JP2951740B2 - Optical fiber type temperature distribution measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber type temperature distribution measuring device for measuring a temperature distribution along an optical fiber using the optical fiber as a sensor.

[0002]

2. Description of the Related Art An optical fiber type temperature distribution detecting device utilizes the fact that the intensity of scattered light, such as Raman scattered light or Rayleigh scattered light, generated in an optical fiber changes with temperature. Time Domain Reflectomet
ry), the temperature distribution along the longitudinal direction of the optical fiber is measured.

The measurement concept of the optical fiber type temperature distribution measuring device using Raman scattered light will be described below with reference to FIGS.

First, when the pulse light (pulse width Tw, pulse period Tp) shown in FIG. 4 is guided from the light source to the sensor fiber, scattered light such as anti-Stokes light or Stokes light is generated in the optical fiber. Some of them return to the measuring device as backscattered light as shown in FIG. The pulse incident time of this backscattered light is set to t = 0, and the sampling time interval T
When measured with s, the time functions Ia '(t) and Is' (t) of the light intensity of the anti-Stokes light and the Stokes light are obtained for t which is an integral multiple of the sampling time interval ts. These received light intensities Ia '(t) and Is' (t) are calculated as follows: the time required for the scattered light generated at the distance x in the optical fiber to return to the light pulse incident end (measurement unit) Using the expression, the functions Ia ′ (x) and Is ′ (x) of the distance x along the optical fiber are obtained.
Can be replaced by

[0005]

## EQU1 ## t = 2 × x / v v: speed of light in the optical fiber Transmission loss αa of the measuring light from each distance x to the measuring unit
If (x) and αs (x) are known, the intensity of the scattered light generated at each distance can be obtained by Expressions (2) and (3).

[0006]

(Equation 2)

[0007]

(Equation 3)

The ratio Ia (x) / Is (x) is a pure function of temperature if the wavelength of the light source and the material of the optical fiber are determined, as shown in the following equation (4). Temperature T
(x) can be obtained.

[0009]

(Equation 4)

Next, an outline of an optical fiber type temperature distribution measuring apparatus using Raman scattered light will be described with reference to FIG. This temperature measuring device includes a measuring device 10 and a sensor optical fiber 20.

The pulse light from the light source 2 is transmitted to the sensor fiber 2
0, a part of the scattered light generated in the optical fiber returns to the measuring device 10 side as back scattered light,
Incident on. In the optical demultiplexer 31, the anti-Stokes light having the wavelength λa and the Stokes light having the wavelength λs are demultiplexed from the incident backscattered light, and the anti-Stokes light having the wavelength λa is
An anti-Stokes light OTDR measurement circuit 30a composed of a photodetector 4a, an amplifier 5a, and an averaging processing circuit 6a passes through an optical fiber 23a, and a time function Ia (t) of the anti-Stokes light intensity is obtained from the light intensity. Is required. Similarly, the time function Is' (t) of the Stokes light intensity of the Stokes light is obtained by the Stokes light OTDR measurement circuit 30s including the light receiver 4s, the amplifier 5s, and the averaging circuit 6s via the optical fiber 23s. Desired. The period adjustment of the pulse light source 2 and the averaging processing circuits 6a and 6s is performed by the synchronization signal of the trigger circuit 1, and the sampling of the reflected light is performed in the averaging processing circuits 6a and 6s at a constant time interval Ts shown in FIG. Will be The light receivers 4a and 4s normally use an APD (avalanche photo diode) in order to increase the light receiving sensitivity.

This Raman-type temperature measuring device can be used for controlling a power transmission capacity, for example, by laying an optical fiber for a sensor along a power cable so as to know a temperature distribution in a longitudinal direction of the power cable. For example, it is possible to detect a part where the temperature is high due to deterioration of the cable or the like. Further, when used for detecting a fire in a building, a tunnel, or the like, a fire occurrence position can be located.

The Raman-type temperature distribution measuring device can measure the temperature distribution in the longitudinal direction by the method described above. However, in the backscattered light returning from the optical fiber, in addition to the Raman scattered light to be measured (anti-Stokes light of wavelength λa and Stokes light of wavelength λs), Rayleigh scattered light of the same wavelength λ0 as the light source The intensity of the Rayleigh scattered light is 1000 times stronger than the Raman scattered light, and the difference between λ0 and λa, λs is as small as about 30 nm.・ OTDR measurement circuit 3 for Stokes light
It is difficult to prevent the Rayleigh scattered light from entering the optical signal incident on 0a at all, and in reality, some Rayleigh scattered light is mixed. In other words, the received light signal contains a small amount of Rayleigh scattered light in addition to the anti-Stokes light, and the true anti-Stokes light generation amount Ia (x) cannot be obtained at the distance x as it is. . Therefore, the ratio R of the amount of Rayleigh scattered light mixed into the optical signal incident on the OTDR measurement circuit for anti-Stokes light 30a is R
Using a, the true anti-Stokes light reception intensity Ia '(t) is obtained by equation (5), and from this, the true anti-Stokes light generation amount Ia (x) at the distance x is obtained. Equation 5 is similarly used for the Stoke light.

[0014]

(Equation 5)

[0015] By performing the correction of the above equation (5), the influence of the Rayleigh scattered light on the temperature distribution measurement result is 2K.
m was not detected in the temperature distribution measurement.

[0016]

However, the need for temperature distribution measurement of 2 km or more was examined, and the accuracy of the measured temperature was examined. It was found that the temperature tended to be measured lower than the temperature of.

An object of the present invention is to solve the above-mentioned problems and to provide an optical fiber type temperature distribution measuring apparatus which minimizes a measurement error even when measuring a temperature distribution of 2 km or more.

[0018]

In order to achieve the above object, the present invention provides an optical fiber for a sensor, in which a light pulse is incident from a light source in a measuring device, and backscattered light generated in the optical fiber is measured by the measuring device. The temperature of the optical fiber is calculated from the light intensity of the backscattered light, and the position of the backscattered light is calculated from the difference between the incident time of the light pulse and the time when the backscattered light reaches the measurement system. , The temperature and the position are measured simultaneously, and the temperature distribution T (x) of the optical fiber is measured.
In the optical fiber type temperature distribution measuring apparatus for measuring a post
Of Raman scattered light generated in optical fiber as
The anti-Stokes light and the Stokes light were measured, and
Using 0 and Equation 11, Ira "(x), Ir
s "(x)

(Equation 10)

[Equation 11] The measured value of anti-Stokes light Ia "(x)
From the measured light values of Isx (x) to Ira "(x) and Irs" (x).
Is subtracted to satisfy Equation 8, and

(Equation 8) By correcting the transmission loss αr (x), the distance x
True anti-Stokes light intensity Ia (x) and Stokes light emission
The raw intensity Is (x) is obtained, and the anti-Stokes light intensity Ia (x) and
And Stokes light intensity Ins (x)

## EQU9 ## A temperature distribution calculator for calculating the temperature by substituting into T (x) = K ₂ / [ln [{Ia (x) / Is (X)} / K ₁ ]] is provided. is there.

[0019]

[0020]

The gist of the present invention is that the ratio of the Rayleigh scattered light contained in the measurement signal is made a function of the distance x, the true Raman scattered light intensity is calculated using a correction coefficient different for each distance, and then the temperature conversion is performed. Temperature distribution calculator.

The reason for performing such a correction will be described with reference to FIG.

The insertion loss at the anti-Stokes light wavelength λa from the sensor fiber 20 to the anti-Stokes light OTDR measurement circuit 30a is La, and the Rayleigh scattered light wavelength λ
Assuming that the insertion loss at 0 is Lra, the optical signal intensity Ia ″ (x) incident on the anti-Stokes light OTDR measurement circuit 30a converted to each distance and the distance returned from the sensor fiber 20 are converted to each distance. The relationship between the obtained anti-Stokes optical signal intensity Ia '(x) and the Rayleigh scattered light intensity Ir' (x) is expressed by the following equation (6).
It can be represented by an equation. The same applies to the optical signal intensity Is ″ (x) of the Stokes light incident on the Stokes light OTDR measurement circuit 30s converted into each distance.

[0023]

(Equation 6)

On the other hand, the respective backscattered light intensities Ia (x), Is (x), Ir (x) generated at the distance x and the respective scattered light intensities Ia ′ (x), Is reaching the optical demultiplexer 31. The relationship between '(x) and Ir (x) can be expressed by equation (7).

[0025]

(Equation 7)

Accordingly, the scattered light intensity Ia generated at the distance x
(x) and Is (x) can be expressed by the following equation (8) using equations (6) and (7).

[0027]

(Equation 8)

By using the above equation (8), the measured values Ia "(x) and Is" (x) are each a function of the distance Ia "(x) and Is" (x).
After subtracting ra "(x), Irs" (x), correction of transmission loss and the like are performed to obtain true anti-Stokes light and Stokes light generation intensity Ia (x), Is (x) at distance x. By substituting this into equation (4), the temperature distribution T
(x) can be obtained without being affected by the Rayleigh scattered light.

That is, in the conventional correction method, the mixing ratio of the Rayleigh scattered light was subtracted as being constant irrespective of the distance. This is because the transmission loss between the Rayleigh scattered light and the Raman scattered light from the distance x is slightly different. Attention is paid, and the equation 8 in which this difference is accurately evaluated is calculated and used for calculating the temperature distribution.

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIG.

FIG. 1 has almost the same configuration as that of the conventional example shown in FIG. 3, except that an improved temperature distribution calculator 71 having a different Rayleigh scattered light correction calculation method is used as the temperature distribution calculator.

Since the improved temperature distribution calculator 71 has a calculation function in accordance with the above equation (8), the temperature distribution T (x) is calculated by the equation (9) derived from the equation (4), whereby the light to be measured is calculated. The temperature distribution along the sensor optical fiber 20 can be measured over a distance of 2 km or more without being affected by Rayleigh scattered light at all.

[0033]

Equation 9] T (x) = K ₂ / [ln [{Ia (x) / Is (X)} / K 1] ] used in equation (8) Rayleigh scattered light correction term and Ira "(x) and Irs"
(x) is obtained by using Equations (7) and (8) and Equation (11) using Equation (10) expressing the relationship between the Rayleigh scattered light intensity at the distance x and the Rayleigh scattered light intensity at the distance of zero (the entrance). Can be expressed as

[0034]

(Equation 10)

[0035]

[Equation 11]

Rayleigh scattered light wavelength λ in optical demultiplexer 31
0, the insertion loss Lra, Lrs from the sensor fiber 20 to the anti-Stokes light OTDR measurement system 30a and the Stokes light OTDR measurement system 30s, the Rayleigh scattered light intensity Ir (o) at the incident light, If the transmission loss αr (x) of Rayleigh scattered light in the fiber is known, it can be obtained.

The Rayleigh scattered light intensity Ir (O) at the distance 0 and the transmission loss αr (x) of the Rayleigh scattered light can be calculated by using the existing OTDR measurement technique having the same incident light wavelength and detected light wavelength.
It can be easily measured.

In the above embodiment, the correction of the Rayleigh scattered light is corrected by using the anti-Stokes light and Stokes light intensity measurement data Ia "(t), Is" (t) as distance functions Ia "(x), Is" (x). ), And the functions of the distance x, Ira "(x), Irs" (x)
(See equation 8), but at the stage of the time function Ia "(t), Is" (t) before conversion into the distance function, the same method as in the previous embodiment is used. Correction of Rayleigh scattered light mixing may be performed.

That is, Ira "(x), Irs" (x), αa (x), αs
(x) is corrected using the following equation (12) in which each is replaced by a function of time using the relation of equation (1), and the correction result is replaced by a function of distance using the relation of equation (1) to obtain Ia
(x) and Is (x) are obtained, and the temperature distribution T (x) is obtained by Expression (9).

[0040]

(Equation 12)

As another embodiment, the present invention may be applied to a system using only anti-Stokes light to obtain a temperature distribution. Also, the present invention may be applied to a method of obtaining a temperature distribution by using one or both of Rayleigh scattered light, anti-Stokes light, and Stokes light.

[0042]

In summary, according to the present invention, even when the measurement distance exceeds 2 km, the temperature distribution measurement error due to the mixing of Rayleigh scattered light into the light to be measured is eliminated, and high accuracy over long distances is achieved. A measurable optical fiber type temperature distribution measuring device can be realized.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a sensor system showing an embodiment of an optical fiber type temperature distribution measuring device of the present invention.

FIG. 2 is a schematic diagram for explaining the present invention.

FIG. 3 is a sensor system configuration diagram showing one embodiment of a conventional optical fiber type temperature distribution measuring device.

FIG. 4 is an explanatory diagram of pulsed light from a light source.

FIG. 5 is an explanatory diagram of backscattered light returning from a sensor fiber to a measurement device.

FIG. 6 is a diagram showing a temperature distribution measurement example for explaining a problem in a conventional optical fiber type temperature distribution measurement device.

[Explanation of symbols]

 Reference Signs List 1 trigger circuit 2 pulse light source 4a, 4s light receiver 5a, 5s amplifier 6a, 6s averaging processing circuit 7 temperature distribution calculator 10 measuring device 20 sensor optical fiber 21, 23a, 23s optical fiber 30a anti-Stokes light OTDR measurement Circuit 30s OTDR measurement circuit for Stokes light 31 Optical demultiplexer 72 Shows temperature distribution display. 71 Improved temperature distribution calculator

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-201129 (JP, A) JP-A-58-92926 (JP, A) JP-A-2-145933 (JP, A) (58) Field (Int.Cl. ⁶ , DB name) G01K 11/32

Claims (1)

(57) [Claims] 1. A light pulse is incident on a sensor optical fiber from a light source in a measuring device, and backscattered light generated in the optical fiber is guided to a photodetector of a measuring system in the measuring device. The temperature and position of the optical fiber are determined from the light intensity, and the position where the backscattered light is generated is determined from the difference between the incident time of the light pulse and the time when the backscattered light reaches the measurement system. Fiber temperature distribution T
Raman scattering generated in the optical fiber as backscattered light in the optical fiber type temperature distribution measuring device for measuring (x)
Measures stray anti-Stokes light and Stokes light
And Ira ", which is a function of distance, using Equations 10 and 11
(x), Irs "(x), and [Equation 11] The measured value of anti-Stokes light Ia "(x)
From the measured light values of Isx (x) to Ira "(x) and Irs" (x).
Is subtracted so as to satisfy Equation 8, and By correcting the transmission loss αr (x), the distance x
True anti-Stokes light intensity Ia (x) and Stokes light emission
The raw intensity Is (x) is obtained, and the anti-Stokes light intensity Ia (x) and
Temperature by substituting the fine Stokes light generated intensity Is (x) Equation 9] equation (9) to T (x) = K ₂ / [ln [{Ia (x) / Is (X)} / K 1] ] calculating optical fiber type temperature distribution measuring apparatus characterized in that a temperature distribution calculator for.