JP2516613B2 - Optical fiber temperature measurement method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of use) The present invention relates to a temperature measuring method using a change in light transmission characteristics of an optical fiber itself.

(Prior Art) In recent years, an optical fiber temperature sensor has been actively developed as a part of application technology of optical fibers. As an optical fiber temperature sensor, a radiation type optical fiber thermometer has been developed which guides radiated light (infrared ray) from a target object through an optical fiber and measures the temperature from the light intensity value.

Recently, a fiber type temperature type in which the optical fiber itself has a function of a transformer has been studied.

As the latter type of sensor, rare-earth element ions such as Nd ³⁺ , Sm ³⁺ , Eu ³⁺ , and Ho ³⁺ are doped into the matrix glass of the optical fiber, and the temperature change of the optical absorption performance of these rare-earth element ions is measured. Detected as an increase or decrease in the loss value in the fiber,
Means have been devised to measure the temperature distribution of the ambient environment along the optical fiber.

As a concrete example, Fig. 5 shows the loss spectrum of the silica-based optical fiber doped with Nd ^3+, and Fig. 6 shows the change of the loss value with respect to the temperature of the absorption peak near 900 nm present in Fig. 5. . 590nm, 750nm observed in Fig. 5,
The optical absorption band centered at 890 nm is the absorption based on the transition between electronic levels peculiar to Nd ³⁺ , and in these absorption bands, for example, at the wavelength 600 nm included in the absorption band centered around 590 nm. As shown in FIG. 6, the absorption characteristic changes monotonously with a rise in temperature. Therefore, the temperature can be measured by detecting the amount of light absorption, that is, the amount of light loss.

For optical fiber, Optical Time Domain Refrect
An optical fiber loss evaluation method called the meter (OTDR) method has been established. This method measures the backward Rayleigh scattered light intensity in the optical fiber of the optical pulse input into the optical fiber, and determines the loss value at the wavelength of the input optical pulse along the longitudinal direction of the optical fiber with a spatial resolution within 1 m. This is an evaluation method. Therefore, by inputting an optical pulse of a wavelength contained in the Nd ³⁺ absorption band using this method and examining the change in the absorption loss at that wavelength in the distance direction, it is possible to detect near the fiber along the installed optical fiber. The temperature distribution of can be measured.

(Problems of Prior Art) As shown in the example of Nd ³⁺ in FIG. 6, generally, absorption characteristics of rare earth element ions are uniquely changed in a temperature range of about 100 ° C. or lower. By selecting various rare earth elements as additives, it is possible to measure temperatures between -200 ° C and 100 ° C. In this case, since the optical fiber itself is a sensor, it is necessary to place the optical fiber itself in its temperature environment. However, in doing so, the various coating layers (generally polymers) applied to the optical fiber for protection also reach that temperature,
Temperature change (contraction, expansion, etc.) of the coating layer occurs,
This also changes the loss of the optical fiber. Therefore, in the loss change due to the temperature environment around the optical fiber,
Factors other than the change due to the absorption characteristics of the rare earth element corresponding to the temperature are included, and the change becomes an error factor in the temperature measurement.

(Object of the Invention) The object of the present invention is to determine the loss change caused by the structure of the optical fiber cable due to the ambient temperature of the optical fiber and the absorption characteristics of the additive added to the optical fiber to have the function as a transducer. It is to realize a method for obtaining an accurate temperature by separating the change.

(Means for Solving the Problems) As shown in FIG. 1, a temperature measuring method using an optical fiber according to the present invention is a method in which an impurity having a light absorption band whose light absorption characteristic changes with temperature is added to a glass base material. From one end or both ends of the fiber 6, the wavelength λ ₁ included in the above optical absorption band
Of the light pulse of wavelength λ ₂ which is not included in the light absorption band, and is emitted as backscattered light from the time of input to the optical fiber 6 and the backscattering intensity of the light pulse of each wavelength. The loss value at each wavelength with respect to the length from one end of the optical fiber 6 is obtained by measuring the time difference until the temperature is measured, and the loss value at both wavelengths is compared and calculated to measure the temperature of the measurement target. It is the one.

(Embodiment 1) FIG. 1 shows an embodiment of the present invention. 6 in this figure
Is an optical fiber cable that serves as a temperature measuring transducer, and the glass of the matrix of the optical fiber 6 contains an additive having an absorption band whose light absorption characteristic changes with ambient temperature, such as rare earth element ions. ing.

1 in FIG. 1 is an optical pulse light source having a wavelength component λ ₁ included in the above absorption band, and 2 is an optical pulse light source having a wavelength λ ₂ not included in the above absorption band. These light pulse light sources 1,
The light pulse generated from 2 is made into one light beam 4 by the multiplexer 3, and is incident on the optical fiber 6 through the beam splitter 5, the condenser 15 such as a lens or a spherical mirror. The condenser 15 is not an essential component in the present invention.

Two lights having different wavelengths input to the optical fiber 6 are
Due to Rayleigh scattering inside the optical fiber 6, they are also scattered backward in the direction opposite to the incident direction. In this case, the scattered light is output from the incident end face of the optical fiber 6 while receiving a loss according to the length of the returning optical fiber while returning to the input end.

A part of the output light 7 is reflected by the beam splitter 5, and further separated by the demultiplexer 8 according to the wavelength. One of them is the light of the wavelength component λ ₁ included in the absorption band, and the other is in the absorption band. The light has a wavelength component λ ₂ that is not included. Those lights are detected by photodetectors 9 and 10, respectively, and the relationship of the light intensity value with respect to time as shown in FIG. 2 is obtained through an amplifier and an averaging processor which are not shown.

Since the time in this case is the transit time of the optical pulse in the optical fiber, it corresponds to the length of the optical fiber, that is, the position of the measurement point. The loss value of the optical fiber at that position is given by differentiating the value of the light intensity with respect to time (that is, the length of the optical fiber). In other words, the slope of the curve represents the loss. If this slope is constant, the loss value is constant, but the abrupt change is at a position where the loss value differs from others due to some reason, and the loss value can be calculated from the magnitude of the gradient (differential value). .

In the present invention, such characteristics of the relationship of the light temperature value with respect to time depend on the light pulse of the wavelength component λ ₁ included in the absorption band of the additive and the light pulse of the wavelength component λ ₂ not included in the absorption band. You can get two kinds of things. Of these, the latter characteristic does not include information on changes in temperature due to absorption of additives added for temperature measurement of rare earth ions, and other effects such as contraction and expansion due to changes in the ambient temperature of the coating material of the optical fiber. It is only affected by the loss change that is difficult to uniquely correspond to the ambient temperature based on the change of. In addition to the latter characteristic, the former characteristic includes the influence of the temperature characteristic of absorption of the additive.
Therefore, it is possible to extract only the temperature characteristic of the absorption of the additive by comparing the two types of characteristics of the former and the latter and performing correction, for example, subtraction or division.

(Embodiment 2) FIG. 3 shows another embodiment of the present invention. In FIG. ₁ , both the optical pulse of wavelength component λ ₁ included in the absorption band of the additive and the optical pulse of wavelength component λ ₂ not included in the absorption band are input from the same incident end face of the optical fiber 6. 3, the optical pulse having the wavelength component λ _{1 and} the optical pulse having the wavelength component λ ₂ not included in the absorption band are input from the opposite direction of the optical fiber 6.

(Embodiment 3) FIG. 4 shows still another embodiment of the present invention. This is because a reflection mirror 14 is provided on one side of the beam splitter 5 when observing the backward Rayleigh scattered light intensity with respect to time, and the optical fiber 6
The optical pulse immediately before being input to is input to the photodetectors 9 and 10 by the reflection mirror 14 and the time sweep is performed by using this signal as a trigger. In this method, by changing the position of the reflection mirror 14, it is possible to measure the temperature by time-sweeping only the portion of the optical fiber 6 that is particularly desired to be observed. By doing so, the response time can be shortened. A time delay device can also be used when the time subtraction is started by using the external output of the optical pulse oscillator as a trigger.

(Effect of the invention) The temperature measurement method using the optical fiber of the present invention is a loss change based on the temperature characteristic of absorption of an additive added for temperature measurement in the optical fiber 6, and an optical fiber loss due to other causes. By observing the change and extracting the absorption loss with respect to the temperature due to the additive from the difference between them, it becomes possible to measure the accurate temperature along the longitudinal direction of the optical fiber.

[Brief description of drawings]

FIG. 1 is an explanatory view showing an embodiment of the temperature measuring method of the present invention, FIG. 2 is a characteristic explanatory view of time difference-backscattered light intensity by the temperature measuring method of FIG. 1, FIG. 3, and FIG. FIG. 5 is an explanatory view of another embodiment of the present invention, FIG. 5 is an explanatory view of a loss spectrum of a silica optical fiber doped with Nd ^3+, and FIG.
It is explanatory drawing which shows the relationship between the output change by 0 nm absorption loss, and temperature. 1 and 2 are optical pulse light sources 3 are multiplexers 5 are beam splitters 6 are optical fibers 7 are backscattered light 8 are demultiplexers 9 and 10 are photodetectors 14 are reflection mirrors

Claims (1)

(57) [Claims] 1. An optical pulse having a wavelength contained in the above optical absorption band from one or both ends of an optical fiber in which an impurity having an optical absorption band whose optical absorption characteristic changes depending on temperature is mixed in a glass matrix, Input the light pulse of the wavelength not included in the light absorption band, measure the backscattering intensity of the light pulse of each wavelength, and the time difference from the time of input to the optical fiber until the light is emitted as backscattered light. The optical fiber is characterized in that the loss value at each wavelength with respect to the length from one end of the optical fiber is obtained, and the loss value at both wavelengths is compared and calculated to measure the temperature of the measurement target. How to measure temperature.