JP5150445B2 - Optical fiber sensor device, temperature and strain measurement method, and optical fiber sensor - Google Patents

Description

  The present invention relates to an optical fiber sensor device that measures physical quantities such as temperature and strain using an optical fiber, a temperature and strain measurement method, and an optical fiber sensor, and more particularly to a technique that can simultaneously measure temperature and strain.

Sensors that measure physical quantities such as temperature and strain using optical fibers have a long life, are lightweight, and have a small diameter and flexibility, so they can be used in narrow spaces, and optical fibers are insulated. Therefore, it is expected to be used for the evaluation of the soundness of aerospace equipment such as huge structures such as bridges and buildings, passenger planes and artificial satellites.
The performance required for optical fiber sensors for evaluating the soundness of these structures includes high strain resolution, a multipoint sensor (wide detection range), and real-time measurement capability. .

Various optical fiber sensors have been proposed so far, and the most promising one that sufficiently satisfies the required performance is an optical fiber sensor comprising a fiber Bragg grating (hereinafter abbreviated as FBG).
The FBG is an optical fiber type device in which the core of the optical fiber has a periodic refractive index change, and has a characteristic of reflecting light of a specific wavelength determined by the core refractive index change period and the effective refractive index. . This reflected light is Bragg reflected light, and the reflected wavelength is called Bragg wavelength.
When a temperature change or strain occurs in the FBG, the period of the refractive index change of the core and the effective refractive index change accordingly, and the Bragg wavelength shifts. By measuring the relationship between the Bragg wavelength shift amount and the temperature change amount and strain amount in advance, the temperature change and strain can be measured from the Bragg wavelength shift amount. The optical fiber sensor made of FBG has a feature that it can measure temperature change and strain with extremely high resolution, depending on the measurement accuracy of the means for measuring the shift amount of the Bragg wavelength.

  In addition, the following methods are known as means for measuring temperature change and strain using an optical fiber sensor made of FBG. As a first method, a plurality of FBGs having different Bragg wavelengths are arranged for one optical fiber, and measurement light over the Bragg reflection wavelength region of all the FBGs is continuously incident, so that the Bragg wavelengths from each FBG are incident. A wavelength division multiplexing (Wavelength Division Multiplexing; hereinafter abbreviated as WDM) method for measuring the shift amount can be exemplified. As a second method, a plurality of Bragg wavelengths that are substantially the same for one optical fiber are used. FBGs are arranged at intervals of a certain distance or more, measurement light over the Bragg wavelength range of all FBGs is incident in pulses, and the position of each FBG is determined based on the difference in propagation delay time of Bragg wavelength light from each FBG. A time division multiplexing (Time Division Multiplexing; hereinafter abbreviated as TDM) method for measuring the amount of shift of the Bragg wavelength can be illustrated as a third method. As described above, a plurality of FBGs having substantially the same Bragg wavelength are arranged at an arbitrary interval with respect to one optical fiber, and the period of the interference intensity between the Bragg reflected light from each FBG and the reflected light from the reference reflection end An optical frequency domain reflection measurement (Optical Frequency Domain Reflectometry; hereinafter abbreviated as “OFDR”) system that specifies the position of each FBG using a change and measures the shift amount of the Bragg wavelength can be exemplified.

Among these measurement means, Patent Document 1 discloses details of the OFDR measurement method. According to Patent Document 1, the OFDR measurement method has an advantage that the strain detected by a plurality of strain detection FBGs arranged in one optical fiber can be measured in real time.
As described above, the optical fiber sensor made of FBG has the greatest feature that it can measure temperature changes and strains generated in the FBGs with high resolution, and has an advantage that there are many means for measuring these temperature changes and strains. have.

As an optical fiber sensor other than the optical fiber sensor made of FBG, as disclosed in Patent Document 2, an optical fiber sensor made of an optical fiber and a mirror provided at the end thereof is disclosed. According to Patent Document 2, this optical fiber sensor is used as a person detection sensor. When a person passes over the optical fiber sensor, the optical fiber is bent and the intensity of the measurement light varies. This optical fiber sensor measures the intensity of measurement light reflected by a mirror provided on the end face of the optical fiber, and determines the presence or absence of a person from the intensity.
However, since the optical fiber sensor described in Patent Document 2 does not have a quantitative correlation between the strain amount and the intensity fluctuation (bending of the optical fiber) of the measurement light, it is like an optical fiber sensor using FBG as a sensor. There is a problem that accurate strain measurement cannot be performed. From the comparison with Patent Document 2, the usefulness of the optical fiber sensor made of FBG is clear.

On the other hand, as a general problem of the optical fiber sensor made of FBG, when a plurality of physical quantities such as temperature and strain change, it is not possible to individually measure and measure these changes. For this reason, for example, when used as a strain detection sensor, an optical fiber sensor capable of measuring strain without capturing a temperature change of the detection unit as a change in strain and a measurement method thereof are considered necessary. It is done.
As such an optical fiber sensor and a measuring method thereof, in the technique described in Patent Document 1, one of a plurality of FBGs is placed in a state in which no strain acts (no strain), and the Bragg wavelength of the FBG is set. By measuring the shift amount, instability of the wavelength sweep of the light source is removed, and only when all the FBGs are at the same temperature, temperature correction of the strain detection FBG (that is, occurs in the strain detection FBG). It is described that temperature change can be measured).

Further, in Patent Document 3, since the displacement of the diaphragm is detected from the FBG (displacement amount applied to the FBG) that serves as a displacement detection sensor for measuring the displacement of the diaphragm, the FBG is actually used as a strain detection sensor. FBG, which is a sensor for temperature detection, is placed on the same base as before, and the temperature of the base is measured in advance by the sensor for temperature detection, and the change in Bragg wavelength of the sensor for displacement detection is changed to the temperature change. A technique for measuring the displacement of the diaphragm by subtracting the corresponding wavelength shift amount is disclosed. Further, Patent Document 4 discloses a technique in which an FBG serving as a strain detection sensor and a temperature compensation member are combined so that the strain detection sensor does not cause a shift of the Bragg wavelength due to a temperature change.
Japanese Patent No. 3740500 Japanese Patent Laid-Open No. 2005-184772 JP 2000-221085 A JP-A-2005-091151

However, the strain compensation FBG temperature compensation method described in Patent Document 1 has a problem that temperature compensation of strain detection FBGs cannot be performed when the temperature change occurring in each strain detection FBG is different. In addition, in the method of providing an FBG serving as a temperature detection sensor described in Patent Document 2, two FBGs are required to detect temperature change and strain (diaphragm displacement). There is an expensive problem.
Furthermore, in the strain detection FBG described in Patent Document 3, an optical fiber sensor is expensive because of a temperature-independent structure, and a temperature change occurring in the strain detection FBG cannot be measured. is there.

  The present invention has been made in view of the above circumstances, and has high resolution with respect to temperature change and strain, is capable of multipoint measurement, is excellent in real-time measurement, and is inexpensive and an optical fiber for measuring temperature and strain. An object is to provide a sensor, a method for measuring temperature and strain, and an optical fiber sensor device.

The optical fiber sensor device of the present invention uses an FBG formed in the core of an optical fiber as a sensor, and from the periodic change in the interference intensity of the Bragg reflected light from the sensor and the reflected light from the reference reflection end, An optical fiber sensor device that is applied to an optical frequency domain reflection measurement system that specifies a position and measures the distortion of the sensor unit from the shift amount of the wavelength of Bragg reflected light from the sensor, and is formed in the core of the optical fiber A light having a strain detection sensor unit made of FBG and a temperature detection sensor unit made of a temperature detection optical fiber connected to the strain detection sensor unit and provided with a reflection part at its end. and the fiber sensor, a referential reflecting end, a light source, the light receiver and is provided with a total of a temperature change from the fiber length variation of the sensor portion to become the optical fiber for the temperature sensing In doing so, by utilizing the periodic change in the interference intensity of the reflected light from the reflection portion provided on the end face of the optical fiber serving as the temperature detection sensor portion and the reflection light from the reference reflection end, the temperature detection identifies the reflection portion position of the optical fiber end face serving as a sensor unit measures the optical path length of the optical fiber, the temperature change is measured from the change amount of the optical path length of this, the shift amount of the Bragg wavelength of the FBG The strain is measured by subtracting the shift amount of the Bragg wavelength corresponding to the measured temperature change.

In the present invention, the optical fiber serving as the temperature detecting sensor section has a fiber length of 40 m or more.
In the present invention, the optical fiber serving as the temperature detecting sensor part is loosely inserted into a tube.
In the present invention, one end or both ends of the FBG serving as the strain detecting sensor unit and the optical fiber serving as the temperature detecting sensor unit are attached to a planar body.
In the present invention, the planar body may be a thin metal plate.

In the present invention, one end or both ends of the optical fiber to be the strain detecting sensor part and one end or both end parts of the optical fiber to be the temperature detecting sensor part are fixed to the planar body by an adhesive layer. The structure made may be sufficient.
In the present invention, one end portion or both end portions of the FBG serving as the strain detection sensor portion and one end portion or both end portions of the optical fiber serving as the temperature detection sensor portion are fixed to the planar body by an adhesive layer. It may be a structure.
In the present invention, based on the measurement result of the optical path length of the optical fiber, the temperature change amount is measured from the change of the optical path length of the optical fiber to be a temperature detection sensor, and the Bragg of the FBG to be a strain detection sensor. The control device that measures the distortion by subtracting the shift amount of the Bragg wavelength corresponding to the temperature change measured by the optical fiber serving as a temperature detection sensor from the shift amount of the wavelength is connected to the light receiver and the light source. The structure provided may be sufficient.

The temperature and strain measurement method of the present invention uses any one of the optical fiber sensor devices described above, measures the temperature change from the amount of change in the fiber length of the optical fiber serving as the temperature detection sensor section, The distortion is measured by subtracting the change amount of the Bragg reflection wavelength due to the temperature change measured by the temperature detection sensor unit from the shift amount of the Bragg reflection wavelength of the FBG serving as the detection sensor .

The optical fiber sensor of the present invention uses the FBG formed in the core of the optical fiber as a sensor, and the position of the sensor is detected from the periodic change in interference intensity between the Bragg reflected light from the sensor and the reflected light from the reference reflection end. And an optical fiber sensor used in an optical frequency domain reflection measurement method for measuring strain and temperature of the sensor unit from the shift amount of the wavelength of the Bragg reflected light from the sensor,
A temperature detecting sensor comprising a strain detecting sensor portion made of FBG formed in the core of the optical fiber and a temperature detecting optical fiber connected to the strain detecting sensor portion and provided with a reflection portion at an end thereof. An optical fiber sensor comprising a portion,
When measuring the temperature change from the change in the fiber length of the optical fiber serving as the temperature detecting sensor part, the reflected light from the reflecting part provided on the end face of the optical fiber serving as the temperature detecting sensor part and the reference using a periodic variation of the interference intensity of the reflected light from the reflecting end, the identify the reflection portion position of the optical fiber end face serving as a sensor unit for temperature detection, to measure the optical path length of the optical fiber, this The temperature change is measured from the change amount of the optical path length, and the strain is measured by subtracting the shift amount of the Bragg wavelength corresponding to the measured temperature change from the shift amount of the Bragg wavelength of the fiber Bragg grating. To do.

  In the optical fiber sensor device of the present invention, the temperature detection sensor unit is composed of an optical fiber and a reflection unit formed at the end thereof. Therefore, an optical fiber sensor using FBG as the temperature detection sensor unit, Compared to an optical fiber sensor having a dependency structure, it can be provided as an inexpensive optical fiber sensor. Therefore, it is possible to provide an optical fiber sensor device that can use an inexpensive optical fiber for temperature detection, measure strain with the FBG in a temperature compensated state, and perform accurate strain measurement.

In the optical fiber sensor device of the present invention, when an optical fiber having a length of 40 m or more is used as the temperature detection optical fiber, the optical path length change of the optical fiber constituting the temperature detection sensor unit by the OFDR method is used. When calculating temperature changes, accurate temperature measurement is possible.
In the optical fiber sensor device of the present invention, when an optical fiber serving as a temperature detection sensor is loosely inserted into the tube, even if the temperature detection sensor section is distorted, only the tube expands and contracts. Since the strain hardly acts on the optical fiber serving as the temperature detection sensor housed in the optical fiber, the optical fiber serving as the temperature detection sensor section does not change the optical path length due to the strain. Thereby, accurate temperature detection can be performed.
Furthermore, in the optical fiber sensor device of the present invention, the temperature measurement is performed without being affected by the strain because the optical fiber to be a temperature detection sensor is fixed to the detection unit or the planar body while being loosely inserted into the tube. be able to.

Since the optical fiber sensor device of the present invention has a structure in which an optical fiber serving as a temperature detection sensor and an FBG for strain detection are fixed to a planar body made of a thin metal plate, the attachment to the detection unit is facilitated. In addition, in the planar body for fixing the temperature detection optical fiber and the strain detection FBG, an adhesive layer is provided on a surface different from the surface on which these are fixed, thereby facilitating attachment to the detection unit.
Since the optical fiber sensor device of the present invention has a structure in which a plurality of optical fiber sensors are connected in series using connectors, temperature and strain sensing over multiple points can be easily performed. In addition, the number of points to be sensed can be easily adjusted by attaching / detaching the connector.

  According to the temperature and strain measurement method using the optical fiber sensor device of the present invention, since the OFDR type measuring instrument is used, the fiber length of the optical fiber serving as a temperature detection sensor can be shortened.

Further, in the optical fiber sensor of the present invention, the temperature detection sensor part is composed of the optical fiber and the reflection part formed at the end thereof, so that the optical path length of the optical fiber of the temperature detection sensor part in the OFDR method. It can be used to determine the temperature change from the change in temperature, and can also be used to measure strain in a temperature compensated state when measuring strain with FBG based on the temperature measurement result. .
Therefore, according to the optical fiber sensor of the present invention, it can be provided as an inexpensive optical fiber sensor as compared with an optical fiber sensor using FBG as a temperature detecting sensor part or an optical fiber sensor having a temperature-independent structure. . That is, it is possible to provide an optical fiber sensor that can measure strain with an FBG in a state where temperature is compensated using an inexpensive optical fiber as a sensor for temperature detection.

Hereinafter, the basic structure of an optical fiber sensor for measuring temperature change and strain according to the present invention will be described with reference to the drawings, but the present invention is of course not limited to the embodiments described below.
FIG. 1 is a configuration diagram showing a basic structure of an optical fiber sensor (temperature strain measurement sensor) for measuring temperature change and strain according to the present invention. In FIG. 1, an optical fiber sensor S of the present embodiment includes a strain detection sensor unit 3 composed of an optical fiber 2 having an FBG (fiber Bragg grating) 1 in its core, and an optical fiber optically connected to the sensor unit 3. 5 is provided with a temperature detecting sensor portion 7 formed by forming a reflecting portion 6 at an end portion.

  The FBG 1 is configured such that the refractive index change distribution in the longitudinal direction of the optical fiber core changes at regular intervals. For example, the FBG 1 has a structure in which a high refractive index portion and a low refractive index portion are repeated at regular intervals. . The FBG 1 desirably has an optical characteristic capable of measuring strain with high resolution (measuring the shift amount of the Bragg wavelength with high resolution). The optical fiber 5 constituting the temperature detecting sensor unit 7 is optically connected at one end thereof to one end side of the strain detecting sensor unit 1 and has an appropriate reflectance at the other end surface. And having a length such that the optical path length between the FBG 1 and the reflecting portion 6 changes greatly with respect to a temperature change. As an example, the optical fiber 5 is preferably an optical fiber having a length of 40 m or more.

Next, a first optical fiber sensor device (temperature strain measurement sensor device) including the optical fiber sensor S having the basic structure shown in FIG. 1 and employing an optical frequency reflection measurement method (hereinafter abbreviated as OFDR method) is applied. An embodiment is shown in FIG. 2. An optical fiber sensor device 10 of this embodiment includes an optical coupler (fiber coupler) 11, a tunable laser (TLS; tunable light source) 12, and a photodiode (PD; photodetector) 13. And a reflection end 16 for reference and an optical fiber sensor S having the above structure, and the tunable laser 12 is optically connected to the optical coupler 11 via the optical fiber 14 and the photodiode 13 via the optical fiber 15. The reference reflection end 16 is optically connected to the optical coupler 11 via the optical fiber 17 and the optical fiber sensor S is connected to the optical coupler 11 via the optical fiber 18. That is, the tunable laser 12, the photodiode 13, the reference reflection end 16, and the optical fiber sensor S are connected to each other through the optical coupler 11 by optical fibers 14, 15, 17, and 18. The measurement light emitted from the optical fiber 12 can enter the optical fiber 17 and the optical fiber sensor S through the optical coupler 11. In addition, since part of the measurement light incident on the optical fiber 17 and the optical fiber sensor S is reflected, it can be measured as interference light by the photodiode 13 via the optical coupler 11.

  In the optical fiber sensor device 10 having the above-described structure, the tunable laser 12 includes an FBG 1 in which the measurement light emitted from the tunable laser 12 serves as a strain detection sensor unit 3 and an optical fiber 5 that serves as a temperature detection sensor unit 7. It is desirable to have a coherent length longer than the optical path length that is reflected by the reflecting portion 6 on the end face and enters the photodiode 13. The photodiode 13 can detect intensity modulation of optical interference obtained from the two reflection points of the reference reflection end 16 and the reflection part 6 when the wavelength of the measurement light emitted from the tunable laser 12 is changed. Those having a cut-off frequency are desirable.

In order to perform the temperature and strain measurement method using the optical fiber sensor device 10 having the structure shown in FIG. 2, the measurement light emitted from the tunable laser 12 is incident on the FBG 1 and the optical fiber 5 and reflected by the reflection unit 6. The measurement light detected by the photodiode 13 and emitted from the tunable laser 12 is reflected from the reference reflection end 16 and detected by the photodiode 13. At this time, the following measurement is performed.
In the temperature and strain measurement method according to the present invention, the position of the FBG 1 is determined using the periodic change in the interference intensity of the Bragg reflected light from the strain detecting FBG 1 and the reflected light from the reference reflecting end 16. At the same time, the amount of shift of the Bragg wavelength is measured, and at the same time, the periodic change in the intensity of the interference light from the reflecting portion 6 and the reflecting end 16 of the reference end of the optical fiber 5 serving as the sensor portion 7 for temperature detection. Is used to identify the position of the reflection part of the end face of the optical fiber 5 that becomes the temperature detecting sensor part 7 and measure the optical path length of the optical fiber 5.

Further, based on the measurement result, the temperature change amount is measured from the change in the optical path length of the optical fiber 5 serving as the temperature detecting sensor unit 7, and the Bragg wavelength shift of the FBG 1 serving as the strain detecting sensor unit 3 is measured. The strain can be measured by subtracting the shift amount of the Bragg wavelength corresponding to the temperature change measured by the temperature detection sensor unit 7 from the amount.
The outline of the present invention has been described above, but specific examples of the present invention will be described in more detail in the following embodiments.

"Example 1"
FIG. 3 is a schematic configuration diagram of an optical fiber sensor for temperature strain measurement according to the first embodiment of the present invention, FIG. 3A is a plan view, and FIG. 3B is an AB view of FIG. It is sectional drawing which follows a line.
An optical fiber sensor 20 shown in FIG. 3 includes an optical fiber sensor S having the same structure as that of the optical fiber sensor S described above, and a connector. That is, in the optical fiber sensor 20 of the present embodiment, the FBG 22 for strain detection, the optical fiber 23 for temperature detection optically connected thereto, and the end face optically connected to the end side which is the input side of the FBG 22 are PC-polished. A connector 25 composed of an FC connector (hereinafter abbreviated as FC / PC connector), a reflective portion 26 composed of an FC / PC connector optically connected to the terminal end of the optical fiber 23, and a resin for fixing the FBG 22 at the center thereof And a loose tube 27 into which the optical fiber 23 for temperature detection is loosely inserted. The optical fiber 23 is wound in a loop shape and installed on the upper surface side of the planar body 21. Further, the FBG 22 portion becomes the strain detection sensor unit 30, and the optical fiber 23 portion becomes the temperature detection sensor unit 31.

  In the optical fiber sensor 20 of the present embodiment, the FBG 22 serving as a strain detection sensor is manufactured by a general exposure method using a krypton fluoride (KrF) excimer laser and a uniform phase mask. In the example, the grating length (sensor length) is 98 mm, the Bragg wavelength is about 1549.5 nm, and the reflectance at the Bragg wavelength is 1%.

An optical fiber 23 serving as a temperature detecting sensor unit applied to the present embodiment is a general single mode fiber (hereinafter referred to as an SM fiber), and a cladding made of quartz glass and germanium (Ge) are made of doped silica glass. The refractive index difference of the core is about 0.3%.
In this embodiment, the fiber length of the optical fiber 23 made of this SM fiber is about 40 m. The reflection part 26 formed on the end face of the optical fiber 23 is subjected to a fiber terminal and a connector terminal for Fresnel reflection (approximately 3.4% reflectivity) and a metal coating for total reflection (reflectance of 99% or more). It is desirable that the end surface of the fiber end has an appropriate reflectance.
The appropriate reflectance is preferably a reflectance of 0.001% or more, although it depends on the performance of the measuring instrument that measures the temperature strain sensor of the present invention. In the present embodiment, the FC / PC connector 26 is connected to the end face of the optical fiber 23 serving as a temperature detection sensor.

  When the connector 26 is not connected, its end face has a characteristic of Fresnel reflection. The entire length of the optical fiber 23 serving as the temperature detecting sensor unit 31 was inserted into the loose tube 27. By inserting the optical fiber 23 to be the temperature detecting sensor unit 31 into the loose tube 27, even if the temperature detecting sensor unit 31 is distorted, the loose tube 27 only expands and contracts. Does not expand or contract. That is, the optical path length of the optical fiber 23 serving as the temperature detecting sensor unit 31 does not change due to strain. The other part of the FBG 22 serving as the strain detection sensor unit 30 to which the optical fiber 23 serving as the temperature detection sensor unit 31 is connected can be connected to a measuring instrument that measures the temperature strain sensor of the present invention. It is desirable to have a connection point. In this embodiment, such a connection point is provided as the FC / PC connector 25.

  In the temperature strain sensor of this example, one surface or both surfaces are fixed to the planar body 21 so that the temperature strain sensor can be easily attached to a measurement target portion (hereinafter referred to as a detection portion) for measuring temperature and strain. The planar body 21 for fixing the temperature strain sensor on one or both surfaces may be any flat plate such as a metal material, a plastic material, or a glass material. In order to accurately detect strain, a metal-based material having high thermal conductivity and high elastic modulus is desirable, and it is desirable that the thickness be such that the planar body has sufficient elastic characteristics. In this embodiment, the planar body 21 is made of a stainless steel plate (JIS regulation SUS303) having a thickness of about 0.3 mm.

  The temperature strain sensor is fixed to the planar body 21 using an adhesive made of a thermosetting resin or an ultraviolet curable resin. In this embodiment, the loose tube 27 into which the FBG 22 serving as the strain detection sensor unit 30 and the optical fiber 23 serving as the temperature detection sensor unit 31 are inserted is fixed on the planar body 21 by the polyimide resin adhesive layer 33. . More specifically, after applying a polyamic acid resin containing N-methyl-2pyrrolidinone as a solvent, the planar body 21 and the temperature strain sensor are heated to about 200 ° C. to evaporate the N-methyl-2pyrrolidinone solvent. At the same time, the crosslinking of the polyamic acid resin was promoted and chemically changed to a polyimide resin to be cured and fixed. Here, the loose tube 27 into which the FBG 22 serving as the strain detection sensor unit 30 and the optical fiber 23 serving as the temperature detection sensor unit 31 are inserted is fixed with polyimide resin, which has high heat resistance and high elastic modulus. Since it is a material, it can be used even when the detection unit is at a high temperature, and the strain of the detection unit can be reliably transmitted to the FBG 22 serving as a strain detection sensor.

  In addition, the optical fiber sensor 20 has a surface on the other side that is the sensor fixing surface of at least one of the planar bodies 21 that fixes one or both sides so that the optical fiber sensor 20 can be easily attached to the detection unit. Is provided with an adhesive layer 33a. As the adhesive layer 33a, an adhesive made of a thermosetting resin or an ultraviolet curable resin, or a tape having an adhesive component can be used. In this example, an adhesive layer 33a was formed by attaching an arbitrary surface of a silicone resin tape having an adhesive component on both sides to a planar body.

Next, a method for measuring temperature and strain by using the optical fiber sensor 20 having the basic structure and applying an optical frequency reflection measurement method (OFDR method) will be described more specifically.
FIG. 4 is a schematic configuration diagram showing a specific example of an optical fiber sensor device for carrying out the temperature and strain measurement method of the present invention. The optical fiber sensor device 40 of this example includes three fiber couplers (optical couplers) 41, 42, and 43, a tunable laser (wavelength variable light source; 8164A manufactured by Agilent) 44, and two photodiodes (photodetectors; New). Focus 2117FC) 45, 46, three reference reflection ends R1, R2, R3, and the optical fiber sensor 20 having the above-described configuration, which are optical fibers 51, 52, 53, 54, 55, 56, 57, 58, 59, 60 are connected. These optical fibers 51 to 60 are not particularly limited as long as they can propagate in the single mode with respect to the wavelength of the measurement light, and are general SM fibers or PANDA (Polarization-maintaining AND Absorption-reducing) fibers. A polarization maintaining fiber such as can be used. In this example, the same type of SM fiber as the optical fiber 23 was used.

  The tunable laser 44 is connected to and controlled by a system controller (control device: PXI-8106 manufactured by National Instruments) 63 via a general-purpose interface bus (GPIB). Further, the signals from the two photodiodes 45 and 46 are sampled by an A / D converter (PXI-6115 made by National Instruments) 64, and the sampling data is short-time Fourier transform (STFT) by the system controller 63. (Fourier transform) analysis. The tunable laser 44, the system controller 63, and the A / D converter 64 constitute an OFDR measuring instrument.

In the OFDR measuring instrument of the configuration of this embodiment, the tunable laser 44 emits measurement light that is swept (monotonically increased or monotonously decreased) at a certain constant speed and in a certain wavelength range. In this example, measurement light was swept in a wavelength range of 1545 to 1555 nm at a speed of 10 nm / s.
The measurement light emitted from the tunable laser 44 enters the optical coupler 41, and the optical power is branched by the optical coupler 41 to enter the two optical interferometers.
One optical interferometer is composed of a fiber coupler 42, an optical fiber 57 and its reflection end R1, an optical fiber 58 and its reflection end R2, and optical fibers 53 and 54 and a photodiode 45. The fiber of the reflection end R1 and the reflection end R2 is provided. A trigger corresponding to the length difference (optical path length difference) is generated. In this embodiment, the fiber length difference between the reflection end R1 and the reflection end R2 is 200 m. This trigger is generated by the following method.

When measurement light swept from the tunable laser 44 at a certain speed and in a certain wavelength range is incident on the optical interferometer, the measurement light is reflected by the reflection end R1 and the reflection end R2, and the interference light is reflected by the photodiode. Measured at 45. The voltage signal output from the photodiode 45 is sampled and converted into a digital signal by the A / D converter 64, and the digital signal is taken into the system controller 63 and analyzed.
Since the wavelength of the measurement light emitted from the tunable laser 44 changes at a constant speed, the voltage signal output from the photodiode 45 becomes a sine function that varies at constant light wave number intervals. Therefore, a certain voltage value is set as a threshold, and the timing at which the threshold is exceeded on the system controller 63 (the timing at which the threshold is exceeded from the value below the threshold or the value above the threshold is set). By generating a trigger at a timing (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 44 is not constant. Further, when the instability of the wavelength sweep of the tunable laser 44 cannot be removed even by this trigger generation method, an FBG for wavelength correction as described in Patent Document 1 may be used. (However, temperature correction is not performed from this FBG.)

The other optical interferometer includes a photodiode 43, an optical fiber 59 and its reflection end R 3, an optical fiber sensor 20, optical fibers 55 and 56, and a photodiode 46. In this optical interferometer, the interference light of the Bragg reflected light from the optical fiber sensor 20 and the reference light from the reflection end R 3 is incident on the photodiode 46.
Here, the reflected light from the optical fiber sensor 20 mainly includes an FC / PC connector 25 connected to an OFDR measuring instrument, an FBG 22 serving as a strain detection sensor, and a terminal FC of the optical fiber 23 serving as a temperature detection sensor. / Starts at three locations of the PC connector 26.

The optical interferometer of the present embodiment, the optical interference signal D ₂ that enters the photodiode 46, are expressed by the following equation (1).
D ₂ = R _FBG cos (k2nL _FBG ) + R _connect cos (k2nL _connect ) (1) However, the interference light by the FC / PC connector 25 connected to the measuring instrument of the OFDR system and the reflection end R3 is the present invention. Ignore it because it is not used in.
Here, R _FBG is the interference light intensity between the _FBG 22 and the reflection end R3, R _connect is the interference light intensity between the terminal FC / PC connector 26 and the reflection end R3 of the optical fiber 23 serving as the temperature detection sensor unit 31, and k is the wave number. , N is the effective refractive index of the SM fiber, L _FBG is the fiber length difference between the _FBG 22 and the reflection end R3, and L _connect is the fiber of the terminal FC / PC connector 26 and the reflection end R3 of the optical fiber 23 serving as the temperature detection sensor unit 31. Indicates the length difference. In this embodiment, L _FBG is about 2 m and L _connect is about 42 m. That is, the fiber length of the optical fiber 23 serving as the temperature detecting sensor unit 31 is about 40 m.

Then, by STFT analysis of the obtained optical interference signal _{D 2} by the system controller 63, R _FBG, R _connect and nL _FBG, seek nL _{the connect.} Here, nL _FBG and nL _connect indicate the optical path length because they are the effective refractive index of the optical fiber multiplied by the optical fiber length. In the present embodiment, the obtained optical interference signal is extracted with a window width corresponding to an interval of about 40 ms (the tunable laser 44 is swept at a speed of 10 nm / s, so that the wavelength is converted to a wavelength of about 400 pm). The data was assigned to 16 bits (2 ¹⁶ ) and analyzed. When the sweep speed of the tunable laser 44 is not constant, 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. Finally, a known value is substituted for n to obtain optical fiber length differences L _FBG and L _connect . In this example, n = 1.44772, which is a typical value of the used SM fiber, was used.

The results of measuring the state of the temperature strain sensor using the OFDR measuring instrument of this example are shown in FIGS. In addition, the detection part at this time was 20 degreeC and 0 microepsilon (no distortion state). In the measuring instrument of the present embodiment, the Bragg reflected light from the FBG 22 and the reflected light from the end of the optical fiber 23 are displayed as spectrograms.
In the spectrograms shown in FIGS. 5 and 6, the horizontal axis indicates the wavelength, the vertical axis indicates the fiber position (fiber length difference from the position corresponding to the length of the optical fiber having the reflection end R3), and the color tone indicates the reflection intensity. FIG. 5 shows a range of fiber positions 1.9 to 2.1 m among the results of analyzing the D ₂ signal, and FIG. 6 shows fiber positions 42.0 to of the results of analyzing the D ₂ signal. The range of 42.2m is displayed.
The reflection of 98 mm in total length obtained in FIG. 5 is due to the FBG 22, and the reflection in the wide wavelength band obtained in FIG. 6 is due to the FC / PC connector 26 at the end of the optical fiber 23. Since the reflection by the FBG 22 showed the maximum intensity at 1549.52 nm, the wavelength is the Bragg wavelength. Further, it was found from this measurement result that the fiber length difference between the FBG 22 and the FC / PC connector 26 is 40.050 m (this value is defined as the distance from the end of the FBG 22 to the FC / PC connector 26).

Next, a measuring method using the optical fiber sensor 20 of the present invention will be further described.
In the following description, the state of the detection unit measured by the optical fiber sensor 20 is defined as a reference temperature (20 ° C.) and a reference strain (0 με).
In the optical fiber sensor 20 of the present embodiment, the temperature is first measured from the length of the optical fiber represented by the OFDR measuring instrument of the optical fiber 23 that is a sensor unit for temperature detection. For example, the reference temperature is 40.050 m as described above. When this optical fiber length is defined as the reference optical fiber length, the relationship of the change amount of the optical fiber length with the temperature change of the detection unit in the OFDR measuring instrument of the present embodiment depends on the optical fiber length.
FIG. 7 is a graph showing the relationship between the temperature of the detector and the change in optical fiber length in the OFDR measuring instrument. This change in the optical fiber length depends on the temperature dependence of the effective refractive index of the optical fiber and the elongation of the optical fiber due to thermal expansion, that is, the change in the optical path length. The change amount of the optical fiber length due to these can be obtained from the shift amount of the Bragg wavelength due to the temperature change of the FBG manufactured in the same type of optical fiber. FIG. 8 is a graph showing the shift amount of the Bragg wavelength of the FBG due to temperature change.

In FIG. 8, the Bragg wavelength of the FBG is expressed by the following equation (2).
λ _B = 2nΛ (2) where λ _B is the Bragg wavelength of FBG, n Represents the refractive index of the optical fiber, and Λ represents the period of refractive index change.
In equation (2), when FBG changes in temperature, n changes due to the temperature dependence of the effective refractive index of the optical fiber, and Λ changes due to fiber elongation due to thermal expansion of the optical fiber. That is, FIG. 8 shows the result of obtaining the total amount of change of n and Λ in the equation (2) as the central wavelength shift amount of the FBG.
On the other hand, since the optical fiber length L _FBG and L _connect are calculated from a certain n, the optical fiber length calculated by the OFDR measuring instrument is regarded as the fiber elongation due to the thermal expansion of the optical fiber. Will be calculated. That is, FIG. 7 shows the result of calculating the total amount of changes in n, L _FBG , and L _connect as changes only in L _FBG and L _connect .

The relationship between the temperature of the detection unit and the change in optical fiber length in FIG.
ΔL = 6.4536 × 10 ⁻⁶ · ΔT · L (3) where ΔL is the change in optical fiber length (mm) calculated by the OFDR measuring instrument, and ΔT is the difference between the detector temperature and the reference temperature. (° C.) and L indicate the optical fiber length (mm). The coefficient of 6.4536 × 10 ⁻⁶ indicates the amount of change in the optical path length per unit temperature change of the used SM fiber, and has a different value for each optical fiber.
In the optical fiber sensor of the present embodiment, an OFDR type measuring instrument is used to measure the change in the fiber length of the optical fiber 23 serving as a temperature detection sensor unit, and ΔT is obtained by substituting the measurement into equation (3). Thus, the temperature of the detection unit is measured.

  Next, strain measurement is performed by subtracting the Bragg wavelength shift amount corresponding to the temperature change measured by the temperature detection sensor from the Bragg wavelength shift amount of the FBG serving as the strain detection sensor unit 30. Note that the shift amount of the Bragg wavelength of the FBG in the temperature change obtained from the change in the optical fiber length can be obtained from FIG. FIG. 9 is a graph showing the shift amount of the Bragg wavelength of the FBG due to the strain change.

In FIG. 9, the shift amount of the Bragg wavelength due to the strain change is expressed by the following equation (4).
Δλ _B = 1.1206 · ε (4)
Here, Δλ _B is the Bragg wavelength shift amount (nm) due to strain, and ε is the strain amount (με). The coefficient 1.1206 indicates the Bragg wavelength shift amount per unit strain amount of the used SM fiber, and has a different value for each optical fiber.

  By performing the above arithmetic processing, it is possible to measure the temperature and strain of the detection unit using the optical fiber sensor 20 of the present invention. These calculations can be easily performed using the system controller 63 provided in the OFDR measuring instrument described in the present embodiment.

  The strain measurement accuracy required in general strain sensing is about 10 με. That is, the Bragg wavelength shift amount of the FBG due to the strain change needs to be able to be measured with an accuracy of about 0.01 nm. Further, the temperature change amount by which the Bragg wavelength of the FBG is shifted by 0.01 nm is about 1 ° C. That is, by measuring the temperature with an accuracy of 1 ° C. and subtracting the shift amount of the Bragg wavelength corresponding to the temperature change, the strain can be measured to satisfy the required strain measurement accuracy.

In this embodiment, the optical interference signal obtained by the OFDR measuring instrument is analyzed by assigning data extracted at a window width corresponding to an interval of about 40 ms to 16 bits. The spectrogram at this time is The wavelength is obtained in increments of 0.0011 nm and fiber positions in increments of 0.24 mm. In order to measure the temperature with an accuracy of 1 ° C. in the optical fiber sensor of the present invention, the fiber length needs to change by 0.24 mm or more when the temperature of the detection unit changes by 1 ° C. The required fiber length is 40 m or more.
When the optical fiber 23 serving as the temperature detection sensor unit 31 is 40 m or longer, if the strain is measured by subtracting the Bragg wavelength shift amount due to temperature change, the Bragg wavelength shift amount of the FBG 22 due to strain change is measured with an accuracy of 0.01 nm. Can do. That is, in the optical fiber sensor 20 of the present invention, it is desirable that the optical fiber 23 serving as the temperature detection sensor unit 31 be 40 m or longer.

  As described above, the length of the optical fiber 23 serving as the temperature detection sensor unit 31 is determined by the optical fiber position measurement resolution of the measuring instrument. More specifically, the higher the optical fiber position measurement resolution of the measuring instrument, the shorter the required optical fiber length. Since the OFDR type measuring instrument has a very high fiber position measuring resolution of 0.24 mm, it is very effective as a means for measuring the optical fiber sensor 20 of the present invention.

In the first embodiment of the present invention, an example is shown in which one optical fiber sensor (temperature strain sensor) 20 is connected to an OFDR measuring instrument. However, an OFDR measuring instrument is provided by connecting the optical fiber sensor 20 in series. A plurality of optical fiber sensors 20 may be connected.
In such a configuration, when the optical fiber sensor 20 connected to the OFDR measuring instrument is the first optical fiber sensor, and the optical fiber sensor connected to the first optical fiber sensor is the second optical fiber sensor, In the second optical fiber sensor, it is desirable to connect the connection point connected to the FBG 22 serving as a strain detection sensor to the reflection point of the end of the temperature detection optical fiber 23 in the first optical fiber sensor 20. .
That is, in the optical fiber sensor 20 in the first embodiment of the present invention, the first optical fiber sensor and the second optical fiber sensor are connected by the FC / PC connector. The reflectance of this FC / PC connector connection point is 0.01%, and can be measured as a reflection point with an OFDR measuring instrument. That is, it is possible to measure a temperature change in the first optical fiber sensor 20 even if the second optical fiber sensor is connected. Thus, according to the optical fiber sensor and the measurement method of the present invention, it is possible to simultaneously measure a plurality of optical fiber sensors.

Since the optical fiber sensor of the present embodiment has a plurality of optical fiber sensors connected in series using a connector, it is easy to sense temperature and strain across multiple points. In addition, the number of points to be sensed can be easily adjusted by attaching / detaching the connector.
The optical fiber sensor according to the present invention described above is the same as the technique described in Patent Document 1 in that the measurement is performed by the OFDR method, but the temperature change is not measured from the shift amount of the Bragg wavelength of the FBG. It can be said that the invention differs from the technique described in Patent Document 1 in that the temperature change is measured from the change in the optical path length of the fiber.
Moreover, although the point which measures a physical quantity using the reflection part formed in the optical fiber end surface is equivalent to the technique of patent document 2, in the technique of patent document 2, distortion (passage of a person) is carried out. Since these are means for qualitative measurement and the present invention is a means for quantitatively measuring temperature changes, these are different inventions. This is clear even in view of each measurement method already described.

FIG. 1 is a block diagram showing the basic structure of an optical fiber sensor according to the present invention. FIG. 2 is a configuration diagram illustrating a first embodiment of an optical fiber sensor device including the optical fiber sensor illustrated in FIG. 1. FIG. 3 is a configuration diagram showing a state where the optical fiber sensor shown in FIG. 1 is mounted on a planar body. FIG. 4 is a block diagram showing a first embodiment of an optical fiber sensor device including the optical fiber sensor shown in FIG. FIG. 5 is a diagram showing a spectrogram of analysis results at fiber positions 1.9 to 2.1 m obtained in the example. FIG. 6 is a diagram illustrating a spectrogram of an analysis result obtained at the fiber positions of 42.0 to 42.2 m obtained in the example. FIG. 7 is a diagram illustrating a relationship when an optical path length change with respect to a temperature change of the optical fiber used in the example is regarded as an optical fiber length change. FIG. 8 is a diagram showing a Bragg wavelength shift characteristic with respect to a temperature change of the FBG formed in the optical fiber used in the example. FIG. 9 is a diagram showing a shift characteristic of the Bragg wavelength with respect to a change in strain of the FBG formed in the optical fiber used in the example.

Explanation of symbols

  S, 20 ... Optical fiber sensor, 1, 22 ... Fiber Bragg grating (FBG), 3, 30 ... Sensor unit for strain detection, 5, 23 ... Optical fiber, 6, 26 ... Reflection unit, 7, 31 ... Temperature detection Sensor unit 10, 40 ... optical fiber sensor device 11, 41, 42, 43 ... optical coupler, 12, 44 ... tunable laser (wavelength variable light source), 13, 45, 46 ... photodiode (receiver) , 16, R3 ... Reflection end for reference, 14, 15, 17, 18, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60 ... Optical fiber, 21 ... Metal thin plate, 25, 26 ... FC / PC connector, 27 ... loose tube, 33 ... polyimide resin adhesive layer, 33a ... adhesive layer.

Claims (10)

Using the fiber Bragg grating formed in the core of the optical fiber as a sensor, the position of the sensor is specified from the periodic change in the interference intensity of the Bragg reflected light from the sensor and the reflected light from the reference reflection end, and An optical fiber sensor device applied to an optical frequency domain reflection measurement method for measuring distortion of a sensor unit from a shift amount of a wavelength of Bragg reflected light from a sensor,
A sensor unit for strain detection made of a fiber Bragg grating formed in the core of the optical fiber, and a temperature detection optical fiber made of a temperature detection optical fiber connected to the sensor unit for strain detection and provided with a reflection part at the end. An optical fiber sensor comprising a sensor unit, a reference reflection end, a light source, and a light receiver,
When measuring the temperature change from the change in the fiber length of the optical fiber serving as the temperature detecting sensor part, the reflected light from the reflecting part provided on the end face of the optical fiber serving as the temperature detecting sensor part and the reference using a periodic variation of the interference intensity of the reflected light from the reflecting end, the identify the reflection portion position of the optical fiber end face serving as a sensor unit for temperature detection, to measure the optical path length of the optical fiber, this The temperature change is measured from the change amount of the optical path length, and the strain is measured by subtracting the shift amount of the Bragg wavelength corresponding to the measured temperature change from the shift amount of the Bragg wavelength of the fiber Bragg grating. An optical fiber sensor device.  The optical fiber sensor device according to claim 1, wherein a fiber length of the optical fiber serving as the temperature detecting sensor unit is 40 m or more.  The optical fiber sensor device according to claim 1, wherein an optical fiber serving as the temperature detection sensor unit is loosely inserted into a tube.  4. The fiber Bragg grating as the strain detection sensor part and one or both ends of an optical fiber as the temperature detection sensor part are attached to a planar body. 2. An optical fiber sensor device according to item 1.  The optical fiber sensor device according to claim 4, wherein the planar body is a thin metal plate.  One end portion or both end portions of the fiber Bragg grating serving as the strain detection sensor portion and one end portion or both end portions of the optical fiber serving as the temperature detection sensor portion are fixed to the planar body by an adhesive layer. The optical fiber sensor device according to claim 4 or 5, wherein:  A plurality of optical fiber sensors including the strain detecting sensor unit and the temperature detecting sensor unit are connected via a connector, and the connected optical fiber sensors individually measure strain and temperature. The optical fiber sensor device according to any one of claims 1 to 6.  Based on the measurement result of the optical path length of the optical fiber, the temperature change amount is measured from the change in the optical path length of the optical fiber serving as the temperature detection sensor, and the Bragg wavelength of the fiber Bragg grating serving as the strain detection sensor is measured. A control device for measuring strain by subtracting the shift amount of the Bragg wavelength corresponding to the temperature change measured by the optical fiber serving as a temperature detection sensor from the shift amount is provided in a state of being connected to the light receiver and the light source. The optical fiber sensor device according to claim 1, wherein the optical fiber sensor device is an optical fiber sensor device.  A fiber that uses the optical fiber sensor device according to any one of claims 1 to 8 to measure a temperature change from a change amount of a fiber length of the optical fiber that becomes the temperature detection sensor unit, and that becomes a strain detection sensor A temperature and strain measuring method, comprising: subtracting a shift amount of a Bragg reflection wavelength due to a temperature change measured by a temperature detection sensor unit from a shift amount of a Bragg reflection wavelength of a Bragg grating to measure strain. Using the fiber Bragg grating formed in the core of the optical fiber as a sensor, the position of the sensor is specified from the periodic change in the interference intensity of the Bragg reflected light from the sensor and the reflected light from the reference reflection end, and An optical fiber sensor applied to an optical frequency domain reflection measurement method for measuring a strain of a sensor unit from a change in wavelength of Bragg reflected light from a sensor,
A sensor unit for strain detection made of a fiber Bragg grating formed in the core of the optical fiber, and a temperature detection optical fiber made of a temperature detection optical fiber connected to the sensor unit for strain detection and provided with a reflection part at the end. The sensor part is provided,
When measuring the temperature change from the change in the fiber length of the optical fiber serving as the temperature detecting sensor part, the reflected light from the reflecting part provided on the end face of the optical fiber serving as the temperature detecting sensor part and the reference using a periodic variation of the interference intensity of the reflected light from the reflecting end, the identify the reflection portion position of the optical fiber end face serving as a sensor unit for temperature detection, to measure the optical path length of the optical fiber, this The temperature change is measured from the change amount of the optical path length, and the strain is measured by subtracting the shift amount of the Bragg wavelength corresponding to the measured temperature change from the shift amount of the Bragg wavelength of the fiber Bragg grating. Optical fiber sensor.

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