JPS6147513A - Optical fiber sensor - Google Patents

Abstract

PURPOSE:To make it possible to measure the physical force to be measured among various physical forces without receiving the effect of the other physical forces, by directly connecting a compensation constant polarization optical fiber, which has a characteristic substantially equal to that of a sensor constant polarization optical fiber, to the sensor constant polarization optical fiber. CONSTITUTION:A polarizer 3 is arranged with respect to a compensation constant polarization optical fiber 2 so that linear polarized light is excited in a ratio of 1:1 to the X'- and Y'- directions of a constant polarization optical fiber 2. The constant polarization optical fiber 1 and the constant polarization optical fiber 2 are constituted so as to be made substantially equal in a double refraction index characteristic and same in phase difference. The main axes of both optical fibers are mutually rotated by 90 deg. to be connected so as to be crossed at right angles so that the linear polarized light propagated through the constant polarization optical fiber 2 in the X'-direction is propagated to a Y-direction in the constant polarization optical fiber 1 and that propagated through the constant polarization optical fiber 2 in the Y'-direction is propagated to an X-direction in the constant polarization optical fiber 1 to an X-direction. By this method, when the same temp. is imparted to both of the constant polarization optical fibers 1, 2, the phase difference generated between the polarizations of the sensor constant polarization optical fiber 1 in the X- and Y-directions is cancelled out by that generated between the polarizations of the compensation constant polarization optical fiber 2 in the X'- and Y'-directions.

Description

Detailed Description of the Invention (a) Industrial Application Field This invention utilizes the fact that when physical forces such as temperature, pressure, and tension are applied to a constant polarization optical fiber, its birefringence changes. This invention relates to improvements in optical fiber sensors for measurement.

(b) Conventional technology It has been known that the birefringence of a polarization-controlled optical fiber depends on temperature, tension,
Since it changes depending on pressure, etc., various optical fiber sensors that utilize this have been proposed. For example, it is constructed as shown in FIG. 5 using a polarization-controlled optical fiber 1 as shown in FIG.

This constant polarization optical fiber l has two stress applying parts 13 and 13 arranged on both sides of the core 11 in a cladding 12 formed around a central core 11, and has anisotropy in residual strain. It has double refraction. As shown in FIG. 5, the polarizer 3 and analyzer 4 are brought into a spatially parallel or orthogonal Nicol state, and during this time, the main axis of the polarization-constant optical fiber l is at 45° with respect to the incident polarization. The polarization-constant optical fiber 1 is rotated so that the fixed polarization optical fiber 1 is rotated so that the fixed polarization optical fiber 1 is rotated so as to With this configuration, when linearly polarized light is excited at l:1 with respect to the X and Y directions, which are the main axes of the constant polarization optical fiber 1, the polarization state at the end changes due to the phase difference between the two polarized lights. Since the power of the light passing through the analyzer 4 changes accordingly, it becomes possible to measure temperature, tension, pressure, etc.

By the way, optical fiber sensors that utilize the change in birefringence of polarized optical fibers are generally highly sensitive, but they react to many measurement targets such as temperature, tension, and pressure; If you only want to measure the temperature, there are drawbacks such as the need to keep the ambient temperature constant and the environmental conditions for use limited.

(c) Purpose This invention enables the measurement of only the physical forces to be measured, such as temperature, tension, pressure, etc., without being influenced by other physical forces that are not to be measured. It is an object of the present invention to provide an optical fiber sensor that can be used in a wider range of applications by removing restrictions such as conditions.

(d) Configuration According to this invention, the polarization constant optical fiber is heated.

In an optical fiber sensor that measures the above-mentioned physical force by utilizing the change in birefringence when a physical force such as pressure or tension is applied, a compensating constant having substantially the same characteristics as a constant polarization optical fiber for the sensor is used. A polarized optical fiber is orthogonally connected to the sensor polarization constant optical fiber. For example, when tension is the object to be measured, temperature changes in the surrounding environment are applied to both the sensor polarization-constant optical fiber and the compensation polarization-constant optical fiber, causing birefringence changes in both in the same way. Therefore, by orthogonally connecting defined polarization optical fibers, the change in birefringence of the polarization constant optical fiber for the sensor due to the temperature that is not the object of measurement is compensated for by the birefringence change of the compensation polarization constant optical fiber, and the polarization constant optical fiber for the sensor is Measures only the tension that is applied to the object.

(E) Embodiment In FIG. 1, the optical fiber sensor according to the present invention is
It consists of a constant polarization optical fiber 1 for a sensor, a constant polarization optical fiber 2 for compensation that is orthogonally connected to the constant polarization optical fiber 1, a polarizer 3, and an analyzer 4. The polarizer 3 is configured such that the linearly polarized light in the Xo and Y'' directions of the constant polarization optical fiber 2 is l
:1. The constant polarization optical fiber 1 and the constant polarization optical fiber 2 are made by cutting the same constant polarization optical fiber to the same length, so that the birefringence characteristics are substantially equal and the phase difference is the same. Make it. Then, the linearly polarized light in the X′ force direction that propagated through the constant polarization optical fiber 2 moves in the Y direction in the constant polarization optical fiber 1, and the linearly polarized light in the Y′ direction that propagated in the constant polarization optical fiber 2 moves in the X direction in the constant polarization optical fiber 1. For propagation, they are orthogonally connected with their main axes rotated by 90°. In other words, when the same temperature is applied to both polarized optical fibers 1 and 2, the phase difference that occurs between the polarized light in the X and Y directions of the polarized optical fiber 1 for sensor is Xo of fiber 2,
It is canceled out by the phase difference that occurs between the polarized lights in the Y° force direction.
In this way, even if there is a temperature change in the surrounding environment, it acts on both polarization constant optical fibers 1.2, so that the phase difference between the two polarizations can be ignored.

Therefore, physical forces that do not act on both polarization-controlled optical fibers 1.2, for example, the polarization-controlled optical fiber for sensor l.
A phase difference between polarized lights due to a change in birefringence appears only due to the tension or pressure applied to the measurement object, and a change in optical power occurs through the analyzer 4.

It can be measured by closing.

FIG. 2 shows a second embodiment. In this figure, the fixed polarization optical fiber 1 for sensor, the fixed polarization optical fiber 2 for compensation, the polarizer 3, the analyzer 4, and their arrangement are the same as in FIG. 1. However, the compensation polarization constant optical fiber 2 and the polarizer 3
A polarization-constant optical fiber 5 is inserted between the two. This polarization-constant optical fiber 5 serves to transmit linearly polarized light in the X" direction or Y" direction, which is excited through the polarizer 3 at the input end, to the compensation polarization-constant optical fiber 2. have, and
The constant polarization optical fiber 5 is connected so that its main axis rotates by 45 degrees with respect to the main axis of the a-credit constant polarization optical fiber 2, so that the polarization constant optical fiber 2 is rotated in the The polarized light component in the force direction is excited at a ratio of 1:1.By using the constant polarization optical fiber 5 for transmission in this way, J14 constant is possible at a remote location.

FIG. 3 shows a third embodiment. In this embodiment, light from a laser oscillator 7 is incident on one end of a transmission polarization-controlled optical fiber 5 through a polarization splitting prism 6, and the linearly polarized light is converted into a fixed-polarization light. The fiber 5 is excited in the X'' direction or Y'' direction. Five fixed polarization optical fibers are connected to the fixed polarization optical fiber 2 so that their main axes are rotated by 45 degrees with respect to the main axis of the fixed polarization optical fiber 2 for compensation, and thereby the X′ direction of the fixed polarization optical fiber 2 is , Y' force direction polarization component is excited by 1:IK. As in FIGS. 1 and 2, the sensor polarization-constant optical fiber 1 and the compensation polarization-constant optical fiber 2 are connected with their principal axes rotated by 90 degrees. Furthermore, a mirror 9 made of metal such as AI or Ag is formed at the end of the polarization-constant optical fiber 1 for the sensor to fold back the optical path. The connection point between optical fiber 1 and optical fiber 2 is fixed by fixing member 10, the terminal end of optical fiber 1 is pulled with tension F, and the tension is measured.

The birefringence changes due to the tension applied to the fixed polarization optical fiber 1 for the sensor, which causes a change in the phase difference, and the light reflected by the mirror 9 returns to the fixed polarization optical fiber 1, 2, 5.
is propagated and returns to the input end of the polarization-constant optical fiber 5. Since this returned light has a component orthogonal to the initially incident polarized light, it is sent to the photoelectric converter 8 via the polarization separation prism 6 and can be observed as a change in optical power. Therefore, it is possible to measure tension with high precision from a remote location, with the influence of environmental temperature changes negligible.

Next, specifically, a fixed polarization optical fiber with a diameter of 125 gm and a beat length of 1.8 mm at a wavelength of 0.633 lm was coated with epoxy acrylate to have an outer diameter of 400 lm.
I made a reflection type optical fiber sensor as shown in Fig. 3 using the strand of wire, so I will explain it. The fixed polarization optical fiber 1 for sensor and the fixed polarization optical fiber 2 for compensation each have a length of 5 cm, and the fixed polarization optical fiber 5 for transmission has a length of 10 m, and they are fusion spliced, and the connection part is made of epoxy resin. Reinforced with acrylate.

In general, phase change Δ when pulling a constant polarization optical fiber
φ is expressed as Δφ′, (2π/in)·118OII l·Δez. Here, BO is the mode birefringence at room temperature, ■
is the fiber length, Δε2 is the change in tensile strain, and K
is a constant determined by the structural parameters of the polarization constant optical fiber, and was 18 in this specific example.

When the tension was actually measured, the tensile strain that produced a 2π phase difference was 10-3.

The temperature change that would cause the same change would be 14.4°C without the compensation polarization-controlled optical fiber 5, which would result in a 7% error for a 1°C change, as shown in Figure 3. By connecting the compensation polarization optical fiber 5 to 0.
We were able to keep it down to 1%.

(f) Effects According to the present invention, only the physical force to be measured out of various physical forces such as temperature, tension, and pressure can be measured without being influenced by other physical forces that are not to be measured. For example, when measuring tension, only the tension can be measured without being affected by temperature changes in the surrounding environment, and the applicable temperature range can be expanded. In this way, restrictions such as usage environmental conditions can be removed and the range of usage can be expanded.

[Brief explanation of the drawing]

ft51 is a schematic diagram showing one embodiment of the present invention, FIGS. 2 and 3 are schematic diagrams showing other embodiments, respectively, and FIG.
The figure is a sectional view showing an example of a constant polarization optical fiber, and FIG. 5 is a schematic diagram of a conventional example.

Claims (1)

[Claims] (1) In an optical fiber sensor that measures the above-mentioned physical forces by utilizing the change in birefringence when physical forces such as temperature, pressure, tension, etc. are applied to a polarization-controlled optical fiber, the polarization-controlled optical fiber for sensors and A compensation polarization constant optical fiber having substantially the same characteristics is orthogonally connected to the sensor polarization constant optical fiber, and a physical force that is not the object of measurement is applied to both the sensor polarization constant optical fiber and the compensation polarization constant optical fiber. The change in birefringence of the fixed polarization optical fiber for the sensor is compensated by the change in birefringence of the fixed polarization optical fiber for compensation, and only the physical force that is the object of measurement that is applied to the fixed polarization optical fiber for the sensor is measured. optical fiber sensor.