JPH0370167B2 - - Google Patents

Description

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

[Prior Art] Temperature sensors using polarization-maintaining fibers have already been developed, and it has been shown that these temperature sensors are significantly affected by strain. An example of a strain sensor using such a polarization-maintaining fiber (for example, 1985 Tohoku Branch Federation Conference of Electrical Related Societies, Document No. 2F17) is shown in FIG. 2 and will be described.

In FIG. 2, 10 is a polarization-maintaining fiber used as a strain 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 component 1:
1) is a polarizer for polarizing light, 23 is a polarizer 22
24 is an analyzer that acts on the light emitted from the fiber, 25 is a collimator for collecting the light that has passed through the analyzer 24, and 26 is a lens for the light from the collimator. Photomultiplier tube, photomultiplier tube for detecting intensity
A photodetector 27 includes a diode or an avalanche photodiode, and 27 is a display device such as a cathode ray tube display or a recorder for displaying the output of the photodetector 26.

Although the X-axis component and the X-axis component of the polarized light incident on the polarization-maintaining fiber 10 by the polarizer 22 are equal, due to strain (tension in the axial direction of the polarization-maintaining fiber 10), The transit time (delay time) of the X-axis component and Y-axis component of the polarized light passing through the polarization-maintaining fiber 10 changes, and the amount of change is
The axis component and the Y-axis component show different values. 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 the 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 27.

[Problem 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 brought about only by strain, which is tension in the axial direction of the polarization-maintaining fiber 10. It has been shown that this phenomenon is caused by temperature changes as well as by temperature changes.
It was not possible to separate the effects of the two.

Additionally, temperature changes can 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 is caused only by the strain. However, since it also detects temperature changes at the same time, it is not possible to accurately measure strain.

Furthermore, fiber strain sensors are expected to be used in places with poor environments, and in such places, surrounding temperature changes will be large, further degrading the accuracy of strain measurement.

[Means for Solving the Problems] The present invention has been made to solve these problems, and it uses two types of polarization-maintaining fibers that have different strain characteristics and the same temperature 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, if the plane of polarization is rotated clockwise by the polarization-maintaining fiber at the input end, the plane of polarization will be rotated counterclockwise by the polarization-maintaining fiber at the output end, which is less sensitive to temperature changes. Therefore, two types of polarization-maintaining fibers were selected so that the clockwise and counterclockwise rotation angles exactly cancel each other out.

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

[Example] A fiber strain 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,
A first fiber having a diameter of 125 μm including a core 13 having a core diameter of 4 μm and an elliptical cladding 14 surrounding it; and a diameter including a core 17 having a core diameter of 4 μm and an elliptical cladding 18 surrounding it. A second fiber having a fiber diameter of 125 .mu.m is connected with the long axes of the elliptical claddings 14 and 18 orthogonal in order to shift their polarization axes by 90 DEG. 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 12 and the second fiber 16 are made of different materials, and temperature changes and strain (tension in the axial direction of the first fiber 12 and the second fiber 16) can cause changes in the phase difference. The influence on the two fibers is different.

Therefore, Δφ is the change in the phase difference of the first fiber 12, A ₁ is the temperature coefficient for temperature change that affects 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 16 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).

The first fiber and the second fiber are selected so that A ₁ L ₁ =A ₂ L ₂ (3).

That is, the amount of change in the phase difference caused by the temperature change of 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 amount of change Δφ in the phase difference between both 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=B ₁ ΔL ₁ −B ₂ ΔL ₂ (5) can be obtained.

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

As is clear from the above description, instead of the polarization-maintaining fiber 10 shown in FIG. 2 used as a conventional strain sensor, the polarization plane maintaining fiber 10 shown in FIG.
If a fiber in which two types of polarization-maintaining fibers having the characteristics explained using equation (5) are connected in series, the absence of distortion can be accurately measured without being affected by temperature changes.

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 strain without being affected by temperature changes, especially in places with large temperature changes in poor environments. Since the present invention provides a fiber strain sensor suitable for strain measurement, its effects are extremely large.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a fiber strain sensor of the present invention, and FIG. 2 is a diagram showing a conventional example. 10, 12, 16...Polarization plane maintaining fiber, 1
3, 17... Core, 14, 18... Elliptical cladding, 1
9... 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 holding fiber and the first fiber have the same amount of change in phase difference between the X-axis component and the Y-axis component of polarized light caused by temperature change, and The first fiber consists of a second fiber, which is a polarization plane-maintaining fiber, which differs in the amount of change in phase difference between the X-axis component and the Y-axis component of polarized light caused by strain, and the polarization of the first fiber and the second fiber is different from each other. A fiber strain sensor characterized by being connected with its axes shifted by 90°.