JPH0380251B2 - - Google Patents

Description

[Detailed Description of the Invention] (1) Industrial technical field This invention involves making a light pulse enter from one end into an optical fiber equipped with a sensor section provided with a temperature-dependent reflective film, and the light pulse being reflected from the sensor section. The present invention relates to an improvement in an optical fiber temperature sensor that extracts reflected light and detects a temperature change in a sensor section from a change in its level.

(2) Prior art Figure 3 shows a conventional example of this type of temperature sensor, where 1 is an optical fiber, and 2, 3, and 4 are connected to three points in the length direction of the optical fiber 1 via couplers. sensor section A, which is a short fiber with a temperature-dependent reflective film at its tip;
5 is an optical pulse generator;
6 is a directional coupler, and 7 is an optical pulse tester that detects the reflected pulses of the optical pulses incident on the optical fiber 1 from the respective sensor sections A, B, and C. Therefore, when a temperature change occurs in each sensor section A, B, and C, light is absorbed by the temperature-dependent reflective film, and the level of reflected light detected by the optical pulse tester 7 decreases. It is possible to know the temperature changes at points A, B, and C. However, according to this type of temperature sensor, even if a temperature change occurs at a location other than each sensor section A, B, C in the optical fiber 1, each sensor section A, B, C
There is a problem in that the level of reflected light from the source changes. In other words, FIG. 4 is a graph of the back reflected light level (vertical axis) against the time detected by the optical pulse tester 7 (horizontal axis), and the reflected light level at times t ₁ , t ₂ , and t ₃ is shown as a graph. This corresponds to those from each sensor section A, B, and C.

For example, if a temperature change occurs for some reason at point D of the optical fiber between sensor sections A and B, light loss will occur here, and the level of reflected light at points A, B, and C of each sensor section will be The result is as shown in Figure 5. In other words, in the figure, the level corresponding to point A t ₁ does not change, but the level corresponding to point D t' decreases, and the levels at points B t ₂ and C t ₃ are also compared with those in Figure 4. Then it is decreasing. This means that even though there was a temperature change at point D, the effect was also occurring on sensor sections B and C. This means that the temperature change occurred not only at the sensor section but also at other locations. In this case, changes in the sensor section cannot be accurately known.

(3) Purpose In view of the above, the present invention provides an optical fiber temperature sensor that can accurately detect temperature changes in a sensor portion without being affected by temperature changes in locations other than the sensor portion. It is.

(4) Structure The optical fiber temperature sensor according to the present invention includes an optical fiber, the optical fiber is optically coupled to the optical fiber at predetermined intervals in the length direction, and a reflective film having temperature dependence is applied to one end of the optical fiber. a plurality of short fibers each having a sensor portion having the same characteristics as the optical fiber; and a plurality of short fibers having the same characteristics as the optical fiber; It consists of a plurality of short fibers each having a reference part coated with a neutral reflective film, and the level of backscattered light from the sensor part and the reference part of the light pulse incident from one end of these optical fibers is measured. The method is characterized in that a temperature change in the sensor section is detected by comparison. Note that reflective films that exhibit temperature dependence include GaAs, GaP,
Examples include CdTe.

(5) Embodiment FIG. 1 shows an optical fiber temperature sensor according to the present invention, and the same reference numerals indicate the same parts as in FIG. 3. In the figure, 11 is an optical fiber having the same characteristics as optical fiber 1, and 2
2, 33, 44 are 3 in the length direction of the optical fiber 1.
It is a short fiber connected via a coupler to the corresponding position, and its tip is equipped with reference parts a, b, and c, each of which is provided with a temperature-independent reflective film made of metal vapor deposition, etc. be. Note 8
66 is a directional coupler; 77 is a reference section for each optical pulse input to optical fiber 11; 66 is a directional coupler; This is an optical pulse tester that detects reflected pulses from a, b, and c.

Although it appears from FIG. 1 that the optical fibers 1 and 11, their sensor parts A, B, and C and the reference parts a, b, and c are separated from each other, they are actually close to each other and the corresponding parts are It is designed to be almost equally sensitive to changes in the surrounding environment.

In the above configuration, the optical pulses that are simultaneously input from the optical pulse generator 5 to both optical fibers 1 and 11 via the demultiplexer 8 and the directional couplers 6 and 66 are as follows.
In a steady state, the sensor portions and reference portions of the optical fibers 1 and 11 display information at approximately the same level as in FIG. 4, respectively.

Now, if the environment near point D changes and a temperature change occurs in the surrounding area, the level of reflected light from each sensor section A, B, and C of optical fiber 1 will show a state as shown in FIG. 5, as described above. However, this alone makes it impossible to grasp the conditions of the sensor sections B and C that should actually be observed. However, by comparing the level of the reflected light from the optical fiber 11, the status of the sensor sections B and C can be known. That is, since the reference parts b and c of the optical fiber 11 are formed with a reflective film that has no temperature dependence, the reference parts b and c
Furthermore, if there is no environmental change in the vicinity of sensor sections B and C, only the level drop corresponding to the optical loss at point D will be directly expressed as that level.

Therefore, the reference parts a, b, c of the optical fiber 11
Observe the level of reflected light from the sensor and if it is almost the same as in Figure 5, it means that in this case a temperature change has occurred only near point D, and no temperature change has occurred in sensor sections B and C. It happens.

Then, at time _t2 , that is, the reflected light level of the sensor part B is as shown in FIG. 2, and the reference part b shown in FIG.
The reflected light level is further reduced compared to the reflected light level of
The reflected light level of sensor part C and reference part c is 5th
If the value is almost the same as that shown in the figure, it means that a temperature change has occurred in the sensor section B.

In this way, even though a temperature change occurs at point D, the temperature change at sensor section B can be confirmed without being influenced by the temperature change.

If you measure the temperature characteristics of the optical fiber and the temperature characteristics of the reflective film that show temperature dependence in advance, you can quantify the temperature change at each sensor section by comparing the reflected light level of each sensor section and each reference section. You can also ask for it.

(6) Effects According to the present invention, a plurality of short fibers are attached at predetermined intervals in the length direction of one optical fiber, and a reflective film having temperature dependence is applied to one end of the short fibers. A plurality of short fibers are attached at predetermined intervals in the length direction of the other optical fiber, and one end of the short fibers is coated with a reflective film that is not temperature dependent, so that temperature changes in each sensor section can be easily monitored. Stable detection is possible without being affected by temperature changes from other locations.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a graph showing measurement results according to the present invention, FIG. 3 is a block diagram showing a conventional optical fiber sensor, and FIG. 4 is a block diagram showing an optical fiber sensor. A graph showing the measurement results when the sensor unit is in a steady state, Figure 5 is D
It is a graph showing measurement results when a temperature change occurs at a point. In the figure, 1, 11: optical fiber, A, B,
C: Sensor part, a, b, c: Reference part, 7, 77:
Optical pulse tester.

Claims (1)

[Claims] 1. an optical fiber, and a plurality of short fibers that are optically coupled at predetermined intervals in the length direction of the optical fibers, and each of which has a sensor section coated with a temperature-dependent reflective film on one end; An optical fiber having the same characteristics as the optical fiber, and a reference part that is optically coupled at predetermined intervals in the length direction of the optical fiber and has a temperature-independent reflective film on one end thereof. The sensor is composed of a plurality of short optical fibers, and the temperature change in the sensor section is detected by comparing the levels of backscattered light from the sensor section and the reference section of a light pulse incident from one end of both of these optical fibers. Features an optical fiber temperature sensor.