JPH0769222B2 - Temperature measurement method and distributed optical fiber temperature sensor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a distributed optical fiber temperature sensor and a temperature measuring method, and in particular, in an optical fiber to be measured which becomes a problem when the ratio of Stokes light to anti-Stokes light is taken. The present invention relates to a distributed optical fiber temperature sensor and a temperature measuring method capable of correcting the influence of the difference in the attenuation rate and accurately measuring the temperature distribution of a long-distance optical fiber.

[Prior Art] A block diagram of a conventional distributed optical fiber temperature sensor is shown in FIG. The laser pulse oscillated from the laser pulser 10 of the light source unit is incident on the optical fiber 12 to be measured, and the Raman scattered light generated in the optical fiber 12 returns to the incident end. The Raan scattered light is guided to the measuring device by the optical directional coupler 11, the Stokes light and the anti-Stokes light in the Raman scattered light are separated and detected by the filter 13, and their intensities are detected by the photoelectric conversion units 14 and 14 ', respectively. Is converted into an electric signal proportional to. The electric signals are preamplifiers 15 and 1, respectively.
The signal is amplified by 5 ', and averaged by the averager 16 a predetermined number of times. The averaged signal is transmitted to the signal processing unit 17, where the delay time and intensity ratio of the Stokes light and anti-Stokes light are calculated, and the temperature distribution is output. When calculating the intensity ratio of Stokes light and anti-Stokes light, it is difficult to find the absolute intensity ratio, so the relative intensity ratio is used to determine Dakin (Dakin, JP, Pratt, DJ, Bibby, GW, Ros
s, JN “Temperature distribution measurement using
Raman ratio thermometry ", SPIE Vol 566 Fiber Optic
and Laser Sensors III 249 (1985)).

In the above equation, T is the measured temperature, Θ is the reference temperature, and R '
(T) is the relative intensity ratio of the measured part, R '(?) Is the relative intensity ratio of the reference temperature part, k is Boltzmann's constant, h is Planck's constant, C is high speed, and v is Raman shift amount.

[Problems to be Solved by the Invention] An incident laser pulse oscillated from a light source such as an LD is naturally attenuated by propagating in an optical fiber to be measured. However, the effect of this attenuation can be avoided by taking the ratio of Stokes light to anti-Stokes light, but the attenuation rate of Stokes light and anti-Stokes light when Raman scattering is generated by the incident wave and the scattered light propagates backward The effect of the difference cannot be avoided only by taking the ratio. Therefore, it is necessary to make a correction in consideration of the influence of the difference in the attenuation rate. If this correction is not made, even if a uniform temperature distribution is measured, a greatly inclined temperature distribution as shown in FIG. 3 is output. Further, the attenuation rate difference itself has temperature dependence, and the influence thereof becomes large when measuring a long distance, which makes accurate measurement difficult.

[Means for Solving the Problems] The present invention has been made to solve the above-mentioned problems, and makes a laser pulse incident on an optical fiber to be measured,
In the temperature measuring method for measuring the temperature distribution of the optical fiber from the intensity ratio of the Stokes light and anti-Stokes light contained in the return light from the optical fiber and the delay time, T is the measurement temperature, Θ is the reference temperature, and R ′ (T) is R '(Θ) is the relative intensity ratio of the reference temperature part, k is the Boltzmann constant, h is the blank constant, C is the speed of light, ν is the Raman shift amount, α is the Stokes light and anti-Stokes light. The expression introducing a correction term of an exponential function whose exponent part is the product of the above-mentioned attenuation rate difference α and the distance x, where x is the distance, and x is the distance in the optical fiber. And a light source section for injecting a laser pulse into an optical fiber to be measured, and an optical directional coupler for guiding return light from the optical fiber to a signal processing device. And a signal processor for detecting the Stokes light and anti-Stokes light contained in the return light and measuring the temperature distribution of the optical fiber from the intensity ratio and delay time thereof, in the distributed optical fiber temperature sensor, T is the measurement temperature, Θ is the reference temperature, R '(T) is the relative intensity ratio of the measured part, R'
(Θ) is the relative intensity ratio of the reference temperature part, k is the Boltzmann constant, h is the blank constant, C is the speed of light, ν is the Raman shift amount, α is the difference in attenuation rate between the Stokes light and the anti-Stokes light in the optical fiber, x Is the distance α
An expression that introduces a correction term for the exponential function whose exponent is the product of x and distance x The present invention provides a distributed optical fiber temperature sensor characterized by measuring a temperature distribution according to.

The above attenuation rate difference α (m ^-1 ) is the attenuation coefficient of Stokes light. Anti-Stokes light attenuation coefficient Then, α = α _S −α _AS .

A major problem in determining the correction term is that the attenuation rate difference α itself has a temperature characteristic in a strict sense. Therefore, it is necessary to select a condition in which the amount of change in the attenuation rate difference α is small. Therefore, as a result of intensive studies by the present inventors, as a condition that the attenuation rate difference α is substantially constant in the temperature range (about 0 ° C. to about 80 ° C.) to which the present sensor is applied, the optical fiber to be measured is made a core wire. It was found that the state. The core state is a state as shown below.

Usually, an optical fiber has a core and a clad made of a material such as quartz or silica, and a first coating material made of nylon or fluororesin which is coated on the outside of the core and the clad,
Further, the outermost coating material such as a synthetic resin coated on the outside of the first coating material is referred to as a core wire up to the first coating material, and the cord up to the outermost coating material.

Explaining in more detail, the diameter of the core (quartz) is 100μ.
m, clad (quartz) 140 μm, core diameter 0.9 mm, cord diameter 2.9 mm. In the core state, thin semi-solid silicon resin is used as the primary coating and nylon is used as the secondary coating. In general, in a cord state, the outside of the core wire is covered with a Kepler (carbon fiber) and the outside is covered with PVC resin.

The core state as referred to in the present invention is a state including the above-mentioned first covering material, and nylon is preferable as the covering material.

[Operation] In the present invention, the purpose of eliminating the temperature dependence of the attenuation rate difference α by introducing a correction term for the attenuation rate difference α of the Stokes light and the anti-Stokes light into the Dakin's equation used for measuring the temperature distribution of the optical fiber. As a result of using the optical fiber in the state of a core wire, even if the temperature distribution of a long-distance optical fiber is measured, the problem that the curve of the temperature distribution greatly inclines with distance is solved, and high-precision measurement becomes possible. Is.

Example An example of the present invention is shown in FIGS. 1 and 2. FIG. 1 is a graph showing the results of measuring the temperature dependence of the attenuation factor difference α before measuring the temperature distribution of the optical fiber. . The amount of temperature change when α is the minimum optical fiber temperature change, that is, when the optical fiber is in the form of a core and nylon is used as the coating material is shown. Under this condition, the change is 0.26 dB in the range of 0 to 60 ° C, and the deterioration of the temperature measurement accuracy due to this is the maximum at the position of 1 km.
It will be 4.3 ℃. This value is improved by about half when compared with the measurement result by the coded optical fiber. If this value is 0.3 dB or less in the range of 0 to 60 ° C, sufficiently accurate measurement can be performed.

FIG. 2 is a graph showing the results of processing using an optical fiber in the above-described core wire state and introducing a correction term of an exponential function whose exponent part is the product of the attenuation rate difference α and the distance x in the signal processing device. Is.

Here, the sensor device of the present invention means that in the conventional device of FIG. 4, the optical fiber 12 to be measured is in the core state, and the signal processing device, that is, the filter 13, the photoelectric conversion units 14, 14 ',
Preamplifiers 15 and 15 ', averager 16, signal processor 17
Is a device for collectively introducing the above-mentioned correction term, but more specifically, the process in which the above-mentioned correction term is introduced is a signal processing unit 17 or a microcomputer connected to the signal processing unit 17. Etc.

The results in Fig. 2 show that α is 9.3 × 10 ^-5 (m ^-1 ), and Θ is 25.
(℃), R '(T) and R' (Θ) are experimental values, ν = 44
It was measured as 0 (cm ⁻¹ ), and the temperature measurement accuracy in the long distance portion was improved without inclination of the entire temperature distribution curve as compared with the conventional example of FIG.

Note that ν may show a changed value, and an effective value obtained experimentally may be used.

Further, by using a new correction term δ as shown in the following equation, the adverse effect on the relative intensity ratio of the Stokes light and the anti-Stokes light, which is caused by the optical coupling between the respective devices in the sensor system of the present invention, is used. In some cases, the signal processing device may be offset and corrected before measurement.

[Effects of the Invention] The present invention performs processing based on an expression in which a correction term is introduced to the conventionally used Dakin's expression, and further selects a condition for minimizing the temperature fluctuation of the attenuation rate difference α during the correction operation. By this, there is an excellent effect that the problem that the entire curve of the temperature distribution inclines greatly according to the distance is solved and that the temperature measurement accuracy at a long distance is improved.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 show an embodiment of the present invention, FIG. 1 is a graph showing the temperature dependence of the damping ratio difference α, and FIG. 2 is the method and sensor of the present invention. FIG. 3 is a graph of measurement results obtained by using, FIG. 3 is a graph of conventional measurement results, and FIG. 4 is a block diagram of a conventional sensor.

Claims (2)

[Claims] 1. A temperature measuring method in which a laser pulse is incident on an optical fiber to be measured, and a temperature distribution of the optical fiber is measured from an intensity ratio and a delay time of Stokes light and anti-Stokes light included in return light from the optical fiber. ,
T is the measurement temperature, Θ is the reference temperature, R '(T) is the relative intensity ratio of the measured portion, R' (Θ) is the relative intensity ratio of the reference temperature portion, k
Where B is the Boltzmann constant, h is Planck's constant, C is the speed of light, ν is the Raman shift amount, α is the attenuation rate difference between the Stokes light and anti-Stokes light in the optical fiber, and x is the distance, the attenuation rate difference α and the distance. An expression that introduces a correction term for the exponential function whose exponent is the product of x A temperature measuring method characterized by measuring a temperature distribution by means of. 2. A light source section for injecting a laser pulse into an optical fiber to be measured, an optical directional coupler for guiding return light from the optical fiber to a signal processing device, and Stokes light included in the return light. In a distributed optical fiber temperature sensor comprising a signal processing device for detecting anti-Stokes light and measuring the temperature distribution of the optical fiber from their intensity ratio and delay time, said signal processing device comprises T as a measurement temperature, Θ as a reference temperature and R ′ (T) is the relative intensity ratio of the measured part, R ′
(Θ) is the relative intensity ratio of the reference temperature part, k is the Boltzmann constant, h is the Planck's constant, C is the speed of light, ν is the Raman shift amount, α is the difference in attenuation rate between the Stokes light and the anti-Stokes light in the optical fiber, x Is the distance α
An expression that introduces a correction term for the exponential function whose exponent is the product of x and distance x A distributed optical fiber temperature sensor characterized by measuring the temperature distribution by means of.