JPS5927232A - Optical fiber pressure sensor - Google Patents

Abstract

PURPOSE:To make it possible to simply measure the change of pressure by using one optical fiber. CONSTITUTION:Light emitted from a light source 1 is passed through a polarizer to be converted to linear polarized light and a lambda/2 plate 33 is rotated to align an incident polarization angle theta130 to a desired value (usually 45 deg.). In this case, the change of light output generated when external force F27 is added to a pressure sensitive part 37 with a length L is measured by an analyser 38 by a motor 39. A polarization degree E129 is shown by a formula I from the max. output¦EM¦<2> and the min. light receiving power¦Em¦<2> of light output. The linear polarization E129 obtained by projecting the polarization angle thetai30 to an angle other than 0 deg. or 90 deg. is represented by the vector sum of linear polarization components¦Ex¦<2>,¦Ey¦<2> in two axes X, Y crossing at right angles. When the phase difference of both of them is phi, the polarization degree P is shown by a formula II. Especially, when thetai is 45 deg., the polarization degree P is shown by a formula III. Therefore, the phase difference between polarization components in an optical fiber 35 is calculated.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a highly accurate pressure sensor using optical fiber.

Conventionally, a pressure sensor using an optical fiber as shown in FIG. 1 has been proposed. In FIG. 1, light source 1
In this method, the light from 1 is split into two by a beam splitter I2, and each of the lights is input to an optical fiber 13 including a pressure sensitive part 15 and a reference optical fiber 14, thereby making use of interference between two linearly polarized waves. Two linearly polarized waves polarized in the same direction interfere with each other, and the difference in phase between the two waves causes brightness and darkness of the interference fringes 16. When pressure is applied to one optical fiber 13, the phase of the propagating light changes, and interference fringes 16 are generated between it and the output light from the other optical fiber 14. The amount of pressure fluctuation can be measured based on the displacement of the interference fringes 16. This method uses two optical fibers, and the polarization plane of the reference optical fiber 14 is affected by various external disturbances (temperature changes, vibrations, etc.).
This has the disadvantage that the measurement system becomes complicated, such as the reference optical fiber being required to be placed in a thermostatic oven or the like in order to improve measurement accuracy.

The present invention provides an optical fiber pressure sensor that can easily measure pressure changes by using just one optical fiber.

Hereinafter, the present invention will be explained in detail with reference to the F1 drawing.

Figure 2 shows an elliptic curve as an example of a polarization-preserving Noah/Iba.
This shows the coordinate system of an optical fiber with a doped polarization plane of 1 and 2.
1 is a core, 22 is an internal cladding, 23 is an elliptical cladding,
24 indicates an external club. Here, the elliptical cladding 23
Let the long and short axes of 2 be the X axis 25 and the y axis 26, respectively. θ
F28 is the inclination angle of the applied force direction of external force F27 with respect to the X axis,
Ei 29 indicates the amplitude of the incident wave, and θi30 indicates the incident polarization angle, that is, the inclination angle of the incident wave with respect to the X axis. bx
, by represent the major and minor axes of the ellipses, and the cladding ellipticity ε is defined as follows.

'bx -by ・・・・・・・・・
...(1) bx + by FIG. 3 is a diagram showing the principle of the present invention, where 31 is a light source, 32 is a polarizer, 33 is a λ/2 plate, 34 is a condensing lens, and 35 is a polarization-preserving light beam. 36 is a fiber support and rotation mechanism, 37 is a pressure sensitive section, 38 is an analyzer, 39 is an analyzer rotation mechanism, 310 is a light receiver, and 311 is an output display section. The principle of measuring pressure will be explained below. The light emitted from the light source 31 becomes linearly polarized light by passing through the polarizer 32,
It is rotated by the λ7/2 plate 33 to adjust the incident polarization angle θi30 to a desired value (usually 45°).

Therefore, the analyzer 38 is rotated 39 to measure the change in the light output when the external force F27 is applied to the long pressure sensitive part 38. Maximum power of optical output IEMI2 and minimum received power IE
From m12, the degree of polarization P is expressed as follows.

FIG. 4 shows the coordinate system of the polarization plane of the elliptical polarization plane at the output end of the fiber 35. In FIG. 4, 29 represents the amplitude Ei of the incident polarized wave, and 30 represents the incident ll1i wave angle θ1142, the inclination angle ψ of the elliptically polarized wave surface 41 with respect to the X axis.

43 and 44 represent the amplitudes of the major axis 4 and minor axis Em of the elliptically polarized plane 41.

In Figures 2 and 4, the incident polarization angle θ430 is set to 00 or 9.
The linearly polarized wave Ei 29 incident at an angle other than 0° can be expressed as a vector sum of linearly polarized wave components IEx12.1Ey+'' in two orthogonal axial directions, for example, the X-axis and y-axis directions. If the phase difference is ψ, the degree of polarization P is expressed by the following equation.

Especially when θ・=45°, it becomes F-45°, so the formula (
The polarization degree P of 3) is P = cns(P (')
It is expressed as

Therefore, by measuring the degree of polarization P from equation (3) or equation (4), the phase difference ψ between the orthogonal polarization components HE blue mode and HE steam mode propagating in the optical fiber <35
is required. The degree of polarization P expressed by equation (2) can be easily measured by rotating the analyzer 38. An example of this measurement is shown in FIG. In FIG. 5, the vertical axis represents the change in the light reception level, and the horizontal axis represents time and corresponds to the load (F, L). From this figure, the load (F.

It can be seen that when L) is added, the received light level changes sinusoidally.

FIG. 6 is an actual measurement example in which the change in light intensity as shown in FIG. 5 is expressed by the polarization degree P of equation (2). When the incident polarization angle θi is 45°, when the inclination angle θ of the application direction of the external force F is 45°, the degree of polarization P hardly changes, but θ is 45°.
It can be seen that when the angle is 00 or 90 degrees, the degree of polarization P changes greatly from 0 to 1 in response to a change in the magnitude of the external force F.

In FIG. 7, the magnitude of the external force F received by the pressure sensitive part 37 is plotted on the horizontal axis.
The phase difference ψ (Equation (4)) at that time is taken on the vertical axis,
It can be seen that when θ is other than 45° or 90°, the magnitude of the external force F and the phase difference ψ are almost linearly proportional. From FIG. 7, it was found that this was due to the pressure sensitivity of external force.

Stress difference Δσef (=σe−σ
y) is simply expressed by the following formula.

Here, Δσ. represents the difference between the residual stress components in the y-axis and y-axis directions of the polarization-maintaining optical fiber due to the elliptical cladding 23, and Δσf represents the stress difference newly generated within the fiber by the external force F. ε is the ellipticity < b” by
>, E is Young's modulus, ν is Poisson's ratio, bx-1-by Δα (=α24-α23) is the difference in thermal expansion coefficient between media 24 and 23, and ΔT' (=T-To) is the difference between room temperature T and fiber represents the difference between the softening point temperature To and the softening point temperature To. A numerical example of the stress difference Δσef is shown in FIG. From Figure 8, the external force application direction is 00≦θ2
<45°, the stress difference becomes zero (Δσef
-o). Furthermore, the response J difference Δσef when the external force F is zero is the stress difference Δσ inherent to the fiber. By changing the external force F, the inherent residual stress Δσ generated in the manufacturing process of the polarization-maintaining optical fiber can be easily calculated. Estimating 7% B1 ability.

Based on the photoelastic effect and equation (5), the phase difference ψ between the y-axis and the light waves polarized in the y-axis direction is expressed by the following equation.

Here, Δβ (−β knee βy) represents the propagation constant difference between the polarization modes in the X and y axis directions. Lo and L represent the total fiber length and the fiber length of the pressure sensitive part, and CP is the photoelastic constant (≧
3.50X 10-5 mA/Kq), λ represents the wavelength.

From equation (6), the phase change due to external force F, that is, the pressure sensitivity (
− collection) is LdF F triviality ・f(θF ) (7) LdF
It is expressed as λb. Here, b represents the outer cladding radius of the fiber. From equation (7), the change machine of pressure (external force) can be easily determined.

Figure 9 shows the outer cladding radius of the fiber, the photoelastic constant C
P (3.50 x 10'i/9), fiber pressure sensitive length length In Figure 9, the horizontal axis is the direction of external force application. From the figure, it was found that the sensitivity is greater when the external force application direction θ2 is near 00 or 90° (=I. From equation (7) and Figure 9, the sensitivity is determined by the length of the polarization-maintaining optical fiber to which the external force is applied. Since it is proportional to the length, the length L of the fiber can be selected depending on the required sensitivity.

Further, in FIG. 3, as a means for detecting a phase difference, in addition to the configuration in which the analyzer 38 is rotated 39 as described above, there is a device as shown in FIGS. 109 and 11.

FIG. 1O shows a method of measuring by fixing the analyzer 38 in the 45° (or -45°) direction with respect to the optical axis of the λ/2 plate 33 without rotating it.

In addition, as shown in FIG. 11, by using a birefringent crystal 111 such as a Wollaston prism at the fiber output end, the output elliptically polarized wave is separated into two linearly polarized waves, and the two photodetectors 1
12, 113 to measure the optical power i, l I2, 11
The configuration is such that the degree of polarization is measured by the output processing section No. 4.

Although we have described an elliptical clad fiber as a polarization maintaining fiber, it is also possible to use a side pit type, a panda fiber, etc., which are already well-known.
It goes without saying that any fiber that preserves the plane of polarization in its axis is sufficient. Alternatively, it is also possible to embed it in a cable or repeater during a voltage resistance test of a repeater, and it can be applied to various voltage resistance tests.

Further, remote measurement is also possible by installing the output display section 311 apart from other parts.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing an example of a conventional pressure sensor, FIG. 2 is a coordinate system diagram for explaining an example of a polarization maintaining fiber used in the present invention, and FIG. 3 is a configuration diagram for explaining the present invention in detail. ,
FIG. 4 is a coordinate system diagram for detailed explanation of the present invention, FIG.
Figure 6. 71 FIGS. 8 and 9 are characteristic diagrams showing measurement results in the present invention, and FIGS. 10 and 11 are block diagrams showing an example of detecting a phase difference in the present invention. 11... Light source, 12... Beam splitter, 13.
... Optical fiber, 14 ... Reference peripheral optical fiber, 15 ... Pressure sensitive part, 16 ... Interference fringe, 21 ... Core, 22 ... Internal cladding, 23 ... Elliptical cladding, 24 ...External cladding, 25...y axis, 26...
・Y axis, 27...Outer collar 28...Tilt angle, 29...
・Amplitude, 30...Incidence polarization angle, 31...Light source, 32
... Polarizer, 33 ... λ/2 plate, 34 ... Condensing lens, 35 ... Polarization maintaining optical fiber, 36 ...
Fiber support and rotation mechanism, 37...pressure sensing section, 38.
...Analyzer, 39...Analyzer rotation mechanism, 310...
Light receiver, 311... Output display unit, 41... Elliptical polarization plane, 42... Tilt angle, 43... Amplitude of long axis EM, 4
4...Short axis E. amplitude, 111...birefringent crystal, 112, 113...
- Photodetector, 114...output processing section. Patent applicant International Telegraph and Telephone Co., Ltd. agent Inuzuka 1 off-campus person 13! ]1 Figure 2 6 Daso diagram 6 Tato force F (Q/CIη) Figure 7 External force F (g/cm) Figure 8 External force F (kg/cm) Figure 9 Negativity of external force application θF (deg)

Claims (1)

[Claims] means for inputting polarized light that is not orthogonal to the long axis or short axis of the polarization-maintaining optical fiber into the polarization-maintaining optical fiber; and pressure (external force) in a direction other than 45° with respect to the long axis (or short axis).
a polarization-maintaining optical fiber having a pressure-sensitive portion that receives the pressure; and means for detecting a phase difference between the polarized light in the long axis direction and the short axis direction, and detecting the magnitude of the pressure from the phase difference. An optical fiber pressure sensor featuring: