WO2002016909A1 - Procede et appareil de detection de charge transversale a l'aide d'une fibre optique decalee en phase pi - Google Patents

Procede et appareil de detection de charge transversale a l'aide d'une fibre optique decalee en phase pi Download PDF

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Publication number
WO2002016909A1
WO2002016909A1 PCT/US2001/025935 US0125935W WO0216909A1 WO 2002016909 A1 WO2002016909 A1 WO 2002016909A1 US 0125935 W US0125935 W US 0125935W WO 0216909 A1 WO0216909 A1 WO 0216909A1
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WO
WIPO (PCT)
Prior art keywords
fiber
grating
bragg grating
birefringence
loading
Prior art date
Application number
PCT/US2001/025935
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English (en)
Inventor
Michael Leblanc
Sandeep Vohra
Tsung Tsai
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The Government Of The United States Of America, As Represented By The Secretary Of The Navy
Priority date (The priority date 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 date listed.)
Filing date
Publication date
Application filed by The Government Of The United States Of America, As Represented By The Secretary Of The Navy filed Critical The Government Of The United States Of America, As Represented By The Secretary Of The Navy
Priority to AU2001288310A priority Critical patent/AU2001288310A1/en
Publication of WO2002016909A1 publication Critical patent/WO2002016909A1/fr

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Classifications

    • 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
    • G01D5/35383—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 multiple sensor devices using multiplexing techniques

Definitions

  • Fiber Bragg gratings for sensing strain in the direction parallel to an optical fiber is well known. It is also possible to use Bragg gratings to measure strain, e.g. in a direction transverse to the optical fiber in a host material in which the fiber is embedded, or from a load applied to the optical fiber. This approach is based on the principle that a nonaxisymmetric load applied to the optical fiber will create birefringence in the optical fiber that one can observe by splitting the refection spectrum of the Bragg grating into two peaks (having maxima at ⁇ , and ⁇ 2 respectively), corresponding to the fast and the slow axes of the grating.
  • n is the average index of refraction of the grating core
  • ⁇ " ( ⁇ , + ⁇ 2 )/2 is the average wavelength of the grating.
  • microstrain resolution which normally implies changes in fiber birefringence smaller than 10 "6 .
  • This level of birefringence corresponds to wavelength separations much smaller than the typical bandwidth of a Bragg grating.
  • Bragg gratings written in polarization maintaining fiber has been proposed.
  • Such fiber has an initial birefringence that is sufficient to split completely the grating's spectrum into two separate spectral peaks, and what is monitored are small changes in the spectral separation already present.
  • polarization maintaining fiber is unduly expensive.
  • an object of the invention is to permit measurement of strain, loading, or the like in an optical fiber directed transverse to the fiber's optical axis.
  • Another object is to do the foregoing in a manner which is relatively temperature insensitive.
  • Another object is to do the foregoing with micro strain resolution.
  • Another object is to do the foregoing without the need for polarization maintaining optical fiber.
  • the invention concerns a method and apparatus which employs an optical fiber with at least some birefringence, and in which is a ⁇ -phase-shifted fiber Bragg grating, along with a detector disposed to detect, responsive to light incident on the grating, the spectral position of each transmission peak of the grating.
  • peaks are unique to ⁇ -phase-shifted Bragg gratings, and by monitoring the position of, or change in separation of, the spectral positions of the peaks, one can infer fiber birefringence, or change of birefringence over time.
  • Figure 1 is a schematic view illustrating an embodiment according to the invention.
  • Figure 2 is a graph having three curves, each illustrating aspects of the spectral response of a ⁇ -phase-shifted Bragg grating.
  • Figure 3 is a schematic of an embodiment according to the invention used to test features of the invention.
  • Figure 4 is a schematic of the apparatus used in the test to load a ⁇ -phase-shifted Bragg grating transverse to its optical axis.
  • Figure 5 is a graph of data taken in the.
  • Figure 6 is a graph of data taken with an apparatus similar to that of figure 3, illustrating temperature stability of the apparatus.
  • figure 1 is a schematic showing an embodiment according to the invention.
  • Laser 10 outputs a relatively broadband signal to optical coupler 12 and ultimately to a length of optical fiber 14 in which is inscribed a ⁇ -phase-shifted Bragg grating 19.
  • Fiber 14, and hence grating 19 has a mechanical load 16 directed on it, e.g. from stressing object in which fiber 14 is embedded or by placing a load on fiber 19, which physically distorts the glass of fiber 14 in a direction transverse to direction 15 .
  • This distortion causes corresponding distortion in the fast and slow birefringent axes of fiber 14, changing the fiber's net birefringence.
  • Members 13 and 17 are the individual grating elements of the Bragg grating, illustrated schematically in figure 1 as vertical lines, and are periodically spaced portions of fiber 14 at which the fiber's index of refraction has been changed to vary sufficiently from surrounding glass to set up an optical grating in fiber 14.
  • Forming elements 13, 17, can be done in any conventional manner, e.g. by doping fiber 14 with material having a differing index of refraction, or, more commonly, by inscription using a laser.
  • Disposed between grating elements 13, 17, is a portion 15 of fiber 14 containing no gratings.
  • Spacing 15 is characteristic of a ⁇ -phase-shifted Bragg grating, and distinguishes it from an ordinary Bragg grating by setting up, in effect, a narrow bandwidth Fabry-Perot filter within the Bragg grating.
  • Detector 18 receives and detects light transmitted through fiber 14 as a function of optical wavelength, and detector 26 does the same for light reflected from fiber 14 though optical coupler 24 via optical tap 22.
  • Detectors 18, 26 can be any device which can detect optical power or intensity as a function of wavelength, and preferably are simple and conventional spectrum analyzers or oscillographs.
  • Direction 15 represents for sake of illustration the optical axis of fiber 14, and is the direction through which light flows through grating 19.
  • FIG. 1 illustrates the response of a ⁇ -phase-shifted Bragg fiber.
  • Curves a, b, and c represent the transmission response of the grating, normalized to a magnitude of one, for light polarized at three different orientations to the gratings fast and slow axes.
  • Each of the curves shows a transmission notch 27 corresponding to the effective bandwidth of the Bragg grating, disposed about a center wavelength 32, which is characteristic of all Bragg gratings.
  • Within bandwidth 27 is a narrowband portion 28, 30 of high transmissivity characteristic of a ⁇ -phase-shifted Bragg.
  • Curves 28 and 30 illustrate grating response for light which is polarized directly along the respective fast and slow axes of the optical fiber in which the grating resides.
  • Curve b illustrates grating response for input light which is polarized transverse to the fast and slow axes, resulting in two peaks 28' and 30' narrowly spaced from one another in wavelength, each on the order of half the magnitude of peaks 28, 30, indicating that peaks 28' and 30' resulted from optical energy which split between the two birefringent axes of the fiber carrying the Bragg grating.
  • the wavelength spacing between axes 28 'and 30' thus reflects the amount of birefringence in fiber 14 and permits a means by which to measure it. Because the spacing peaks 28', 30' is much narrower than bandwidth 27 of the Bragg grating, this permits a more sensitive measure of birefringence than is obtainable by ordinary, non- ⁇ -phase-shifted, Bragg gratings.
  • Bragg grating 19 reflects a portion of the light, which couples through member 24 to detector 26, and transmits a portion of the light, which couples via member 20 to detector 18.
  • the light transmitted to detector 18, and that transmitted to detector 26 are complementary to one another: the spectrum detected at 18 is like the curves of figure 2, with bandwidth 27 appearing as a notch removed from the spectrum of laser 12, and peaks 28, 28', 30, 30' appearing as passband peaks; whereas the spectrum detected at 26 is like the curves of figure 2 flipped about the wavelength axis, with a peak within bandwidth 27, and a notched trough corresponding to 28, 28', 30, 30'.
  • the spectral responses at either of detectors 18 or 26 is hereinafter generically called the spectral response of ⁇ -phase-shifted Bragg grating 19, and the bands 28, 28', 30, 30' referred to generically as transmission peaks, with the understanding that the term comprehends as well the corresponding reflection notch as viewed at detector 26.
  • a Phonetics Tunics 1550 tunable laser 10' was used to interrogate grating 19'.
  • Laser 10' had a step accuracy of 1pm, but could be controlled with high resolution over a approximately 30 pm range by use of an external voltage applied to the laser's fine scanning input (FSC in figure 3) (2.0pm/V at 300 Hz).
  • the spectrum of the selected 30 pm region of laser 10's output was displayed as an x-y trace on oscilloscope 31 (TEKl) (the x and y axes defining a plane orthogonal to the optical axis of Bragg grating 19').
  • Detector 26' Dl provided the signal for the y-axis.
  • Polarization controller 32 (PC) was used to adjust the polarization of input light, and phase shifter 34 ( ⁇ ( ⁇ ) was used to compensate for the laser's voltage-to-wavelength delay. This method of interrogation was selected because of the availability of the components in the test's laboratory, and does not represent an optimal system in terms of either achievable resolution or overall system cost.
  • the ⁇ -phase-shifted grating 19' was written in Naval Research Laboratory photosensitive fiber by use of the ultraviolet re-exposure approach, and figure 2 is the resultant spectral response of grating 19'.
  • the center wavelength of the free grating was measured to be 1566.872 nm, with polarization peaks 28', 30' separated by 6.0 pm and each having a full-wave-half-maximum bandwidth of 3.6 pm.
  • the separation of the peaks was measured by use of oscilloscope 30's built in cursor-voltmeter features, with a 0.1 pm (0.05 V) resolution, corresponding to a 6(10) "8 birefringence resolution.
  • polarization controller 32 was manually adjusted to alternate between the peaks.
  • Figure 5 shows the measured birefringence as a function of the applied load (per unit fiber length) for three different orientations of the ⁇ -phase-shifted grating 19'.
  • the scale at the top of figure 5 gives the strain difference (6 x -£ y ) at the core of fiber 14', with a scale factor of 339 ⁇ £f (N/mm), as provided by theory.
  • the wavelength separation ⁇ between peaks 28', 30' is:
  • F is the force on grating 19' per unit length
  • E f 70.3 and is Young's modulus of optical fiber 14'
  • d f 1.25 ⁇ m and is the diameter of fiber 14'
  • V O.17 Poisson's ration for fiber 14'.
  • the applied load either increases of decreases birefringence.
  • the initial effect of the load is to rotate the direction of the principal axes, with no change in the magnitude of the birefringence.
  • the slope of the linear portions of the data is 73 ⁇ 2pm/(N/mm), whereas theory predicts 78.5 pm/(N/mm). This discrepancy, it is believed, can be attributed to the lack of precise knowledge during the test of the optical properties of grating 19', and P ⁇ and P 12 in particular, at the wavelength used.
  • the 0.1 pm accuracy of the setup corresponds to 1.4 (10) "3 N/mm, or 0.5 ⁇ £- of strain difference.
  • the maximum separation of the transmission peaks is approximately 100 pm for grating 10', corresponding to a -500 ⁇ C principal strain difference in the optical fiber, or a 1.4 N/mm transverse load. This is sufficient for many practical applications, particularly so because the transverse strain differences in optical fiber is typically 20 to 100 times smaller than those in a host.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optical Transform (AREA)

Abstract

L'invention concerne un procédé et un appareil permettant de déterminer la biréfringence d'une fibre optique (14) à réseau de Bragg (19) décalé en phase PI, ainsi que des changements de biréfringence de la fibre causés, par exemple, par une charge mécanique (16) appliquée sur la fibre de manière transversale par rapport à l'axe optique (15) de la fibre. Un réseau optique (19) de ce type comprend une bande de transmission étroite dans une bande de réflexion plus large, dont la pointe est divisée en deux si le réseau (19) est dans une fibre biréfringente, la magnitude de la division étant proportionnelle à la magnitude de biréfringence. En plus de permettre de mesurer directement la biréfringence de la fibre, le contrôle des changements d'espacement entre les deux pointes permet d'indiquer les changements correspondants de biréfringence de la fibre. Ceci peut être le résultat d'une déformation mécanique de la fibre, par exemple par chargement ou application d'une contrainte, permettant de déterminer cette charge, contrainte, etc., par détermination du changement d'espacement entre les deux pointes.
PCT/US2001/025935 2000-08-18 2001-08-20 Procede et appareil de detection de charge transversale a l'aide d'une fibre optique decalee en phase pi WO2002016909A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU2001288310A AU2001288310A1 (en) 2000-08-18 2001-08-20 A method and apparatus for transverse load sensing by use of pi-phase shifted optical fiber

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US22601500P 2000-08-18 2000-08-18
US60/226,015 2000-08-18

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN102829806A (zh) * 2012-08-23 2012-12-19 中国科学院半导体研究所 基于相移光纤光栅的光纤传感系统
CN103335979A (zh) * 2013-07-16 2013-10-02 山东省科学院激光研究所 基于复合腔光纤激光器的高灵敏度内腔气体检测仪
GB2540430A (en) * 2015-07-17 2017-01-18 Airbus Operations Ltd Calibration of transducers
CN114235743A (zh) * 2021-12-20 2022-03-25 武汉理工大学 基于相移光栅温度补偿技术的氢气检测装置

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5401956A (en) * 1993-09-29 1995-03-28 United Technologies Corporation Diagnostic system for fiber grating sensors
US5564832A (en) * 1993-01-29 1996-10-15 United Technologies Corporation Birefringent active fiber laser sensor
US5641956A (en) * 1996-02-02 1997-06-24 F&S, Inc. Optical waveguide sensor arrangement having guided modes-non guided modes grating coupler

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5564832A (en) * 1993-01-29 1996-10-15 United Technologies Corporation Birefringent active fiber laser sensor
US5401956A (en) * 1993-09-29 1995-03-28 United Technologies Corporation Diagnostic system for fiber grating sensors
US5641956A (en) * 1996-02-02 1997-06-24 F&S, Inc. Optical waveguide sensor arrangement having guided modes-non guided modes grating coupler

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN102829806A (zh) * 2012-08-23 2012-12-19 中国科学院半导体研究所 基于相移光纤光栅的光纤传感系统
CN102829806B (zh) * 2012-08-23 2014-12-10 中国科学院半导体研究所 基于相移光纤光栅的光纤传感系统
CN103335979A (zh) * 2013-07-16 2013-10-02 山东省科学院激光研究所 基于复合腔光纤激光器的高灵敏度内腔气体检测仪
GB2540430A (en) * 2015-07-17 2017-01-18 Airbus Operations Ltd Calibration of transducers
CN114235743A (zh) * 2021-12-20 2022-03-25 武汉理工大学 基于相移光栅温度补偿技术的氢气检测装置

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