JPH0421130B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a temperature sensor using fiber. In particular, the present invention relates to a temperature sensor that measures temperature by passing a laser beam through a polarization-maintaining fiber.

[Prior Art] An example of a temperature sensor using a polarization-maintaining fiber (for example, 1985 Tohoku Branch Federation Conference of Electrical Related Associations, Document No. 2F17) is shown in FIG. 2 and will be described.

In FIG. 2, 10 is a polarization plane-maintaining fiber used as a temperature sensor, 21 is a laser device such as a gas laser or a semiconductor laser, and 22 is an output light of the laser device 21 with a polarization plane of 45° (orthogonal components 1:1).
23 is a lens for making the light that has passed through the polarizer 22 enter the fiber 10; 24 is an analyzer that acts on the light emitted from the fiber; 25 is a light that has passed through the analyzer 24; 26 is a photomultiplier tube for detecting the intensity of light from the collimator, a photodetector whose photodiode includes an avalanche photodiode, and 27 is a photodetector for displaying the output of the photodetector 26. This is a display device such as a cathode ray tube display or a recorder.

The X-axis component and Y-axis component of the polarized light incident on the polarization-maintaining fiber 10 by the polarizer 22 are equal, but due to temperature changes, the X-axis component of the polarized light passing through the polarization-maintaining fiber 10 changes. The transit time (delay time) of the component and the Y-axis component changes, and the amount of change shows different values for the X-axis component and the Y-axis component.
That is, at the output end of the polarization-maintaining fiber 10, the phase shift between the X-axis component and the Y-axis component of the polarized light changes, and the polarization plane of the light that combines the X-axis and Y-axis components is a straight line. Since polarized waves change from polarized waves to elliptically polarized waves and circularly polarized waves, when this is detected by the photodetector 26 through the analyzer 24, it is obtained as a change in the intensity of the light, and this intensity change is displayed on the display device 2.
7 is displayed.

[Problems to be Solved by the Invention] Here, the change in the angle of the polarization plane at the output end of the polarization-maintaining fiber 10 is not brought about only by temperature change, but also due to changes in the axial direction of the polarization-maintaining fiber 10. It is also caused by strain, which is the tension against.

In general, temperature changes will affect the polarization maintaining fiber 10.
It causes expansion or contraction of itself,
Furthermore, if there is a coating for protecting the polarization-maintaining fiber 10, the strain caused by its expansion or contraction is applied to the polarization-maintaining fiber 10, so the rotation of the polarization plane of the light emitted from the polarization-maintaining fiber 10 changes with temperature. Not only that, but it also detects strain at the same time, making it impossible to accurately measure temperature.

Furthermore, fiber temperature sensors are often used in locations with poor environments, and in such locations, distortions such as vibrations are added from the surroundings, further deteriorating the accuracy of temperature measurement.

[Means for Solving the Problems] The present invention has been made to solve the above problems, and it uses two types of polarization-maintaining fibers that have different temperature characteristics and the same strain characteristics to adjust the polarization of the fibers. If the axes are shifted by 90 degrees and light is incident on this through a polarizer, the effect of the polarization maintaining fiber on the input end side on the phase shift of the X-axis component and Y-axis component of polarized light, and the influence of the polarization plane-maintaining fiber on the output end side. The influence of polarization maintaining fibers operates with opposite polarity. In other words, when the plane of polarization is rotated clockwise by the polarization-maintaining fiber at the input end, the plane of polarization is rotated counterclockwise by the polarization-maintaining fiber at the output end, which causes distortion. Two types of polarization-maintaining fibers were selected so that the clockwise and counterclockwise rotation angles exactly cancel each other out.

[Effect] As a result, the rotation of the plane of polarization due to strain is canceled out for the light that passes through the two types of polarization-maintaining fibers, and only the rotation of the plane of polarization due to temperature changes is obtained, allowing for highly accurate temperature measurement. became possible.

[Example] A fiber temperature sensor according to the present invention is shown in FIG. 1 and will be described.

12 is a polarization-maintaining fiber (first fiber) with length L ₁ , 16 is a polarization-maintaining fiber (second fiber) with length L ₂ , and both fibers are connected to connection point 1.
At 9, they are connected by fusion bonding or by connectors. At this connection point 19, for example,
For example, a first fiber having a fiber diameter of 125 μm includes a core 13 having a core diameter of 4 μm and an elliptical clad 14 surrounding the core 13;
A second fiber having a fiber diameter of 125 μm, which includes a core 17 having a core of 50 m and an elliptical cladding 18 surrounding it, has its long axes of the elliptical claddings 14 and 18 orthogonal in order to shift its polarization axis by 90°. At least it's connected. As polarized light passes through a polarization-maintaining fiber, temperature changes and strains have different delay time effects on the X-axis and Y-axis components; As a result, a phase difference occurs between the X-axis component and the Y-axis component, and the plane of polarization obtained by combining these two components becomes rotated. The rotation of the plane of polarization, which is a result of this phase difference, is detected by a photodetector (26 in FIG. 2) as the intensity of the light when the light passes through an analyzer (24 in FIG. 2).

Here, the first fiber and the second fiber are made of different materials, and the influence of temperature change and strain on the fluctuation of the phase difference is different between the two fibers.

Therefore, ΔΦ ₁ is the change in the phase difference of the first fiber, A ₁ is the temperature coefficient for temperature change on the phase difference, B ₁ is the strain coefficient for strain, and ΔL is the change in the length of the first fiber 12 due to strain. ₁ , and similarly, the change in phase difference of the second fiber is ΔΦ ₂ , the temperature coefficient is A ₂ , the strain coefficient is B ₂ , the change in length due to strain is ΔL ₂ , and the temperature change is ΔT
Then, the following relationships are obtained: ΔΦ ₁ =A ₁ L ₁ ΔT+B ₁ ΔL ₁ (1) ΔΦ ₂ =A ₂ L ₂ ΔT+B ₂ ΔL ₂ (2). Here, the first fiber and the second fiber are selected so that B ₁ ΔL ₁ =B ₂ ΔL ₂ (3).

That is, the amount of change in phase difference caused by strain in the first fiber 12 and the second fiber 16 is made equal.

Then, the first fiber 12 and the second fiber 16
Since the polarization axes of the first fiber 12 and the second fiber 16 are shifted by 90 degrees, the change ΔΦ in the phase difference between the X-axis and Y-axis components of the light that has passed in series is ΔΦ=ΔΦ ₁ −ΔΦ ₂ (4). Therefore, from equations (1) to (4), ΔΦ=(A ₁ L ₁ −A ₂ L ₂ )ΔT (5) can be obtained.

As is clear from equation (5), the change in phase difference ΔΦ is not affected by strain at all, and the temperature change ΔT
Only the change in phase difference due to is obtained.

As is clear from the above explanation, instead of the polarization maintaining fiber 10 shown in FIG. 2 used as a conventional temperature sensor, the polarization maintaining fiber 10 shown in FIG. 1 and explained using equations (1) to (5) If a fiber is used in which two types of polarization-maintaining fibers having different characteristics are connected in series, only temperature changes can be accurately measured without the influence of distortion.

Polarization maintaining fibers 12, 16 in FIG.
The explanation was given using an example of an elliptic cladding.
It will be apparent from the above description that similar results can be obtained with other polarization maintaining fibers, such as panda-type polarization maintaining fibers.

[Effects of the Invention] As is clear from the above explanation, according to the present invention, it is possible to accurately measure temperature changes without being affected by distortion, and it is particularly useful in places with a lot of vibrations and poor environments. Since it provides a fiber temperature sensor suitable for temperature measurement, its effects are extremely large.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a fiber temperature sensor of the present invention, and FIG. 2 is a diagram showing a conventional example. 10, 12, 16... polarization maintaining fiber, 1
3, 17... Core, 14, 18... Elliptical cladding, 19... Connection point, 21... Laser device, 22
...Polarizer, 23...Lens, 24...Analyzer,
25...collimator, 26...photodetector, 27...
...Display device.

Claims (1)

[Scope of Claims] 1. A first fiber that is a polarization plane-maintaining fiber, and the first fiber have the same amount of change in the phase difference between the X-axis component and the Y-axis component of polarized light caused by strain, and The fiber consists of a second fiber, which is a polarization plane-maintaining fiber, in which the change in the phase difference between the X-axis component and the Y-axis component of polarized light that occurs due to temperature changes is different, and the polarization of the first fiber and the second fiber is different from each other. A fiber temperature sensor characterized in that the axes are connected at an angle of 90°.