JP4441624B2 - Strain / temperature distribution measuring method and measuring apparatus using optical fiber - Google Patents

Description

  The present invention relates to a method and apparatus for measuring strain and temperature distribution with high accuracy using an optical fiber.

  Research and development related to optical fiber sensing technology that measures strain or temperature distribution using optical fibers has been actively promoted for application to health monitoring of structures such as high-rise buildings, bridges, dams, airplanes, ships, and tunnels. Trial experiments are also being conducted.

  Optical fiber sensing techniques for measuring strain or temperature distribution using an optical fiber include [1] a method using Brillouin scattering in an optical fiber and [2] a method using Rayleigh scattering. As described in Non-Patent Document 1, the method of [1] is a distortion in which the frequency distribution of Brillouin scattered light, which is one of backscattered light generated when an optical pulse is incident on an optical fiber, is applied to the optical fiber. Alternatively, the strain or temperature distribution is measured by utilizing the characteristic of shifting in proportion to the temperature. In the method [2], as described in Non-Patent Document 2 and Non-Patent Document 3, a Rayleigh scattering waveform obtained by coherent OTDR (Optical Time Domain Reflectometer) with optical frequency controlled exhibits a zigzag waveform, This is a method for measuring the strain or temperature distribution by utilizing the fact that the zigzag waveform changes when the strain or temperature applied to the optical fiber changes.

  Furthermore, as another method using Rayleigh scattering, Non-Patent Document 4 reports a strain or temperature distribution measurement method using a Rayleigh scattering waveform obtained by coherent OFDR (Optical Frequency Domain reflectometer).

  However, when the method using the Brillouin scattering in the optical fiber of [1] is applied to the health monitoring of the structure, there is a problem that the measurement accuracy is insufficient as described in Non-Patent Document 5. . In addition, in the method using [2] coherent OTDR, in principle, the strain or temperature change can be detected with higher sensitivity and higher accuracy than the measurement method using Brillouin scattering in [1]. Absent. Furthermore, the coherent OFDR method, which is another method using Rayleigh scattering, can determine the amount of change in strain or temperature in the same manner as the measurement method using Brillouin scattering, but is strict with respect to the phase of the measurement light. Since the conditions are imposed, it is difficult to measure a long-distance optical fiber. Therefore, what is reported in Non-Patent Document 4 is a result of measurement with respect to an optical fiber of 20 m.

T.A. Kurashima, et. al. "Brillouin optical fiber time domain reflectometry", IEICE Trans. Comun. , Vol. E76-B, no. 4, pp. 382-390, 1994. Oyamada, “Proposal of high-sensitivity strain distribution measurement method for optical fiber using Rayleigh scattering”, IEICE Technical Report, OFT 98-23, 1998. Y. Koyamada, et. al. "Novel fiber-optical strain and temperature sensor with very high resolution", IEICE Trans. Comun. , Vol. E89-B, no. 5, pp. 1722-1725, 2006. B. J. et al. Soller, et. al. , “Measurement of localized heating in fiber optical components with millimeter spatial resolution”, OFC 2006, OFN 3, 2006. Jin Wu, “Recent Trends of Optical Fiber Sensing in Structural Health Monitoring,” IEICE Optical Fiber Applied Technology Research Group 2nd Class Study Material, OFT 2004-2-04, 2004.

  As described above, in order to apply the strain or temperature distribution method using the conventional optical fiber to the health monitoring of the structure, any method has problems in practicality, accuracy, and measurable distance, There is a need for a practical, highly accurate and long-range measurement method.

  The present invention has been made in view of the above circumstances. Strain / temperature distribution measurement using an optical fiber that can measure strain and temperature changes over a long distance as a practical distribution state along the axial direction of the optical fiber. It is an object to provide a method and a measuring device.

  In order to achieve the above object, the strain / temperature distribution measuring method using the optical fiber of the present invention is the optical fiber for sensing that repeats optical pulses while changing the optical frequency before and after the strain or temperature change is applied to the sensing optical fiber. A first step of acquiring and accumulating Rayleigh scattered light data returned from the sensing optical fiber and obtaining a correlation peak frequency based on the data acquired and accumulated in the first step, and the correlation peak frequency The second step of calculating the axial strain change and temperature change of the optical fiber using the relationship between the strain change amount, temperature change amount, and optical frequency change amount of the optical fiber.

を計算し、相関のピークの周波数を求め、あらかじめ求めておいた歪変化量Δε_ｃおよび温度変化量ΔＴと光周波数の変化量ｆ_ｃの関係を用いて光ファイバの軸方向の歪変化Δεおよび温度変化ΔＴを算出することを特徴とする。 In the strain / temperature distribution measuring method using the optical fiber of the present invention, the optical pulse is repeatedly incident on the sensing optical fiber while changing the optical frequency ν before and after the strain or temperature change is applied to the sensing optical fiber. Data p _u (ν, z) (before change) and p _v (ν, z) obtained by acquiring Rayleigh scattered light returned from the sensing optical fiber and storing the scattered light power as a function of optical frequency ν and distance z Cross-correlation on the frequency axis from (after change)

Was calculated to obtain the frequency of the peak of the correlation, strain changes [Delta] [epsilon] and the axial direction of the optical fiber using the relationship in advance the distortion change amount had been determined [Delta] [epsilon] _c and the temperature variation ΔT and the optical frequency variation f _c A temperature change ΔT is calculated.

According to the present invention, in the distribution measuring method of the strain and temperature with the optical fiber, axial cross-correlation R _{uv (f,} z) is the optical fiber than the peak frequency f _c to be 0.7 or more at each position z The strain change Δε and the temperature change ΔT are calculated.

  The strain / temperature distribution measuring apparatus using the optical fiber according to the present invention causes the optical fiber to repeatedly enter the sensing optical fiber while changing the optical frequency before and after the strain or temperature change is applied to the sensing optical fiber, and the sensing optical fiber. A first means for acquiring and accumulating data of Rayleigh scattered light returned from the optical fiber; a correlation peak frequency is obtained based on the data acquired and accumulated by the first means; and a distortion change of the optical fiber is calculated from the correlation peak frequency. And a second means for calculating an axial strain change and a temperature change of the optical fiber using the relationship between the amount, the temperature change amount, and the optical frequency change amount.

Further, the strain / temperature distribution measuring apparatus using the optical fiber of the present invention can set the optical frequency stably and accurately, can shift the optical frequency by a predetermined amount, can output continuous light, and the light source unit. A first optical coupler that divides the emitted continuous light into two, a modulator that pulses the continuous light emitted from one of the first optical couplers, and an optical pulse from the modulator that enters the sensing optical fiber And a second optical coupler that guides Rayleigh scattered light returned from the sensing optical fiber to a light receiving unit, and a polarization that scrambles the polarization of local light that is continuous light emitted from the other of the first optical couplers. A wave controller, a third optical coupler for mixing the Rayleigh scattered light and the local light, a heterodyne detector for detecting the Rayleigh scattered light and the local light mixed by the third optical coupler, An A / D converter for converting the detection signal from the heterodyne detector into a digital signal, the output signal from the A / D converter is inputted, the Rayleigh scattered light before and after the strain or temperature change is applied to the sensing optical fiber The peak frequency of cross-correlation on the frequency axis between the Rayleigh scattered light power before the strain or temperature change can be accumulated and the Rayleigh scattered light power after the strain or temperature change, and the strain change of the optical fiber And a computer that calculates a strain change and a temperature change in the axial direction of the optical fiber using the relationship between the temperature change amount and the optical frequency change amount.

Further, the strain / temperature distribution measuring apparatus using the optical fiber of the present invention can set the optical frequency stably and accurately, can shift the optical frequency by a predetermined amount, can output continuous light, and the light source unit. A modulator for pulsing the emitted continuous light; an optical coupler for causing the optical pulse from the modulator to enter the sensing optical fiber and guiding the Rayleigh scattered light returned from the sensing optical fiber to a light receiving unit; and A photo-counting light-receiving unit that detects Rayleigh scattered light from an optical coupler, and a detection signal from the photo-counting light-receiving unit are input , and Rayleigh scattered light data before and after a strain or temperature change is applied to the sensing optical fiber. Between the Rayleigh scattered light power before the strain or temperature change is applied and the Rayleigh scattered light power after the strain or temperature change is applied. Comprising: a peak frequency of the cross-correlation, and a computer using the relationship between the strain variation and temperature variation, the optical frequency variation of an optical fiber for calculating the distortion change or the temperature change in the axial direction of the optical fiber on the Number of axes It is characterized by doing.

  The strain and temperature distribution measuring method and measuring apparatus using the optical fiber of the present invention can easily measure the strain or temperature change applied to the optical fiber with high accuracy and in the axial direction of the long-distance optical fiber. Is possible. The strain / temperature distribution measuring method and measuring apparatus using the optical fiber of the present invention can be applied to health monitoring of structures such as high-rise buildings, bridges, dams, airplanes, ships, and tunnels.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIGS. 1A and 1B are configuration explanatory views showing a strain / temperature distribution measuring apparatus using an optical fiber according to an embodiment of the present invention. In order to perform measurement with a distance resolution of about 10 cm, it is necessary to use a highly sensitive light receiver, and use of a heterodyne detector (FIG. 1 (a)) or a photocounter (FIG. 1 (b)) can be considered.

  First, a system using the heterodyne detector shown in FIG. As shown in FIG. 1 (a), as an example of the configuration of a light source unit that can stably and accurately set an optical frequency, shift an optical frequency by a predetermined amount, and output continuous light, here, an optical frequency stabilized DFB- The structure which combined LD and the SSB modulator is shown.

  DFB-LD (distributed feedback laser diode) is used as a light source (center frequency fluctuation range: <10 MHz) 1 that stabilizes the optical frequency by utilizing molecular absorption lines such as hydrogen cyanide gas, and the optical frequency is changed at regular intervals. In order to obtain a Rayleigh scattering waveform, a predetermined amount of frequency shift is performed through the SSB modulator 2 as a frequency shifter of continuous light emitted from the DFB-LD. A high frequency signal is input to the SSB modulator 2 from a high frequency transmitter 3. The light source unit shown here is an example, and any other configuration may be used as long as the optical frequency can be accurately set by stabilization, the optical frequency can be shifted by a predetermined amount, and continuous light can be output.

Further, the continuous light emitted from the SSB modulator 2 the first optical coupler 4 ₁ by divided into two parts, to one of the signal light, and the other local light. The signal light is pulsed by an electro-optic (EO) modulator as the optical modulator 6 and is incident on the sensing optical fiber 9 via the _second optical coupler 42. A pulse signal is input from the pulse generator 5 to the optical modulator 6. In the case where the Rayleigh scattering wave is affected by polarization dependence performs polarization scrambling using the first polarization controller (PC) 7 ₁ in order to remove the influence, also the power of the pulse If it is insufficient, an optical amplifier 8 must be inserted and amplified. FIG. 1A shows the case where these are inserted.

Further, after Rayleigh scattered light returning from the sensing optical fiber 9 through the second optical coupler 4 _2, are mixed with divided local light by the first optical coupler 4 ₁ 3dB optical coupler 10, a heterodyne light Heterodyne detection is performed by the detector 11. In this case, the local light is polarization scrambled by the _second polarization controller 72 in order to remove noise due to polarization mismatch between the Rayleigh scattered light and the local light. The detection signal from the heterodyne photodetector 11 is converted from an analog signal to a digital signal by the A / D converter 12, and then sent to the computer 13, where the data is accumulated after the mean square process. The average number of times per frequency is 1000 to 10,000. The above measurement is repeated step by step while changing the optical frequency. The computer 13 calculates the axial strain change and temperature change of the optical fiber using the relationship between the correlation peak frequency of the Rayleigh scattered light power and the strain change amount, temperature change amount, and optical frequency change amount of the optical fiber. .

  Next, a system using the photo counter shown in FIG. As shown in FIG. 1B, as an example of the configuration of the light source unit that can set the optical frequency stably and accurately, can shift the optical frequency by a predetermined amount, and can output continuous light, here, the optical frequency stabilized DFB- The structure which combined LD and the SSB modulator is shown.

  DFB-LD (distributed feedback laser diode) is used as a light source (center frequency fluctuation range: <10 MHz) 1 that stabilizes the optical frequency by utilizing molecular absorption lines such as hydrogen cyanide gas, and the optical frequency is changed at regular intervals. In order to obtain a Rayleigh scattering waveform, a predetermined amount of frequency shift is performed through the SSB modulator 2 as a frequency shifter of continuous light emitted from the DFB-LD. A high frequency signal is input to the SSB modulator 2 from a high frequency transmitter 3. The light source unit shown here is an example, and any other configuration may be used as long as the optical frequency can be accurately set by stabilization, the optical frequency can be shifted by a predetermined amount, and continuous light can be output.

  Further, the continuous light emitted from the SSB modulator 2 is pulsed by an electro-optic (EO) modulator as the optical modulator 6 and is incident on the sensing optical fiber 9 through the optical coupler 4. A pulse signal is input from the pulse generator 5 to the optical modulator 6. If the Rayleigh scattering waveform has a polarization-dependent effect, polarization scrambling is performed using the polarization controller (PC) 7 to eliminate the influence, and the pulse power is insufficient. In this case, it is necessary to amplify by inserting the optical amplifier 8. FIG. 1B shows the case where these are inserted.

  Further, Rayleigh scattered light returning from the sensing optical fiber 9 is input to the photocounting light receiving unit 16 including the photocounting light detection unit 14 and the counting module 15 via the optical coupler 4, and Rayleigh scattered by the photocounting light receiving unit 16. Light is detected. A detection signal from the photocounting light receiving unit 16 is sent to a computer 13, which performs data accumulation and cross-correlation processing of Rayleigh scattered light. The average number of times per frequency is 1000 to 10,000. The above measurement is repeated step by step while changing the optical frequency. The computer 13 calculates the axial strain change and temperature change of the optical fiber using the relationship between the correlation peak frequency of the Rayleigh scattered light power and the strain change amount, temperature change amount, and optical frequency change amount of the optical fiber. .

  The basic operation of the system of FIG. 1B using the photocounting light receiving unit is the same as that of the system using the heterodyne detector of FIG. The difference is that mixing of Rayleigh scattered light and local light is not performed, and the detection signal is averaged without being squared. Further, the above-described constituent optical component is an example, and other components may be used as long as the above functions are satisfied.

ここで、ａ_ｉとτ_ｉはそれぞれｉ番目の散乱光の振幅と遅延時間、Ｎは光ファイバ中の全散乱点数、αは光ファイバの損失係数、ｃは真空における光速、ｎ_ｆは光ファイバの屈折率である。また、−１／２≦（ｔ−τ_ｉ）／Ｗ≦１／２のときｒｅｃｔ（（ｔ−τ_ｉ）／Ｗ）＝１であり、その他のときはｒｅｃｔ（（ｔ−τ_ｉ）／Ｗ）＝０とする。遅延時間τ_ｉはｉ番目の散乱点の入射端からの距離ｚ_ｉとτ_ｉ＝２ｎｆｚ_ｉ／ｃなる関係で結ばれる。項ｒｅｃｔ（（ｔ−τ_ｉ）／Ｗ）は光パルスが伝搬するにしたがって、散乱点の分布が変化することを表している。後方散乱光パワーｐ（ｔ）は次式で与えられる。 Next, the measurement principle of the present invention will be described in detail. First, it is assumed that the amplitude of the optical pulse is 1, the pulse width is W, the optical frequency is ν, and the pulse center enters the optical fiber at time t = 0. At this time, the electric field e (t) of the light wave that is Rayleigh scattered at each point in the optical fiber and returns to OTDR at time t, and the Rayleigh scattering waveform of the coherent OTDR is analyzed using an impulse response model. Can do.

Here, a _i and τ _i are the amplitude and delay time of the i-th scattered light, N is the total number of scattering points in the optical fiber, α is the loss factor of the optical fiber, c is the speed of light in vacuum, and n _f is the optical fiber. Is the refractive index. Also, when -1 / 2 ≦ (t−τ _i ) / W ≦ 1/2, rect ((t−τ _i ) / W) = 1, and at other times rect ((t−τ _i ) / W) = 0. The delay time τ _i is connected to the distance z _i from the incident end of the i-th scattering point and the relationship τ _i = 2nfz _i / c. The term rect ((t−τ _i ) / W) represents that the distribution of scattering points changes as the light pulse propagates. The backscattered light power p (t) is given by the following equation.

で与えられ、φ_ｉｊ＝２πν（τ_ｉ−τ_ｊ）である。φ_ｉｊはｉ番目の散乱点から戻る光波とｊ番目の散乱点から戻る光波の位相差を表す。なお、式（１）〜式（４）において、重要でない係数は１と置いている。 _{^{p (t) = | e (}} t) | 2 = p a (t) + p b (t) (2)
Where p _a (t) and p _b (t) are

And φ _ij = 2πν (τ _i −τ _j ). φ _ij represents the phase difference between the light wave returning from the i-th scattering point and the light wave returning from the j-th scattering point. In the equations (1) to (4), an unimportant coefficient is set to 1.

The right side of Equation (3) indicates the light power scattered at each scattering point, and p _a (t) is the sum. When p _a (t) is displayed as a function of time t, an OTDR waveform obtained by transmitting an incoherent optical pulse is obtained, which is almost independent of the strain and temperature of the optical fiber and the frequency of the optical pulse. Therefore, p _a (t) does not change depending on the strain and temperature of the optical fiber and the frequency of the optical pulse.

On the other hand, p _b (t) indicates interference between light waves scattered at each scattering point. This term is zero for incoherent light pulses. On the other hand, when a coherent light pulse is incident, the OTDR waveform has an effect of making a zigzag waveform. Includes a cos [phi _ij on the right side of the equation _{(4), φ ij = 4πν} (n f s ij / c), s ij = z by i -z _j, φ _ij is the optical frequency [nu, refractive index _{n f} , And the scattering point spacing s _ij . Therefore, p _b (t) is a function of ν, n _f , and s _ij . Furthermore, since the refractive index n _f and the scattering point interval s _ij depend on the strain and temperature of the optical fiber, p _b (t) also depends on the strain and temperature of the optical fiber. Therefore, the waveform measured by coherent OTDR varies depending on the strain or temperature applied to the optical fiber. Thus, the OTDR waveform obtained by injecting a coherent light pulse into the optical fiber exhibits amplitude fluctuation due to interference of light returning from each scattering point. This fluctuation waveform depends on the amplitude of the scattered light returning from each point and the phase difference between the scattered lights, and the phase difference is determined by the delay time difference and the optical frequency. When a strain or temperature change occurs in the optical fiber, the optical path length and the refractive index change, so the delay time difference between the scattered lights changes, and the OTDR waveform changes accordingly.

The Rayleigh scattering waveform is repeatedly acquired while changing the optical frequency ν, and the scattered light power is accumulated as a function p (ν, z) of the optical frequency ν and the distance z. Here, z is the amount converted from the time t by z = ct / 2n _f, represents the distance to the scattering center point of the scattered light back the OTDR at time t. The scattered light power measured at the first time point u is defined as p _u (ν, z), and the scattered light power measured at the time point v when strain or temperature change occurs in the optical fiber is defined as p _v (ν, z). A mutual function R _uv (f, z) on the frequency axis of p _u (ν, z) and p _v (ν, z) is obtained by the following equation.

For simplicity, ignore the noise. If there is no strain or temperature change between the first time point u and the next time point v, then R _uv (f, z) is maximum at f = 0, ie R _uv (f, z) = 1. If there is a change in strain or temperature, f = _{f c} in _R uv (f, z) is maximum, i.e. _{R uv (f, z) =} 1 and becomes. Here, f _c represents the distortion (and temperature) variation of only the optical frequency return Rayleigh scattering waveform in the waveform at time u to compensate for changes in the. The relationship between the strain change amount Δε _c and the temperature change amount ΔT and the optical frequency change amount f _c greatly depends on the coating material coated on the outer circumference of the optical fiber to be used and the shape and diameter of the coating. Therefore, it is necessary to grasp in advance using a coated optical fiber that uses the relationship between the strain change amount Δε _c and the temperature change amount ΔT and the optical frequency change amount f _c . Here, as an example, a case where the influence of the coating on the outer periphery of the optical fiber is very small will be described. In this case, the optical frequency change amount f _c for compensating for the strain change amount Δε _c and the temperature change amount ΔT is given by the following equation.

f _c / ν = − (1−p _e ) Δε≈−0.78 × Δε (6)
_{f c / ν = - (γ} s + γ n) ΔT ≒ - (9.2 × 10 -6) ΔT (7)
Where p _e = n ² f / 2 {p ₁₂ −σ (p ₁₁ + p ₁₂ )} (8)
Here, p ₁₁ and p ₁₂ are photoelastic coefficients, σ is a Poisson ratio, γ _s = (1 / s _ij ) (ds _ij / dT) is a thermal expansion coefficient, and γ _n = (1 / n _f ) (dn _f / DT) is the light temperature coefficient. As can be seen from the above description, the cross-correlation R _uv (f, z) is calculated from the scattered light powers p _u (ν, z) and p _v (ν, z) measured at the time points u and v, and the maximum value thereof is calculated. The strain change amount Δε _c or the temperature change amount ΔT can be obtained from the change amount f _{c of the} optical frequency that gives the value using Equation (6) or Equation (7).

Computer simulation was performed for the above method.
FIG. 2 is a configuration explanatory view showing a test optical fiber in a simulation according to the embodiment of the present invention. As shown in FIG. 2, from the seven sections A (50 cm), B (10 cm), C (90 cm), D (20 cm), E (80 cm), F (100 cm), and G (50 cm) for the simulation. Consider a test optical fiber having a total length of 4 m. The test optical fiber temperature is always constant. In the initial state (time point u), it is assumed that the strain in all the sections is zero, and at the time point v, a strain of 100 με is added to the sections B, D, and F. It is assumed that a strain distribution measurement is performed by entering an optical pulse having a wavelength of 1.5 μm and a width of 1 ns into the test optical fiber. The optical frequency of the pulse is changed over 100 GHz in 500 MHz steps, and the Rayleigh scattered waveform is acquired at each frequency. The scattered light power obtained by computer simulation is a function of the optical frequency ν and the distance z to the scattering point. As shown in FIG.

  FIG. 3 is an explanatory diagram showing the Rayleigh scattered light power p (ν, z) from the test optical fiber according to the embodiment of the present invention. As shown in FIG. 3, the Rayleigh scattering waveform is like a random number, and it can be distinguished even by comparing the power distribution (a) in the initial state with the power distribution (b) after applying the strain with the human eye. Absent. FIG. 4 shows the result of calculating the cross-correlation using the formula (5) for the two power distributions.

  FIG. 4 is an explanatory diagram showing the cross-correlation obtained from the Rayleigh scattered light power of FIG. As shown in FIG. 4, in the section A (z = 0-50 cm), C (z = 60-150 cm), E (z = 170-250 cm), and G (z = 350-400 cm), f = 0, the section In B (z = 50-60 cm), D (z = 150-170 cm), and F (z = 250-350 cm), it can be seen that a correlation peak appears in the vicinity of f = −15.6 GHz. FIG. 5 shows the strain distribution in the test optical fiber obtained by converting the correlation peak frequency into strain using Equation (6) and comparing it with the actual value.

FIG. 5 is a characteristic diagram showing the strain distribution (when there is no noise) in the test optical fiber obtained based on the results of FIG. 4, where the thick line is the simulation value and the thin line is the actual strain application state value. Note that the correlation value is meaningless only at the maximum value (peak), and is considered to be meaningful only when the value exceeds a certain value. Therefore, here, at the position (Z) where the correlation peak exceeds 0.7, the amount of distortion converted from the peak frequency is the magnitude of distortion at that position, and the magnitude of distortion at a position below 0.7 is: It is obtained as an intermediate value of the magnitude of distortion at the positions before and after that. It can be seen that the simulation value closely follows the actual value and can be measured with a distance resolution of 10 cm. Thus, the light that gives the maximum value by calculating the cross-correlation R _uv (f, z) from the scattered light powers p _u (ν, z) and p _v (ν, z) measured at the time points u and v. It was confirmed by simulation that the strain change amount Δε _c or the temperature change amount ΔT can be obtained from the frequency change amount f _{c using the} equation (6) or the equation (7).

  Finally, since the simulation ignored the noise, in order to know the influence of the noise, a simulation was performed when the signal-to-noise ratio (SNR) was 10 dB and 5 dB. The results are shown in FIG. 6 and FIG.

  FIG. 6 is a characteristic diagram showing a strain distribution (in the case of SNR = 10 dB) along the test optical fiber according to the embodiment of the present invention, where a bold line is a simulation value, and a thin line is an actual strain application state value. FIG. 7 is a characteristic diagram showing a strain distribution (in the case of SNR = 5 dB) along the test optical fiber according to the embodiment of the present invention, where a bold line is a simulation value and a thin line is an actual strain application state value.

  FIG. 6 shows that when SNR = 10 dB, the influence of noise is not so much. The difference between the simulation result and the actual value is 1 με or less. The measurement error by the strain distribution measuring device using Brillouin scattering is reported to be 10 to 50 με, and it can be expected that the error by this measurement method is 1 to 2 orders of magnitude smaller than this. FIG. 7 shows that when SNR = 5 dB, the strain in the section B (z = 50-60 cm) is not measured.

From these results, it is considered that this measurement method requires an SNR of approximately 10 dB or more. For heterodyne detection type (FIG. 1 (a)), the pulse width 1 ns (distance resolution 10cm corresponding), the receiver bandwidth 1 GHz, the average number per 25 dBm, 1 frequency input power to the optical fiber 10 ⁴ Then, the SNR is expected to be 15 dB. Therefore, there is a margin of 5 dB for the required SNR = 10 dB, and if this is directed to the optical fiber loss (0.25 dB / km, considering round trip loss), it is possible to measure an optical fiber having a length of 10 km It is. In the photo counting type (FIG. 1B), further performance improvement can be expected.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. Further, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, you may combine suitably the component covering different embodiment.

1 is a configuration explanatory view showing a strain / temperature distribution measuring apparatus using an optical fiber according to an embodiment of the present invention. FIG. It is composition explanatory drawing which shows the test optical fiber in the simulation which concerns on embodiment of this invention. It is explanatory drawing which shows the Rayleigh scattered light power p ((nu), z) from the test optical fiber which concerns on embodiment of this invention. It is explanatory drawing which shows the cross correlation calculated | required from the Rayleigh scattered light power of FIG. It is a characteristic view which shows the distortion distribution (when there is no noise) in the test optical fiber calculated | required based on the result of FIG. It is a characteristic view which shows the distortion distribution (in the case of SNR = 10 dB) along the test optical fiber which concerns on embodiment of this invention. It is a characteristic view which shows the distortion distribution (in the case of SNR = 5 dB) along the test optical fiber which concerns on embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Optical frequency stabilization light source, 2 ... SSB modulator, 3 ... High frequency transmitter, 4 ₁ ... 1st optical coupler, 4 ₂ ... 2nd optical coupler, 5 ... Pulse generator, 6 ... Optical modulator, 7 DESCRIPTION OF SYMBOLS ₁ ... 1st polarization controller (PC), 7 ₂ ... 2nd polarization controller, 8 ... Optical amplifier, 9 ... Optical fiber for sensing, 10 ... 3dB optical coupler, 11 ... Heterodyne photodetector, 12 ... A / D converter, 13 ... Computer, 14 ... Photo counting light detection unit, 15 ... Counting module, 16 ... Photo counting light receiving unit.

Claims (6)

First, the light pulse is repeatedly made incident on the sensing optical fiber while changing the optical frequency before and after the strain or temperature change is applied to the sensing optical fiber, and data of Rayleigh scattered light returned from the sensing optical fiber is acquired and accumulated. And the steps
The correlation peak frequency is obtained based on the data acquired and accumulated in the first step, and the axial direction of the optical fiber is calculated from the correlation peak frequency using the relationship between the strain change amount, temperature change amount, and optical frequency change amount of the optical fiber. A strain / temperature distribution measuring method using an optical fiber, comprising a second step of calculating a strain change and a temperature change. The light pulse is repeatedly incident on the sensing optical fiber while changing the optical frequency ν before and after the strain or temperature change is applied to the sensing optical fiber, and the Rayleigh scattered light returned from the sensing optical fiber is acquired, and the scattered light power is obtained. On the frequency axis from data p _u (ν, z) (before change) and p _v (ν, z) (after change) that are stored as a function of optical frequency ν and distance z

Was calculated to obtain the frequency of the peak of the correlation, strain changes [Delta] [epsilon] and the axial direction of the optical fiber using the relationship in advance the distortion change amount had been determined [Delta] [epsilon] _c and the temperature variation ΔT and the optical frequency variation f _c A strain or temperature distribution measuring method using an optical fiber, characterized by calculating a temperature change ΔT. The cross-correlation R _{uv (f,} z) at each position z is in claim 2, characterized in that to calculate the distortion change Δε and the temperature variation ΔT in the axial direction of the optical fiber than the peak frequency f _c to be 0.7 or more A strain or temperature distribution measuring method using the described optical fiber. First, the light pulse is repeatedly made incident on the sensing optical fiber while changing the optical frequency before and after the strain or temperature change is applied to the sensing optical fiber, and data of Rayleigh scattered light returned from the sensing optical fiber is acquired and accumulated. Means of
The correlation peak frequency is obtained based on the data acquired and accumulated by the first means, and the axial direction of the optical fiber is calculated from the correlation peak frequency using the relationship between the strain variation, temperature variation, and optical frequency variation of the optical fiber. A strain / temperature distribution measuring apparatus using an optical fiber, comprising: a second means for calculating a strain change and a temperature change. A light source that can stably and accurately set the optical frequency, shift the optical frequency by a predetermined amount, and output continuous light; and
A first optical coupler that divides the continuous light emitted from the light source unit into two parts;
A modulator for pulsing the continuous light emitted from one of the first optical couplers;
A second optical coupler that causes the optical pulse from the modulator to enter the sensing optical fiber and guides Rayleigh scattered light returned from the sensing optical fiber to a light receiving unit;
A polarization controller for scrambling the polarization of local light that is continuous light emitted from the other of the first optical couplers;
A third optical coupler that mixes the Rayleigh scattered light and the local light;
A heterodyne detector for detecting Rayleigh scattered light and local light mixed by the third optical coupler;
An A / D converter that converts a detection signal from the heterodyne detector into a digital signal;
The output signal from the A / D converter is inputted , and the Rayleigh scattered light data before and after the strain or temperature change is applied to the sensing optical fiber can be accumulated. Or the axis of the optical fiber using the relationship between the peak frequency of the cross-correlation between the Rayleigh scattered light power after the temperature change and the strain variation, temperature variation, and optical frequency variation of the optical fiber. A strain / temperature distribution measuring apparatus using an optical fiber, comprising: a computer that calculates a strain change in a direction and a temperature change. A light source that can stably and accurately set the optical frequency, shift the optical frequency by a predetermined amount, and output continuous light; and
A modulator for pulsing the continuous light emitted from the light source unit;
An optical coupler that causes the optical pulse from the modulator to enter a sensing optical fiber and guides Rayleigh scattered light returned from the sensing optical fiber to a light receiving unit;
A photocounting light receiving unit for detecting Rayleigh scattered light from the optical coupler;
The detection signal from the photo-counting light receiving unit is input , and Rayleigh scattered light data before and after the strain or temperature change is applied to the sensing optical fiber can be accumulated, and the Rayleigh scattered light power and distortion before the strain or temperature change is applied. Or the axis of the optical fiber using the relationship between the peak frequency of the cross correlation on the frequency axis between the Rayleigh scattered light power after the temperature change and the strain variation, temperature variation, and optical frequency variation of the optical fiber. A strain / temperature distribution measuring apparatus using an optical fiber, comprising: a computer that calculates a strain change in a direction and a temperature change.

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