JP2864825B2 - Physical quantity distribution detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a physical quantity distribution sensor for measuring a physical quantity distribution along an optical fiber using the optical fiber itself as a sensor, and more particularly to an optical sensor capable of diagnosing self-abnormality based on the measurement result. The present invention relates to a physical quantity distribution detection sensor using a fiber.

[0002]

2. Description of the Related Art As a physical quantity detection sensor using an optical fiber, a temperature distribution detection sensor for measuring a temperature distribution along an optical fiber is generally known. In this temperature distribution detecting sensor, as shown in FIG. 2, an optical pulse generated from one end of the optical fiber 4 by the light source 1 is transmitted to the optical fiber 4 via an optical branching / demultiplexing means 3 such as an optical coupler or an optical demultiplexer. Make it incident.
Then, of the scattered light generated at each position of the optical fiber as the light pulse incident on the optical fiber travels through the optical fiber, the scattered light returning to the light incident end is called back scattered light. The light branching and branching means 3, of the backscattered light, taken out by separating the light of wavelength lambda _s and anti-Stokes light of wavelength lambda _a Stokes light of the Raman scattered light, these respective light receiving circuit 5s, 5a To convert it into an electric signal. Then, these electric signals are sampled at time intervals ts, and the time functions g _s (t) and g _a (t) of the Stokes light and the anti-Stokes light are measured. The amount of scattered light generated in the optical fiber is weak, and the SN ratio of the time functions g _s (t) and g _a (t) is poor. Is repeated a number of times to obtain a time function g _s (t), g
_a G with improved SN ratio by averaging of (t)
_s (t) and G _a (t) are obtained. Here, if the speed of light v in the optical fiber is known, G _s (t) and G _a (t) measured as a function of time are plotted along the optical fiber defined for each sampling point at the sampling distance interval xs. It is possible to replace the distance functions G _s (x) and G _a (x). The distance function G _s thus obtained
(X) and G _a (x) are the measured backscattered light intensities of the Stokes light and the anti-Stokes light at one end of the optical fiber, respectively. Therefore, the probability R _s (x ) And R _a (x),
Equations (1) and (2) shown in equations (1) and (2) have the relationship shown in equations (1) and (2).

[0003]

(Equation 1)

[0004]

(Equation 2)

[0005] Here, P₀; Optical fiber incident light intensity α₀; Optical fiber transmission loss at the wavelength of incident light (dB / m) γ; backscattering coefficient α_s, Α_aLight of Stokes light and anti-Stokes light
Fiber transmission loss (dB / m) M_s, M_a; Stokes light and anti-light in the light receiving circuit
O / E conversion efficiency of Stokes light Therefore, Stokes light and anti-Stokes light at x point
The occurrence probabilities are shown in Equations 3 and 4, respectively (3) and (4)
Expressed by the equation, K_s, K_aIf you know the measured value G
_s(X), G_aIt can be determined using (x).

[0006]

(Equation 3)

[0007]

(Equation 4)

On the other hand, there is a relationship between the occurrence probabilities R _s and R _{a of} the Stokes light and the anti-Stokes light generated in the optical fiber and the temperature T as shown in Expression (5), and the distance x The temperature T (x) can be obtained by using Expression (6) shown in Expression 6.

[0009]

(Equation 5)

[0010]

(Equation 6)

Where k ₁ and k ₂ are the light source to be used and
This is a constant determined by the optical fiber.

The constant K can be obtained if the temperature T (xr) at the specific point xr of the optical fiber is known.
The transmission losses α _s and α _a of the optical fiber at the wavelengths of the optical fiber and the anti-stokes light can be measured in advance. Therefore, the temperature distribution along the optical fiber can be obtained by measuring the backscattered light intensity distributions G _s (x) and G _a (x) at the stokes light and anti-stokes light wavelengths. .

As can be seen from equation (6), even if the transmission losses α _s and α _a of the optical fiber change by Δα due to microbending loss or the like caused by external pressure on the optical fiber, the change is And the temperature distribution T (x) is
It can be obtained without being affected by this.

[0014]

However, the prior art has the following problems.

(1) Equation (6) for calculating the temperature distribution T (x) includes the transmission losses α _s and α _a of the optical fiber, and the values of the transmission losses α _s and α _a are measured in advance. Obtained by: When measuring the temperature distribution, when these values are different from the previously measured values, or when the change Δα
Is different depending on the wavelength, the changes Δα _s and Δα _a are not canceled out, so that the measurement of the temperature distribution is not accurate.

(2) If the wavelength of the light source used for measurement deviates from the initially set wavelength, the stokes light and anti-stokes light wavelengths also deviate from their initial values. For this reason, the backscattered light intensity distributions G _s (x) and G _a (x) of the stokes light and the anti-stokes light cannot be measured correctly, and the measurement of the temperature distribution becomes inaccurate. On the other hand, since the wavelength of the light source is easily changed depending on its temperature, temperature control is usually performed in the light source so that a constant wavelength can be obtained. If the temperature control is malfunctioning due to a failure or the like, the wavelength of the light source changes, and the measurement of the temperature distribution will not be accurate.

(3) When the amount of light incident on the optical fiber exceeds a certain level, stimulated Raman scattering, which is a non-linear phenomenon, occurs at the wavelength of the stokes light. In this case, the backscattered light intensity distribution G _s (x) of the stokes light cannot be measured correctly. On the other hand, as the amount of light incident on the optical fiber increases, the accuracy of temperature distribution measurement improves. Therefore, the amount of light is increased to the last so that stimulated Raman scattering does not occur. However, the output of the light source fluctuates depending on the ambient temperature and the like, so that the amount of light is increased to the bare limit so that stimulated Raman scattering does not occur.
Stimulated Raman scattering occurs due to this fluctuation, and the backscattered light intensity distribution G _s (x) of the Storks light cannot be measured correctly.

As described above, an abnormal case may occur in which the measurement of the temperature distribution is not accurate. However, conventionally, the correctness of the measured temperature distribution has to be compared with the temperature measured by another method. Was. In addition, even if the temperature distribution is incorrectly measured, the measurement result does not indicate that an abnormality has occurred, so that there is a problem that the temperature distribution is erroneously determined to be correct.

It is an object of the present invention to solve the above-mentioned problems and to provide a physical quantity distribution detection sensor using an optical fiber capable of performing self-diagnosis based on a measurement result.

[0020]

In order to achieve the above object, the present invention calculates the apparent occurrence probabilities of Stokes light and anti-Stokes light at one or more sampling points from measurement results of physical quantity distributions. An abnormality is detected based on a difference in apparent occurrence probability.

[0021]

Attention is paid to the fact that when the physical quantity distribution is measured accurately, the difference between the Stokes light occurrence probability and the anti-Stokes light occurrence probability is within a certain range. When the difference between the apparent two occurrence probabilities is out of a certain range, it can be determined that an abnormality has occurred that makes the measurement result inaccurate.

More specifically, the true probability of occurrence of Stokes light and the true probability of occurrence of anti-Stokes light R _0s (x) R _0a (x) at the sampling point x at the temperature T (x) are _{expressed by the following} equations. , Eq. (8) and (8).

[0023]

(Equation 7)

[0024]

(Equation 8)

Taking the difference between the two sides of equations (7) and (8) gives equation (9) shown in equation 9.

[0026]

(Equation 9)

Here, K ₀ (x) is a constant determined by the wavelength of the light source to be used, the material of the optical fiber, and the like, and is considered to be strictly different depending on the distance x, but takes a substantially constant value.

As shown in equation (9), the difference between the true occurrence probability of the Stokes light at the distance x and the true occurrence probability of the anti-Stokes light is constant.

On the other hand, G _s (x) and G _a (x) of the measurement results
From Equations (3) and (4), R _s (x), R
_a (x) also satisfies the relational expression equivalent to Expression (9), that is, Expression (10) shown in Expression 10 if these values are correct.

[0030]

(Equation 10)

However, when the amount of change in the transmission loss differs between the Stokes light and the anti-Stokes light, the wavelength of the light source shifts, or stimulated Raman scattering occurs, the apparent scattered light occurrence probability R obtained from the measurement result is obtained. _s (x), R
_a (x) does not satisfy the expression (10). Therefore, whether or not the measurement becomes inaccurate due to such an abnormality of the optical fiber or the apparatus is determined by the apparent scattered light occurrence probability R.
_It is sufficient to check whether _s (x) and _Ra (x) satisfy the expression (10).

Based on the above findings, the apparent occurrence probabilities of Stokes light and anti-Stokes light at the sampling point are calculated from the measurement results, and the difference between the apparent occurrence probabilities is calculated. Occurs, the difference between the two apparent occurrence probabilities deviates from a certain range, so that the abnormality can be detected.

[0033]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below in detail with reference to the accompanying drawings.

As shown in FIG. 1, the measuring system is a light source 1,
It comprises optical branching / demultiplexing means 3, optical fiber 4, light receiving circuit 5, sampling circuit 6, averaging processing circuit 7, control circuit 8, temperature distribution calculation circuit 9, temperature distribution display device 10, and abnormality diagnosis circuit 11. I have. The light receiving circuit 5, the sampling circuit 6, and the averaging processing circuit 7 are light receiving circuits 5s, 5a, sampling circuits 6s, 6a, and averaging processing circuits 7s, 7a for Stokes light and anti-Stokes light, respectively.
The light branching / demultiplexing means 3 guides the light from the light source 1 to the optical fiber 4, separates the Stokes light and the anti-Stokes light from the optical fiber 4, separates the light into light receiving circuits 5s, 5a
It is configured to lead to. In addition, a temperature measuring device 42 for measuring the temperature T (xr) is provided in an area 41 that defines a specific point xr of the optical fiber. Area 41 is
The temperature is kept constant by being surrounded by a soaking box. The configuration of FIG. 1 is the same as the conventional configuration of FIG.

The abnormality diagnosis circuit 11 includes an averaging processing circuit 7
The outputs of s and 7a and the output of the temperature measuring device 42 are input, and an abnormality is determined from the calculation result, which is transmitted to the temperature distribution display device 10. The details of this calculation will be described below.

First, in applying equation (10), the apparent occurrence probabilities R _s (x), R _{s of} Stokes light and anti-Stokes light are obtained from the measured values G _s (x) and G _a (x).
_a A method for obtaining (x) will be described.

When Expressions (3) and (4) are expressed for the specific point xr, Expressions (11) and (12) shown in Expressions 11 and 12 are obtained.

[0038]

[Equation 11]

[0039]

(Equation 12)

The equations (3) and (11), and the equations (4) and (12), taking the ratios of both sides thereof, can be rearranged into equations (13) and (14). Becomes

[0041]

(Equation 13)

[0042]

[Equation 14]

Next, the occurrence probability R _s (x
r) and R _a (xr) are calculated from the temperature T (xr) by (7),
A true value that can be obtained using equation (8), that is,
5, Equations (15) and (16) shown in Equation 16 are used, and Equation (17) shown in Equation 17 is further defined.

[0044]

(Equation 15)

[0045]

(Equation 16)

[0046]

[Equation 17]

Therefore, equations (13) and (14) can be expressed by equations (18) and (19) shown in equations (18) and (19).

[0048]

(Equation 18)

[0049]

[Equation 19]

The apparent occurrence probabilities R _s (x) and R _a (x) expressed by the equations (18) and (19) can be obtained from the measurement results.

Hereinafter, a method of determining an abnormality using the apparent occurrence probabilities R _s (x) and R _a (x) obtained by the equations (18) and (19) will be described.

When the left side of equation (10) is calculated using equations (18) and (19), equation (20) shown in equation 20 is obtained.

[0053]

(Equation 20)

[0054] Here, as a constant corresponding to equation (10) K ₀ (x), K _0min and (x) per K _0max (x) and the respective sampling points, determined in advance. It is determined whether Equation (21) shown in Equation 21 holds between R _s (x) -R _a (x), which can be obtained by Equation (20), and these constants.

[0055]

(Equation 21)

[0056] K _0min (x) and K _0max (x), taking into account the measured acceptable error is a constant that is set for each sampling point.

When there is a measurement result that does not satisfy the expression (21), the abnormality diagnosis circuit 11 judges an abnormality of the apparatus,
The temperature distribution display device 10 is informed of this, and the temperature distribution display device 10 is informed of the abnormality.

As described above, in the present invention,
The self-abnormality diagnosis based on the measurement result can be performed, and it is not necessary to confirm the measurement by another method, and the temperature distribution measured incorrectly is not mistaken as being correct.

Next, another embodiment of the present invention will be described.

In the above embodiment, the temperature distribution calculation circuit 9 and the abnormality diagnosis circuit 11 function completely independently, but the data processed by the abnormality diagnosis circuit 11 is used by the temperature distribution calculation circuit 9 to calculate the temperature. It is also possible to calculate the distribution. That is, in equation (22) shown in equation 22 derived from equation (5),
The occurrence probabilities R _s (x), R obtained by the equations (18) and (19)
_a (x) is substituted.

[0061]

(Equation 22)

The calculation functions of the temperature distribution calculation circuit 9 and the abnormality diagnosis circuit 11 can be replaced by software executed by a personal computer or a microcomputer.

In this embodiment, the temperature distribution detecting device is described in detail. If the temperature distribution detecting device measures and uses the distance distribution of Stokes light and anti-Stokes light generated in an optical fiber, other devices may be used. It is needless to say that the present invention can be applied to measurement of physical quantities such as radiation distribution.

[0064]

The present invention exhibits the following excellent effects.

(1) The self-diagnosis diagnosis based on the measurement result becomes possible, and it is not necessary to confirm the measurement by another method.

(2) Since an abnormality is displayed by the self-abnormality diagnosis, the temperature distribution measured incorrectly is not erroneously recognized as correct, and the reliability is improved.

[Brief description of the drawings]

FIG. 1 is a block diagram of a measurement system showing one embodiment of the present invention.

FIG. 2 is a block diagram of a measurement system showing a conventional example.

[Explanation of symbols]

 Reference Signs List 1 light source 4 optical fiber 5 light receiving circuit 9 temperature distribution calculation circuit 11 abnormality diagnosis circuit

Claims (1)

(57) [Claims] 1. A physical quantity distribution detector for detecting Stokes light and anti-Stokes light generated in an optical fiber, and calculating and measuring a physical quantity distribution along the optical fiber from a distribution of a probability of occurrence of these lights in a distance direction. The apparatus calculates the apparent occurrence probabilities of Stokes light and anti-Stokes light at one or more sampling points from the measurement results of the physical quantity distribution, and detects an abnormality based on the difference between the two apparent occurrence probabilities. Characteristic physical distribution detector.