JPH07218354A - Sensor for detecting distribution of physical quantity using optical fiber - Google Patents

Abstract

PURPOSE:To eliminate a measurement error due to transmission loss change and to detect the distribution of physical quantity with less measurement error by optically connecting a same side by providing two optical fibers and then obtaining transmission loss based on the ratio of rear scattered light measurement result at two locations. CONSTITUTION:Two optical fibers 10 connected to a detection device 20 are provided and are subjected to fusing connection at the remote edges. The near edges of the optical fibers 10 are connected to detectors and are inserted into a soaking case 40 which is constituted so that the internal temperature becomes constant. One out of the optical fibers 10 connected to the burnt case is connected to an OTDR measurement circuit 30 and a temperature sensor 50 is provided at the case 40. Then, the temperature distribution information from the circuit 30 and temperature information of the sensor 50 are input to a temperature distribution operation circuit 80 and a transmission loss (An) and a temperature (Tn) between sampling sections of the optical fiber 10 are obtained, thus eliminating a measurement error caused by the difference in the amount of transmission loss change at two wavelengths due to the influence of the surrounding environment, a measurement error due to the transmission loss change at the wavelength of a light source, and a measurement error in distance direction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a physical quantity detection sensor for detecting a physical quantity using light, and more particularly to a physical quantity distribution detection sensor for measuring a distribution of a physical quantity along an optical fiber by using the optical fiber itself as a sensor. It is a thing.

[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 the optical fiber is generally known. In this temperature distribution detecting sensor, as shown in FIG. 2, an optical pulse generated from the light source 1 from one end of the optical fiber 4 is sent 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, as the light pulse incident on this optical fiber travels in the optical fiber, the backscattered light returning to the light incident end among the scattered light generated at each position of the optical fiber is branched off.
The wavelength of the first scattered light and the wavelength of the second scattered light which are desired to be measured are separated and taken out by the demultiplexing means 3, and the O / E is obtained respectively.
The electric signals are converted by the converters 5s and 5a, sampled at time intervals ts, and the time functions g ₁ (t) and g ₂ (t) of the first and second scattered lights are measured. Since the amount of scattered light generated in the optical fiber is weak and the SN ratio of the time functions g ₁ (t) and g ₂ (t) is poor, in order to improve this, the operation from the incidence of the optical pulse to the sampling is improved. Is repeated many times, and time functions g ₁ (t), g ₂
G in which the SN ratio is improved by performing the averaging process of (t)
₁ (t) and G ₂ (t) are obtained, respectively. Here, if the speed of light v in the optical fiber is known, G ₁ (t) and G ₂ (t) measured as a function of time are measured along the optical fiber defined for each sampling point of the sampling distance interval xs. The distance functions G ₁ (x) and G ₂ (x) can be replaced. The distance function G ₁ thus obtained
Since (x) and G ₂ (x) are the backscattered light intensities of the first and second wavelengths, respectively, measured at one end of the optical fiber, the probability R of occurrence of scattered light at the point xm is R. ₁
The relations (x) and R ₂ (x) have the relationships shown in the equations (1) and (2) shown in the equations 1 and 2, respectively.

[0003]

[Equation 1]

[0004]

[Equation 2]

Where P ₀ : optical fiber incident light intensity α ₀ ; incident light wavelength optical fiber transmission loss dB / m γ; backscattering coefficient α ₁ , α ₂ ; first and second wavelength optical fiber transmission Loss M ₁ , M ₂ ; O / E conversion efficiency of the first and second wavelengths Therefore, the probability of occurrence of the first and second scattered light at the xm point is
Expressions (3) and (4) shown in Equations 3 and 4 respectively represent K
_{If 1} and K ₂ are known, it can be determined using the measured values G ₁ (x) and G ₂ (x).

[0006]

[Equation 3]

[0007]

[Equation 4]

On the other hand, there is a relationship between the occurrence probabilities R1 and R2 of the scattered light of the first and second wavelengths generated in the optical fiber and the temperature T as shown in the equation (5). The temperature T (X) at the distance x can be obtained by using the equation (6) shown in Equation 6.

[0009]

[Equation 5]

[0010]

[Equation 6]

In this way, the temperature distribution T (X) along the optical fiber can be obtained. The details of these relationships are as follows. That is, the Stokes light of Raman scattered light is used as the scattered light of the first wavelength, and the anti-stock light of Raman scattered light is used as the scattered light of the second wavelength. The occurrence probabilities Rs and Ras of these scattered lights and the temperature T have the relationship shown in the equation (7) shown in the equation 7. Therefore, the temperature at the distance x is expressed by the equation (8)
It is represented by.

[0012]

[Equation 7]

Where k ₁ and k ₂ are the light sources used and
It is a constant determined by the optical fiber.

[0014]

[Equation 8]

The constant K can be determined by knowing the temperature T (xr) of the specific point xr of the optical fiber, and the constant
The transmission loss α ₁ and α ₂ of the optical fiber at the wavelengths of the cox light and the anti-stock 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 ₁ (x) and G ₂ (x) at the wavelengths of the stokes light and the anti-stokes light. .

Further, as can be seen from the equation (8), even if the transmission losses α ₁ and α ₂ of the optical fiber are changed by Δα due to the microbend loss or the like caused by the external pressure on the optical fiber, this change is offset. , The temperature distribution T (X) can be obtained without being affected by this.

However, the equation (8) gives the transmission losses α ₁ , α ₂
When these values are different from the previously measured values, or when these Δα are different depending on the wavelength, the temperature distribution T (X) cannot be accurately measured.

As a method for solving such a problem,
JP-A-62-110160 is invented. According to this patent, the backscattered light from the optical fiber having a single wavelength or adjacent wavelengths is measured from both ends of the optical fiber, and the transmission loss of the optical fiber is measured by using the measurement results from the both ends. It is said that the physical quantity distribution along the optical fiber can be obtained without being affected by. this is,
When the backscattered light is measured from both ends of the optical fiber, for example, the result G _A (X) of the backscattered light intensity from one point X of the optical fiber measured from one end A of the optical fiber is given by the formula (9) shown in Formula 9. Also, the result G _B (X) obtained from the other end B is the formula (10) shown in Formula 10, and the formula (10) obtained by multiplying both sides of Formulas (9) and (10) ( (11) is divided by the equation (12) shown in the equation 12 corresponding to the equation (11) when the physical quantity is known at the Xr point, and thus, at the X point as in the equations (13) and (14). It is based on the fact that the scattered light generation probability can be transformed into a form that is not affected by the transmission loss of the optical fiber.

[0019]

[Equation 9]

[0020]

[Equation 10]

[0021]

[Equation 11]

[0022]

[Equation 12]

[0023]

[Equation 13]

[0024]

[Equation 14]

Here, a (x) = α ₀ (x) + α
₁ (x) R (x); Probability of scattered light generation at point X of optical fiber L; Optical fiber length K _A , K _B ; Constant (corresponding to K ₁ and K ₂ in equations (3) and (4))

[0026]

However, in reality, the measurement result of the backscattered light of a single wavelength slightly includes the signal of the wavelength of the light source in addition to the signal of the wavelength to be measured. The transmission loss from the scattered light generation point to the measurement end differs depending on the transmission distance, and since the wavelength of the light source and the wavelength of the measured light are different and the transmission loss is also different, the scattered light generated at the same point is not The ratio of the light of the wavelength of the light source included in the measured light differs between the case of measuring at the A end and the case of measuring at the B end of the optical fiber. Therefore, the physical quantity distribution obtained from the signals measured from both ends of the optical fiber contains an error. To correct this error, it is necessary to obtain the transmission loss of the wavelength of the light source, but this transmission loss changes under the influence of the ambient environment such as radiation, so it is impossible to correct it, and the measurement error It has the drawback that it cannot be avoided.

Further, in the OTDR method for obtaining the distance distribution of the backscattered light intensity from the optical fiber, it is inevitable that noise generated in the measurement system is mixed into the measurement result to some extent, and the influence of this noise is the distance of the measurement result. They differ in the direction, and this appears as a measurement error of different magnitude in the distance direction in the result of measuring the distance distribution of the physical quantity. With this conventional method, this problem cannot be avoided.

Therefore, an object of the present invention is to provide the above-mentioned conventional drawbacks, that is, a measurement error caused by a difference in transmission loss change amount at two wavelengths due to the influence of the surrounding environment of the optical fiber, and a transmission loss change at the wavelength of the light source. It is an object of the present invention to provide a physical quantity distribution detection sensor with a small measurement error by completely overcoming or partially improving the measurement error in the distance direction caused by the OTDR measurement and the measurement error.

[0029]

In order to solve the above-mentioned problems, according to the present invention, first, two optical fibers are provided side by side, and one end on the same side of each optical fiber is optically connected to each other. .

Secondly, the ends of the two optical fibers installed side by side, which are opposite to the ends connected to each other, are connected to the detection device, respectively.

Thirdly, the scattered light of two or more wavelengths generated in the optical fiber is measured and the distribution information of the physical quantity along the optical fiber is detected.

Fourth, the physical quantity of a part of the two optical fibers provided side by side is measured by another physical quantity detection sensor, and the optical quantity is measured by using this measurement result and the measurement result of scattered light generated in the optical fiber. The physical quantity distribution along the fiber was measured.

Fifth, the light is incident only from one end of the two optical fibers provided side by side connected to the detector, and only the backscatter from this same end is measured as the scattered light from the optical fiber. I decided to do it.

Sixth, the transmission loss of each part of the optical fiber is obtained from the measurement result of the backscattered light intensity from the optical fiber, and the physical quantity distribution along the optical fiber is measured from this result and the backscattered light intensity measurement result. I chose

Seventh, in detecting the physical quantity, the transmission loss of each part of the optical fiber obtained from the measurement result of the backscattered light from the two ends connected to the detecting device for the two optical fibers provided side by side, and the detecting device. The backscattered light measurement result obtained by measuring only one optical fiber connected to is used.

[0036]

With the above construction, in the two optical fibers installed side by side, the temperatures of the parts installed side by side are equal to each other, and the transmission losses caused by the surrounding environment and the installed state are equal. Therefore, first, the backscattered light measurement result in the two optical fibers provided side by side at that portion is obtained. Based on the ratio of the backscattered light measurement results at these two locations,
The transmission loss here can be calculated. In this way, the optical fiber transmission loss of each part is obtained, and then the optical fiber transmission loss of each part and the physical quantity distribution along the optical fiber can be detected by removing the influence of the change. That is, physical quantity distribution detection with less measurement error is achieved.

Further, it is possible to detect a change in the ambient environment such as radiation from the change in the optical fiber transmission loss of each part thus obtained.

[0038]

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

As shown in FIG. 1, two optical fibers 10 connected to a detection device 20 for detecting the temperature distribution are provided at the far end 11 by fusion splicing. The far ends 11 may be optically connected to each other by a connector connection, a reflecting mirror, or the like, or the same optical fiber may be folded and configured by a bent portion. The two near ends of the optical fiber 10 are connected to the detection device 20, respectively, and are inserted into a soaking case 40 that is provided in the detection device 20 and that is configured to have a uniform internal temperature. There is. One of the two optical fibers 10 inserted into the soaking case 40 is connected to the OTDR measurement circuit 30. The soaking case 40 is provided with a temperature sensor 50 such as a platinum resistor, a thermocouple, and a thermistor. The temperature information in the soaking case 40 from the temperature sensor 50 is configured to be sent to the temperature distribution calculation circuit 80 together with the temperature distribution information from the OTDR measurement circuit 30. The temperature distribution calculation circuit 80 is provided with the soaking case 40.
An optical fiber transmission loss A (n) for each sampling section of the optical fiber 10 is obtained from the internal temperature information and the temperature distribution information of the optical fiber 10, and the temperature T (n) at each sampling point is obtained using this. Is.

Next, the operation of the embodiment will be described.

First, the calculation contents of the temperature distribution calculation circuit 80 will be described. The backscattered light intensity of the specific wavelength from the optical fiber 10 at the n-th sampling point measured by the OTDR measurement circuit 30 is expressed by Expression (15) shown in Expression 15.

[0042]

[Equation 15]

K = constant On the other hand, O of the optical fiber 10 including the inside of the detection device 20
The sampling point closest to the TDR measurement circuit 30 is designated as No. 1 and the farthest sampling point is designated as 2N, and the far end 11
When the sampling point is provided such that the sampling point of No. 1 is the address N, the address of the sampling point in the other optical fiber 10 closest to the address n in one optical fiber 10 is (2N-n).

Here, when the ratio U (n) of the backscattered light intensity of the corresponding address n and the backscattered light intensity of the address (2N-n) is taken, the temperature of the address n and that of the address (2N-n) are calculated. Since the temperature and the temperature are considered to be equal to each other, the expression (16) shown in Expression 16 is obtained. Further, by transforming the equation (16), the sum of the transmission loss at the wavelength of the light source and the wavelength of the measured light in the section from the address n to the address (2N-n) is calculated as the equation (17). To be

When the same operation is performed for the corresponding addresses (n-1) and {2N- (n-1)}, (n-1)
The sum of the transmission loss at the wavelength of the light source and the wavelength of the light under measurement in the section from the address to the address {2N- (n-1)} is obtained as in Expression (18). Further, taking the difference between the equations (17) and (18), the section from the address (n-1) to the address n and from the address {2N- (n-1)} to (2N-
The sum of the transmission loss at the wavelength of the light source and the wavelength of the light under measurement in the section up to the address n) is obtained as in Expression (19).

The optical fiber 10 in the section from the (n-1) th address to the nth address and from the (2N- (n-1)) th address to (2N)
Since the optical fiber 10 in the section up to -n) address is co-located with the optical fiber 10 and it is considered that the influence of the change in transmission loss caused by the ambient environment such as radiation is the same, the addresses from (n-1) to n The sum A (n) of the transmission loss at the wavelength of the light source and the wavelength of the measured light in the section and the section from the address {2N- (n-1)} to the address (2N-n) is shown in Formula 20. It will be represented by equation (20).

[0047]

[Equation 16]

[0048]

[Equation 17]

[0049]

[Equation 18]

[0050]

[Formula 19]

[0051]

[Equation 20]

In this way, all transmission losses between the sampling points of the optical fiber 10, that is, the sum of the transmission loss at the wavelength of the light source and the wavelength of the light under measurement can be obtained.

Next, a method for obtaining the temperature at each sampling point using this result will be described.

As the backscattered light from the optical fiber 10,
The Stokes light of the Raman scattered light and the anti-Stocks light are measured, and the sum of the transmission loss at the wavelength of the light source between the sampling points and the wavelength of the Stokes light As ( n) and the sum of transmission loss Aas (n) at the wavelength of the light source and the wavelength of the anti-stalk light. Formula (14)
Using, as shown in equations 21 and 22,

[0055]

[Equation 21]

And

[0057]

[Equation 22]

Α _s : Transmission loss at wavelength of stock light per unit length α _as ; Transmission loss at wavelength of anti-stock light per unit length Δx; Distance between sampling points. -Measurement result of backscattered light intensity distribution Gs (n) and Gas
The temperature distribution T (x) can be obtained from the equation (8) using (n).

In this embodiment, since the short-wavelength side stokes light and the long-wavelength side anti-stocks light of the wavelength of the light source are measured, the wavelength of the light source which is an intermediate wavelength of these wavelengths (laser light) is measured.
The transmission loss of each section of (re-scattered light) is As (n) and As
By taking the average with (n), it can be obtained with high accuracy. Therefore, the measurement error due to the mixing of the light of the wavelength of the light source, which was a problem in the above-mentioned JP-A-62-110160, is caused by using the transmission loss of each section of the wavelength of the light source obtained by such a method. For example, the problem can be solved by using the method described in Japanese Patent Application No. 3-59027 previously proposed by the applicant, and the problem can be solved.

Further, in this embodiment, the backscattered light intensity distributions of two wavelengths of the stokes light and the anti-stocks light are obtained, and the temperature distribution is obtained by taking the ratio of these, so that the above-mentioned JP-A-62-62 is used. It is possible to reduce the error in the distance direction due to the noise generated in the OTDR measurement circuit, which is a problem in Japanese Patent Laid-Open No. 110160. That is, the error in the distance direction is the distance distribution of the backscattered light intensity of the stock light and the distance distribution of the backscattered light intensity of the anti-stock light,
This is because they are included with the same distance dependence, so that the noise component is canceled by taking the ratio of the stokes light and the anti-stocks light.

As described above, the measurement error occurs due to the difference in the transmission loss change amount at the two wavelengths due to the influence of the surrounding environment of the optical fiber, the measurement error due to the transmission loss change at the wavelength of the light source, and the OTDR measurement. Measurement error in the distance direction can be eliminated. That is, in the physical quantity distribution detection sensor according to the present invention, physical quantity distribution detection with less measurement error is achieved.

As (n) and Aa obtained at time t1
s (n) and As (n) and Aas obtained at time t2
By comparing with (n), the radiation dose in each section from time t1 to t2 can be obtained.

The transmission loss of each section can also be obtained from the measurement result of the backscattered light intensity distribution measured from both ends of the optical fiber. Scattered light at the X point obtained from the equation (9) indicating the backscattered light intensity from the A end of the both ends of the optical fiber to the X point of the optical fiber and the backscattered light intensity measurement result measured from both ends of the optical fiber. When the equation (14) of occurrence probability is transformed into an equation between sampling points, equations (23) and (24) shown in equations 23 and 24 are obtained.

[0064]

[Equation 23]

[0065]

[Equation 24]

Substituting equation (24) by transforming equation (23), as shown in equation 25,

[0067]

[Equation 25]

When the equation (25) is taken into consideration the transmission loss up to the address (nX-1), as shown in the equation 26,

[0069]

[Equation 26]

When dividing both sides of the equation (25) by the equation (26),
As shown in equations 27 and 28,

[0071]

[Equation 27]

[0072]

[Equation 28]

In this way, the transmission loss of each section can be obtained by using the measurement results from both sides. In this method, the corresponding section [(n- 1) to n] and [{2N- (n-1)} to (2N
-N)] is not equal to the transmission loss.

Using the result of equation (28), the temperature distribution along the optical fiber can be obtained in the same manner as shown in the embodiment. However, in this method, in order to perform measurement from both ends, it is necessary to switch the optical path in the OTDR measurement system or prepare two sets of light sources.
It becomes necessary to consider the life of the switch used for switching the optical path, or two sets of light sources are prepared, which is expensive and the device becomes large.

A method for combining the measurement from both ends and the measurement from one end using an optical switch for solving this problem will be described below.

First, measurement from both ends is first performed to obtain the transmission loss in each section, which is designated as A ₀ (n). next,
Using only one of the transmission loss of each section obtained by measuring both ends, or the transmission loss of each section obtained by measuring only from one end, or by averaging these, Ask for. At the same time that the temperature distribution is obtained, the transmission loss A (n) of each section obtained by the measurement from only one end is compared with the transmission loss A ₀ (n) of each section obtained by the both-end measurement obtained previously. performed, a if the difference (n) and a ₀ and (n) is out of range a predetermined, was measured from both ends again, again the transmission loss a ₀ the transmission loss of each was determined from the results section (N)
And Obtaining such a temperature distribution and comparing the transmission loss are repeated. By adopting such a method, it is possible to reduce the number of switching operations of the optical switch while using the transmission loss of each section obtained based on the measurement from both ends with high accuracy.

As a slight modification of this method,
Instead of comparing (n) with A ₀ (n), after performing measurement from one end for a certain number of times or for a certain period, measurement from both ends is performed and A ₀ (n) is updated periodically. It is also possible.

A combination of both of these is also possible.

It goes without saying that the above-mentioned measuring method by the combination of both-ends measurement and one-ends measurement can be applied to the measurement of only one wavelength or the measurement of a plurality of wavelengths.

As the two optical fibers to be installed side by side, in order to influence the temperature and the surrounding environment of the side-by-side section, the tension at the time of installation, etc. equally on the two optical fibers installed side by side A cable accommodating two or more optical fibers is provided, and two of these optical fibers are used, a structure in which a plurality of optical fibers are arranged in a tape shape, or a space.
It is desirable to use an optical fiber of a type in which two or more optical fibers such as a saw-type optical fiber cable are covered with a common sheath.

[0081]

The present invention exhibits the following excellent effects.

(1) It becomes possible to detect the distribution of physical quantities under the special influence of radiation or the like.

(2) It is possible to detect the physical quantity distribution with high accuracy by removing the transmission loss from a loss generation factor having a small wavelength dependency such as a microbend.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is a configuration diagram of a conventional temperature distribution measuring sensor.

[Explanation of symbols]

 10 optical fiber 11 optical fiber far end (fusion connection part) 20 detection device 50 temperature sensor

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location G02B 6/02 A

Claims (7)

[Claims] 1. A sensor for measuring a distance distribution of scattered light generated in an optical fiber and detecting distribution information of a physical quantity along the optical fiber, wherein two optical fibers are provided side by side and each of the optical fibers A physical quantity distribution detection sensor using an optical fiber, wherein one ends on the same side are optically connected to each other. 2. A physical quantity distribution using an optical fiber according to claim 1, wherein two ends of the two optical fibers installed side by side are connected to a detecting device at ends opposite to one end connected to each other. Detection sensor. 3. A physical quantity distribution using the optical fiber according to claim 1, wherein scattered light of two or more wavelengths generated in the optical fiber is measured to detect distribution information of the physical quantity along the optical fiber. Detection sensor. 4. A physical quantity of a part of two optical fibers installed side by side is measured by another physical quantity detection sensor, and the measurement result and the measurement result of scattered light generated in the optical fiber are used to measure the physical quantity in the optical fiber. The physical quantity distribution detection sensor using the optical fiber according to claim 1, wherein a physical quantity distribution along the physical quantity is measured. 5. The light is incident only from one end of the two optical fibers provided side by side connected to the detection device, and as the scattered light from the optical fiber, only the backscatter from this same end is measured. The physical quantity distribution detection sensor using the optical fiber according to any one of claims 1 to 4, wherein the physical quantity distribution along the optical fiber is measured. 6. The transmission loss of each part of the optical fiber is obtained from the measurement result of the backscattered light intensity from the optical fiber, and the physical quantity distribution along the optical fiber is measured from this result and the backscattered light intensity measurement result. A physical quantity distribution detection sensor using the optical fiber according to claim 1. 7. To detect a physical quantity, the transmission loss of each part of the optical fiber obtained from the measurement result of the backscattered light from the two ends connected to the detection device for the two optical fibers provided side by side, and the connection to the detection device The backscattered light measurement result obtained by measuring only one of the optical fibers that has been measured is used.
A physical quantity distribution detection sensor using the optical fiber according to any one of 4 to 6 above.