇ (x) ⁇ o + — x .
  • the refractive index can be assumed to have the following behaviour:
  • phase shift caused by strain and temperature in a Fabry-Perot interferometer with common straight Bragg grating reflectors has been simulated over the range +1 % and -50° - +100°C in Fig. 2, the phase being expressed in degrees.
  • phase shift in an optional interferometer with at least one Bragg grating as reflector as a function of the strain, ⁇ , temperature change, ⁇ T, and the combined shifting of the centres of the reflections in the gratings, ⁇ C X can be expressed as:
  • the relative error caused by omitting the terms of the higher order is less than 1 l ⁇ in the maximum temperature range.
  • the refractive index of course varies with strain and tem-
  • n r ⁇ n n ( ⁇ ,AT) n 0 - ⁇ ⁇ [P . i - . (P.. +P ]*+ L T (8) ⁇ T
  • the invention concerns the technique of utilising the internal residual tension state in a fibre-optic Bragg grating for separating temperature signals and different strain signals. This is achieved by controlling the chirp of a Bragg grating, such that a fibre-optic interferometer, in which at least one reflector consists of a chirped Bragg grating, is made invariant to one arbitrary measurand of either temperature change or mechanical strain in any direction. This is achieved by the chi ⁇ function of the grating being balanced against one non-desired measurand, such that the shifting of the centre of the reflection which is a result of the variation of the non-desired measurand exactly compensates for the optical path difference which this meas ⁇ urand generates in the interferometer.
  • the combined change of the optical path difference in an interferometer will be zero, when influence is effected by one arbitrary measurand of either temperature changes or mechanical strain in any direction, changes of the refractive index of the glass caused by this arbitrary measurand and the change of the position of the centre of the reflective area in a chi ⁇ ed grating caused by the arbitrary measurand.
  • this can take place by the grating pitch of the grating approximatively following a chirp function of the form
  • A, B and C are constants which can be determined analytically or experi ⁇ mentally.
  • the constants are determined for each different case of desired invari ⁇ ance.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optical Transform (AREA)
  • Measuring Temperature Or Quantity Of Heat (AREA)
  • Length Measuring Devices By Optical Means (AREA)

Abstract

Cette invention porte sur un capteur à fibre optique se présentant sous l'aspect d'un interféromètre de Fabry-Pérot comportant un ou deux éléments réfléchissants de faisceau conçus comme un réseau de Bragg (BG). L'invention a ceci d'original que la fonction de compression-expansion d'impulsions d'au moins un réseau de Bragg est équilibrée par rapport à une mesurande non souhaitée de sorte que le décalage du centre (xr) de la réflexion qui résulte de la variation de la mesurande non souhaitée corrige exactement la différence de trajet optique qu'entraîne cette mesurande dans l'interféromètre. Ceci peut être mené à bien à l'aide de la fonction de compression-expansion répondant à l'expression suivante: Μ0(x) = μ0/2n0 (1+Ax/BL0-Cx), dans laquelle A, B et C sont des constantes déterminées de manière analytique ou expérimentale pour chaque cas différent d'une invariance souhaitée.
PCT/SE1996/001731 1995-12-22 1996-12-20 Capteur a fibre optique se presentant sous l'aspect d'un interferometre de fabry-perot comportant un ou deux elements reflechissants conçus comme un reseau de bragg Ceased WO1997023766A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU12198/97A AU1219897A (en) 1995-12-22 1996-12-20 Fibre-optic sensor in the form of a fabry-perot interferometer having one or both reflecting elements made as a bragg grating

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
SE9504601-7 1995-12-22
SE9504601A SE504772C2 (sv) 1995-12-22 1995-12-22 En fiberoptisk sensor i form av en Fabry-Pérot-interferometer med det ena eller båda strålreflekterande elementen utförda som ett Bragg-gitter

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Publication Number Publication Date
WO1997023766A1 true WO1997023766A1 (fr) 1997-07-03

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SE (1) SE504772C2 (fr)
WO (1) WO1997023766A1 (fr)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2000039549A1 (fr) * 1998-12-24 2000-07-06 Bae Systems Plc Jauge de contrainte a reseau de bragg module a fibre fournissant une valeur absolue des contraintes
US20180135971A1 (en) * 2015-05-12 2018-05-17 The Board Of Regents Of The University Of Neberaska Fiber optic sensor and methods for highly-sensitive detection of strain in large frequency range
CN116839643A (zh) * 2023-07-12 2023-10-03 武汉理工大学 一种基于光纤光栅法布里珀罗的干涉型温湿度传感装置及方法

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN105180977B (zh) * 2015-05-27 2017-08-25 三峡大学 一种单光纤迈克尔逊干涉传感器及传感系统
CN113686368A (zh) * 2021-08-24 2021-11-23 中北大学 用于获得相位变化参数的光纤传感器解调系统和解调方法

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1986001286A1 (fr) * 1984-08-13 1986-02-27 United Technologies Corporation Jauge de contrainte a fibre optique a resolution spatiale pour mesurer des repartitions de contraintes
US5042898A (en) * 1989-12-26 1991-08-27 United Technologies Corporation Incorporated Bragg filter temperature compensated optical waveguide device
US5345519A (en) * 1993-09-07 1994-09-06 Canadian Marconi Company Temperature-compensated fiber optic external cavity strain sensors and an intensity-based fiber optic sensor system
US5420948A (en) * 1993-08-12 1995-05-30 Northern Telecom Limited Chirped optical fibre filter
WO1995024614A1 (fr) * 1994-03-08 1995-09-14 United Technologies Corporation Capteur optique integre pouvant mesurer les contraintes et la temperature a l'aide d'un seul reseau de diffraction

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1986001286A1 (fr) * 1984-08-13 1986-02-27 United Technologies Corporation Jauge de contrainte a fibre optique a resolution spatiale pour mesurer des repartitions de contraintes
US5042898A (en) * 1989-12-26 1991-08-27 United Technologies Corporation Incorporated Bragg filter temperature compensated optical waveguide device
US5420948A (en) * 1993-08-12 1995-05-30 Northern Telecom Limited Chirped optical fibre filter
US5345519A (en) * 1993-09-07 1994-09-06 Canadian Marconi Company Temperature-compensated fiber optic external cavity strain sensors and an intensity-based fiber optic sensor system
WO1995024614A1 (fr) * 1994-03-08 1995-09-14 United Technologies Corporation Capteur optique integre pouvant mesurer les contraintes et la temperature a l'aide d'un seul reseau de diffraction

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2000039549A1 (fr) * 1998-12-24 2000-07-06 Bae Systems Plc Jauge de contrainte a reseau de bragg module a fibre fournissant une valeur absolue des contraintes
US6417507B1 (en) 1998-12-24 2002-07-09 Bae Systems Plc Modulated fibre bragg grating strain gauge assembly for absolute gauging of strain
US20180135971A1 (en) * 2015-05-12 2018-05-17 The Board Of Regents Of The University Of Neberaska Fiber optic sensor and methods for highly-sensitive detection of strain in large frequency range
US10731969B2 (en) * 2015-05-12 2020-08-04 Nutech Ventures, Inc. In-line fiber sensing, noise cancellation and strain detection
CN116839643A (zh) * 2023-07-12 2023-10-03 武汉理工大学 一种基于光纤光栅法布里珀罗的干涉型温湿度传感装置及方法

Also Published As

Publication number Publication date
SE9504601L (sv) 1997-04-21
SE504772C2 (sv) 1997-04-21
AU1219897A (en) 1997-07-17
SE9504601D0 (sv) 1995-12-22

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