JPS6285808A - Fiber strain sensor - Google Patents

Abstract

PURPOSE:To measure strain with high accuracy by connecting two kinds of polarization plane maintaining fibers which differ in strain characteristics, but have the same temperature characteristics so that their axes of polarization shift by 90 deg.. CONSTITUTION:The polarization plane maintaining fibers 12 and 16 are connected 19 by welding, etc. At this connection point 19, the major axes of elliptic clads 14 and 18 cross each other at right angles so as to make a 90 deg. shift between the axes of polarization of the fibers. Consequently, when polarized light is propagated in the fibers 12 and 16, there are difference in the influence of delay time between an X-axial and a Y-axial component affected by temperature variation and strain. Therefore, the X-axial and Y-axial components have a phase difference as the polarized light is propagated in the fibers 12 and 16 and the plane of polarization obtained by mixing both components rotates. The rotation of the plane of polarization which is the result of the phase difference is detected in the intensity of light by passing the polarized light through an analyzer. Consequently, the strain is detected accurately without being affected by temperature variation.

Description

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

[Prior Art] A temperature sensor using a polarization-maintaining fiber has already been developed, and it has been shown that this temperature sensor is significantly affected by strain. An example of a strain sensor using such a polarization-maintaining fiber (for example, 1985 Tohoku Branch Union Conference of Electrical Related Associations, 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 that is a gas laser or semiconductor laser, and 22 is the output light of the laser device 21 with a polarization plane of 45° (orthogonal components 1:1). A polarizer 23 is used to polarize the light that has passed through the polarizer 22 through the fiber 10.
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 for detecting the intensity of the light from the collimator. 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 and Y-axis components of the polarized light incident on the polarization-maintaining fiber 10 by the polarizer 22 are equal, the polarization-maintaining fiber 10 is distorted due to strain (tension in the axial direction of the polarization-maintaining fiber 10). The passage time (delay time) of the X-axis component and Y-axis component of the polarized light passing through 10 changes,
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 the polarized wave changes from polarized wave to elliptically polarized wave and then to circularly polarized wave, when this is detected by the photodetector 26 through the analyzer 24, it is detected as a change in the intensity of light 1q, and this intensity change is displayed on the display device 27.

[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 caused only by strain, which is the tension in the axial direction of the polarization-maintaining fiber 10. However, it has been revealed that this effect is also brought about by temperature change, and it has not been possible to separate the two effects.

Furthermore, temperature changes cause the polarization-maintaining fiber 10 itself to expand or contract, and if there is a coating to protect the polarization-maintaining fiber 10, the strain caused by the expansion or contraction of the coating protects the polarization-maintaining fiber 10. Since the rotation of the polarization plane of the light emitted from the polarization-maintaining fiber 10 is applied to the fiber 10, not only the strain but also the temperature change can be detected at the same time, making it impossible to accurately measure the 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. When connected with the axes shifted by 90' and incident light through a polarizer,
The influence of the polarization-maintaining fiber on the input end side and the influence of the polarization-maintaining fiber on the output end side on the phase shift of the X-axis component and the Y-axis component of polarized light act with opposite polarity. In other words, if the plane of polarization is rotated, for example, 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. Two types of polarization-maintaining fibers were selected so that the angles of rotation and counterclockwise rotation exactly canceled each other out.

[Function] As a result, the rotation of the polarization plane due to temperature change is overcome for the light that has passed through the two types of polarization plane holding filters, and only the rotation of the polarization plane due to strain is reduced by 1q, allowing highly accurate temperature measurement. It has become 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 a length Ll
, 16 is a polarization maintaining fiber (second fiber) having a length L2, and both fibers are connected at a connection point 19 by fusion splicing or by a connector. This connection point 19
For example, a core 13 having a core diameter of 4 μm
and a diameter 12 including the elliptical cladding 14 surrounding it.
Because the first fiber having a fiber diameter of 5 μm and the second fiber having a fiber diameter of 125 μm including a core 17 having a core of 4 μm and an elliptical cladding 18 surrounding the core 17 have their polarization axes shifted by 90′. The elliptical claddings 14 and 18 are connected to each other with their long axes perpendicular to each other. As polarized light passes through a polarization-maintaining fiber, the delay time effects of temperature changes and strains on the X-axis component and the Y@acid component are different; A phase difference is generated between the X-axis component and the Y-axis component, and the plane of polarization obtained by combining these two components becomes a rotated edge. 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 by passing it through an analyzer (24 in FIG. 2).

Here, the first fiber 12 and the second fiber 16 are made of different materials, and changes in temperature and strain (tension in the axial direction of the first fiber 12 and the second fiber 16) cause changes in the phase difference. The influence on the fibers is different for both fibers.

Therefore, the change in the phase difference of the 177th fiber 12 is calculated by ΔΦ1
, the temperature coefficient for temperature change on the phase difference is A1, the distortion coefficient for strain is 81, and the first) 1 due to strain
Let the change in the sign of Iba 12 be 8L1, and do the same for the second
The change in the phase difference of the fiber 16 is ΔΦ2, and the temperature coefficient is A.
2. If the strain coefficient is 82, the change in length due to strain is ΔL2, and the temperature change is ΔT, then ΔΦ1 = Δ1L1ΔT + B1ΔL1 (1) ΔΦ2
= A2 L2ΔT1828L2 (2> A relationship is obtained. Here, the first fiber and the second fiber are selected so that A1L1=A2L2 (3').

In other words, the amount of change in phase difference that occurs with respect to temperature changes in the first fiber 12 and the second fiber 16 is made equal.

Then, the change Δ in the phase difference between the X-axis and YINI components of the light that has passed through the 177th fiber 12 and the second fiber 16 in series is the difference between the first fiber 12 and the second fiber 16.
Since the polarization axis is shifted by 90', ΔΦ=ΔΦ1−ΔΦ2 (4).

Therefore, from equations (1)2 to (4), ΔΦ=B1ΔL1−
B2ΔL2 (5) is obtained.

As is clear from this equation (5), the change in phase difference ΔΦ
There is no effect of temperature change on the graph, and only the change in phase difference due to strain can be obtained.

As is clear from the above description, the polarization-maintaining fiber 10 shown in FIG. 2 has been used as a conventional strain sensor.
Instead, if we use a fiber made by connecting two types of polarization-maintaining fibers in series, which have the characteristics shown in Figure 1 and explained using equations (1) to (5), the strain will be reduced without being affected by temperature changes. It is possible to measure only with high accuracy.

In Fig. 1, the J3 polarization maintaining fiber 12.16 is explained using an elliptical clad as an example, but similar results can be obtained with other polarization maintaining fibers such as a panda-type polarization maintaining fiber. It is clear from the above explanation that this can be obtained.

[Advantageous Effects of the Invention 1] 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 maintaining fiber 13.17
... 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] The first fiber, which 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 a temperature change, and The fiber consists of a second fiber, which is a polarization-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 in response to strain is different, and the polarization axes of the first fiber and the second fiber are different from each other. is 90°
A fiber strain sensor characterized in that the fiber strain sensor is connected in a staggered manner.