JP5065187B2 - Optical fiber sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fiber optic sensor having measuring sensitivity higher than before and using guided acoustic-wave Brillouin scattering light. <P>SOLUTION: The fiber optic sensor includes: a light source for outputting testing light; at least one optical fiber for measurement; and a detection means. The at least one optical fiber is made of quartz glass, includes a core part and a cladding part having an outer diameter of 90 &mu;m or less formed in the outer circumference of the core part, receives the testing light at least at one end part, and outputs guided acoustic-wave Brillouin scattering light generated by the testing light from the other end part. The detection means receives the guided acoustic-wave Brillouin scattering light outputted from the optical fiber for measurement, measures the peak frequency of the guided acoustic-wave Brillouin scattering light on its frequency spectrum, and detects the ambient temperature of the optical fiber for measurement or stress on the optical fiber for measurement on the basis of a measured peak frequency. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to an optical fiber sensor that measures temperature and distortion of an object to be measured using an optical fiber.

  Conventionally, there has been disclosed an optical fiber sensor that uses an optical fiber to measure temperature and distortion of an object to be measured without electrical contact. There are various types of optical fiber sensors. For example, when test light is input to the optical fiber, the interaction between the longitudinal acoustic wave generated by the thermal energy in the optical fiber and the light wave of the test light occurs. There is a method of generating Brillouin scattered light and measuring the frequency shift amount of the Brillouin scattered light. Note that such Brillouin scattered light is scattered backward with respect to the light propagation direction, and is hereinafter referred to as back Brillouin scattered light. The frequency shift amount of the backward Brillouin scattered light varies depending on the temperature of the optical fiber and the applied stress. Therefore, by measuring the frequency shift amount, it is possible to measure the ambient temperature of the optical fiber and the distortion of the measurement object to which the optical fiber is attached. This method of measuring the frequency shift amount of backward Brillouin scattered light has the advantage that the distribution state of temperature and strain can be measured over a long distance using an optical fiber normally used in transmission lines (for example, patents) Reference 1).

However, the method of measuring the frequency shift amount of the back Brillouin scattered light has the following advantages, but has the following problems. That is,
In order to detect backscattered light, a configuration using an OTDR device (Optical Time Domain Reflectometer) is required. As a result, the apparatus configuration is complicated.
In the case of an optical fiber made of ordinary silica glass, the frequency shift amount of backward Brillouin scattered light is around 10 GHz. Therefore, in order to measure the frequency shift amount, a light receiving element having a measurable frequency band of 10 GHz or more is required, and a corresponding high frequency circuit is also required for the corresponding electric circuit. Therefore, the measuring apparatus becomes expensive. .

On the other hand, guided acoustic wave Brillouin scattering in which Brillouin scattered light having a very low scattering efficiency of 10 ⁻¹⁰ m ⁻¹ is generated by the interaction between the transverse acoustic wave generated by the thermal energy in the optical fiber and the light wave. A scattering phenomenon called (Guided Acoustic Wave Brillouin Scattering, GAWBS) is known (for example, see Non-Patent Documents 1 to 3). The scattered light by the guided acoustic wave type Brillouin scattering (guided acoustic wave type Brillouin scattered light) travels in the same direction as the propagation direction of the test light, and is also referred to as forward Brillouin scattered light.

This guided acoustic wave type Brillouin scattered light is modulated by a transverse acoustic wave, but the modulation frequency changes according to the temperature of the optical fiber and the applied stress, so the temperature and strain are measured. (See, for example, Non-Patent Documents 4 and 5). In addition, the method using guided acoustic wave type Brillouin scattered light has the following characteristics as compared with the method using backward Brillouin scattered light. That is,
-An OTDR device is not required to detect forward scattered light. As a result, the apparatus configuration is simplified.
A plurality of peaks exist in the modulation frequency of scattered light. Since the peak frequency occurs in the vicinity of 20 to 800 MHz, about 1 GHz is sufficient as the frequency band characteristics of the light receiving element. Therefore, the measuring device becomes cheaper.

JP 2007-178346 A Nishizawa, Mori, Goto, Miyauchi “Characteristics of GAWBS in Constant Polarization Fiber” IEICE Technical Report LQE95-103 (1995-11) K. Shiraki and M. Ohashi “Sound velocity measurement based on guided acoustic-wave brillouin scattering” IEEE Photn. Technol. Lett. Vol. 4, pp. 1177-1180, 1992 A.Melloni, M.Martinelli and A.Fellegara “Frequency characterization of the nonlinear refractive index in optical fiber” Fiber and Integrated Optics, 18, pp1-13, 1999 Tanaka, Komine “Optical fiber temperature sensor using GAWBS” IEICE General Conference 1997, C-3-142 Tanaka, Nunokawa, Kominato “Optical Fiber Tensile Strain Sensor Using GAWBS” The Institute of Electronics, Information and Communication Engineers Electronics Society 1997, C-3-4

  However, the conventional optical fiber sensor using guided acoustic wave type Brillouin scattered light has a problem that measurement sensitivity is low because the scattering efficiency of guided acoustic wave type Brillouin scattered light is small.

  The present invention has been made in view of the above, and an object of the present invention is to provide an optical fiber sensor using guided acoustic wave Brillouin scattered light having higher measurement sensitivity than conventional ones.

  In order to solve the above-described problems and achieve the object, an optical fiber sensor according to the present invention includes a light source that outputs test light and quartz glass, and an outer portion formed on a core portion and an outer periphery of the core portion. At least one measurement having a clad portion having a diameter of 90 μm or less, receiving the test light at at least one end, and outputting guided acoustic wave Brillouin scattered light generated by the test light from the other end Receiving the guided acoustic wave type Brillouin scattered light output from the optical fiber for measurement, and measuring the peak frequency on the frequency spectrum of the guided acoustic wave type Brillouin scattered light, and measuring the measured peak frequency And detecting means for detecting the ambient temperature of the measuring optical fiber or the stress applied to the measuring optical fiber based on the above.

  The optical fiber sensor according to the present invention is characterized in that, in the above-mentioned invention, the measurement optical fiber has a maximum value of a relative refractive index difference of the core portion relative to the cladding portion of 1.0% or more. .

  The optical fiber sensor according to the present invention includes a plurality of measurement optical fibers in the above invention, and the plurality of measurement optical fibers generate guided acoustic wave Brillouin scattered light having different peak frequencies. At the same time, the connection is made via a connection optical fiber having a clad diameter larger than that of the plurality of measurement optical fibers.

  The optical fiber sensor according to the present invention is characterized in that, in the above-mentioned invention, the plurality of measurement optical fibers have different outer diameters of the clad portion.

  In the optical fiber sensor according to the present invention as set forth in the invention described above, the plurality of measurement optical fibers have different maximum relative refractive index differences between the core portion and the clad portion.

  The optical fiber sensor according to the present invention is characterized in that, in the above-mentioned invention, the optical fiber sensor further comprises a plate-like member held on the surface in a state in which the measurement optical fiber is wound.

  The optical fiber sensor according to the present invention is characterized in that, in the above invention, the plate member has flexibility.

  In the optical fiber sensor according to the present invention as set forth in the invention described above, the measurement optical fiber is a polarization maintaining optical fiber.

  According to the present invention, there is an effect that it is possible to realize an optical fiber sensor using guided acoustic wave Brillouin scattered light having higher measurement sensitivity than conventional ones.

  Hereinafter, embodiments of an optical fiber sensor according to the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments. Moreover, in the following drawings, the same code | symbol is attached | subjected suitably to the same or corresponding element. Further, guided acoustic wave type Brillouin scattering is abbreviated as GAWBS as appropriate.

(Embodiment 1)
FIG. 1 is a block diagram showing a configuration of an optical fiber sensor according to Embodiment 1 of the present invention. As shown in FIG. 1, the optical fiber sensor 100 includes a light source device 1, a measurement optical fiber 2 connected to the light source device 1, and a detection device 3 connected to the measurement optical fiber 2. Note that the measurement optical fiber 2 is placed at a place where the temperature is to be measured, or attached to an object to be measured whose temperature or strain is to be measured.

  The light source device 1 includes, for example, a distributed feedback type semiconductor laser diode, and outputs laser light that is continuous light as test light. The wavelength of the laser light is, for example, 1500 to 1600 nm used for optical fiber communication, but is not particularly limited as long as the wavelength propagates through the silica glass optical fiber without excessive loss.

  FIG. 2 is a diagram showing a schematic cross section of the measurement optical fiber 2 shown in FIG. 1 and a corresponding refractive index profile. As shown in FIG. 2, the measurement optical fiber 2 includes a central core portion 211, an outer core portion 212 formed on the outer periphery of the central core portion 211, and a cladding portion 213 formed on the outer periphery of the outer core portion 212. And a resin coating portion 22 formed on the outer periphery of the clad portion 213.

  The glass portion 21 is made of quartz glass, germanium (Ge) is added to the central core portion 211, and fluorine (F) is added to the outer core portion 212. The clad 213 is made of pure quartz glass that does not contain a refractive index adjusting dopant. As a result, the optical fiber for measurement 2 has a refractive index profile shaped like the profile P in the glass portion 21. Further, the maximum value of the relative refractive index difference with respect to the clad portion 213 of the central core portion 211 is Δ1, and the relative refractive index difference with respect to the clad portion 213 of the outer core portion 212 is Δ2. The outer diameter of the cladding part 213, that is, the cladding diameter is 90 μm or less.

  FIG. 3 is a diagram showing an example of characteristics of the measurement optical fiber 2 shown in FIG. In FIG. 3, “outer diameter” means the outer diameter of the resin coating portion 22, and “MFD” means the mode field diameter. As shown in FIG. 3, the measurement optical fiber 2 has, for example, a cladding diameter of 60 μm and an outer diameter of 125 μm. Δ1 and Δ2 are 2.0% and −0.6%, respectively.

  FIG. 4 is a block diagram showing the configuration of the detection device 3 shown in FIG. As shown in FIG. 4, in this detection device 3, a polarizer 31, a photodiode (PD) 32 as a light receiving element, an electric signal amplifier 33, a spectrum analyzer 34, and a control display 35 are sequentially connected. It has a configuration. In FIG. 4, as for the lines connecting the components, the solid line means optical connection using a lens, an optical fiber or the like as appropriate, and the broken line means electrical connection.

  Next, the operation of the optical fiber sensor 100 will be described. When test light is output from the light source device 1, the measurement optical fiber 2 receives the test light at one end, and the test light propagates through the measurement optical fiber 2.

Here, a transverse acoustic wave is generated in the glass portion 21 of the measurement optical fiber 2 by the thermal energy. FIG. 5 is a diagram showing the vibration direction of the transverse acoustic wave generated in the glass portion 21 of the measurement optical fiber 2 shown in FIG. 2 by arrows. As shown in FIG. 5, the transverse acoustic wave includes an R _0m mode (radial mode) that vibrates centrally at a predetermined resonance frequency and _{m of an} orthogonal xy axis, where m is an integer representing an order. There are two modes called TR _2m mode (torsional / radial mode) that vibrate at a predetermined resonance frequency so that one expands and the other compresses in the direction. Since the R _0m mode modulates the refractive index of the glass portion 21, phase modulation is induced. For this reason, the GAWBS resulting from the R _0m mode is called a polarized GAWBS. Therefore, the polarized GAWBS light generated by the polarized GAWBS is phase-modulated at a predetermined resonance frequency. On the other hand, since the TR _2m mode induces birefringence in the glass portion 21, it mainly induces polarization modulation. For this reason, GAWBS resulting from the TR _2m mode is called depolarized GAWBS. Therefore, the depolarized GAWBS light generated by the depolarized GAWBS is polarization-modulated at a predetermined resonance frequency.

  In this manner, in the measurement optical fiber 2, polarized GAWBS light and depolarized GAWBS light having the same modulation frequency as a predetermined resonance frequency are generated by the interaction between the transverse acoustic wave of each mode and the light wave of the test light. Each generated GAWBS light propagates in the same direction as the test light and is output from the other end of the measurement optical fiber 2.

  Next, the detection device 3 receives each GAWBS light output from the measurement optical fiber 2. In the detection device 3, each GAWBS light is sequentially input to the polarizer 31 and the PD 32, and is converted into an electric signal by the PD 32. The converted electrical signal is amplified by the electrical signal amplifier 33 and input to the spectrum analyzer 34. Here, of the GAWBS light, the depolarized GAWBS light is transmitted through the polarizer 31, and the polarization modulation is converted into intensity modulation. As a result, in the spectrum analyzer 34, the modulation frequency of the intensity-modulated depolarized GAWBS light is detected as a plurality of peaks according to the order of the mode. The transmission polarization direction of the polarizer 31 is appropriately adjusted so that the intensity of the depolarized GAWBS light transmitted through the polarizer 31 is maximized. Next, the control indicator 35 detects the ambient temperature of the measurement optical fiber 2 or the distortion of the object under measurement based on a predetermined one of the peak frequencies detected by the spectrum analyzer 34. The detection of the control indicator 35 is performed by calculation processing based on a relational expression between a peak frequency measured in advance and temperature or stress. Further, it may be performed based on a correspondence table of peak frequency and temperature or stress stored in the memory inside the control display 35. The control indicator 35 is realized by a personal computer, for example.

  Here, in the first embodiment, since the cladding diameter of the measurement optical fiber 2 is 90 μm or less, the peak intensity of the modulation frequency of the depolarized GAWBS light is stronger than that of the conventional one. As a result, this optical fiber sensor 100 has higher measurement sensitivity than before. This will be specifically described below.

  As described above, the conventional optical fiber sensor using the GAWBS light has a problem that the measurement sensitivity is low because the scattering efficiency of the GAWBS light is small. Therefore, the present inventors have examined the relationship between the resonance frequency of each GAWBS and the characteristics of the optical fiber in order to improve the measurement sensitivity.

(Relational expression of resonance frequency of depolarized GAWBS)
First, consider the resonant frequency of the depolarized GAWBS generated from the light wave HE ₁₁ mode and the TR _2m mode. According to Non-Patent Document 2, the resonance frequency f _{Tm in the} TR _2m mode is expressed by the following formula (1), where V _S is the acoustic wave velocity, y _{m is} the eigenvalue, and d is the cladding diameter of the optical fiber. The

Note that the eigenvalues _{y m} is from the boundary conditions at the cladding portion surface of the optical fiber in the _{TR 2m} mode, the ratio between the shear wave velocity _{V S} and the longitudinal wave velocity _{V L} of the acoustic wave, i.e., the _V S / _{V L} alpha, The second-order and third-order Bessel functions are determined as J ₂ and J ₃ using the following equation (2).

(Relational expression of the resonance frequency of Polarized GAWBS)
Next, the resonance frequency of the polarized GAWBS generated from the light wave HE ₁₁ mode and the R _0m mode will be considered. According to Non Patent Literature 3, the resonance frequency f _{Rm in the} R _0m mode is expressed by the following equation (3), where the eigenvalue is μ _m .

Note that the eigenvalues mu _m, when ignoring the boundary conditions of the cladding portion of the optical fiber and the resin-coated portion, the 0-order Bessel function as J _0, determined using the following equation (4).

From Equations (1) and (3), in two types of optical fibers, in order to generate different frequencies f for the same mode order, a) different cladding diameters d, b) different V _S or V _L The following two conditions can be considered.

Further, regarding the relationship between the acoustic wave velocities V _S and V _L and the concentration of Ge to be added, when the Ge concentration is w _g (mass%), the following equations (5) and (6) are established.

V _L = 5944 (1-7.2 × 10 ⁻³ w _g ) (5)
V _S = 3749 (1-6.4 × 10 ⁻³ w _g ) (6)

(Note that the above equations (5) and (6) are "Simulating and Designing Brillouin Gain Spectrum In Single-Mode Fibers" Y. Koyamada, S. Sato, S. Nakamura, H. Sotobayashi and W. Chujo, Journal of Lightwave. Technology Vol.22 No.2 2004). Since GAWBS represented by the equations (1) and (3) is a traveling wave in the radial direction of the optical fiber, the acoustic wave propagates through both the core portion and the cladding portion of the optical fiber. Therefore, the acoustic wave velocities V _S and V _L considered here are the acoustic wave velocities in both the core part and the clad part of the optical fiber. Is basically the cladding portion Ge is not added, because the V _S and V _L change would Ge concentration of the core is changed, it will change the results in GAWBS peak frequency. That is, as is clear from the equations (1), (3), (5), and (6), the peak frequency varies depending on the cladding diameter and the composition of the optical fiber.

In the case of a normal quartz glass-based optical fiber, V _S = 3740 m / s and V _L = 5910 m / s at a temperature of 20 ° C. Then, for example, the primary resonance frequency of the polarized GAWBS is 29.85 MHz, and other high-order resonance frequencies are generated at intervals of V _L / (πd) ≈48 MHz. Further, the velocity of the acoustic wave varies depending on the temperature of the optical fiber or the applied stress.

  As described above, in both the depolarized GAWBS and the polarized GAWBS, the resonance frequency peaks occur at periodic frequency intervals. The upper limit of the generated frequency band is about 800 MHz as described above, and it is known that the peak intensity is attenuated in a further high frequency band.

  Here, in the optical fiber sensor 100, the cladding diameter of the measurement optical fiber 2 is 90 μm or less, which is smaller than 125 μm, which is the cladding diameter of the conventional measurement optical fiber. As a result, as is clear from the equations (1) and (3), the resonance frequency interval of the measurement optical fiber 2 is wider than in the conventional case. As a result, the number of resonance frequencies included up to the upper limit of 800 MHz decreases, so that the amplitude intensity per mode becomes strong, and the mode existing in the frequency region up to about 300 MHz becomes particularly strong. As a result, since the intensity per frequency peak detected by the spectrum analyzer 34 is also increased, the peak detection accuracy is improved, so that the optical fiber sensor 100 has higher measurement sensitivity than before.

FIG. 6 is a diagram showing a frequency spectrum measured by the spectrum analyzer 34 when the measurement optical fiber 2 having the characteristics shown in FIG. 3 is used. In FIG. 6, the horizontal axis indicates the frequency, and the vertical axis indicates the relative intensity when the base noise level is 0 dB. As shown in FIG. 6, in the spectrum analyzer 34, peaks of a plurality of modulation frequencies corresponding to the resonance frequency _fTm in the TR _2m mode are observed, and the upper limit is about 800 MHz.

  Next, FIG. 7 is a diagram showing the relationship between the temperature around the measurement optical fiber 2 and the frequency of each peak for the peak P1 having a frequency of 223 MHz and the peak P2 having a frequency of 660 MHz in FIG. is there. In FIG. 7, “◯” and “●” indicate measured values for the peaks P1 and P2, respectively, and lines L1 and L2 indicate approximate straight lines of the measured values for the peaks P1 and P2, respectively. When the temperature is T (° C.) and the peak frequency is f (MHz), the line L1 is represented by f = 222.83 + 0.01048T, and the line L2 is represented by f = 658.01 + 0.05462T. Therefore, the control indicator 35 can calculate the temperature using the above relational expression representing the line L1 from the frequency of the peak P1 among the peak frequencies detected by the spectrum analyzer 34, for example.

  As described above, the optical fiber sensor 100 according to the first embodiment has a higher measurement sensitivity than the prior art because the peak intensity of the modulation frequency of the detected depolarized GAWBS light is increased.

(Embodiment 2)
Next, a second embodiment of the present invention will be described. The optical fiber sensor according to the second embodiment includes a plurality of measurement optical fibers.

  FIG. 8 is a block diagram showing the configuration of the optical fiber sensor according to the second embodiment. As shown in FIG. 8, the optical fiber sensor 200 includes a light source device 1 and a detection device 3 similar to those shown in FIG. 1, and two measurement optical fibers 2a and 2b. The light source device 1 and the measurement optical fiber 2a are connected to each other at the connection portion 5a via the connection optical fiber 4a. Similarly, the measurement optical fibers 2a and 2b and the connection optical fiber 4b are connected at a connection portion 5b and a connection portion 5c, respectively. That is, the measurement optical fiber 2a and the measurement optical fiber 2b are connected via the connection optical fiber 4b. The detection device 3 and the measurement optical fiber 2b are connected to each other at the connection portion 5d via the connection optical fiber 4c. The connection parts 5a to 5d are, for example, an optical connector connection part, a fusion connection part, or the like.

  In this optical fiber sensor 200, the test light output from the light source device 1 propagates sequentially through the connection optical fiber 4a, the measurement optical fiber 2a, the connection optical fiber 4b, the measurement optical fiber 2b, and the connection optical fiber 4c. To do. In each optical fiber, depolarized GAWBS light is generated and input to the detection device 3.

Here, the measurement optical fiber 2a and the measurement optical fiber 2b have the same structure and refractive index profile as the measurement optical fiber 2 shown in FIG. 2, and both have a cladding diameter of 90 μm or less. Therefore, since the peak intensity of the modulation frequency of the depolarized GAWBS light is increased, the measurement sensitivity is higher than the conventional one. Further, the measurement optical fiber 2a and the measurement optical fiber 2b have different clad diameters. Therefore, as shown in the equation (1), in the measurement optical fiber 2a and the measurement optical fiber 2b, the resonance frequencies f _{Tm in the} TR _2m mode are different from each other even if they have the same order, that is, m. As a result, the peak frequencies of the depolarized GAWBS light generated in the measurement optical fibers 2a and 2b by the test light output from the light source device 1 do not overlap even if they have the same order. Therefore, for example, each measurement optical fiber 2a, 2b is attached to a different object to be measured, and each of the peak frequencies of the depolarized GAWBS light generated in each measurement optical fiber 2a, 2b is measured. The temperature and strain of two objects to be measured can be simultaneously measured with high sensitivity.

  As the connection optical fibers 4a to 4c, optical fibers having a cladding diameter larger than that of the measurement optical fibers 2a and 2b are used. As a result, the depolarized GAWBS light generated in the connection optical fibers 4a to 4c has a peak frequency that does not overlap with that of the measurement optical fibers 2a and 2b, and has a peak intensity higher than that of the measurement optical fibers 2a and 2b. Since it becomes smaller, it does not adversely affect the temperature measurement. As the connection optical fibers 4a to 4c, for example, ITU-T (International Telecommunication Union) G.I. A standard single mode optical fiber (SMF) having a clad diameter defined by 652 of 125 μm and a relative refractive index difference of about 0.3% with respect to the clad portion of the core portion can be used.

  FIG. 9 is a diagram showing an example of characteristics of the measurement optical fibers 2a and 2b shown in FIG. FIG. 10 is a diagram showing a frequency spectrum measured by the spectrum analyzer 34 when the measurement optical fibers 2a and 2b having the characteristics shown in FIG. 9 are used. In FIG. 10, the horizontal axis indicates the frequency, and the vertical axis indicates the relative intensity when the base noise level is 0 dB. The peak P3 is the peak of the primary modulation frequency due to the measurement optical fiber 2a, and the peak P4 is the peak of the primary modulation frequency due to the measurement optical fiber 2b. As shown in FIGS. 9 and 10, in the measurement optical fibers 2a and 2b, since the clad diameters are different from each other, the peaks P3 and P4 are different from each other. Moreover, the peak P3 of the measurement optical fiber 2a having a smaller cladding diameter has a higher frequency than the peak P4.

In addition, if Ge is added so that the maximum refractive index difference is 1% or more, which is sufficiently larger than 0.3% of the standard SMF conventionally used, the bending diameter of the optical fiber can be reduced and compact. Even when housed in the housing, light confinement in the core can be increased, which is effective. For example, if Δ1 is 1% or more, an optical fiber standard (ITU-T G.657.B, radius R = 15 mm, 0.03 dB / 10 turns at a wavelength of 1550 nm, which has a small increase in loss with respect to bending as currently defined. The following can be fully satisfied.
In addition, when an optical fiber having a total length of 50 m of 2.8% and a cutoff wavelength of 1400 nm is wired on a fiber sheet having a radius R = 15 mm (about 500 turns), the increase in transmission loss at a wavelength of 1550 nm is 0.08 dB. Met. This is the standard ITU-T G. 657. Even when compared with B, the bending loss is smaller, indicating that compact storage is possible. In the above, one turn is a state where the optical fiber is wound once with a predetermined radius. The fiber sheet is a thin sheet formed by winding an optical fiber for a predetermined turn.

(Embodiment 3)
Next, a third embodiment of the present invention will be described. The optical fiber sensor according to the third embodiment has substantially the same configuration as the optical fiber sensor 100 according to the first embodiment, but further includes a plate-like member that is held on the surface in a state where the measurement optical fiber is wound. I have.

  FIG. 11 is a block diagram showing the configuration of the optical fiber sensor according to the third embodiment. As shown in FIG. 11, the optical fiber sensor 300 includes a light source device 1, a measurement optical fiber 2, and a detection device 3 similar to those shown in FIG. 1. The light source device 1 and the measurement optical fiber 2 are connected to each other at the connection portion 5e through the connection optical fiber 4d. Similarly, the detection device 3 and the measurement optical fiber 2 are connected to each other at the connection portion 5f via the connection optical fiber 4e. The connection optical fibers 4d and 4e are optical fibers having a cladding diameter larger than that of the measurement optical fiber 2, for example, standard SMF.

  Further, the optical fiber sensor 300 includes a plate-like member 6, and the measurement optical fiber 2 is bonded to the surface of the plate-like member 6 in a state of being wound with the maximum winding diameter D1 and the minimum winding diameter D2. It is stuck with. Thus, since the plate-like member 6 is held on the surface in a state where the measurement optical fiber 2 is wound, the winding diameter is smaller than that of a bobbin having a constant winding diameter normally used for storing the optical fiber. Therefore, the entire optical fiber sensor 300 becomes small and thin, and is suitable for measuring a narrow place or a small object to be measured. The measurement optical fiber 2 has a smaller cladding diameter than the conventional SMF and a smaller outer diameter including the resin coating portion 22, so that not only the measurement sensitivity can be increased, but also the accommodation property is higher. Yes. Therefore, even if the plane area is the same, the measurement optical fiber 2 with a longer length can be accommodated, and the interaction length between the test light and the transverse acoustic wave can be increased. Can be high.

  The material of the plate-like member 6 is not particularly limited. For example, if a flexible material such as polyethylene terephthalate is used and formed into a thin sheet shape, the object to be measured has a curved outer shape, for example. In addition, the temperature and strain can be measured more accurately.

  Further, as a modification of the optical fiber sensor 300, another protection plate is provided so as to cover the measurement optical fiber 2 wound in the configuration shown in FIG. A shaped member may be used.

  FIG. 12 shows the relationship between the load and the primary peak frequency of the depolarized GAWBS light when a load is applied from above the protective plate member in the configuration of the modification of the optical fiber sensor 300 according to the third embodiment. FIG. The value of the maximum winding diameter D1 of the measurement optical fiber 2 was 9.5 cm, and the value of the minimum winding diameter D2 was 3 cm. In FIG. 12, “◯” indicates measurement data, and the solid line indicates an approximate straight line. As shown in FIG. 12, since the load and the peak frequency have a proportional relationship, the stress applied to the measurement optical fiber 2 can be calculated from the peak frequency. Therefore, the strain of the object to be measured can be measured using the optical fiber sensor 300.

  By the way, when the cladding diameter is reduced in the optical fiber, there is a risk that leakage loss in which light leaks from the core portion occurs. This leakage loss becomes a problem particularly when the optical fiber is bent. FIG. 13 is a diagram showing the relationship between the cladding diameter of the optical fiber and the leakage loss. Note that FIG. 13 is based on the result of simulation calculation by the finite element method (FEM) (for example, K. Saitoh and M. Koshiba, “Full-Vectorial Imaginary-Distance Beam Propagation Method Based on Finite Element Scheme: Application to Photonic Crystal Fibers ”, IEEE Journal of Quantum Electronics, vol. 38, No. 7, July, 2002.). FIG. 13 shows a case where the cladding diameter is reduced in the standard SMF and a case where the cladding diameter is reduced in the measurement optical fiber 2 having the characteristics shown in FIG. As shown in FIG. 13, for any optical fiber, the leakage loss increases as the cladding diameter decreases. However, since the optical fiber for measurement 2 having a larger maximum relative refractive index difference in the core part has a strong light confinement in the core part, even if the clad diameter is 40 μm, the leakage loss is smaller than the SMF with the clad diameter of 125 μm. There is no practical problem.

  In order to keep the plate-like member 6 serving as the housing portion of the measurement optical fiber 2 in a compact shape, it is effective to reduce the winding diameter of the measurement optical fiber 2, but the measurement is performed as the winding diameter is reduced. The bending strain applied to the optical fiber 2 will be increased. Here, the bending strain applied to the optical fiber is proportional to the cladding diameter. Therefore, the measurement optical fiber 2 having a small cladding diameter is preferable because it can be wound with a small winding diameter while suppressing bending distortion applied to the optical fiber.

Specifically, the relationship between the cladding diameter d and the bending strain ε applied to the optical fiber is expressed by the following equation (7), where R is the bending radius of the optical fiber and D is the outer radius of the coated portion of the optical fiber. expressed.
ε = d / (R + D) (7)

  Further, since the bending strain applied to the optical fiber is reduced, the breaking rate of the optical fiber can be reduced. FIG. 14 is a diagram showing the relationship between the wrapping radius of the optical fiber, that is, the bending radius, and the breaking rate in 20 years for optical fibers having cladding diameters of 60, 90, and 125 μm. As shown in FIG. 14, the rupture rate increases as the wrapping radius decreases. However, if the wrapping radius is the same, the rupture rate decreases as the cladding diameter decreases. Further, the smaller the cladding diameter, the same breaking rate can be realized with a smaller winding radius. In other words, the optical fiber sensor 300 uses the measurement optical fiber 2 to achieve the same or lower breakage as the conventional one, and is more compact and more suitable for measurement in a narrow place. And an optical fiber sensor with high measurement sensitivity.

(Embodiment 4)
Next, a fourth embodiment of the present invention will be described. The optical fiber sensor according to the fourth embodiment has substantially the same configuration as that of the optical fiber sensor 200 according to the second embodiment, but includes a plate-like member that is held on the surface in a state where each measurement optical fiber is wound. It has more.

  FIG. 15 is a block diagram showing the configuration of the optical fiber sensor according to the fourth embodiment. As shown in FIG. 15, this optical fiber sensor 400 is similar to the optical fiber sensor 200 shown in FIG. 8 in that the light source device 1, the measurement optical fibers 2a and 2b, the detection device 3, and the connection optical fiber 4a. To 4c and connecting portions 5a to 5d, and plate-like members 6a and 6b similar to the plate-like member 6 shown in FIG. The measurement optical fiber 2a and the measurement optical fiber 2b are connected to each other via a connection optical fiber 4b. Further, the plate-like members 6a and 6b are held on the surface by a bonding agent in a state where the measurement optical fibers 2a and 2b are wound, respectively. As a result, this optical fiber sensor 400 can measure the temperature of two places at the same time, while having a simple configuration, and has a higher capacity and a smaller size while realizing the same or lower breaking rate as the conventional one. There is an optical fiber sensor suitable for measurement in a narrow place and having high measurement sensitivity.

(Embodiment 5)
Next, a fifth embodiment of the present invention will be described. The optical fiber sensors 100 to 400 according to the first to fourth embodiments described above use depolarized GAWBS light, but the optical fiber sensor according to the fifth embodiment uses polarized GAWBS light. .

  FIG. 16 is a block diagram showing the configuration of the optical fiber sensor according to the fifth embodiment. As shown in FIG. 16, the optical fiber sensor 500 includes the light source device 1 and the measurement optical fiber 2, the optical coupler 7, and the detection device 3a similar to those shown in FIG.

  The optical coupler 7 is a directional coupler type and includes four input / output ports 71 to 74. The branching ratio of the optical coupler 7 is 1: 1. That is, for example, the optical coupler 7 branches light received at the input / output port 71 at an intensity ratio of 1: 1 and outputs the branched light to the input / output ports 73 and 74, respectively. The input / output port 71 is connected to the light source device 1, and the input / output port 72 is connected to the detection device 3. The input / output ports 73 and 74 are connected to both ends of the measurement optical fiber 2. Therefore, the optical coupler 7 and the measurement optical fiber 2 constitute an optical fiber loop interferometer.

  On the other hand, the detection device 3a has a configuration in which a PD 32, an electric signal amplifier 33, a spectrum analyzer 34, and a control display 35 are sequentially connected. That is, the detection device 3a has a configuration in which the polarizer 31 is removed from the detection device 3 shown in FIG.

  Next, the operation of the optical fiber sensor 500 will be described. When test light is output from the light source device 1, the optical coupler 7 receives the test light at the input / output port 71, branches at a 1: 1 intensity ratio, and outputs it to the input / output ports 73 and 74, respectively. Next, the measurement optical fiber 2 receives the test light output from the input / output ports 73 and 74 at each end, and each test light propagates through the measurement optical fiber 2 in the opposite direction.

  Next, in the measurement optical fiber 2, polarized GAWBS light having a predetermined modulation frequency is generated by the interaction between the transverse acoustic wave of each mode and the light wave of each test light, and propagates in the same direction as each test light. . Each polarized GAWBS light reaching the input / output ports 73 and 74 is coupled by the optical coupler 7. At this time, since the polarized GAWBS light is phase-modulated, it is converted into intensity modulation by the interference effect of the optical coupler 7, and a part thereof is output from the input / output port 72.

  Next, the detection device 3 a accepts the polarized GAWBS light output from the input / output port 72 of the optical coupler 7. The polarized GAWBS light is input to the PD 32 and converted into an electric signal by the PD 32. After that, the ambient temperature of the measurement optical fiber 2 or the distortion of the object to be measured is detected by the spectrum analyzer 34 and the control display 35 in the same manner as described in the first embodiment. Also in this optical fiber sensor 500, since the measurement optical fiber 2 has a clad diameter of 90 μm or less, the measurement sensitivity is higher than that of the conventional optical fiber sensor according to each of the above embodiments.

  As a modification of the optical fiber sensor 500 using the polarized GAWBS light, a configuration including a plurality of measurement optical fibers as in the optical fiber sensor 200 according to the second embodiment may be used. It is good also as a structure further equipped with the plate-shaped member hold | maintained in the state which wound the optical fiber for a measurement like the optical fiber sensors 300 and 400 which concern on form 3 and 4. FIG.

  In the optical fiber sensor using the depolarized GAWBS light according to the first to fourth embodiments, the measurement optical fibers 2, 2a and 2b are optical fibers having a normal structure, but polarization maintaining optical fibers are used. May be.

  FIG. 17 is a schematic cross-sectional view of an example of a polarization maintaining type measurement optical fiber. As shown in FIG. 17, the measurement optical fiber 2c is a stress-applying polarization maintaining optical fiber, and has the same core core portion 211 and outer core portion 212 as those shown in FIG. 2 has a glass part 21c made up of a clad part 214 formed on the outer periphery of the steel sheet, stress applying members 215 and 215 located inside the clad part 214, and a resin coating part 22 similar to that shown in FIG. The clad portion 214 is the same as the clad portion 213 shown in FIG. 2 except that the clad portion 214 includes stress applying members 215 and 215 therein. The stress applying members 215 and 215 are made of, for example, quartz glass to which boron (B) is added, and the central axes of the central core portion 211 and the outer core portion 212 are positioned substantially on a straight line connecting the central axes thereof. Has been placed. As a result, birefringence occurs in the measurement optical fiber 2c with the polarization axis in the direction connecting the central axes and the direction orthogonal thereto.

When this measurement optical fiber 2c is used instead of the measurement optical fibers 2, 2a, 2b, the polarization direction of the linearly polarized light of the test light of the light source device 1 and any polarization of the measurement optical fiber 2c The axis and the transmission polarization direction of the polarizer 31 of the detection device 3 are made to coincide with each other. Then, since the test light propagates through the measurement optical fiber 2c while maintaining the polarization direction, the interaction with the transverse acoustic wave of the TR _2m mode is strong and stable over the longitudinal direction of the measurement optical fiber 2c. Maintained. As a result, since the intensity of the generated depolarized GAWBS light is increased, the peak intensity on the frequency spectrum is also increased, so that the measurement sensitivity is further increased and stabilized.

  Moreover, in the optical fiber sensor using the depolarized GAWBS light according to the first to fourth embodiments, a detection device having the following configuration may be used instead of the detection device 3.

  FIG. 18 is a block diagram showing a configuration of another example of a detection apparatus that can be used in the optical fiber sensors according to Embodiments 1 to 4. As shown in FIG. 18, this detection device 3b includes polarizers 31a and 31b, PDs 32a and 32b, an electric signal amplifier 33, a spectrum analyzer 34, a control display 35, half-wave plates 36a and 36b, A polarization beam splitter 37 and a multiplexer 38 are provided.

  Next, the operation of the detection device 3b will be described. First, the detection device 3b accepts depolarized GAWBS light output from the measurement optical fiber 2, for example. In the detection device 3b, the polarization state of the depolarized GAWBS light is adjusted by the half-wave plate 36a, and is separated into two linearly polarized light beams whose polarization directions are orthogonal to each other by the polarization beam splitter 37. One of the separated linearly polarized light is further adjusted in its polarization state by the half-wave plate 36b, sequentially input to the polarizer 31a, PD32a, or the polarizer 31b, PD32b, and converted into an electric signal by the PD32a or PD32b. Is done. Each electrical signal converted by the PD 32 a or PD 32 b is multiplexed by the multiplexer 38, input to the electrical signal amplifier 33, amplified, and input to the spectrum analyzer 34. By doing so, even if the polarization state of the light output from the measurement optical fiber 2 fluctuates, the power of the electrical signal input to the spectrum analyzer 34 can be made constant. In the spectrum analyzer 34, the modulation frequency of one of the depolarized GAWBS lights separated by the polarization direction is detected as a plurality of peaks. Next, the control indicator 35 detects the ambient temperature of the measurement optical fiber 2 or the distortion of the object under measurement based on a predetermined one of the peak frequencies detected by the spectrum analyzer 34.

  That is, since the measurement optical fiber 2 is an optical fiber having a normal structure, the polarization state of the test light propagating through the measurement optical fiber 2 may vary with time. Therefore, the polarization state of the generated depolarized GAWBS light may also vary.

  On the other hand, in the detection device 3b, the depolarized GAWBS light is separated into two linearly polarized light by the polarization beam splitter 37. The spectrum analyzer 34 detects the peak by adding the separated lights converted into electric signals. Therefore, by using the detection device 3b, more reliable and stable measurement can be realized regardless of fluctuations in the polarization state of the test light propagating through the measurement optical fiber 2.

  In each of the above embodiments, the light source device 1 outputs continuous light as test light, but may output pulsed light as test light. In Embodiments 2 and 4, the number of optical fibers for measurement is two, but three or more optical fibers for measurement may be provided.

  Further, the structure of the measurement optical fiber is not limited to the one having a two-layer core portion, and may have only a central core portion or a further multilayer structure. Further, the clad portion may be formed with holes. In addition, the polarization maintaining optical fiber is not limited to the stress-applying optical fiber, but other polarization maintaining optical fiber, for example, an elliptical core type whose core section has an elliptical cross section is used. May be.

It is the block diagram which showed the structure of the optical fiber sensor which concerns on Embodiment 1 of this invention. It is the figure which showed the typical cross section of the optical fiber for a measurement shown in FIG. 1, and a corresponding refractive index profile. It is the figure which showed an example of the characteristic of the optical fiber for measurement shown in FIG. It is the block diagram which showed the structure of the detection apparatus shown in FIG. It is the figure which showed the vibration direction of the transverse acoustic wave which has generate | occur | produced in the glass part of the optical fiber for a measurement shown in FIG. 2 with the arrow. It is the figure which showed the frequency spectrum measured in a spectrum analyzer at the time of using what has the characteristic shown in FIG. 3 as a measurement optical fiber. FIG. 7 is a diagram showing the relationship between the ambient temperature of the measurement optical fiber and the frequency of each peak for a peak with a frequency of 223 MHz and a peak with a frequency of 660 MHz in FIG. 6. It is the block diagram which showed the structure of the optical fiber sensor which concerns on Embodiment 2 of this invention. It is the figure which showed an example of the characteristic of the optical fiber for a measurement shown in FIG. It is the figure which showed the frequency spectrum measured in a spectrum analyzer at the time of using the measurement optical fiber which has the characteristic shown in FIG. It is the block diagram which showed the structure of the optical fiber sensor which concerns on Embodiment 3 of this invention. In the structure of the modification of the optical fiber sensor which concerns on Embodiment 3, it is the figure which showed the relationship between the load and the primary peak frequency of depolarized GAWBS light at the time of applying a load from on the plate member for protection. . It is the figure which showed the relationship between the clad diameter of an optical fiber, and leakage loss. It is the figure which showed the relationship between the winding radius of an optical fiber, and the fracture rate in 20 years about the optical fiber whose clad diameter is 60, 90, and 125 micrometers. It is the block diagram which showed the structure of the optical fiber sensor which concerns on Embodiment 4 of this invention. It is the block diagram which showed the structure of the optical fiber sensor which concerns on Embodiment 5 of this invention. It is a typical sectional view of an example of a polarization maintaining type measurement optical fiber. It is the block diagram which showed the structure of another example of the detection apparatus which can be used in the optical fiber sensor which concerns on Embodiment 1-4.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source device 2, 2a-2c Optical fiber for measurement 3, 3a, 3b Detector 4a-4e Optical fiber for connection 5a-5f Connection part 6, 6a, 6b Plate member 7 Optical coupler 21, 21c Glass part 22 Resin coating Part 31, 31a, 31b Polarizer 32, 32a, 32b PD
33 Electric Signal Amplifier 34 Spectrum Analyzer 35 Control Display 36a, 36b Half Wave Plate 37 Polarization Beam Splitter 38 Multiplexer 71-74 Input / Output Ports 100-500 Optical Fiber Sensor 211 Central Core Portion 212 Outer Core Portion 213, 214 Cladding Part 215 Stress applying member D1 Maximum winding diameter D2 Minimum winding diameter L1, L2 Line P Profile P1-P4 Peak

Claims (8)

A light source that outputs test light;
It is made of quartz glass and has a core part and a clad part having an outer diameter of 90 μm or less formed on the outer periphery of the core part. The test light is received at at least one end part, and is guided by the test light. At least one measurement optical fiber that outputs the wave acoustic wave Brillouin scattered light from the other end;
The guided acoustic wave type Brillouin scattered light output from the measurement optical fiber is received, the peak frequency on the frequency spectrum of the guided acoustic wave type Brillouin scattered light is measured, and the measurement is performed based on the measured peak frequency. Detecting means for detecting ambient temperature of the optical fiber for measurement or stress applied to the optical fiber for measurement;
An optical fiber sensor comprising:   2. The optical fiber sensor according to claim 1, wherein the measurement optical fiber has a maximum value of a relative refractive index difference of the core portion relative to the cladding portion of 1.0% or more.   A plurality of measurement optical fibers, the plurality of measurement optical fibers generate guided acoustic wave type Brillouin scattered light having different peak frequencies, and have a larger cladding diameter than the plurality of measurement optical fibers; 3. The optical fiber sensor according to claim 1, wherein the optical fiber sensor is connected via a connecting optical fiber.   The optical fiber sensor according to claim 3, wherein the plurality of measurement optical fibers have different outer diameters of the cladding portion.   5. The optical fiber sensor according to claim 3, wherein the plurality of measurement optical fibers have different maximum relative refractive index differences between the core portion and the clad portion.   The optical fiber sensor according to any one of claims 1 to 5, further comprising a plate-like member that is held on the surface in a state in which the measurement optical fiber is wound.   The optical fiber sensor according to claim 6, wherein the plate-like member has flexibility.   The optical fiber sensor according to claim 1, wherein the measurement optical fiber is a polarization maintaining optical fiber.

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