Regenerated phase shift fiber bragg grating, manufacture intermediate and manufacturing method of the same

Abstract

To provide a regenerated phase shift fiber Bragg grating having target characteristics, a manufacturing method of the same, and a manufacturing intermediate from which the regenerated phase shift fiber Bragg grating is obtained.SOLUTION: A regenerated phase shift FBG is manufactured by annealing a grating body 31. In the grating body 31, a grating region 31a is formed in a core 22a of an optical fiber 22. The grating region 31a periodically modulates a refractive index of the core 22a in its axial direction, and half-cycle shifts the phase of refractive index modulation at the center. In order to satisfy features of a target regenerated phase shift FBG, a grating length L in an axial direction and a refractive index amplitude of the grating region 31a are specified, and a grating body 31 having the specified grating length L and the refractive index amplitude is manufactured.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a regenerative phase-shifted fiber Bragg grating, its manufacturing intermediate and manufacturing method. [Background technology]

[0002] Fiber Bragg gratings (hereinafter referred to as FBGs) are known in which a grating region with periodic refractive index modulation is formed in the core of an optical fiber. When broadband light is incident on an optical fiber, an FBG reflects only light in a narrow wavelength range centered on the Bragg wavelength specific to the grating region, and transmits light components of other wavelengths. Strain (expansion and contraction) occurring in the optical fiber changes the period of the refractive index modulation in the grating region, shifting the Bragg wavelength. Taking advantage of this property, FBGs are often used as sensors. For example, they are also used as sensors for acoustic emission (AE) measurement to detect elastic waves (sound waves and ultrasonic waves) generated when the inside of a structural member is deformed or destroyed.

[0003] Phase-shifted FBGs are also known, in which the phase of the refractive index modulation is shifted by a half period at the center of the grating region. Phase-shifted FBGs have a sharp spectral dip centered at the center of the reflection spectrum, i.e., the Bragg wavelength. This phase-shifted FBG is used as a sensor with higher sensitivity than the above-mentioned FBGs, taking advantage of the fact that this spectral dip is shifted by strain generated in the optical fiber.

[0004] When fabricating FBGs and phase-shifted FBGs, for example, optical fibers made of silica-based glass are used, and a light-induced refractive index change is induced in a core doped with, for example, Ge (germanium) to increase the refractive index by irradiating it with ultraviolet light, and the refractive index of the core is periodically changed in the axial direction.

[0005] A regenerated phase-shifted FBG is known in which a phase-shifted FBG is annealed at high temperature (see, for example, Patent Document 1 and Non-Patent Document 1). When the phase-shifted FBG fabricated as described above is heated to high temperature (for example, 900°C or higher), the phase-shifted FBG properties are lost, but when the high temperature is maintained for a certain period of time, the grating region is regenerated. The regenerated phase-shifted FBG is thus regenerated after the properties as a phase-shifted FBG are once lost at high temperature. The regenerated phase-shifted FBG does not lose its properties as a phase-shifted FBG even if it is cooled and then heated again to, for example, 900°C, and therefore can be used as a sensor with excellent heat resistance. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2013-142667 A [Non-patent literature]

[0007] [Non-Patent Document 1] Feng-ming Yu, Yoji Okabe "Regenerated Fiber Bragg Grating Sensing System for Ultrasonic Detection in a 900℃ Environment" ASME J Nondestructive Evaluation. Feb 2019, 2(1): 011006-011006-8 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the regenerative phase-shifted FBG has problems such as a significant change in characteristics from the original phase-shifted FBG, that is, a change in the reflection spectrum and a significant decrease in sensitivity.

[0009] Therefore, an object of the present invention is to provide a regenerated phase-shifted fiber Bragg grating having desired characteristics, a manufacturing method thereof, and a manufacturing intermediate from which the regenerated phase-shifted fiber Bragg grating can be obtained. [Means for solving the problem]

[0010] The method for producing a regenerative phase-shifted fiber Bragg grating of the present invention includes the following steps: a specifying step of specifying, from the characteristics of a regenerative phase-shifted fiber Bragg grating, a grating length and a refractive index amplitude of a grating region that satisfies the characteristics of a regenerative phase-shifted fiber Bragg grating, for a grating body that is the basis of a desired regenerative phase-shifted fiber Bragg grating and has a periodic refractive index modulation in the core of an optical fiber and a grating region having a half-period phase shift in the center of the refractive index modulation; a grating body fabricating step of fabricating a grating body by changing the refractive index of the core of the optical fiber by a light-induced refractive index change to form a grating region having the grating length and the refractive index amplitude specified in the specifying step; and an annealing step of annealing the grating body to eliminate the refractive index modulation in the grating region and then regenerating it to form a regenerative phase-shifted fiber Bragg grating.

[0011] The intermediate manufacturing product of the regenerative phase-shifted fiber Bragg grating of the present invention is an intermediate manufacturing product that is turned into a regenerative phase-shifted fiber Bragg grating by annealing, in which the core of the optical fiber has a periodic refractive index modulation and a grating region with a half-period phase shift in the center of the refractive index modulation, and the grating length of the grating region is at most 17 mm.

[0012] The intermediate product of the regenerative phase-shifted fiber Bragg grating of the present invention is an intermediate product that is annealed to become a regenerative phase-shifted fiber Bragg grating, in which the core of the optical fiber has a periodic refractive index modulation, and a grating region with a half-period phase shift in the center of the refractive index modulation, and the grating length of the grating region is at least 5 mm.

[0013] The intermediate product of the regenerated phase-shifted fiber Bragg grating of the present invention is a regenerated phase-shifted fiber Bragg grating that is annealed. The intermediate product has a core of an optical fiber having a periodic refractive index modulation and a grating region having a half-period phase shift in the center of the refractive index modulation. The refractive index amplitude of the grating region is at most 15×10 -4 This is what we have decided.

[0014] The regenerated phase-shifted fiber Bragg grating of the present invention is obtained by removing the refractive index modulation of the intermediate product by annealing and then recovering it.

[0015] The regenerative phase-shifted fiber Bragg grating of the present invention is formed in the core of an optical fiber, and has a grating region having a periodic refractive index modulation, a half-period phase shift in the center of the refractive index modulation, and the refractive index modulation does not disappear at least in an environment of 900°C. The grating region has a grating length of 5 mm or more, and the refractive index amplitude of the grating region is 2.5 × 10 -4 The following is the result. Effect of the Invention

[0016] According to the present invention, a regenerative phase-shifted fiber Bragg grating having the desired characteristics can be obtained. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is an explanatory diagram showing a configuration of an acoustic emission measuring device according to an embodiment. [Diagram 2] FIG. 2 is an explanatory diagram showing a regenerative phase-shifted fiber Bragg grating. [Diagram 3] FIG. 2 is an explanatory diagram illustrating a typical reflection spectrum of a regenerative phase-shifted fiber Bragg grating. [Figure 4] FIG. 2 is an explanatory diagram showing a grating body that is the basis of a regenerative phase-shifted fiber Bragg grating. [Diagram 5] 1 is a graph showing the refractive index in a grating region of a grating body. [Figure 6] 1 is a graph showing a spectral dip obtained by a numerical simulation of a regenerative phase-shifted fiber Bragg grating having different refractive index amplitudes. [Figure 7] 1 is a flow chart showing a procedure for fabricating a regenerative phase-shifted fiber Bragg grating. [Figure 8] FIG. 1 illustrates a regenerative phase-shifted fiber Bragg grating modeled as being divided into multiple segments for numerical simulation. [Figure 9] 1 is a graph showing examples of the reflection spectra of a phase-shifted fiber Bragg grating that was numerically simulated and actually fabricated. [Figure 10] 13 is a graph showing reference data showing the relationship between the grating length and the refractive index amplitude and the maximum reflectance and the gradient of the gradient region obtained by a numerical simulation. [Figure 11] 1 is a graph showing a reflection spectrum by a numerical simulation when the grating length and the refractive index amplitude of a phase-shifted fiber grating are changed. [Figure 12] 1 is a graph showing the reflection spectra at room temperature of samples of grating bodies having different grating lengths and refractive index amplitudes, and samples of regenerated phase-shifted fiber Bragg gratings made from those grating bodies. [Figure 13] 1 is a graph showing an enlarged view of a spectral dip. [Figure 14]1 is a graph showing a detection signal obtained when a chirped ultrasonic wave is propagated through a sample of a regenerative phase-shifted fiber Bragg grating. [Figure 15] 1 is a graph showing a frequency spectrum of a detection signal obtained when a chirp ultrasonic wave is propagated through a sample of a regenerative phase-shifted fiber Bragg grating. [Figure 16] 13 is a graph showing the reflection spectrum, the detection signal, and the frequency spectrum of the detection signal measured by changing the temperature of one sample of a regenerative phase-shifted fiber Bragg grating. [Figure 17] 1 is a graph showing the signal-to-noise ratios of two regenerated phase-shifted fiber Bragg gratings fabricated from grating bodies with grating lengths of 5 mm and 10 mm. [Figure 18] 1 is a graph showing the change in the reflection spectrum with respect to the temperature change of two regenerative phase-shifted fiber Bragg gratings made from grating bodies having grating lengths of 10 mm and 20 mm. [Figure 19] 1 is a graph showing the reflection spectra at room temperature of samples of grating bodies having grating lengths of 14 mm and 17 mm and samples of regenerated phase-shifted fiber Bragg gratings fabricated from those grating bodies. [Figure 20] 1 is a graph showing the change in the reflection spectrum with respect to the temperature change of a regenerated phase-shifted fiber Bragg grating made from a grating body having a grating length of 14 mm. [Figure 21] 1 is a graph showing an enlarged view of the reflection spectra at 900° C. and 100° C. of a regenerated phase-shifted fiber Bragg grating made from a grating body having a grating length of 14 mm. [Figure 22] 1 is a graph showing the change in the reflection spectrum with respect to the temperature change of a regenerated phase-shifted fiber Bragg grating made from a grating body having a grating length of 17 mm. [Diagram 23]1 is a graph showing an enlarged view of the reflection spectra at 900° C. and 100° C. of a regenerated phase-shifted fiber Bragg grating made from a grating body having a grating length of 17 mm. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] In FIG. 1, an AE (acoustic emission) measuring device 10 detects elastic waves (sound waves or ultrasonic waves) generated when a specimen 11 is deformed or destroyed using a regenerated phase-shifted fiber-optic Bragg grating (hereinafter referred to as a regenerated phase-shifted FBG) 12 as a sensor.

[0019] The AE measurement device 10 is composed of a laser light source 14, a circulator 15, a photodetector 16, optical fibers 21 to 23, etc. in addition to the regenerative phase-shifted FBG 12. One end of the optical fiber 21 is connected to the laser light source 14 and the other end is connected to the circulator 15, and one end of the optical fiber 22 is connected to the circulator 15. The regenerative phase-shifted FBG 12 is formed as a fiber sensor in this optical fiber 22. One end of the optical fiber 23 is connected to the circulator 15 and the other end is connected to the photodetector 16.

[0020] Laser light from a laser light source 14 serving as a light source is sent to the circulator 15 via an optical fiber 21. The circulator 15 sends the laser light incident via the optical fiber 21 to an optical fiber 22, and makes the laser light incident on the regenerated phase-shifted FBG 12. The circulator 15 also sends the reflected light from the regenerated phase-shifted FBG 12 returning from the optical fiber 22 to the photodetector 16 via the optical fiber 23. The reflected light from the regenerated phase-shifted FBG 12 is the laser light reflected by the regenerated phase-shifted FBG 12, and the light intensity is modulated by inputting an elastic wave to the regenerated phase-shifted FBG 12.

[0021] The laser light source 14 used is one that outputs laser light having a wavelength within the wavelength range of the spectral dip of the regenerated phase-shifted FBG 12 (more specifically, the wavelength range of the linearly inclined region of the spectral dip). The spectral dip of the regenerated phase-shifted FBG 12, which will be described in detail later, is a portion in the reflection spectrum where the reflectance drops to a minimum at the Bragg wavelength of the regenerated phase-shifted FBG 12. In this example, a tunable laser is used as the laser light source 14, so that the wavelength of the laser light can be adjusted or regulated in accordance with the spectral dip of the regenerated phase-shifted FBG 12. Note that the light source is not particularly limited as long as it outputs light of a single wavelength.

[0022] By using a tunable laser as the laser light source 14, the wavelength of the laser light can be increased or decreased to match the spectral dip of the regenerative phase-shifted FBG 12, which shifts due to changes in temperature, i.e., the wavelength range of the changing spectral dip, making it possible to set the wavelength of the laser light optimal for the spectral dip.

[0023] A part of the optical fiber 22 is fixed to the specimen 11 using, for example, a heat-resistant adhesive 26. The regenerated phase-shifted FBG 12 is formed in the vicinity of the fixed part of the optical fiber 22 to the specimen 11. That is, the vicinity of the part of the optical fiber 22 where the regenerated phase-shifted FBG 12 is formed is fixed to the specimen 11. An elastic wave generated in the specimen 11 is transmitted to the optical fiber 22 via the fixed part to the specimen 11, and the elastic wave causes axial distortion (expansion and contraction) in the part of the optical fiber 22 where the regenerated phase-shifted FBG 12 is formed. The distortion of the optical fiber 22 shifts the spectrum dip together with the Bragg wavelength of the regenerated phase-shifted FBG 12. The part of the optical fiber 22 where the regenerated phase-shifted FBG 12 is placed in a high-temperature environment together with the specimen 11, or the part of the regenerated phase-shifted FBG 12 becomes hot due to heat from the specimen 11. The optical fiber 22 in this example is formed of quartz glass, and the regenerated phase-shifted FBG 12 can be used in an environment up to the heat-resistant temperature (1000° C.) of the optical fiber 22.

[0024] The photodetector 16 converts the light incident from the optical fiber 22, i.e., the reflected light from the regenerated phase-shifted FBG 12, into an electrical detection signal according to the light intensity and outputs it. Note that a balanced photodetector may be used as the photodetector 16 to differentially detect the reflected light and transmitted light from the regenerated phase-shifted FBG 12. Deformation, destruction, etc. of the specimen 11 is determined based on the detection signal from the photodetector 16.

[0025] 2, the optical fiber 22 is the same as a single-mode optical fiber made of general silica-based glass, and has a layer structure consisting of a core 22a and a cladding 22b that surrounds the core 22a and has a lower refractive index than the core 22a, and has an outer diameter of, for example, 125 μm and a diameter of the core 22a of about 10 μm. As described above, the regenerative phase-shifted FBG 12 is formed in a part of the optical fiber 22. Note that the optical fiber 22 is not limited to being made of silica-based glass as long as the fiber Bragg grating is regenerated by annealing as described in detail later, and may be, for example, a sapphire optical fiber.

[0026] The regenerative phase-shift FBG 12 has a grating region 12a formed in a core 22a. The grating region 12a is a region in which the refractive index in the axial direction (light propagation direction) of the core 22a is periodically modulated and the phase of the refractive index modulation is shifted by a half period at the center. As will be described in detail later, the regenerative phase-shift FBG 12 is made of a grating body 31.

[0027] FIG. 3 shows a schematic diagram of the reflection spectrum of the regenerative phase-shift FBG 12. The reflection spectrum of the regenerative phase-shift FBG 12 is B There is a spectral dip centered at the Bragg wavelength λ BThis is a region where the reflectance drops between a pair of maximum reflectance peaks centered approximately at . When an elastic wave acts on the regenerated phase-shifted FBG 12 and generates axial distortion in the regenerated phase-shifted FBG 12 portion of the optical fiber 22, the modulation period of the refractive index in the axial direction of the core 22a in the grating region 12a changes. As a result, the reflection spectrum shifts in accordance with the direction and amount of distortion of the core 22a, i.e., the Bragg wavelength λ B The position of the spectral dip moves along with the change in the reflection spectrum. Note that such a shift in the reflection spectrum also occurs due to a change in the temperature of the regenerative phase-shift FBG 12 (optical fiber 22).

[0028] The Bragg wavelength λ of the phase-shifted FBG including the regenerative phase-shifted FBG 12 is B is expressed as "λ" when the period of the refractive index modulation of the core in the grating region (grating period) is n and the effective refractive index is D. B =2nD".

[0029] In the measurement, the wavelength of the laser light is matched to a wavelength corresponding to, for example, the middle position of the slope region H of the spectral dip. The slope region H of the spectral dip is a region where the reflectance increases or decreases monotonically with increasing wavelength, and in order to easily relate the light intensity of the reflected light from the regenerated phase-shifted FBG 12 to the magnitude of the elastic wave, it is preferably set as a region where the reflectance changes almost linearly with respect to the change in wavelength, and more preferably set as a region where the reflectance changes linearly. In the example of Fig. 3, the region where the reflectance decreases linearly with increasing wavelength is set as the slope region H.

[0030] When performing measurement in the AE measurement device 10, the laser light source 14 is adjusted so that the wavelength of the laser light matches a wavelength corresponding to, for example, the middle position of the slope region H of the spectral dip in the reflection spectrum at the temperature at which the measurement is performed. In the measurement, the change in reflectance of the regenerated phase-shifted FBG 12 with respect to the laser wavelength is utilized.

[0031] When an elastic wave generated in the specimen 11 is transmitted to the optical fiber 22, the elastic wave generates an axial distortion in the regenerative phase-shifted FBG 12. This distortion causes the Bragg wavelength λ to change in a shift direction and shift amount according to the direction and amount of distortion, as described above. B The reflection spectrum shifts with the change in the angle of incidence of the reconstructed phase-shifted FBG 12. As a result, the inclined region H shifts with respect to the laser light with a fixed wavelength, and the light intensity of the laser light reflected by the reconstructed phase-shifted FBG 12, i.e., the reflected light from the reconstructed phase-shifted FBG 12, increases or decreases depending on the direction and amount of the shift. That is, the reflected light is intensity-modulated according to the vibration of the elastic wave. The reflected light from the reconstructed phase-shifted FBG 12 is input to the photodetector 16 via the optical fiber 22 and the circulator 15. The reflected light is then converted into a detection signal by the photodetector 16. This provides a detection signal according to the elastic wave transmitted to the optical fiber 22.

[0032] The sensitivity of the regenerative phase shift FBG 12 is the change in reflectance relative to the shift in the reflection spectrum, so the sensitivity of the regenerative phase shift FBG 12 depends on the gradient G of the gradient region H (see FIG. 3). Therefore, the smaller the gradient G of the gradient region H (the gentler the gradient region H), the lower the sensitivity, and the larger the gradient G (the steeper the gradient region H), the higher the sensitivity. The dynamic range of the regenerative phase shift FBG 12 is the width (width of the wavelength range) of the gradient region H, and is smaller the narrower the wavelength range of the gradient region H, and is larger the wider it is. If the height of the gradient region H (the difference between the highest and lowest reflectances of the gradient region H) is the same, the larger the gradient G of the gradient region H, the smaller the dynamic range, and the smaller the gradient G, the larger the dynamic range. The gradient G and width of the gradient region H that can be effectively set depend on the shape of the spectrum dip. Therefore, the sensitivity and dynamic range of the regenerative phase shift FBG 12 depend on the shape of the spectrum dip.

[0033] The regenerative phase-shifted FBG 12 is fabricated by annealing the grating body 31. The grating body 31 is an intermediate product of the regenerative phase-shifted FBG 12. As shown in FIG. 4, the grating body 31 is fabricated by forming a grating region 31a in the core 22a of the portion of the optical fiber 22 that is to become the regenerative phase-shifted FBG 12. The grating region 31a is a region of the core 22a that has a periodic refractive index modulation (refractive index distribution) in the axial direction (Z direction) of the core 22a and has a half-period phase shift in the center of the refractive index modulation, as shown in FIG. 5. That is, in the grating region 31a, the refractive index of the core 22a is periodically modulated with a period Λ in the axial direction, and the phase of the refractive index modulation in the center of the axial direction is shifted by a half period. When the axial length of the grating region 31a is the grating length L, there is a region where the refractive index is inverted (phase shifted) at a position of length L / 2 from the end of the grating region 31a. The grating region 31a of the grating 31 is provided with a refractive index modulation in the axial direction similar to that of a phase-shifted FBG, but it does not matter if a spectral dip is not observed in the reflection spectrum of the grating 31.

[0034] The optical fiber 22 used to fabricate the grating body 31 has a core 22a doped with an additive that increases the refractive index, in this example, Ge (germanium). A photoinduced refractive index change is caused in the core 22a doped with the additive by irradiation with ultraviolet light, and the refractive index of the core 22a is changed to form the grating region 31a. For example, a phase mask method or a two-beam interference method is used to form the periodic refractive index modulation of the grating region 31a. Note that the additive is not limited to Ge as long as it causes a photoinduced refractive index change and the refractive index modulation is regenerated by annealing. Examples of such additives include P (phosphorus) and F (fluorine) in addition to Ge.

[0035] The grating length L and the refractive index amplitude dN of the grating region 31a are determined according to the desired characteristics of the regenerative phase shift FBG 12. As shown in FIG. 5, the refractive index amplitude dN is the amplitude of the refractive index modulation in the grating region 31a, i.e., half the difference between the maximum and minimum refractive indices (|(maximum refractive index)-(minimum refractive index)|). The refractive index amplitude dN can be controlled by increasing or decreasing the irradiation time of ultraviolet light when causing a photoinduced refractive index change. The refractive index amplitude dN, which will be described later, R is the value for the regenerated phase shift FBG 12 corresponding to the refractive index amplitude dN, and the actual regenerated phase shift FBG 12 refractive index amplitude dN R is determined based on experimental results, numerical simulation results, etc.

[0036] By performing the numerical simulations described below while varying various parameters, the inventors have found that in phase-shifted FBGs including the regenerative phase-shifted FBG 12, the grating length and refractive index amplitude are effective parameters in determining the shape of the reflection spectrum, particularly the shape of the spectral dip.

[0037] The grating length L of the grating region 31a in the grating body 31 does not change due to annealing. That is, the grating length L of the grating region 12a of the regenerative phase shift FBG 12 R is the same as the grating length L of the grating body 31 that is the source of it. On the other hand, the refractive index amplitude dN R Regarding the refractive index, the experimental results using the actually fabricated regenerated phase-shift FBG 12 and the results of the numerical simulation described later show that the refractive index amplitude dN of the grating body 31 on which it is based is smaller by a certain percentage. In other words, the refractive index amplitude dN of the regenerated phase-shift FBG 12 relative to the refractive index amplitude dN of the original grating body 31 is R The ratio of the refractive index amplitude ratio (hereinafter referred to as the refractive index amplitude ratio) 1 / k(=dN RThe inventors have found that the refractive index amplitude ratio 1 / k is constant. In the optical fiber 22 used in this example, that is, an optical fiber made of silica glass with Ge doped in the core, the refractive index amplitude ratio 1 / k is found to be 0.15775. Note that the refractive index amplitude dN R When the value is negative, its magnitude (absolute value) is expressed as the refractive index amplitude dN R Treat as such.

[0038] Therefore, characteristics such as sensitivity, which is the rate at which reflected light changes with respect to the distortion of the regenerated phase-shift FBG 12, and dynamic range, which is the range of measurable distortion, depend on the grating length L and refractive index amplitude dN of the grating region 31a in the original grating body 31. Also, based on the characteristics of the regenerated phase-shift FBG 12, the grating length L and refractive index amplitude dN of the grating region 31a to be formed in the original grating body 31 can be specified.

[0039] The larger the grating length L in the grating body 31 and the larger the refractive index amplitude dN, the narrower the spectral dip in the regenerated phase shift FBG 12 and the larger the gradient G of the effective gradient region H, and the higher the sensitivity can be. On the other hand, the smaller the grating length L and the smaller the refractive index amplitude dN, the wider the spectral dip and the gentler the gradient of the effective gradient region H, and the larger the dynamic range can be. In other words, even if the refractive index amplitude dN is small, by increasing the grating length L, it is possible to fabricate a regenerated phase shift FBG 12 with high sensitivity, and even if the grating length L is small, it is possible to fabricate a regenerated phase shift FBG 12 with high sensitivity by increasing the refractive index amplitude dN.

[0040] The grating length L in the grating body 31 is preferably at most 17 mm, i.e., 17 mm or less, and more preferably at most 14 mm, i.e., 14 mm or less. If the length L of the grating region 31a is 17 mm or less, in the regenerative phase shift FBG 12 produced from the grating body 31, the shape of the reflection spectrum changes with temperature change, but the change in the shape of the reflection spectrum due to temperature change in a high temperature range of 300°C or more is small, making it easier to identify the wavelength corresponding to the middle position of the slope region H of the spectrum dip, i.e., the wavelength of the laser light. Then, by adjusting the laser light source 14 so that the wavelength of the laser light matches the specified wavelength, stable measurement can be performed without being affected by temperature change.

[0041] As mentioned above, the grating length L of the regenerative phase-shifting FBG 12 R is the same as the grating length L of the original grating body 31, so the preferred grating length L in the grating body 31 is the preferred grating length L in the regenerative phase-shifted FBG 12. R On the other hand, the refractive index amplitude dN of the grating body 31 is reduced by a refractive index amplitude ratio of 1 / k, and the refractive index amplitude dN of the regenerative phase shift FBG 12 produced from the grating body 31 is R Therefore, the preferable refractive index amplitude dN in the grating body 31 is reduced by a refractive index amplitude ratio of 1 / k (0.15775 times in this example) to obtain the preferable refractive index amplitude dN in the regenerative phase shift FBG 12. R It is.

[0042] Furthermore, if the grating length L of the grating body 31 is 14 mm or less, the shape of the reflection spectrum hardly changes even with temperature changes in the regenerated phase-shift FBG 12 produced from the grating body 31, and the regenerated phase-shift FBG 12 can have stable characteristics from low to high temperatures. The regenerated phase-shift FBG 12 with stable characteristics makes it easy to adjust the wavelength of the laser light according to the temperature of the regenerated phase-shift FBG 12 during measurement, and even if the temperature changes, it is easy to control the wavelength of the laser light, and measurements can be performed with small fluctuations in sensitivity and dynamic range.

[0043] The lower limit of the grating length L is not particularly limited. Under the same refractive index amplitude dN, as the grating length L becomes smaller, the inclination of the inclined region H becomes smaller, and the sensitivity of the regenerative phase shift FBG 12 becomes lower, but the dynamic range can be increased. In order to increase the sensitivity of the regenerative phase shift FBG 12, the grating length L is preferably at least 5 mm, and more preferably at least 10 mm, in order to reduce the disturbance of the overall shape of the reflection spectrum, reduce the spread of the spectrum dip, and ensure an improvement in sensitivity. In other words, the grating length L is preferably 5 mm or more, and more preferably 10 mm or more.

[0044] From the above, in order to increase the sensitivity of the regenerative phase-shifted FBG 12 for detecting weak elastic waves excited by fatigue damage in the specimen 11, the grating lengths L and L R is preferably in the range of 5 mm or more and 17 mm or less, more preferably in the range of 5 mm or more and 14 mm or less or in the range of 10 mm or more and 17 mm or less, and particularly preferably in the range of 10 mm or more and 14 mm or less.

[0045] Even if the grating length L of the grating body 31 is small, by increasing the refractive index amplitude dN, it is possible to fabricate a regenerative phase-shift FBG 12 with improved sensitivity. RLet be 5 mm, and several refractive index amplitudes dN R The spectral dip of the regenerative phase-shifted FBG 12 obtained by the numerical simulation described later is shown in Fig. 1. Theoretically, the refractive index amplitude dN R It can be seen that by increasing the slope G of the inclined region H, the sensitivity of the reproducing phase-shift FBG 12 can be improved.

[0046] On the other hand, the inventors have found through research that a regenerative phase-shifted FBG 12 made from a grating 31 with an excessively large refractive index amplitude dN tends to have a distorted overall shape of the reflection spectrum, with the width of the spectrum dip being wide, i.e., the gradient G of the gradient region H being small. In addition, when the refractive index amplitude dN of the grating 31 is 15e-4 (=15×10 -4 , hereinafter the same) or less, the measured value of the reflection spectrum of the regenerated phase shift FBG 12 produced by annealing the grating body 31 will be almost the same as the result of a numerical simulation of the reflection spectrum. In this example, the refractive index amplitude dN of the regenerated phase shift FBG 12 produced from the grating body 31 with a refractive index amplitude dN of 15e-4 will be R is 2.5e-4 (=(15e-4) 1 / k, 1 / k = 0.15775).

[0047] Since the actual measurement value and the result of the numerical simulation are almost the same, the refractive index amplitude dN for making the regenerative phase shift FBG 12 have the desired characteristics can be calculated using reference data created from the result of the numerical simulation, as will be described later in detail. R and grating length L R Based on these, the refractive index amplitude dN and the grating length L can be easily specified. That is, the refractive index amplitude dN required to obtain the desired characteristics can be calculated. R and grating length L R can be determined from the reference data with a small error, and the final refractive index amplitude dN R and grating length L RBased on these, the number of tests or the like for determining the refractive index amplitude dN and the grating length L can be reduced.

[0048] If the grating length L is 5 mm or more, the regenerated phase-shifted FBG 12 produced from the grating body 31 having a refractive index amplitude dN of 15e-4 or less has a greater slope of the spectral dip and a higher sensitivity than a regenerated phase-shifted FBG produced by annealing a general phase-shifted FBG as shown in Non-Patent Document 1, etc.

[0049] As mentioned above, the grating length L (= L R It is useful and preferable to set the distance dN to 5 mm or more in order to easily specify the refractive index amplitude dN and the grating length L in order to obtain a highly sensitive regenerative phase shift FBG 12. R A regenerative phase-shift FBG 12 having a width of 5 mm or more is preferable in terms of reducing the disturbance in the overall shape of the reflection spectrum and improving the sensitivity.

[0050] If the grating length L is 10 mm or more, the sensitivity can be improved by the size of the grating length L, so that it is easy to obtain a gradient region H having a gradient equal to or close to the spectrum dip of a general phase-shifted FBG or a gradient larger than that without being substantially limited by the refractive index amplitude dN, which is useful for increasing the sensitivity of the regenerative phase-shifted FBG 12. Of course, in such a case, it is preferable that the refractive index amplitude dN is 15e-4 or less.

[0051] The period of the refractive index modulation of the grating region 31a may be appropriately set according to the desired Bragg wavelength, i.e., the wavelength of the spectrum. In this example, the Bragg wavelength of the regenerative phase shift FBG 12 is shifted to the short wavelength side from the Bragg wavelength of the grating body 31.

[0052] The inventors have confirmed that the relationship between the grating length L and refractive index amplitude dN of the grating region 31a and the reflection spectrum of the regenerated phase-shifted FBG 12 can be grasped from theoretical numerical analysis and experimental observation of the reflection spectrum of the regenerated phase-shifted FBG 12. Therefore, based on the grasped relationship, it is possible to specify the optimal grating length L and refractive index amplitude dN of the grating region 31a in the grating body 31 that is the basis for obtaining the desired characteristics of the regenerated phase-shifted FBG 12 that is tailored to the measurement target.

[0053] 7 shows an example of a procedure for fabricating the regenerative phase-shifted FBG 12. First, in a specification step, the grating length L and the refractive index amplitude dN of the grating body 31 that is the source of the regenerative phase-shifted FBG 12 are specified by numerical analysis based on the characteristics of the desired regenerative phase-shifted FBG 12 (step ST1). This specification step will be described in detail later.

[0054] Next, in a grating body fabrication process, a grating region 31a having the specified grating length L and refractive index amplitude dN is formed in the core 22a of the optical fiber 22 by using, for example, a phase mask method or a two-beam interference method as described above, to fabricate the grating body 31 (step ST2). Subsequently, in an annealing process, the fabricated grating body 31 is annealed (step ST3).

[0055] In the annealing step, the grating body 31 is heated and maintained at a trigger temperature that causes the regeneration phenomenon, which is 920°C in this example. For example, the grating body 31 is heated to the trigger temperature at a constant heating rate (e.g., 10°C / min), and then the trigger temperature is maintained for a predetermined time. As a result, the refractive index modulation of the grating body 31 disappears, and then the refractive index modulation is regenerated. That is, the grating region 31a changes to the grating region 12a, and the grating body 31 becomes the regenerated phase-shifted FBG 12.

[0056] When the reflection spectrum of the optical fiber 22 is measured during annealing, the peak reflectance begins to decrease when the temperature of the optical fiber 22 exceeds, for example, 600°C, and the reflection spectrum completely disappears at the trigger temperature (920°C in this example), and the appearance of the reflection spectrum can be confirmed by maintaining the trigger temperature for, for example, about 30 minutes. The regenerated phase-shifted FBG 12 obtained as described above does not lose its reflection spectrum even if it is cooled to room temperature and then heated again to 900°C or higher.

[0057] The details of the determination step will be described below. In this example, the determination step includes a characteristic calculation step of determining the characteristics of the target regenerative phase shift FBG 12 using a computer, and a grating length L, which is a parameter of the regenerative phase shift FBG 12, from the characteristics of the regenerative phase shift FBG 12. R , the refractive index amplitude dN R A parameter specification step for specifying the grating length L R , the refractive index amplitude dN R into the grating length L of the grating body 31 and the refractive index amplitude dN.

[0058] [Characteristic calculation process] In the characteristic calculation process, the slope G of the slope region H and the maximum reflectance R of the spectrum dip are calculated as the characteristics of the target regenerated phase shift FBG 12 from the characteristics required for the AE measurement device 10, which is a measurement system using the regenerated phase shift FBG 12. max That is, the required characteristics of the regenerative phase shift FBG 12 are the gradient G of the gradient region H and the maximum reflectance R of the spectrum dip. max The characteristics required for the AE measurement device 10 are the maximum measurable distortion (dynamic range) ε max , the minimum measurable distortion ε min , Signal to Noise Ratio [S / N] dB The minimum measurable distortion ε min and signal-to-noise ratio [S / N] dB defines the sensitivity of the AE measurement device 10. Note that the gradient G of the gradient region H and the maximum reflectance R of the spectrum dip maxIf it is possible to specify the characteristic, the characteristic calculation step is not necessary.

[0059] As shown in Figure 3, the maximum reflectance R max is the Bragg wavelength λ of the regenerative phase shifter FBG12 B The reflectance is the maximum (local maximum) on both sides of the spectral dip centered on . The slope G is the ratio of the increase in reflectance to the increase in wavelength in the slope region H of the spectral dip. The regenerative phase-shifting FBG 12 shifts the reflection spectrum, i.e., the Bragg wavelength λ, by expanding and contracting in its axial direction, i.e., by strain ε. B The reflectance R changes with the change in max is the Bragg wavelength λ B is the maximum distortion where the reflectance R changes monotonically with respect to the change in min is the Bragg wavelength λ B This is the minimum distortion for which the reflectance R changes monotonically with respect to the change in

[0060] Maximum strain ε max , minimum distortion ε min is the maximum reflectance R max , slope G and signal-to-noise ratio [S / N] dB and the noise level V N a coefficient a indicating the rate at which the reflectance R increases due to an increase in the strain ε, the power P of the laser light incident on the regenerative phase-shift FBG 12, and the response function R of the photodetector 16. PD Using this formula, the maximum reflectance R max and the slope G can be expressed as in equation (B).

[0061]

number

[0062] The strain ε of the regenerative phase-shifted FBG12 and the Bragg wavelength λ due to the strain ε BThe change amount Δλ in is expressed as in formula (1) using a predetermined coefficient a. In addition, in the initial state where no distortion is applied to the regenerative phase shift FBG 12, when the wavelength of the laser light is adjusted to match the wavelength corresponding to the middle position of the tilted region H, the distortion ε (ε min ≦ε≦ε max The reflectance R when a Bragg wavelength λ is applied to the regenerated phase shift FBG 12 is expressed as in formula (2). The second term on the right side of formula (2) is the reflectance R of the laser light in the regenerated phase shift FBG 12 in the initial state, i.e., the reflectance at the middle position of the inclined region H (hereinafter referred to as the initial reflectance), and the first term on the right side is the reflectance R of the laser light in the regenerated phase shift FBG 12 in the initial state, i.e., the reflectance at the middle position of the inclined region H (hereinafter referred to as the initial reflectance). B 14 shows the change in reflectance when the reflectance is changed by an amount Δλ.

[0063]

number

[0064] The maximum reflectance in the slope region H is R Hmax Then, the maximum strain ε max is expressed as Equation (3). The maximum reflectance R in the gradient region H is Hmax is the maximum reflectance R Hmax Therefore, the maximum strain ε max is the maximum reflectance R max Therefore, the maximum strain ε max is expressed as formula (A).

[0065]

number

[0066] Signal to noise ratio [S / N] dB is the detection signal when the laser light reflected by the regenerated phase-shifted FBG 12 is detected by the photodetector 16. S When the noise level V N The detection signal V S , noise level V NThe detection signal V S is the Bragg wavelength λ depending on the strain ε B is a change in the signal output from the photodetector 16 when the change in the laser beam amount Δλ is changed. This is determined by the gradient G of the gradient region H, the power P of the laser beam, and the response function R of the photodetector 16. PD Using the above, it can be expressed as equation (6). In addition, the noise level V N is the noise level of the entire AE measurement device 10. In this example, as shown in equation (7), the noise V OPT , the noise in the photodetector 16 V PD , the environmental noise V ENV From these equations (5) and (6), the minimum strain ε min can be expressed as the above formula (A).

[0067]

number

[0068] As described above, in the characteristic calculation process, the slope G of the slope region H and the maximum reflectance R of the spectrum dip are calculated as the characteristics of the target regenerative phase shift FBG 12 based on the formula (B) from the characteristics required for the AE measurement device 10. max Request.

[0069] [Parameter identification process] In the parameter specification process, the slope G and maximum reflectance R of the slope region H obtained in the characteristic calculation process are max Using the reference data described later, the slope G and maximum reflectance R of the slope region H are calculated. max The grating length L suitable for obtaining R is determined by numerical analysis. The reference data is the grating length L R and the refractive index amplitude dN R and maximum reflectance R max The first reference data showing the relationship between the grating length L R and the refractive index amplitude dNR The first and second reference data are prepared in advance. R and the refractive index amplitude dN R The maximum reflectance R max and the gradient G.

[0070] (Numerical Simulation) The numerical simulation of the reflection spectrum of the regenerative phase-shifted FBG 12 is performed, for example, based on the mode coupling theory and using the matrix transfer method in consideration of the reverse optical coupling of incident light and reflected light. In the numerical simulation, as shown in Fig. 8, the regenerative phase-shifted FBG 12 is divided into m (m is an integer of 2 or more) minute segments, and the core 22a in each segment is modeled to have a uniform refractive index distribution in the axial direction. The segments are all the same length.

[0071] The forward wave propagating in the +z direction and the backward wave propagating in the -z direction that are incident and exit from one end side of the i-th segment, and the transfer matrix F i The relationship between the above is expressed as in Equation (8). In Equation (8), the value R i is the amplitude of the forward wave incident on the i-th segment, the value R i-1 is the amplitude of the forward wave emitted from the i-th segment, and the value S i is the amplitude of the backward wave emitted from the i-th segment, the value S i-1 is the amplitude of the backward wave incident on the i-th segment. Phase shift point P π The transfer matrix P corresponding to π is expressed as in Equation (9). Note that the 0th segment and the (m+1)th segment are optical fibers 22 connected to both ends of the regenerative phase-shifted FBG 12, and the (m+1)th segment is on the laser light source 14 side.

[0072]

number

[0073] As shown in equation (10), the transfer matrix F i The reflection spectrum of the regenerated phase-shifted FBG 12 is calculated by the transfer matrix F of the entire regenerated phase-shifted FBG 12 multiplied by π In the corresponding part, the transfer matrix F π The relationship of the entire regenerative phase shift FBG 12 is expressed as in equation (11) using the transfer matrix F.

[0074]

number

[0075] The inventors have confirmed that the simulation results of the phase-shifted FBG including the regenerative phase-shifted FBG 12 by the above numerical simulation can satisfactorily reproduce the reflection spectra of the phase-shifted FBG and the regenerative phase-shifted FBG 12 that were actually fabricated.

[0076] FIG. 9 shows an example of the reflection spectrum of a phase-shifted FBG that was actually fabricated and that was simulated numerically. FIG. 9(A) shows the reflection spectrum of a grating body 31 that was actually fabricated and the reflection spectrum of a regenerated phase-shifted FBG 12 fabricated from the grating body 31. FIG. 9(B) shows the reflection spectrum of the grating body 31 and the regenerated phase-shifted FBG 12 by numerical simulation, and it can be seen that the spectral dip of the regenerated phase-shifted FBG 12 has been reproduced well compared to the actually fabricated one. In the numerical simulation of the grating body 31, the grating length L (=5 mm) and the refractive index amplitude dN (=6e-4) were the same as those of the actually fabricated grating body 31, and in the numerical simulation of the regenerated phase-shifted FBG 12, the grating length L R On the other hand, the refractive index amplitude dN Ris set to -0.97e-4, which is 0.15775 times the refractive index amplitude dN of the original grating body 31. R Whether positive or negative, if its magnitude is the same, the shape of the reflection spectrum will be the same and only the Bragg wavelength will differ.

[0077] The grating length L mentioned above R and the refractive index amplitude dN R By changing the grating length L obtained by numerical simulation using the transfer matrix F above, R and the refractive index amplitude dN R and maximum reflectance R max The first reference data is created from the relationship between the grating length L R and the refractive index amplitude dN R The second reference data is created based on the relationship between the angle θ and the gradient G. An example of a set of the first reference data and the second reference data is shown in Fig. 10. Fig. 10(A) shows the first reference data, and Fig. 10(B) shows the second reference data. For example, the first reference data and the second reference data are created and prepared in advance.

[0078] From the first and second reference data created as described above, the slope G and maximum reflectance R of the slope region H are calculated. max The grating length L of the FBG12 suitable for obtaining the regenerative phase shift R and the refractive index amplitude dN R Identify.

[0079] Grating length L R and the refractive index amplitude dN R In identifying the maximum strain ε max and the minimum strain ε min , Signal to Noise Ratio [S / N] dB , noise level V N ) and the characteristics of the optical system (the power P of the laser light incident on the regenerative phase-shift FBG 12, the response function R of the photodetector 16 PD ) into equation (B), the slope G of the slope region H and the maximum reflectance R maxNext, the target values ​​of the slope G and maximum reflectance R of the specified slope region H are obtained from the first reference data and the second reference data. max The grating length L satisfies the value of R and the refractive index amplitude dN R Then, from that set, the grating length L R and the refractive index amplitude dN R Select a combination of grating length L from the set. R and the refractive index amplitude dN R When selecting the combination of the grating length L and the refractive index amplitude dN R It is preferable to select a combination of these so that the values ​​for each of these lie within the preferred ranges described below.

[0080] As mentioned above, the grating length L of the regenerative phase-shifting FBG 12 R is the same as the grating length L of the grating body 31 on which it is based. Therefore, from the first reference data and the second reference data, the gradient G of the gradient region H and the maximum reflectance R max The grating length L of the FBG12 is suitable for obtaining the regenerative phase shift R、 Refractive index amplitude dN R When calculating, it is preferable to calculate the grating length L and the refractive index amplitude dN of the grating body 31 within a preferable range. In this case, the first reference data and the second reference data are the grating length L and the refractive index amplitude dN R Each of them may be prepared within the above preferred range.

[0081] That is, the grating length L R The upper limit of the grating length L is preferably set to 17 mm, more preferably 14 mm. R Although there is no particular limitation on the lower limit of the grating length L, in order to increase the sensitivity of the regenerative phase-shift FBG 12, the lower limit is preferably 5 mm or more, more preferably 10 mm or more. RIn order to increase the sensitivity of the regenerative phase-shift FBG 12, it is necessary to obtain the desired grating length L R is preferably in the range of 5 mm or more and 17 mm or less, more preferably in the range of 5 mm or more and 14 mm or less or in the range of 10 mm or more and 17 mm or less, and particularly preferably in the range of 10 mm or more and 14 mm or less.

[0082] Similarly, the refractive index amplitude dN R It is also preferable to determine the refractive index amplitude dN R It is preferable to determine the upper limit as 2.5e-4.

[0083] [Conversion process] In the conversion step, the grating length L of the regenerative phase-shifted FBG12 specified in the parameter specification step is R and the refractive index amplitude dN R is converted into the grating length L and the refractive index amplitude dN of the grating body 31. As described above, the grating length does not change before and after annealing, and the refractive index amplitude decreases at a constant rate after annealing. Therefore, the grating length L calculated as above can be converted into the refractive index amplitude dN as shown in formula (12) by using the inverse (value k) of the refractive index amplitude ratio 1 / k. R is the grating length L, and the refractive index amplitude dN R Let k times the refractive index amplitude dN.

[0084]

number

[0085] As described above, it has been confirmed that the grating length and the refractive index amplitude are effective parameters in determining the shape of the reflection spectrum, particularly the shape of the spectral dip, in the phase-shifted FBG including the regenerative phase-shifted FBG 12. Figure 11 shows the results of a numerical simulation when the grating length and the refractive index amplitude of the phase-shifted FBG are changed, and it can be seen that the reflectance of the spectral dip and the slope of the slope region become larger as the grating length and the refractive index amplitude become larger. For convenience, the grating length is shown as L and the refractive index amplitude as dN in Figure 11.

[0086] Samples A1 to A4 of the regenerated phase shift FBG 12 were prepared from samples a1 to a4 of the grating body 31 having different grating lengths L and refractive index amplitudes dN, and the reflection spectra of these samples a1 to a4 and A1 to A4 were measured at 50°C. The correspondence between samples A1 to A4 and samples a1 to a4 and the grating lengths L and refractive index amplitudes dN of samples a1 to a4 are as shown in Table 1. Among these, sample a1 has the grating length L and refractive index amplitude dN set so as to function as a phase shift FBG having a slope region with a large slope in the spectrum dip in a relatively low temperature environment such as room temperature. Sample A3 has an extremely large refractive index amplitude dN.

[0087] [Table 1]

[0088] The annealing for producing samples A1 to A4 from samples a1 to a4 was performed by heating samples a1 to a4 to 920°C, maintaining the temperature for about 30 minutes, and then gradually lowering the temperature to room temperature. The other production conditions for samples A1 to A4, such as the grating length L and the refractive index amplitude dN, were the same.

[0089] The measurement results of the reflection spectra of samples a1 to a4 and samples A1 to A4 are shown in Figure 12. Moreover, the spectral dips measured near the Bragg wavelength of sample a1, which is the basis of samples A1, A2, A4, and A1, are shown enlarged in Figure 13. Note that in Figure 13, the graph is drawn with the Bragg wavelength set to wavelength "0".

[0090] It is found that the fabricated samples A1 to A4 all have a spectral dip that is minimal at the Bragg wavelength, and function as a phase shift FBG. However, the refractive index amplitude dN of sample A3 fabricated from sample a3 is extremely large, and although it can be used as a regenerative phase shift FBG 12, the shape of the reflection spectrum is distorted. From these results, it is found that while it is useful to increase the refractive index amplitude dN in order to improve the sensitivity of the regenerative phase shift FBG 12, in order to reliably improve the sensitivity, it is preferable to set the refractive index amplitude dN to at most 15e-4.

[0091] Sample A1 has a broad, gentle spectral dip. In contrast, samples A2 and A4 have sharper spectral dips than sample A1, and it can be seen that high sensitivity is obtained by the gradient region H with a steeper gradient than sample A1. Among these, sample A2 has a gradient region H with a gradient similar to that of sample a1, and sample A4 has a gradient region H with a steeper gradient than sample a1, and it can be seen that it has a higher sensitivity than a general phase-shifted FBG such as sample a1.

[0092] At room temperature, a broadband chirp ultrasonic wave (0.1 to 1.5 MHz) generated by an MFC (macro fiber composite) was propagated through each optical fiber 22 forming samples A1, A2, A4, and sample a1, and the chirp ultrasonic wave was detected. The input signal to the MFC that generated the chirp ultrasonic wave used in this detection and the detection signal obtained using samples A1, A2, A4, and sample a1 are shown in FIG. 14. Also, the frequency spectrum of the detection signal obtained using samples A1, A2, A4, and sample a1 is shown in FIG. 15. Note that FIG. 14(A) is the input signal to the MFC, and FIG. 15(A) is its frequency spectrum. Also, FIG. 14(B) and FIG. 15(B) are the detection signal and its frequency spectrum obtained using sample a1. Similarly, Figures 14(C) and 15(C) show the detection signals and their frequency spectra obtained using sample A1, Figures 14(D) and 15(D) show the detection signals and their frequency spectra obtained using sample A2, and Figures 14(E) and 15(E) show the detection signals and their frequency spectra obtained using sample A4.

[0093] From the detection results shown in Figures 14 and 15, it was confirmed that samples A2 and A4 had higher sensitivity than sample A1, corresponding to the shape of the spectral dip described above, and that sample A2 had a sensitivity close to that of sample a1, while sample A4 had a higher sensitivity than sample a1.

[0094] Fig. 16 shows the reflection spectra of sample A2 measured at room temperature, 300°C, 600°C, and 900°C when sample A2 was heated from room temperature to 900°C, the detection signal when the above-mentioned broadband chirp ultrasonic wave was applied, and the frequency spectrum of the detection signal. Fig. 16(A) shows the measurement results at room temperature, Fig. 16(B) at 300°C, Fig. 16(C) at 600°C, and Fig. 16(D) at 900°C.

[0095] Even when heated up to 900°C, the reflection spectrum of sample A2 maintained almost the same shape, with only the central wavelength of the spectral dip, i.e., the Bragg wavelength, shifting with increasing temperature. In addition, the detection signal and the frequency spectrum of the detection signal also showed almost no change. In other words, sample A2 is capable of measuring elastic waves with high stability, with little effect from temperature changes. Furthermore, if the reflection spectrum maintains almost the same shape, it is understood that the regenerative phase-shift FBG12 can measure elastic waves with high stability, with little effect from temperature changes.

[0096] The results of measuring the S / N ratio of Sample A1 and Sample A2 when heated to 800°C are shown in Figure 17. From this measurement, it was confirmed that in a high-temperature environment of 800°C, in the low-frequency range of approximately 600 kHz or less, Sample A2 had an ultrasonic receiving sensitivity approximately 25 dB (approximately 17 times) higher than Sample A1.

[0097] Figure 18 shows the changes in the reflection spectra of sample A2 and sample A4 with respect to temperature changes between room temperature and 900°C. For both sample A2 shown in Figure 18(A) and sample A4 shown in Figure 18(B), the reflection spectra shift with temperature changes in almost the same way. On the other hand, it was found that while the overall shape of the reflection spectrum and the shape of the spectral dip of sample A2 remain almost the same even with temperature changes, the overall shape of the reflection spectrum and the shape of the spectral dip of sample A4 change more greatly with temperature changes than those of sample A2.

[0098] Samples A5 and A6 of the regenerated phase shift FBG 12 were fabricated from samples a5 and a6 of the grating body 31 having different grating lengths L with the refractive index amplitude dN set to the same level as samples A1, A2, and A4, and the reflection spectra of samples A5 and A6 were investigated. The correspondence between samples A5, A6 and samples a5, a6, as well as the grating lengths L and refractive index amplitudes dN of samples a5 and a6, are shown in Table 2. The fabrication conditions for samples A5 to A6 were the same as samples A1 to A4, except for the grating length L and refractive index amplitude dN.

[0099] The reflection spectra of samples A5, A6 and samples a5, a6 at room temperature are shown in Figure 19. Figure 19(A) shows the reflection spectra of samples A5 and a5, and Figure 19(B) shows the reflection spectra of samples A6 and a6. Note that in each graph in Figure 19, the horizontal axis shows the wavelength relative to the Bragg wavelength, and the vertical axis shows the normalized reflectance.

[0100] [Table 2]

[0101] The change in the reflection spectrum with respect to temperature was investigated for the above samples A5 and A6. The change in the reflection spectrum for sample A5 at every 100°C from 100°C to 900°C is shown in Figure 20, and the reflection spectra at 100°C and 900°C are enlarged in Figure 21. The change in the reflection spectrum for sample A6 at every 100°C from 100°C to 900°C is shown in Figure 22, and the reflection spectra at 100°C and 900°C are enlarged in Figure 23.

[0102] As shown in Figures 20 and 21, in sample A5 with a grating length L of 14 mm, the shape of the reflection spectrum hardly changed with temperature changes. Also, as shown in Figures 22 and 23, in sample A6 with a grating length L of 17 mm, the change in the shape of the reflection spectrum due to temperature changes becomes small in the temperature range of 300°C or higher, and it can be seen that stable measurements can be performed without being affected much by temperature changes. From these results, it can be seen that the length L of the grating region 31a is preferably at most 17 mm, and more preferably at most 14 mm.

[0103] Although the above describes an example in which the regenerative phase-shifted FBG is used as a sensor for detecting elastic waves generated when a specimen is deformed or destroyed, the regenerative phase-shifted FBG can be used as a sensor for detecting various types of elastic waves, and can also be used as a sensor for temperature, strain, pressure, etc., just like previous FBGs. [Explanation of symbols]

[0104] 10 AE measurement device 12 Regenerative phase-shifted fiber Bragg grating 12a Grating area 31 Grating body 31a Grating area

Claims

1. a step of identifying a grating length and a refractive index amplitude of a grating region that satisfies the characteristics of a regenerative phase shift fiber Bragg grating, the grating region being the basis of a desired regenerative phase shift fiber Bragg grating and having a periodic refractive index modulation in a core of an optical fiber and a grating region having a half-period phase shift in a central portion of the refractive index modulation, from the characteristics of the regenerative phase shift fiber Bragg grating; a grating body fabrication step of fabricating the grating body by changing the refractive index of a core of an optical fiber by a light-induced refractive index change to form the grating region having the grating length and the refractive index amplitude specified in the specifying step; an annealing step of annealing the grating body to eliminate and then regenerate the refractive index modulation in the grating region to form the regenerated phase-shifted fiber Bragg grating; 2. A method for producing a regenerative phase-shifted fiber Bragg grating comprising the steps of:

2. 2. The method for producing a regenerative phase-shifted fiber Bragg grating according to claim 1, wherein the determining step determines the grating length and the refractive index amplitude by numerical analysis.

3. The method for manufacturing a regenerative phase shift fiber Bragg grating according to claim 2, characterized in that the identifying step determines a grating length and a refractive index amplitude of a grating region that satisfies the characteristics of the regenerative phase shift fiber Bragg grating from the characteristics of the regenerative phase shift fiber Bragg grating and reference data showing a relationship between the grating length and the refractive index amplitude of a grating region in the phase shift fiber Bragg grating obtained by numerical analysis and the characteristics of the phase shift fiber Bragg grating, and converts the determined grating length and the refractive index amplitude of the grating region that satisfies the characteristics of the regenerative phase shift fiber Bragg grating into the grating length and the refractive index amplitude for the grating body.

4. 4. The method for producing a regenerative phase shifted fiber Bragg grating according to claim 1, wherein the length of the grating is specified in the range of 17 mm or less in the specifying step.

5. 4. The method for producing a regenerative phase-shifted fiber Bragg grating according to claim 1, wherein the length of the grating is specified in the range of 5 mm or more in the specifying step.

6. The determining step determines the refractive index amplitude to be 15×10 －４ 4. The method for producing a regenerative phase shift fiber Bragg grating according to claim 1, wherein the range is specified as follows:

7. In the intermediate product to be annealed to produce a regenerated phase-shifted fiber Bragg grating, The core of an optical fiber has a periodic refractive index modulation and a grating region having a phase shift of half a period at a center of the refractive index modulation; The grating length of the grating region is at most 17 mm. 1 is a cross-sectional view of a regenerated phase-shifted fiber Bragg grating according to an embodiment of the present invention;

8. In the intermediate product to be annealed to produce a regenerated phase-shifted fiber Bragg grating, The core of an optical fiber has a periodic refractive index modulation and a grating region having a phase shift of half a period at a center of the refractive index modulation; The grating length of the grating region is at least 5 mm. 1 is a cross-sectional view of a regenerated phase-shifted fiber Bragg grating according to an embodiment of the present invention;

9. In the intermediate product to be annealed to produce a regenerated phase-shifted fiber Bragg grating, The core of an optical fiber has a periodic refractive index modulation and a grating region having a phase shift of half a period at a center of the refractive index modulation; The refractive index amplitude of the grating region is at most 15×10 －４ is 1 is a cross-sectional view of a regenerated phase-shifted fiber Bragg grating according to an embodiment of the present invention;

10. 10. A regenerated phase-shifted fiber Bragg grating in which the refractive index modulation of the intermediate product according to claim 7 is eliminated by annealing and then restored.

11. a grating region formed in a core of an optical fiber, the grating region having a periodic refractive index modulation, a phase shift of a half period at a center of the refractive index modulation, and the refractive index modulation not disappearing at least in an environment of 900° C.; The grating length of the grating region is 5 mm or more, and the refractive index amplitude of the grating region is 2.5×10 －４ is less than or equal to 2. A regenerative phase-shifted fiber Bragg grating comprising:

12. 12. A regenerative phase shifted fiber Bragg grating as claimed in claim 11, characterized in that the grating length is in the range of 5 mm to 17 mm.

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