WO2009107839A1 - 光周波数領域反射測定方式の物理量計測装置、および、これを用いた温度と歪みの同時計測方法 - Google Patents
光周波数領域反射測定方式の物理量計測装置、および、これを用いた温度と歪みの同時計測方法 Download PDFInfo
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- G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
- G01K11/00—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00
- G01K11/32—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres
- G01K11/3206—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres at discrete locations in the fibre, e.g. using Bragg scattering
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- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
- G01B11/18—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge using photoelastic elements
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- 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
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- 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
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- G01D5/35338—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 other arrangements than interferometer arrangements
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- 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
- G01D5/3537—Optical fibre sensor using a particular arrangement of the optical fibre itself
- G01D5/3538—Optical fibre sensor using a particular arrangement of the optical fibre itself using a particular type of fiber, e.g. fibre with several cores, PANDA fiber, fiber with an elliptic core or the like
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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
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- 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
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- G01D5/35393—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 using frequency division multiplexing
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- 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
Definitions
- one or a plurality of fiber Bragg Grating (FBG) sensors are arranged in one Polarization Maintaining (PM) fiber, the position of the FBG sensor, the distortion of the FBG sensor,
- the present invention relates to an optical frequency domain reflectometry (OFDR) type physical quantity measuring apparatus that measures a physical quantity such as temperature, and a temperature and strain simultaneous measurement method using the physical quantity measuring apparatus.
- OFDR optical frequency domain reflectometry
- a sensor that measures a physical quantity such as temperature and strain using an optical fiber has a long life, light weight, a small diameter, and flexibility, and thus can be used in a narrow space.
- this optical fiber has insulation properties, this sensor has a characteristic that it is resistant to electromagnetic noise. For this reason, this sensor is expected to be used for soundness evaluation of huge buildings such as bridges and buildings, and aerospace equipment such as passenger planes and artificial satellites.
- the performance required for sensors for evaluating the soundness of these structures includes high spatial resolution, having a multipoint sensor (wide detection range), and being able to measure in real time.
- optical fiber sensor using an FBG sensor and an OFDR analysis method uses a periodic change in interference intensity between a Bragg reflected light from the FBG sensor and a reflected light from a reference reflection end. Specify the position of.
- this optical fiber system measures the strain and temperature of the detection unit from the amount of change in the wavelength of the Bragg reflected light.
- This optical fiber sensor system has a high spatial resolution of 1 mm or less (see, for example, Non-Patent Document 1), 800 FBG sensors are arranged on an 8 m optical fiber, and a total of 3 optical fibers are used. It is disclosed that distortion measurements of more than 1,000 points can be performed simultaneously (for example, see Non-Patent Document 2), and that the measurement has excellent real-time properties (for example, see Patent Document 1). Furthermore, according to Non-Patent Document 1, it is also possible to measure the strain distribution in the longitudinal direction of the FBG sensor (meaning that the strain amount along the longitudinal direction of the FBG sensor is not uniform). This measurement of strain distribution is also described in Patent Document 3.
- This method uses a PANDA fiber, which is a type of PM fiber, and measures the amount of temperature and strain by measuring the amount of change in the wavelength of Bragg reflected light from two orthogonal polarization axes in an FBG sensor comprising this PANDA fiber. It is a method that can be measured simultaneously. That is, this method is a method capable of realizing a strain sensor that does not require a temperature compensation sensor.
- an optical fiber sensor system using an FBG sensor made of PM fiber and an OFDR analysis method has not been proposed so far. This is because, in order to stably measure Bragg reflected light from two orthogonal polarization axes in an FBG sensor made of PM fiber by an OFDR analysis method, the measurement light is converted into two orthogonal polarization axes. Therefore, it is necessary to split the signal with good controllability and propagate it to the FBG sensor and the reference reflection end. However, the measurement light is usually emitted with a single polarization.
- the position of the FBG sensor is specified from the period of the interference signal between the Bragg reflected light from the FBG sensor and the reflected light from the reference reflection end. That is, by substituting the effective refractive index of an appropriate optical fiber for the obtained interference signal and performing short-time Fourier transform (hereinafter referred to as STFT) analysis, the position of the FBG sensor (to be precise, The fiber length difference between the reference reflection end and the FBG sensor) can be obtained.
- STFT short-time Fourier transform
- the effective refractive indexes of the two orthogonal polarization axes are different, in order to substitute a certain effective refractive index, as a result, from the two polarization axes, The position of the Bragg reflected light is different.
- the present invention has been made in view of the above circumstances.
- One or a plurality of FBG sensors are arranged in one PM fiber, and the position of the FBG sensor and physical quantities such as strain and temperature of the FBG sensor are determined by OFDR.
- an OFDR-type physical quantity measuring apparatus that can measure temperature and strain at the same time and that can measure with high spatial resolution, and this physical quantity measuring apparatus are used. The purpose is to provide a method for simultaneous measurement of temperature and strain.
- An optical frequency domain reflection measurement type physical quantity measuring apparatus includes a tunable laser that emits measurement light; a first polarization maintaining fiber having one end connected to the tunable laser; A polarization maintaining coupler connected to the other end of the polarization maintaining fiber; a second polarization maintaining fiber having one end connected to the polarization maintaining coupler and the other end serving as a reference reflection end; A third polarization maintaining fiber having one end connected to the holding coupler; a sensor comprising a fiber Bragg grating formed in a core of the third polarization maintaining fiber; one end connected to the polarization maintaining coupler A fourth polarization-maintaining fiber; connected to the polarization-maintaining coupler through the fourth polarization-maintaining fiber, and detects Bragg reflected light from the sensor and reference light from the reference reflection end
- a control unit that detects a modulation of interference intensity between the Bragg
- the incident portion has a polarization axis offset angle of 45 ° with respect to the first polarization maintaining fiber when the incident portion is disposed on the first polarization maintaining fiber.
- a fusion splicing portion formed; when the incident portion is arranged in both the second polarization maintaining fiber and the third polarization maintaining fiber, these second polarization maintaining fibers; And a fusion splicing part formed with a polarization axis offset angle of 45 ° in each of the third polarization maintaining fibers.
- the optical path length adjustment unit is a fusion splicing unit formed with a polarization axis offset angle of 90 ° on the third polarization maintaining fiber in which the sensor is formed. .
- the optical path length adjusting unit is provided in the middle of the fiber length from the position corresponding to the length of the second polarization maintaining fiber to the sensor.
- a plurality of the sensors are arranged in the third polarization maintaining fiber.
- the said optical path length adjustment part is each distribute
- an effective refractive index difference between at least two orthogonal polarization axes in the third polarization maintaining fiber is 4.4. It is preferably ⁇ 10 ⁇ 4 or more.
- the method for simultaneously measuring temperature and strain uses the optical frequency domain reflection measurement type physical quantity measuring device according to any one of (1) to (7) above, and the one or more sensors. Measuring the wavelength of Bragg reflected light from two orthogonal polarization axes in FIG .; and, based on the measured wavelength of the Bragg reflected light, the amount of change due to temperature and strain of the wavelength of the Bragg reflected light in the sensor And a step of simultaneously measuring the temperature and strain of the portion where the sensor is arranged based on the calculated amount of change. (9) It is preferable to calculate a temperature distribution and a strain distribution along the longitudinal direction of the portion where the sensor is arranged in the third polarization maintaining fiber.
- a sensor formed in the core of the polarization maintaining fiber and two orthogonal polarization axes of the polarization maintaining fiber in which the sensor is arranged Since it has an incident part for allowing measurement light to enter, it is possible to simultaneously measure the temperature and strain of the sensor.
- it has an optical path length adjustment unit to make the optical path length of Bragg reflected light from two orthogonal polarization axes of the sensor constant since it has an optical path length adjustment unit to make the optical path length of Bragg reflected light from two orthogonal polarization axes of the sensor constant, the position of the sensor can be specified accurately, and physical quantities can be measured with high spatial resolution. Yes.
- one FBG Strain and temperature can be measured simultaneously from the sensor. Furthermore, the temperature distribution and strain distribution along the longitudinal direction of the sensor can be measured simultaneously.
- FIG. 1 is a schematic configuration diagram showing a first embodiment of a physical quantity measuring apparatus of the optical frequency domain reflection measurement method of the present invention.
- FIG. 2 is a schematic configuration diagram showing a modification of the embodiment.
- FIG. 3 is a schematic perspective view showing the polarization axis angle offset fusion splicing when a PANDA fiber is used.
- FIG. 4 is a schematic configuration diagram showing a second embodiment of the physical quantity measuring apparatus of the optical frequency domain reflection measurement method of the present invention.
- FIG. 5 is a schematic configuration diagram illustrating the physical quantity measuring apparatus of the optical frequency domain reflection measurement method according to the first embodiment of the present invention.
- FIG. 6 is a spectrogram showing the result of measuring the state of the sensor using the same example.
- FIG. 1 is a schematic configuration diagram showing a first embodiment of a physical quantity measuring apparatus of the optical frequency domain reflection measurement method of the present invention.
- FIG. 2 is a schematic configuration diagram showing a modification of the embodiment.
- FIG. 3 is a schematic perspective view
- FIG. 7 is a spectrogram showing the result of measuring the state of the sensor using the optical quantity domain reflection measurement type physical quantity measuring apparatus of Comparative Example 1.
- FIG. 8 is a spectrogram showing the result of measuring the state of the sensor using the optical quantity domain reflection measurement type physical quantity measuring apparatus of Comparative Example 2.
- FIG. 9 is a graph showing the result of calculating the positional deviation amount of the Bragg reflected light from the slow axis and the fast axis of the sensor in Comparative Example 2.
- FIG. 10 is a schematic configuration diagram illustrating a physical quantity measuring apparatus of an optical frequency domain reflection measurement method according to the second embodiment of the present invention.
- FIG. 11 is a spectrogram showing the result of measuring the state of the sensor (first sensor) using the second embodiment.
- FIG. 12 is a spectrogram showing the result of measuring the state of the sensor (second sensor) using the second embodiment.
- FIG. 13 is a spectrogram showing the result of measuring the state of the sensor using the optical quantity domain reflection measurement type physical quantity measuring apparatus of Example 3 of the present invention.
- FIG. 14 is a graph showing the relationship between the birefringence of the PANDA fiber and the shift characteristic difference of the Bragg wavelength with respect to the temperature change of the sensor composed of the FBG composed of the PANDA fiber in Example 3.
- FIG. 15 is a spectrogram showing the result of measuring the state of the sensor using the optical quantity domain reflection measurement type physical quantity measuring apparatus of Example 4 of the present invention.
- FIG. 16 is a diagram schematically showing an experimental system for measuring the temperature distribution and strain generated in the sensor in Example 4.
- FIG. 17 is a spectrogram showing the results of measuring the temperature change and strain at the position of the heater A and the position of the heater B in Example 4.
- FIG. 18 is a graph showing the results of measuring the temperature change and strain at the position of the heater A and the position of the heater B in Example 4.
- 10A, 10B, 10C, 10D, 10E, 10F (10) Physical quantity measuring device 11, 31, 32, polarization maintaining coupler 12 tunable laser 13, 35 Photodiode 14, 37, 38 for reference Reflection end 15, 15a, 15b Sensor 16, 17, 18, 19 Polarization maintaining fiber 20 Incident part 21, 21a, 21b Optical path length adjustment part 22 Control part 41, 42, 43, 44, 47, 48 PANDA fiber 53 System controller 54 A / D converter 60 (60A, 60B) PANDA fiber 61 (61A, 61B) Core 62 (62A, 62a, 62B, 62b) Stress applying portion
- FIG. 1 is a schematic configuration diagram showing a first embodiment of a physical quantity measuring apparatus of an optical frequency domain reflection measurement (hereinafter abbreviated as “OFDR”) method of the present invention.
- the OFDR physical quantity measuring apparatus 10A (10) of the present embodiment includes a tunable laser 12 that emits measurement light; a first polarization maintaining fiber 16 having one end connected to the tunable laser 12; A polarization maintaining coupler 11 connected to the other end of one polarization maintaining fiber 16; a second polarization maintaining fiber having one end connected to the polarization maintaining coupler 11 and the other end serving as a reference reflecting end 14; 18; a third polarization maintaining fiber 19 having one end connected to the polarization maintaining coupler 11; a sensor 15 comprising a fiber Bragg grating formed at the core of the third polarization maintaining fiber 19; A fourth polarization maintaining fiber 17 having one end connected to the holding coupler 11; a Bragg from the sensor 15 connected to the polarization maintaining coupler 11 via the fourth polarization maintaining
- a control unit 22 that detects the modulation of the interference intensity between the reference beams; two orthogonal polarization axes of the second polarization maintaining fiber 18 and two orthogonal polarization axes of the third polarization maintaining fiber 19;
- the adjustment part 21 is comprised roughly;
- the polarization maintaining coupler 11 is composed of the same type of PM fiber as the first to fourth polarization maintaining (hereinafter abbreviated as “PM”) fibers.
- a laser having a coherence length longer than the optical path length until the measurement light emitted from the tunable laser 12 is reflected by the sensor 15 and enters the photodiode 13 is preferably used.
- the photodiode 13 when the wavelength of the measurement light emitted from the tunable laser 12 is changed, it is possible to detect intensity modulation of optical interference obtained from two reflection points, that is, the reference reflection end 14 and the sensor 15. Those having a cutoff frequency are preferably used.
- the control unit 22 includes, for example, an A / D converter 54 that samples a signal from the photodiode 13 and a system controller 53 that analyzes the sampling data.
- an A / D converter 54 having a sampling frequency capable of detecting the intensity modulation of optical interference detected by the photodiode 13 is preferably used.
- the A / D converter 54 digitally samples the analog optical interference signal measured by the photodiode 13. This digital interference signal is transmitted to the system controller 53.
- the system controller 53 performs STFT (Short Time Fourier Transform; STFT) analysis using the digital interference signal. The analysis method will be described later.
- STFT Short Time Fourier Transform
- the system controller 53 is not particularly limited as long as the digital interference signal obtained by the A / D converter 54 can be subjected to STFT analysis.
- the system controller 53 is connected to the tunable laser 12 via a general-purpose interface bus (GPIB) and controls the tunable laser 12.
- GPIB general-purpose interface bus
- the incident part 20 is provided in the first PM fiber 16 and demultiplexes the measurement light emitted as a single polarization from the tunable laser 12 to two orthogonal polarization axes of the first PM fiber 16. To do. As the incident part 20, it is sufficient that the measurement light can be incident on both the two orthogonal polarization axes of the second PM fiber 18 and the two orthogonal polarization axes of the third PM fiber 19, as shown in FIG. 2. As described above, both the second PM fiber 18 and the third PM fiber 19 may be disposed.
- the incident portion 20 is a branch portion between the third PM fiber 19 on which the sensor 15 is formed and the second PM fiber 18 having the reference reflection end 14 in that the incident portion 20 only needs to be provided at one place.
- the incident portion 20 includes a method of inserting a ⁇ / 2 plate, a method of providing a polarization axis angle offset fusion splicing, or the polarization of the PM fiber with respect to the single-polarized measurement light from the tunable laser 12.
- the PM fiber is arranged so that the wave axis has an angular offset, and the measurement light of a single polarization, such as a method of coupling the emitted light from the tunable laser 12 to the PM fiber, is obtained by using two orthogonal polarizations of the PM fiber. Any means can be used as long as it can demultiplex to the wave axis.
- the incident portion 20 has a polarization axis offset angle of 45 ° in the first PM fiber 16 because it is simple and the measurement light can be equally split into two polarized waves. It is preferable that the fusion splice is formed (hereinafter referred to as “45 ° offset fusion”).
- the fusion splicing having the polarization axis angle offset means that two PM fibers are fusion spliced so that one polarization axis of the PM fiber has an offset angle that is a fusion point.
- the fact that one polarization axis of a PM fiber has an offset angle that is a fusion point means that the other PM axis perpendicular to the other has a similar offset angle and two PM fibers are fused. Means.
- FIG. 3 is a diagram schematically showing a state of 45 ° offset fusion when a PANDA (Polarization-maintaining and Absorption reducing) fiber is used as the PM fiber.
- the PANDA fiber 60 is a fiber in which a circular stress applying portion 62 is provided on the clad at both ends of the core 61 in order to give the fiber birefringence. Due to the stress applying portion 62, a propagation constant difference (effective refractive index difference) is generated between two orthogonal polarization modes. Therefore, the coupling from each polarization mode to the other polarization mode can be suppressed.
- a propagation constant difference effective refractive index difference
- the polarization axes through which the two orthogonal polarization modes propagate are called the slow axis and the fast axis, and the difference in effective refractive index between the slow axis and the fast axis is called birefringence.
- Straight lines connecting the two stress applying portions 62 and the core 61 that is, the two stress applying portions 62A and 62a of the PANDA fiber 60A and the straight line 63A connecting the core 61A; two stresses of the PANDA fiber 60B
- a fusion splice can be realized.
- Any optical path length adjusting unit 21 may be used as long as the optical path length of Bragg reflected light from two orthogonal polarization axes in the sensor 15 can be adjusted to be constant.
- a method of inserting a birefringent crystal into the PM fiber, a method of providing a fusion splice having a polarization axis angle offset in the PM fiber, and the like can be mentioned.
- optical path length adjustment unit 21 90 ° offset fusion is preferable among the above because it is simple and easy to adjust the optical path length.
- the optical path length adjustment unit 21 corresponds to the length of the PM fiber 18 having the reference reflection end 14 in order to make the optical path length of the Bragg reflected light from the two orthogonal polarization axes in the sensor 15 constant. It is provided in the middle of the fiber length (L 1 shown in FIG. 1 ) from the position to the sensor 15. By providing the optical path length adjusting unit 21 at this position, the optical path length of the Bragg reflected light from the two orthogonal polarization axes in the sensor 15 can be made constant, and the Bragg reflected light from the two orthogonal polarization axes is analyzed. The same measurement position.
- the measurement light emitted as a single polarization from the tunable laser 12 is passed between the tunable laser 12 and the PM coupler 11, and the second PM fiber 18 and An incident portion 20 for branching to two orthogonal polarization axes of the third PM fiber 19 is provided.
- Bragg reflected light from two orthogonal polarization axes in the sensor 15 can be obtained.
- the temperature and strain of the part where the sensor 15 is disposed can be measured simultaneously. As a result, a strain sensor that does not require additional temperature compensation can be realized.
- the optical path length adjustment unit 21 is provided in the middle of the fiber length from the position corresponding to the length of the second PM fiber 18 having the reference reflection end 14 to the sensor 15. It has been. Thereby, the optical path length of the Bragg reflected light from two orthogonal polarization axes in the sensor 15 can be made constant. That is, when the interference signal between the Bragg reflected light from the sensor 15 and the reflected light from the reference reflection end 14 is subjected to STFT analysis, the Bragg reflected light from two orthogonal polarization axes is at the same position.
- the temperature and strain of the detection unit are measured by measuring the amount of change due to temperature and strain in the wavelength of Bragg reflected light from two orthogonal polarization axes in the sensor 15. Can be measured simultaneously.
- R slow and R fast are the intensity of interference light from two orthogonal polarization axes of the PANDA fiber, that is, interference light from the slow axis (X axis) and the fast axis (Y axis).
- k represents the wave number
- n slow and n fast represent the effective refractive indexes of the slow axis (X axis) and the fast axis (Y axis).
- L 1 is the length from the PM coupler 11 to the reference reflection end 14 in the second PANDA fiber (PM fiber) 18 and the length from the PM coupler 11 to the sensor 15 in the third PANDA fiber (PM fiber) 19.
- the difference (fiber length difference) is shown. That is, L 1 indicates the fiber length from the position corresponding to the length of the PA second PANDA fiber 18 having the reference reflection end 14 to the sensor 15 in the third PANDA fiber 19 as shown in FIG. ing.
- the above-mentioned D 1 is obtained using the OFDR physical quantity measuring apparatus 10 A of the present embodiment, and the obtained optical interference signal D 1 is subjected to STFT analysis by the system controller 53.
- (n slow + n fast ) L 1 in the first term and the second term on the right side in Equation (1) indicates the optical path length along which the measurement light emitted from the tunable laser 12 reciprocates the fiber length difference L 1 . That is, the light path length corresponding to L 1 in the third PANDA fiber becomes (n slow + n fast) corresponds to half the L 1 ⁇ (n slow + n fast) / 2 ⁇ L 1.
- an analog optical interference signal corresponding to the above equation (1) measured by the photodiode 13 is digitally sampled by the A / D converter 54 provided in the control unit 22, This digital interference signal is subjected to STFT analysis by the system controller 53 provided in the control unit 22.
- the optical interference signal measured by the photodiode 13 is subjected to STFT analysis by the system controller 53 provided in the control unit 22.
- it means that the same processing is performed.
- the A / D converter 54 has a sampling frequency capable of detecting the intensity modulation of the optical interference detected by the photodiode 13
- the analog optical interference signal and the sampled digital interference signal are the same in principle. Signal.
- portions where the characteristics of the present invention can be more effectively described by using mathematical expressions indicating analog optical interference signals will be described using optical interference signals.
- the known optical path length ⁇ (n slow + n fast ) / 2 ⁇ L 1 is substituted with known n slow and n fast to substitute L 1 .
- the values of n slow and n fast include values obtained from the wavelength of the Bragg reflected light from the sensor 15 and the grating period calculated from the interval of the diffraction grating of the uniform phase mask used to fabricate the sensor 15. A value obtained from pattern measurement can be used.
- the fact that the optical path length of the first term and the second term on the right side the right side of equation (1) is constant, the same optical path length Bragg reflected light with respect to fiber length difference L 1 in the slow axis and the fast axis It means to have.
- Bragg reflected light is obtained from two orthogonal polarization axes in the sensor 15 in this way. Therefore, temperature and strain can be measured simultaneously. As a result, when strain measurement is performed using the OFDR physical quantity measuring apparatus 10A of the present embodiment, a temperature compensation sensor is not required. Further, the Bragg reflected light in the slow axis and the fast axis, to have the same optical path length with respect to the fiber length difference L 1, the position of the sensor 15 can accurately identify, it is possible to measure strain with high spatial resolution.
- This measurement method is a method for obtaining temperature and strain by calculation from the shift amount of the wavelength of the Bragg reflected light from two orthogonal polarization axes of the sensor 15.
- the wavelength of Bragg reflected light from two orthogonal polarization axes of the sensor 15 at a certain reference temperature (for example, 20 ° C.) and reference strain (for example, 0 ⁇ ) is measured in advance.
- the sensor 15 is arranged at a place where the sensor 15 is desired to be detected (hereinafter referred to as “detection unit”), and the wavelength of the Bragg reflected light from the two orthogonal polarization axes of the sensor 15 is measured in this detection unit. Subsequently, the wavelength difference (change amount) of the Bragg reflected light at the reference temperature and the reference strain is calculated. Next, the obtained wavelength difference is substituted into the following equation (2) to obtain the difference between the temperature at the detection unit and the reference temperature, the difference between the strain at the detection unit and the reference strain, and finally the known reference temperature, the reference The actual temperature and actual strain in the detection unit are calculated from the strain.
- ⁇ T represents the difference between the temperature in the detection unit and the reference temperature
- ⁇ represents the difference between the strain in the detection unit and the reference strain
- T is the temperature at the detection unit
- ⁇ is the strain at the detection unit.
- ⁇ slow and ⁇ fast indicate the wavelengths of Bragg reflected light from two orthogonal polarization axes of the sensor 15 in the detection unit.
- ⁇ slow and ⁇ fast are the wavelengths of the Bragg reflected light from the two orthogonal polarization axes of the sensor 15 in the detector, and the Bragg reflected light from the two orthogonal polarization axes of the sensor 15 at the reference temperature and the reference strain. The difference from the wavelength is shown.
- ⁇ slow / ⁇ and ⁇ fast / ⁇ shows Bragg wavelength shift amount of the slow axis and the fast axis per unit strain.
- ⁇ slow / ⁇ T and ⁇ fast / ⁇ T shows Bragg wavelength shift amount of the slow axis and the fast axis per unit of temperature.
- the above-described unit distortion or the amount of shift of the Bragg wavelength per unit temperature is determined by using the OFDR physical quantity measuring device 10A to give distortion to the sensor 15 at the reference temperature (20 ° C.). This is obtained by measuring the strain dependency of the change, giving a temperature change to the sensor 15 at the reference strain (0 ⁇ ), and measuring the temperature dependency of the Bragg wavelength change of the slow axis and the fast axis in the sensor 15.
- these ⁇ slow / ⁇ , ⁇ fast / ⁇ , ⁇ slow / ⁇ T, from the value of ⁇ fast / ⁇ T determine the D value according to the equation (2).
- ⁇ T and ⁇ are obtained by substituting the D value and ⁇ slow and ⁇ fast obtained from the measurement results into the above equation (2).
- the temperature and strain in the detection unit can be obtained.
- FIG. 4 is a schematic configuration diagram showing a second embodiment of the OFDR physical quantity measuring apparatus of the present invention.
- the third PM fiber 19 includes a plurality of sensors 15 (in the illustrated example, two sensors 15a, 15b) It is a point arranged.
- the second optical path length adjustment unit 21b (21) is provided in the middle of the fiber length between adjacent sensors (first sensor 15a, second sensor 15b). Is further arranged.
- the optical path lengths of the Bragg reflected light from the two orthogonal polarization axes in the first sensor 15a and the second sensor 15b can be made constant. That is, when an interference signal between the Bragg reflected light from the first sensor 15a and the second sensor 15b and the reflected light from the reference reflection end is subjected to STFT analysis, Bragg reflection from these two orthogonal polarization axes is performed. Light is detected as the same position at a position unique to the first sensor 15a and the second sensor 15b. As a result, the position of each sensor 15a, 15b can be specified accurately.
- the position of the sensor 15 can be specified and the temperature and strain can be measured in the same manner as in the first embodiment described above.
- the case where two sensors 15 (the first sensor 15a and the second sensor 15b) are provided in the third PM fiber 19 is exemplified, but the OFDR physical quantity measuring apparatus of the present embodiment is It is not limited to this.
- the third PM fiber 19 may be provided with three or more sensors 15. Even in this case, the Bragg reflected light from the two orthogonal polarization axes can be detected at the same position for each sensor 15 as in the present embodiment in which the two sensors 15 are provided. . That is, even if three or more sensors 15 are provided in the third PM fiber 19, the position of each sensor 15 can be accurately specified, and distortion measurement can be performed with high spatial resolution.
- the third PM fiber 19 provided with the sensor 15 has an effective refractive index difference between two orthogonal polarization axes ( It is preferably composed of a PM fiber having a large birefringence. Thereby, the sensitivity difference with respect to the temperature and strain in two orthogonal polarization axes becomes large, and more accurate simultaneous measurement of temperature and strain can be realized. More specifically, the effective refractive index difference between two orthogonal polarization axes is preferably 4.4 ⁇ 10 ⁇ 4 or more.
- the Bragg wavelength shift characteristic difference with respect to the temperature change of the sensor can be made larger than ⁇ 5.0 ⁇ 10 ⁇ 4 nm / ° C., as can be obtained from examples described later. As a result, extremely accurate temperature and strain measurement accuracy of 2 ° C. and strain accuracy of 30 ⁇ can be obtained.
- FIG. 5 is a schematic configuration diagram illustrating an OFDR physical quantity measuring apparatus 10D according to the first embodiment. This example is configured based on the OFDR physical quantity measuring apparatus 10A of the first embodiment described above.
- the OFDR physical quantity measuring device 10D according to the first exemplary embodiment has two PM couplers 31 and 32, a photodiode 35, and two reference reflection ends 37 and 38, in addition to the OFDR physical quantity measuring device 10A shown in FIG. And comprising. These are connected by PANDA fibers 41, 42, 43, 44, 47, and 48, which are one type of PM fiber.
- the first to fourth PM fibers and the PM coupler 11 are also PANDA fibers.
- the tunable laser 12 was connected to the system controller 53 via a general-purpose interface bus (GPIB) and controlled thereby. Signals from the two photodiodes 13 and 35 are sampled by the A / D converter 54, and the sampling data is subjected to STFT analysis by the system controller 53. This analysis method is as described in the first embodiment.
- PTAP-0150-2-B (model) manufactured by Fujikura Corporation was used.
- 8164A (model) manufactured by Agilent was used.
- 2117FC (model) manufactured by New Focus was used.
- the tunable laser 12 emits single-polarized measurement light that is swept (monotonically increased or monotonically decreased) at a certain constant speed and in a certain wavelength range.
- measurement light having a wavelength range of 1545 to 1555 nm was emitted at a speed of 10 nm / s.
- Single-polarized measurement light emitted from the tunable laser 12 propagates through the slow axis of the PANDA fiber 41 and enters the PM coupler 31. Then, the optical power is branched by the PM coupler 31 and enters the two optical interferometers.
- One of the two optical interferometers is schematically constituted by a PM coupler 32, reference reflection ends 37 and 38, and a photodiode 35.
- a trigger corresponding to the fiber length difference (optical path length difference) between the PANDA fiber 47 having the reference reflection end 37 and the PANDA fiber 48 having the reference reflection end 38 is generated.
- the fiber length difference between the PANDA fiber 47 and the PANDA fiber 48 was 50 m.
- This trigger is generated by the following method.
- measurement light swept from the tunable laser 12 at a certain speed and in a certain wavelength range is incident on the optical interferometer, the measurement light is reflected by the reference reflection ends 37 and 38, and the interference light is reflected by the photodiode 35. It is measured by.
- the signal acquired by the photodiode 35 is sampled by the A / D converter 54 and converted into a voltage signal. This voltage signal is taken into the system controller 53.
- the measurement light emitted from the tunable laser 12 has a wavelength that changes at a constant speed. Therefore, the signal measured by the photodiode 35 is a sine function that fluctuates at a constant light wave number interval.
- a certain voltage value is set as a threshold value, and the system controller 53 generates a trigger at a timing exceeding the threshold value (timing exceeding a threshold value or exceeding a threshold value, or timing exceeding a threshold value and falling below the threshold value).
- the generated trigger has a certain light wave number interval.
- This trigger generation method is very effective in that the interval of light wave numbers generated by the trigger is always constant even when the sweep speed of the tunable laser 12 is not constant.
- the other of the two optical interferometers is schematically configured from the first embodiment shown in FIG.
- the sensor 15 was produced by a general exposure method using a KrF excimer laser and a uniform phase mask.
- the grating length (sensor length) was 5 mm.
- the distance L 1 from the position corresponding to the PANDA fiber 14 having the reference reflection end 14 to the sensor 15 was about 20 m.
- 45 ° offset fusion was provided on the PANDA fiber 16.
- optical interference signal D 1 is subjected to STFT analysis by the system controller 53.
- Optical interference signal D 1 of the this time can be expressed also by formula (1) and the first embodiment.
- Example 1 (a rate of the tunable laser 12 10 nm / s, about 400pm distance in terms of wavelength) obtained optical interference signal D 1 approximately 40ms intervals and analyzed by the window width corresponding to.
- the analysis may be performed with a window width corresponding to a certain light wave number interval (that is, a certain wavelength interval) instead of a certain time interval.
- the state of the sensor 15 was measured using the OFDR physical quantity measuring device 10D of the present example.
- the results are shown in FIG.
- the Bragg reflected light from the sensor 15 is displayed as a spectrogram.
- the horizontal axis indicates the wavelength
- the vertical axis indicates the fiber position (fiber length from the position corresponding to the PANDA fiber 18 having the reference reflection end 14)
- the color tone indicates the Bragg reflection intensity.
- the Bragg reflected light of 1550.6 nm is from the slow axis of the sensor 15, and the Bragg reflected light of 1550.2 nm is from the fast axis of the sensor 15. Nearly consistent results were obtained at a position of about 19.672 m.
- Example 1 since Bragg reflected light from two orthogonal polarization axes of the sensor 15 was obtained, it was confirmed that temperature and strain could be measured simultaneously. Accordingly, it was confirmed that a sensor for temperature compensation becomes unnecessary when strain measurement is performed using the OFDR physical quantity measuring device 10D of the present embodiment. In addition, since the position of the sensor 15 can be accurately identified, strain measurement can be performed with high spatial resolution.
- the sensor 15 is distorted at the reference temperature (20 ° C.), and the strain dependency of the Bragg wavelength change of the slow axis and the fast axis in the sensor 15 is measured. did. Further, by using the present embodiment, a temperature change is given to the sensor 15 at the reference strain (0 ⁇ ), and the temperature change dependency of the Bragg wavelength change of the slow axis and the fast axis in the sensor 15 is measured.
- the following formula (3) was obtained.
- ⁇ T and ⁇ are obtained by substituting ⁇ slow and ⁇ fast obtained from the measurement result and the above D into the above equation (2) for calculation. Then, by subtracting the reference temperature and the reference strain from these values, the temperature and strain in the detection unit can be obtained.
- Comparative Examples 1 and 2 performed for verifying the effect of the present invention will be described. Comparative Examples 1 and 2 are not conventional techniques, but are new techniques implemented to verify the effects of the present invention.
- Comparative Example 1 An OFDR-type physical quantity measuring device was produced in the same manner as in Example 1 except that the polarization axis offset angle of the fusion splicing part of the incident part and the optical path length adjusting part was set to 0 °. Using the OFDR physical quantity measuring apparatus of Comparative Example 1, the state of the sensor was measured. The results are shown in FIG. From the result of FIG. 7, in this comparative example 1, only Bragg reflected light from the slow axis of the sensor 15 was obtained. It is impossible to simultaneously measure the temperature and strain of the sensor 15 with only Bragg reflected light from one polarization axis. Therefore, when strain measurement is performed using the OFDR-type physical quantity measurement device of Comparative Example 1, a temperature compensation sensor is required.
- Comparative Example 2 An OFDR physical quantity measuring device was produced in the same manner as in Example 1 except that the polarization axis offset angle of the fusion splicing part of the optical path length adjustment unit was set to 0 °. The state of the sensor was measured using the OFDR type physical quantity measuring apparatus of Comparative Example 2. The results are shown in FIG. From the result of FIG. 8, in Comparative Example 2, Bragg reflected light from the slow axis and the fast axis of the sensor 15 was obtained. Therefore, when strain measurement is performed using the OFDR-type physical quantity measurement device of Comparative Example 2, a sensor for temperature compensation is not required as in Example 1. However, since the position of each Bragg reflected light is different, the position of the sensor 15 cannot be accurately specified, and as a result, distortion measurement cannot be performed with high spatial resolution.
- the optical interference signal D 2 obtained by the photodiode 13 can be expressed by the following equation (4).
- the difference from the optical interference signal D 1 obtained in the first embodiment and Example 1 is that the measurement light emitted from the tunable laser 12 in the first and second terms on the right side reciprocates the fiber length difference L 1 .
- the optical path length to be used is different. This is because the relationship of n slow > n fast always holds between n slow and n fast . That the optical path length of the first term on the right side and the right side second term in equation (4) are different, means having an optical path length Bragg reflected light are different with respect to the fiber length difference L 1 in the slow axis and the fast axis ing. That is, as the result of FIG.
- the position of the Bragg reflected light from the slow axis of the sensor 15 is about 19.629 m, and the position of the Bragg reflected light from the fast axis of the sensor 15 is about 19.624 m. Therefore, the difference is about 5 mm.
- This difference can be detected because the optical fiber sensor system using the sensor and the OFDR analysis method has a spatial resolution of 1 mm or less. In other words, other types of optical fiber sensor systems do not have this level of spatial resolution (or there is no means for specifying the position), so this positional shift cannot be detected. That is, it is effective only for the optical fiber sensor system using the FBG sensor and the OFDR analysis method.
- n slow and n fast were obtained from the wavelength of the Bragg reflected light of the sensor 15 and the grating period calculated from the interval of the diffraction grating of the uniform phase mask used for the production of the sensor 15. Value, value obtained from near field pattern measurement, etc. are used.
- N slow 1.44756
- n fast 1.444720.
- ⁇ slow and ⁇ fast indicate the wavelengths of Bragg reflected light from two orthogonal polarization axes in the sensor 15.
- ⁇ indicates a grating period calculated from the diffraction grating spacing of the uniform phase mask.
- FIG. 10 is a schematic configuration diagram illustrating an OFDR physical quantity measuring apparatus 10E according to the second embodiment.
- the second embodiment is different from the first embodiment in that the second embodiment is manufactured based on the OFDR physical quantity measuring apparatus 10C of the second embodiment described above. That is, this embodiment is different from the first embodiment in that a first sensor 15a and a second sensor 15b are arranged in a third PM fiber (PANDA fiber) 19, and the first sensor 15a and the first sensor 15b
- the second optical path length adjusting unit 21b (90 ° offset fusion) is disposed in the middle of the second sensor 15b.
- the second sensor 15b was provided at a position 5 m from the first sensor 15a.
- the second optical path length adjustment unit 21b was provided at a position of about 2.5 m from the first sensor 15a and the second sensor 15b.
- FIG. 11 shows the result of measuring the state of the first sensor 15a
- FIG. 12 shows the result of measuring the state of the second sensor 15b, using the OFDR physical quantity measuring device 10E of this example.
- the position of the Bragg reflected light from the slow axis of the first sensor 15a and the position of the Bragg reflected light from the fast axis of the first sensor 15a are both approximately 19.672 m. It was confirmed.
- the position of the Bragg reflected light from the slow axis of the second sensor 15b and the position of the Bragg reflected light from the fast axis of the second sensor 15b are both approximately 24.757 m and substantially coincide. It was confirmed.
- Example 3 The third PM fiber 19 provided with the sensor 15 is manufactured in the same manner as in Example 1 except that the third PM fiber 19 is composed of a PANDA fiber having a large effective refractive index difference (birefringence) between the slow axis and the fast axis. This was designated as Example 3.
- the PANDA fiber having a large birefringence can be realized by placing the stress applying portion 62 close to the core 61 as described with reference to FIG. That is, the birefringence of the PANDA fiber can be arbitrarily adjusted by the arrangement of the stress applying portion 62.
- the state of the sensor 15 was measured using the OFDR-type physical quantity measuring apparatus of this example. The results are shown in FIG. In the spectrogram shown in FIG. 13, the Bragg reflected light of 1551.1 nm is from the slow axis of the sensor 15, and the Bragg reflected light of 1550.4 nm is from the fast axis of the sensor 15.
- the Bragg wavelength difference between the slow axis and the fast axis was 0.670 nm.
- the birefringence calculated from this Bragg wavelength difference was 6.22 ⁇ 10 ⁇ 4 .
- the Bragg wavelength difference obtained by analyzing the spectrogram of the sensor 15 obtained in Example 1 in more detail is 0.391 nm, and the birefringence calculated from this Bragg wavelength difference is 3.65 ⁇ 10 ⁇ 4. It was. That is, the PANDA fiber constituting the sensor 15 of Example 3 had birefringence nearly twice as large as that of the PANDA fiber constituting the sensor 15 of Example 1.
- Example 3 the above formula (3) obtained in Example 1 is ⁇ 3.7 ⁇ 10 ⁇ 4 nm / ° C., whereas the above formula (7) obtained in Example 3 is used. Then, it is ⁇ 7.2 ⁇ 10 ⁇ 4 nm / ° C. That is, the sensor 15 of Example 3 had a Bragg wavelength shift characteristic difference with respect to a temperature change nearly twice that of the sensor 15 of Example 1. This is considered to be due to the difference in birefringence of the PANDA fibers constituting each sensor. It is known that the PANDA fiber has a small birefringence generated in the core in proportion to an increase in temperature, and the birefringence becomes almost zero at about 800 to 900 ° C. which is the melting point of the stress applying portion.
- the sensor 15 of the third embodiment has a shift characteristic difference of the Bragg wavelength with respect to a temperature change that is nearly twice that of the sensor 15 of the first embodiment.
- the temperature change from the reference temperature (20 ° C.) is 20 ° C., 40 ° C., 100 ° C. (that is, the set temperature is 40 ° C., 60 ° C., 120 ° C.), and the strain change from the reference strain (0 ⁇ ) is 257 ⁇ , 535 ⁇ , 1056 ⁇ .
- the temperature and strain were measured under a total of nine conditions. As a result, a highly accurate temperature and strain measurement result with a temperature accuracy of 2 ° C. or less and a strain accuracy of 30 ⁇ or less was obtained.
- the sensor made of FBG used in the OFDR physical quantity measuring apparatus of the present invention is composed of a PANDA fiber having a large birefringence.
- a detailed study was conducted on the accuracy of simultaneous measurement of temperature change and strain of a sensor made of FBG.
- the shift characteristic difference of the Bragg wavelength with respect to the temperature change of this sensor is larger than ⁇ 5.0 ⁇ 10 ⁇ 4 nm / ° C. It has been found preferable to have FIG. 14 is a graph showing a result of evaluating the shift characteristic difference of the Bragg wavelength with respect to the temperature change of the birefringence of the PANDA fiber and the temperature sensor of the FBG composed of the fiber. From the results of FIG.
- the birefringence of the PANDA fiber when the birefringence of the PANDA fiber is 4.4 ⁇ 10 ⁇ 4 or more, the shift characteristic difference of the Bragg wavelength with respect to the temperature change of this sensor is larger than ⁇ 5.0 ⁇ 10 ⁇ 4 nm / ° C. Has characteristics. That is, the birefringence of the PANDA fiber is preferably 4.4 ⁇ 10 ⁇ 4 or more. However, if the stress applying portion is too close to the core in order to increase the birefringence, there is a problem that the manufacturing yield of the PANDA fiber is deteriorated. Therefore, the birefringence of the PANDA fiber is preferably 2.0 ⁇ 10 ⁇ 3 or less which can be manufactured with a high yield.
- the birefringence is caused by bringing the stress applying portion closer to the core.
- a large PANDA fiber was used.
- a PANDA fiber having a stress applying portion having a low melting point can be cited. More specifically, when the melting point of the stress-applying portion is 600 ° C. or less, the Bragg wavelength shift characteristic difference can be made larger than ⁇ 5.0 ⁇ 10 ⁇ 4 nm / ° C.
- Example 4 A sensor was manufactured in the same manner as in Example 3 except that the sensor length was set to 100 mm. The state of the sensor 15 was measured using the OFDR-type physical quantity measuring device 10F of this example. The results are shown in FIG.
- the 1549.4 nm Bragg reflected light is from the slow axis of the sensor, and the 1548.7 nm Bragg reflected light is from the sensor fast axis.
- the wavelength difference of the Bragg reflected light obtained by analyzing this spectrogram in more detail was 0.670 nm. This wavelength difference is equivalent to the sensor of Example 3 having a sensor length of 5 mm. Therefore, the PANDA fiber used for the sensor 15 of the present example with a sensor length of 100 mm has the same birefringence as the PANDA fiber used for the sensor 15 of the third example.
- FIG. 16 is a diagram schematically showing an experimental system for measuring the temperature distribution and strain generated in the sensor by the OFDR physical quantity measuring apparatus 10F of the present embodiment.
- the weight W gives a uniform strain along the longitudinal direction of the sensor 15.
- a non-uniform temperature change can be given along the longitudinal direction of the sensor 15 by the heater A and the heater B which can be controlled in temperature independently.
- the state of the sensor 15 was measured by the experimental system shown in FIG. 16 using the OFDR physical quantity measuring device 10F of the present example. The results are shown in FIG. At this time, the strain applied to the sensor 15 by the weight W is 1000 ⁇ , the temperature change applied to the sensor 15 by the heater A is 100 ° C., and the temperature change applied to the sensor 15 by the heater B is 60 ° C.
- the region of the sensor heated by the heater A had a strain of 1000 ⁇ and a Bragg wavelength shift corresponding to a temperature change of 100 ° C.
- a Bragg wavelength shift corresponding to a strain of 1000 ⁇ and a temperature change of 60 ° C. occurred in the non-heated region between the heater A and the heater B.
- a Bragg wavelength shift corresponding to only a strain of 1000 ⁇ occurred in the non-heated region between the heater A and the heater B. That is, in this embodiment, by measuring the change amount of the Bragg wavelength between the slow axis and the fast axis of the sensor 15 along the longitudinal direction of the sensor 15, the temperature distribution and strain along the longitudinal direction of the sensor 15 are measured. Was measured simultaneously.
- the strain applied to the sensor 15 by the weight W is made constant at 1000 ⁇
- the temperature change applied to the sensor 15 by the heater A is made constant at 100 ° C.
- the temperature change and distortion in the position of the heater A and the position of the heater B were measured. The result is shown in FIG.
- the measured strain was constant at 1000 ⁇ .
- the temperature change was also constant at 100 ° C.
- the measured strain was constant at 1000 ⁇ , and the measured temperature change was obtained in correlation with the set temperature of the heater B. That is, the temperature distribution and distortion generated at the position of the heater A and the position of the heater B can be simultaneously measured with high accuracy.
- the present invention can simultaneously measure the temperature distribution and strain along the longitudinal direction of the sensor made of FBG with high accuracy. Further, by using the present invention, even when a temperature distribution and a strain distribution are generated along the longitudinal direction of the FBG sensor, these can be measured simultaneously and with high accuracy.
- the sensor temperature and strain can be measured simultaneously.
- the position of the sensor can be specified accurately, and physical quantities can be measured with high spatial resolution.
- the temperature distribution and strain distribution along the longitudinal direction of the sensor can be measured simultaneously.
Abstract
Description
本願は、2008年2月29日に日本国に出願された特願2008-51343号と、2008年9月18日に日本国に出願された特願2008-239368号とに基づき優先権を主張し、これらの内容をここに援用する。
これら構造物の健全性評価を行うためのセンサに求められる性能としては、空間分解能が高いこと、多点のセンサを有すること(検知範囲が広いこと)、及びリアルタイムで計測できることなどが挙げられる。
FBGセンサとOFDR方式の解析方法とを用いた光ファイバセンサシステムは、FBGセンサからのブラッグ反射光と参照用の反射端からの反射光との干渉強度の周期的変化を利用して、FBGセンサの位置を特定する。また、この光ファイバシステムは、ブラッグ反射光の波長の変化量から、検知部の歪みや温度を計測する。
この問題を解決する手法としては、PMファイバからなるFBGセンサを用いる方法が挙げられる(例えば、特許文献1参照)。この手法は、PMファイバの一種であるPANDAファイバを用い、このPANDAファイバからなるFBGセンサにおける直交する2つの偏波軸からのブラッグ反射光の波長の変化量を測定することにより、温度と歪みを同時に計測できる方法である。
すなわち、この手法は、温度補償用のセンサが不要の歪みセンサを実現し得る方法である。
また、PMファイバからなるFBGセンサとOFDR方式の解析方法とを用いた光ファイバセンサシステムでは、直交する2つの偏波軸の実効屈折率が異なる。そのため、OFDR方式の解析において2つの偏波軸からのブラッグ反射光の位置が異なるという問題がある。そのため、高分解能でFBGセンサの位置を特定することが難しい。
(1)本発明の光周波数領域反射測定方式の物理量計測装置は、測定光を出射するチューナブルレーザと;このチューナブルレーザに一端が接続された第1の偏波保持ファイバと;この第1の偏波保持ファイバの他端に接続された偏波保持カプラと;この偏波保持カプラに一端が接続され、他端が参照用反射端である第2の偏波保持ファイバと;前記偏波保持カプラに一端が接続された第3の偏波保持ファイバと;この第3の偏波保持ファイバのコアに形成されたファイバブラッググレーティングからなるセンサと;前記偏波保持カプラに一端が接続された第4の偏波保持ファイバと;この第4の偏波保持ファイバを介して前記偏波保持カプラと接続され、前記センサからのブラッグ反射光と前記参照用反射端からの参照光とを検出するフォトダイオードと;このフォトダイオードで検出された前記ブラッグ反射光と前記参照光との合波光強度変化に基づき、これらブラッグ反射光及び参照光間の干渉強度の変調を検知する制御部と;前記第2の偏波保持ファイバの直交する2つの偏波軸及び前記第3の偏波保持ファイバの直交する2つの偏波軸の両方に、前記測定光を入射する入射部と;前記第3の偏波保持ファイバに配され、前記センサにおける直交する2つの偏波軸からのブラッグ反射光の光路長を一定にする光路長調整部と;を備え、前記入射部は、前記第1の偏波保持ファイバ、または、前記第2の偏波保持ファイバと前記第3の偏波保持ファイバとの両方に配されている。
(3)前記光路長調整部は、前記センサが形成された前記第3の偏波保持ファイバに、90°の偏波軸オフセット角度を有して形成された融着接続部であるのが好ましい。
(4)前記光路長調整部は、前記第2の偏波保持ファイバの長さに相当する位置から前記センサまでのファイバ長の中間に設けられたのが好ましい。
(5)前記第3の偏波保持ファイバに、前記センサが複数配されているのが好ましい。
(6)前記光路長調整部が、隣接する前記センサ間のファイバ長の中間にそれぞれ配されているのが好ましい。
(7)前記第1の偏波保持ファイバから前記第4の偏波保持ファイバのうち、少なくとも前記第3の偏波保持ファイバにおける直交する2つの偏波軸の実効屈折率差が、4.4×10-4以上であるのが好ましい。
(9)前記第3の偏波保持ファイバの、前記センサが配された部位長手方向に沿った温度分布および歪み分布を算出するのが好ましい。
上記(1)から(7)の何れか1項に記載の光周波数領域反射測定方式の物理量計測装置を用いた上記(8)に記載の温度と歪みの同時計測方法によれば、1つのFBGセンサから歪みと温度を同時に計測できる。さらに、センサの長手方向に沿った温度分布と歪み分布を同時に計測できる。
11,31,32, 偏波保持カプラ
12 チューナブルレーザ
13,35 フォトダイオード
14,37,38 参照用反射端
15,15a,15b センサ
16,17,18,19 偏波保持ファイバ
20 入射部
21,21a,21b 光路長調整部
22 制御部
41,42,43,44,47,48 PANDAファイバ
53 システムコントローラ
54 A/Dコンバータ
60 (60A,60B) PANDAファイバ
61 (61A,61B) コア
62 (62A,62a,62B,62b) 応力付与部
図1は、本発明の光周波数領域反射測定(以下、「OFDR」と略す)方式の物理量計測装置の第一の実施形態を示す概略構成図である。
本実施形態のOFDR方式の物理量計測装置10A(10)は、測定光を出射するチューナブルレーザ12と;このチューナブルレーザ12に一端が接続された第1の偏波保持ファイバ16と;この第1の偏波保持ファイバ16の他端に接続された偏波保持カプラ11と;この偏波保持カプラ11に一端が接続され、他端が参照用反射端14である第2の偏波保持ファイバ18と;偏波保持カプラ11に一端が接続された第3の偏波保持ファイバ19と;この第3の偏波保持ファイバ19のコアに形成されたファイバブラッググレーティングからなるセンサ15と;偏波保持カプラ11に一端が接続された第4の偏波保持ファイバ17と;この第4の偏波保持ファイバ17を介して偏波保持カプラ11と接続され、センサ15からのブラッグ反射光と参照用反射端14からの参照光とを検出するフォトダイオード13と;このフォトダイオード13で検出された前記ブラッグ反射光と前記参照光との合波光強度変化に基づき、これらブラッグ反射光及び参照光間の干渉強度の変調を検知する制御部22と;第2の偏波保持ファイバ18の直交する2つの偏波軸及び第3の偏波保持ファイバ19の直交する2つの偏波軸の両方に、前記測定光を入射する入射部20と;第3の偏波保持ファイバ19に配され、センサ15における直交する2つの偏波軸からのブラッグ反射光の光路長を一定にする光路長調整部21と;から概略構成されている。本実施形態において、偏波保持カプラ11は、第1~第4の偏波保持(以下、「PM」と略す)ファイバと同種のPMファイバで構成されている。
入射部20としては、λ/2板を挿入する方法、偏波軸角度オフセット融着接続を設ける方法、あるいは、チューナブルレーザ12からの単―偏波の測定光に対して、PMファイバの偏波軸が角度オフセットを有するように、PMファイバを配置し、チューナブルレーザ12からの出射光をPMファイバに結合させる方法など、単一偏波の測定光を、PMファイバの直交する2つの偏波軸に分波できる手段であれば、いかなるものでも用いられる。
その中でも、簡便である点、測定光を均等に2偏波に分波できる点から、この入射部20としては、この第1のPMファイバ16に45°の偏波軸オフセット角度を有して形成された融着接続(以下、「45°オフセット融着」と言う)であるのが好ましい。
ここで、PANDAファイバ60とは、ファイバに複屈折を持たせるために、コア61両端のクラッドに、円形の応力付与部62を設けたファイバである。この応力付与部62により、直交する2つの偏波モード間に伝搬定数差(実効屈折率差)が生じる。そのため、それぞれの偏波モードからもう一方への偏波モードへの結合を抑制できる。この直交する2つの偏波モードが伝搬する偏波軸は、スロー軸、ファスト軸と呼ばれ、スロー軸とファスト軸の実効屈折率の差は、複屈折と呼ばれる。
この2つの応力付与部62とコア61を結んだ直線(すなわち、PANDAファイバ60Aの、2つの応力付与部62A,62aと、コア61Aとを結んだ直線63Aと;PANDAファイバ60Bの、2つの応力付与部62B,62bと、コア61Bとを結んだ直線63Bと;)を、2つのPANDAファイバ60A,60Bの間で所望の偏波軸オフセット角度θとなるように接続することで、所望のオフセット融着接続を実現できる。
また、このOFDR方式の物理量計測装置10Aでは、参照用反射端14を有する第2のPMファイバ18の長さに相当する位置からセンサ15までのファイバ長の中間に、光路長調整部21が設けられている。これにより、センサ15における直交する2つの偏波軸からのブラッグ反射光の光路長を一定にできる。すなわち、センサ15からのブラッグ反射光と参照用反射端14からの反射光との干渉信号をSTFT解析すると、直交する2つの偏波軸からのブラッグ反射光は同じ位置となる。
次に、本実施形態のOFDR方式の物理量計測装置10Aを用いたセンサ15の位置特定方法について説明する。第1~第4のPMファイバとして、PANDAファイバを用いた場合を例示する。
本実施形態のOFDR方式の物理量計測装置10Aでは、フォトダイオード13に、センサ15からのブラッグ反射光と参照用反射端14からの反射光との干渉光が入射する。フォトダイオード13に入射するこの光干渉信号D1は、直交する2つの偏波軸の信号の和となり、下記の式(1)で表される。
上記の式(1)において、RslowとRfastはPANDAファイバの直交する2つの偏波軸からの干渉光の強度、すなわち、スロー軸(X軸)とファスト軸(Y軸)からの干渉光強度を示す。kは波数、nslowとnfastはスロー軸(X軸)とファスト軸(Y軸)の実効屈折率を示す。L1は第2のPANDAファイバ(PMファイバ)18におけるPMカプラ11から参照用反射端14までの長さと、第3のPANDAファイバ(PMファイバ)19におけるPMカプラ11からセンサ15までの長さとの差(ファイバ長差)を示す。つまりL1は、図1に示すように、第3のPANDAファイバ19において、参照用反射端14を有するPA第2のPANDAファイバ18の長さに相当する位置からセンサ15までのファイバ長を示している。
なお、本発明の物理量計測装置では、フォトダイオード13において計測した上記式(1)に相当するアナログの光干渉信号を、制御部22に備えたA/Dコンバータ54にてデジタル的にサンプリングし、このデジタル干渉信号を、制御部22に備えたシステムコントローラ53にてSTFT解析するが、本文においては、フォトダイオード13において計測した光干渉信号を制御部22に備えたシステムコントローラ53にてSTFT解析すると略記する場合も、同様の処理をおこなっていることを意味する。前記のとおり、A/Dコンバータ54は、フォトダイオード13で検知した光干渉の強度変調を検知できるサンプリング周波数を有するので、アナログの光干渉信号とサンプリングしたデジタル干渉信号とは、原理的には同じ信号である。また、アナログの光干渉信号を示す数式を用いることで、より効果的に本発明の特徴を説明できる箇所は、光干渉信号を用いて説明する。
このnslowとnfastとしては、センサ15からのブラッグ反射光の波長と、センサ15の作製に使用したユニフォーム位相マスクの回折格子の間隔から計算されるグレーティング周期とから求めた値や、ニアフィールドパターン測定から求めた値などを用いることができる。ここで、式(1)における右辺第1項と右辺第2項の光路長が一定であるということは、スロー軸とファスト軸におけるブラッグ反射光がファイバ長差L1に対して同じ光路長を持つことを意味している。
本実施形態では、このようにしてセンサ15における直交する2つの偏波軸からブラッグ反射光が得られる。そのため、温度と歪みを同時に計測が可能となる。これにより、本実施形態のOFDR方式の物理量計測装置10Aを用いて歪み計測を行なう場合、温度補償用のセンサが不要となる。また、スロー軸とファスト軸におけるブラッグ反射光が、ファイバ長差L1に対して同じ光路長を持つため、センサ15の位置を正確に特定でき、高い空間分解能で歪み計測が可能となる。
次に、本実施形態のOFDR方式の物理量計測装置10Aを用いた温度と歪みの計測方法について説明する。この計測方法は、センサ15の直交する2つの偏波軸からのブラッグ反射光の波長のシフト量から、計算により温度と歪みを求める方法である。
まず、予めある基準温度(例えば、20℃)、基準歪み(例えば、0με)におけるセンサ15の直交する2つの偏波軸からのブラッグ反射光の波長を計測しておく。
次いで、検知部におけるブラッグ反射光の波長と、基準温度、基準歪みでのブラッグ反射光の波長差(変化量)を計算する。
次いで、得られた波長差を、下記の式(2)に代入して、検知部における温度と基準温度の差、検知部における歪みと基準歪みの差を求め、最後に既知の基準温度、基準歪みから検知部における実温度と実歪みを算出する。
これらの演算は、OFDR方式の物理量計測装置10Aのシステムコントローラ53を用いて簡単に行える。
図4は、本発明のOFDR方式の物理量計測装置の第二の実施形態を示す概略構成図である。本実施形態のOFDR方式の物理量計測装置10C(10)が、上述の第一の実施形態と異なる点は、第3のPMファイバ19に、センサ15が複数(図示例では、2つのセンサ15a,15b)配されている点である。
また、この実施形態のOFDR方式の物理量計測装置10Cでは、隣接するセンサ(第1のセンサ15a,第2のセンサ15b)間のファイバ長の中間に、第2の光路長調整部21b(21)が更に配されている。そのため、第1のセンサ15aと第2のセンサ15bにおける、直交する2つの偏波軸からのブラッグ反射光の光路長を、それぞれ一定にできる。すなわち、第1のセンサ15aと第2のセンサ15bのからのブラッグ反射光と参照用反射端からの反射光との干渉信号をSTFT解析すると、これらの直交する2つの偏波軸からのブラッグ反射光は、第1のセンサ15a及び第2のセンサ15bに固有の位置において、それぞれ同じ位置として検出される。その結果、個々のセンサ15a,15bの位置を正確に特定できる。
上述した第一の実施形態~第二の実施形態に関するOFDR方式の物理量測定装置10に関し、センサ15が配された第3のPMファイバ19が、直交する2つの偏波軸の実効屈折率差(複屈折)が大きいPMファイバで構成されているのが好ましい。
これにより、直交する2つの偏波軸における温度と歪みに対する感度差が大きくなり、より高精度の温度と歪みの同時計測を実現できる。より具体的には、直交する2つの偏波軸の実効屈折率差が、4.4×10-4以上であるのが好ましい。この値を満たすことで、後述する実施例から得られるように、センサの温度変化に対するブラッグ波長のシフト特性差を-5.0×10-4nm/℃より大きくできる。その結果、温度精度2℃、歪み精度30μεという、極めて高精度の温度と歪みの計測精度が得られる。
図5は、実施例1のOFDR方式の物理量計測装置10Dを示す概略構成図である。本実施例は、上述した第一の実施形態のOFDR方式の物理量計測装置10Aを基に構成している。図5において、図1に示した第一の実施形態のOFDR方式の物理量計測装置10Aの構成要素と同じ構成要素には同一符号を付して、その説明を省略する。
実施例1のOFDR方式の物理量計測装置10Dは、図1に示すOFDR方式の物理量計測装置10Aに、更に2つのPMカプラ31,32と、フォトダイオード35と、2つの参照用反射端37,38と、を備える。これらはPMファイバの1種であるPANDAファイバ41,42,43,44,47,48によって連設されている。また、第1~第4のPMファイバ及びPMカプラ11にも、PANDAファイバを用いた。
チューナブルレーザ12は、汎用インターフェイスバス(GPIB)を介して、システムコントローラ53に接続し、これにより制御を行なった。
2つのフォトダイオード13,35からの信号は、A/Dコンバータ54によりサンプリングされ、そのサンプリングデータはシステムコントローラ53にてSTFT解析される。この解析方法に関しては、上述した第一の実施形態で記載した通りである。
チューナブルレーザ12としては、Agilent社製の8164A(型式)を用いた。
フォトダイオード13,35としては、New Focus社製の2117FC(型式)を用いた。
PANDAファイバ17,18,19,20,41,42,43,44,47,48としては、フジクラ社製のSM-15-PS-U25A(型式)を用いた。
A/Dコンバータ54としては、National Instruments社製のPXI-6115(型式)を用いた。
この実施例1では、速度10nm/sで、波長範囲1545~1555nmを掃引した測定光を出射した。
チューナブルレーザ12から出射された単―偏波の測定光は、PANDAファイバ41のスロー軸を伝搬してPMカプラ31に入射される。そして、このPMカプラ31にて光パワー分岐されて2つの光干渉計に入射する。
チューナブルレーザ12からある一定速度、ある一定波長範囲で掃引された測定光が、この光干渉計に入射すると、測定光は参照用反射端37,38によって反射され、その干渉光がフォトダイオード35で計測される。フォトダイオード35で取得した信号は、A/Dコンバータ54によりサンプリングされて電圧信号に変換される。この電圧信号は、システムコントローラ53に取り込まれる。チューナブルレーザ12から出射された測定光は、―定速度で波長が変化している。そのため、フォトダイオード35で計測される信号は、一定の光波数間隔で変動する正弦関数となる。したがって、ある一定の電圧値を閾値とし、システムコントローラ53にて、この閾値を超えるタイミング(閾値以下の値から閾値を上回るタイミング、もしくは、閾値以上の値から閾値を下回るタイミング)でトリガを生成することにより、生成されたトリガはある一定の光波数間隔となる。
このトリガの生成方法は、チューナブルレーザ12の掃引速度が一定でない場合でも、トリガが発生する光波数間隔は常に一定となる点で非常に効果的である。
センサ15は、KrFエキシマレーザとユニフォーム位相マスクを用いた一般的な露光方法により作製した。この実施例1では、グレーティング長(センサ長)を5mmとした。また、参照用反射端14を有するPANDAファイバ14に相当する位置からセンサ15までの距離L1は、約20mとした。さらに、L1の中間位置、すなわち、参照用反射端14を有するPANDAファイバ18の長さに相当する位置から約10mの位置に、光路長調整部21として、90°オフセット融着を設けた。入射部20としては、PANDAファイバ16に45°オフセット融着を設けた。
OFDR方式の物理量計測装置10Dでは、センサ15からのブラッグ反射光をスペクトログラムで表示する。このスペクトログラムは、横軸が波長、縦軸がファイバ位置(参照用反射端14を有するPANDAファイバ18に相当する位置からのファイバ長)、色調がブラッグ反射強度を示す。
図6に示すスペクトログラムにおいて、1550.6nmのブラッグ反射光がセンサ15のスロー軸からのものであり、1550.2nmのブラッグ反射光がセンサ15のファスト軸からのものであると考えられ、それぞれの位置が約19.672mでほぼ一致する結果が得られた。
入射部及び光路長調整部の融着接続部の偏波軸オフセット角度を0°としたこと以外は実施例1と同様としてOFDR方式の物理量計測装置を作製し、これを比較例1とした。この比較例1のOFDR方式の物理量計測装置を用いて、センサの状態を計測した。結果を図7に示す。
図7の結果から、この比較例1では、センサ15のスロー軸からのブラッグ反射光しか得られなかった。一方の偏波軸からのブラッグ反射光だけでは、センサ15の温度と歪みを同時に計測することは不可能である。したがって、比較例1のOFDR方式の物理量計測装置を用いて歪み計測を行う場合、温度補償用のセンサが必要となる。
光路長調整部の融着接続部の偏波軸オフセット角度を0°としたこと以外は実施例1と同様としてOFDR方式の物理量計測装置を作製し、これを比較例2とした。この比較例2のOFDR方式の物理量計測装置を用いて、センサの状態を計測した。結果を図8に示す。
図8の結果から、比較例2では、センサ15のスロー軸およびファスト軸からのブラッグ反射光が得られた。そのため、比較例2のOFDR方式の物理量計測装置を用いて歪み計測を行う場合、実施例1と同じく、温度補償用のセンサが不要となる。しかしながら、それぞれのブラッグ反射光の位置が異なるため、センサ15の位置を正確に特定できず、結果として高い空間分解能で歪み計測を行うことができなかった。
第一の実施形態および実施例1で得られる光干渉信号D1と異なるのは、右辺第1項および第2項における、チューナブルレーザ12から出射した測定光が、ファイバ長差L1を往復する光路長が異なる点である。なぜなら、nslowとnfastには、nslow>nfastの関係が常に成り立つためである。式(4)における右辺第1項と右辺第2項の光路長が異なるということは、スロー軸とファスト軸におけるブラッグ反射光がファイバ長差L1に対して異なる光路長を持つことを意味している。すなわち、図8の結果が示すようにそれぞれのブラッグ反射光の位置が異なる。
これは、STFT解析の際、直交する2つの偏波軸の信号に対して、それぞれ別々の実効屈折率(nslowとnfast)を用いなければいけないにもかかわらず、これらの信号は合波されてフォトダイオード13にて光干渉信号D2として計測されるので、ある一定の実効屈折率(比較例2ではnslowを用いた)で計算せざるをえないためである。
この差は、センサとOFDR方式の解析方法とを用いた光ファイバセンサシステムが1mm以下の空間分解能を有するために検出できるものである。言い換えれば、他の方式の光ファイバセンサシステムではこの水準の空間分解能を持たない(あるいは、位置を特定する手段がない)ので、この位置ずれを検出できない。つまり、FBGセンサとOFDR方式の解析方法を用いた光ファイバセンサシステムのみに有効な手段である。
上記の式(5)において、nslowとnfastは、センサ15のブラッグ反射光の波長と、センサ15の作製に使用したユニフォーム位相マスクの回折格子の間隔から計算されるグレーティング周期とから求めた値、ニアフィールドパターン測定から求めた値などを用いる。
この比較例2では、下記の式(6)より、センサ15のブラッグ反射光の波長と、センサ15の作製に使用したユニフォーム位相マスクの回折格子の間隔から計算されるグレーティング周期とから求めた値、nslow=1.44756、nfast=1.44720を用いた。
この計算結果によれば、FBGセンサとOFDR方式の解析方法を用いた光ファイバセンサシステムの空間分解能を1mmとした場合、基準となる位置からセンサ15までの距離が4m以上になると、センサ15におけるスロー軸とファスト軸からのブラッグ反射光の位置ずれが明瞭に確認されると考えられる。
つまり、基準となる位置からFBGセンサまでの距離が4m以上であるとき、本発明は極めて有効となることが分かった。
図10は、実施例2のOFDR方式の物理量計測装置10Eを示す概略構成図である。本実施例2が実施例1と異なる点は、上述した第二の実施形態のOFDR方式の物理量計測装置10Cを基に作製した点である。すなわち、本実施例が実施例1と異なる点は、第3のPMファイバ(PANDAファイバ)19に、第1のセンサ15aと第2のセンサ15bとが配され、この第1のセンサ15aと第2のセンサ15bとの中間には、第2の光路長調整部21b(90°オフセット融着)が配されている点である。第2のセンサ15bは、第1のセンサ15aから5mの位置に設けた。第2の光路長調整部21bは、第1のセンサ15aと第2のセンサ15bとから約2.5mの位置に設けた。
図11の結果から、第1のセンサ15aのスロー軸からのブラッグ反射光の位置と、第1のセンサ15aのファスト軸からのブラッグ反射光の位置とは、ともに約19.672mでほぼ一致することが確認された。
図12の結果から、第2のセンサ15bのスロー軸からのブラッグ反射光の位置と、第2のセンサ15bのファスト軸からのブラッグ反射光の位置とは、ともに約24.757mでほぼ一致することが確認された。
以上の結果から、複数のセンサが配された場合であっても、隣接するこれらセンサの中間にそれぞれ光路長調製部(90°オフセット融着)を設けることにより、FBGセンサ毎に、その直交する2つの偏波軸からのブラッグ反射光を同じ位置にできることが確認された。
センサ15が配された第3のPMファイバ19が、スロー軸とファスト軸の実効屈折率差(複屈折)が大きいPANDAファイバで構成されていること以外は、実施例1と同様に作製し、これを実施例3とした。
この複屈折が大きいPANDAファイバは、図3を用いて説明すると、応力付与部62の配置をコア61に近づけることで実現できる。すなわち、応力付与部62の配置により、PANDAファイバの複屈折を任意に調整できる。
図13に示すスペクトログラムにおいて、1551.1nmのブラッグ反射光がセンサ15のスロー軸からのものであり、1550.4nmのブラッグ反射光がセンサ15のファスト軸からのものである。
具体的には、実施例1で得られた上記の式(3)では-3.7×10-4nm/℃であるのに対して、実施例3で得られた上記の式(7)では-7.2×10-4nm/℃である。すなわち、実施例3のセンサ15は、実施例1のセンサ15よりも2倍近い温度変化に対するブラッグ波長のシフト特性差を有していた。
これは、それぞれのセンサを構成するPANDAファイバの複屈折の差に起因していると考えられる。PANDAファイバは、温度の上昇に比例してコアに生じる複屈折が小さくなり、応力付与部の融点である800~900℃程度で複屈折がほぼ0になることが知られている。つまり、基準温度における複屈折が大きいほど、単位温度上昇当たりの複屈折の減少量が大きくなる。したがって、実施例3のセンサ15は、実施例1のセンサ15よりも2倍近い温度変化に対するブラッグ波長のシフト特性差を有する。
図14は、PANDAファイバの複屈折とこのファイバにより構成されたFBGからなるセンサの温度変化に対するブラッグ波長のシフト特性差を評価した結果を示すグラフである。
図14の結果から、PANDAファイバの複屈折が4.4×10-4以上のとき、このセンサの温度変化に対するブラッグ波長のシフト特性差が-5.0×10-4nm/℃より大きなシフト特性を有する。すなわち、PANDAファイバの複屈折が4.4×10-4以上であることが好ましい。しかしながら、複屈折を大きくするために応力付与部をコアに近づけ過ぎると、PANDAファイバの製造歩留まりが悪くなるという問題がある。ゆえに、PANDAファイバの複屈折は、歩留まりよく製造できる2.0×10-3以下であることが好ましい。
センサ長を100mmとしたこと以外は、実施例3と同様に作製し、これを実施例4とした。本実施例のOFDR方式の物理量計測装置10Fを用いて、センサ15の状態を計測した。結果を、図15に示す。
このスペクトログラムをより詳細に解析して得られたブラッグ反射光の波長差は、0.670nmであった。この波長差は、センサ長が5mmの実施例3のセンサと同等である。したがって、センサ長を100mmとした本実施例のセンサ15に用いたPANDAファイバは、実施例3のセンサ15に用いたPANDAファイバと同等の複屈折である。
この実験系では、分銅Wにより、センサ15の長手方向に沿って均一な歪みを与えている。また、この実験系では、独立して温度制御可能なヒータA及びヒータBにより、センサ15の長手方向に沿って不均一な温度変化を与えることができる。
Claims (9)
- 測定光を出射するチューナブルレーザと;
このチューナブルレーザに一端が接続された第1の偏波保持ファイバと;
この第1の偏波保持ファイバの他端に接続された偏波保持カプラと;
この偏波保持カプラに一端が接続され、他端が参照用反射端である第2の偏波保持ファイバと;
前記偏波保持カプラに一端が接続された第3の偏波保持ファイバと;
この第3の偏波保持ファイバのコアに形成されたファイバブラッググレーティングからなるセンサと;
前記偏波保持カプラに一端が接続された第4の偏波保持ファイバと;
この第4の偏波保持ファイバを介して前記偏波保持カプラと接続され、前記センサからのブラッグ反射光と前記参照用反射端からの参照光とを検出するフォトダイオードと;
このフォトダイオードで検出された前記ブラッグ反射光と前記参照光との合波光強度変化に基づき、これらブラッグ反射光及び参照光間の干渉強度の変調を検知する制御部と;
前記第2の偏波保持ファイバの直交する2つの偏波軸及び前記第3の偏波保持ファイバの直交する2つの偏波軸の両方に、前記測定光を入射する入射部と;
前記第3の偏波保持ファイバに配され、前記センサにおける直交する2つの偏波軸からのブラッグ反射光の光路長を一定にする光路長調整部と;
を備え、
前記入射部は、前記第1の偏波保持ファイバ、または、前記第2の偏波保持ファイバと前記第3の偏波保持ファイバとの両方に配されている
ことを特徴とする光周波数領域反射測定方式の物理量計測装置。 - 前記入射部は、
この入射部が前記第1の偏波保持ファイバに配されている場合には、この第1の偏波保持ファイバに45°の偏波軸オフセット角度を有して形成された融着接続部であり;
前記入射部が前記第2の偏波保持ファイバ及び前記第3の偏波保持ファイバの両方に配されている場合には、これら第2の偏波保持ファイバ及び前記第3の偏波保持ファイバのそれぞれに45°の偏波軸オフセット角度を有して形成された融着接続部である;
ことを特徴とする請求項1に記載の光周波数領域反射測定方式の物理量計測装置。 - 前記光路長調整部は、前記センサが形成された前記第3の偏波保持ファイバに、90°の偏波軸オフセット角度を有して形成された融着接続部であることを特徴とする請求項1または2のいずれか1項に記載の光周波数領域反射測定方式の物理量計測装置。
- 前記光路長調整部は、前記第2の偏波保持ファイバの長さに相当する位置から前記センサまでのファイバ長の中間に設けられたことを特徴とする請求項1ないし3のいずれか1項に記載の光周波数領域反射測定方式の物理量計測装置。
- 前記第3の偏波保持ファイバに、前記センサが複数配されていることを特徴とする請求項1ないし4のいずれか1項に記載の光周波数領域反射測定方式の物理量計測装置。
- 前記光路長調整部が、隣接する前記センサ間のファイバ長の中間にそれぞれ配されていることを特徴とする請求項5に記載の光周波数領域反射測定方式の物理量計測装置。
- 前記第1の偏波保持ファイバから前記第4の偏波保持ファイバのうち、少なくとも前記第3の偏波保持ファイバにおける直交する2つの偏波軸の実効屈折率差が、4.4×10-4以上であることを特徴とする請求項1ないし6のいずれか1項に記載の光周波数領域反射測定方式の物理量計測装置。
- 請求項1ないし7のいずれか1項に記載の光周波数領域反射測定方式の物理量計測装置を用いて、1つまたは複数の前記センサにおける直交する2つの偏波軸からのブラッグ反射光の波長を計測する工程と;
計測した前記ブラッグ反射光の波長に基づいて、前記センサにおける前記ブラッグ反射光の波長の温度と歪みによる変化量を計算する工程と;
計算した前記変化量に基づいて、前記センサが配された部位の温度および歪みを同時に計測する工程と;
を備えることを特徴とする光周波数領域反射測定方式の物理量計測装置を用いた温度と歪みの同時計測方法。 - 前記第3の偏波保持ファイバの、前記センサが配された部位長手方向に沿った温度分布および歪み分布を算出することを特徴とする請求項8に記載の光周波数領域反射測定方式の物理量計測装置を用いた温度と歪みの同時計測方法。
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EP2249127A1 (en) | 2010-11-10 |
US7889332B2 (en) | 2011-02-15 |
JP4420982B2 (ja) | 2010-02-24 |
EP2249127A4 (en) | 2017-05-17 |
CN101680781A (zh) | 2010-03-24 |
EP2249127B1 (en) | 2019-02-13 |
JPWO2009107839A1 (ja) | 2011-07-07 |
CA2696238C (en) | 2013-04-16 |
US20100141930A1 (en) | 2010-06-10 |
CA2696238A1 (en) | 2009-09-03 |
CN101680781B (zh) | 2011-11-23 |
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