JPH076862B2 - Optical fiber pressure sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to an optical fiber pressure capable of optically measuring static pressure or dynamic pressure by exposing an optical fiber having a specific structure as a sensor section to a pressure to be measured. The present invention relates to a sensor, and more particularly to an optical fiber pressure sensor capable of measuring the pressure inside various objects by the thinness and flexibility of the optical fiber serving as the sensor.

[Conventional technology]

Various devices have been proposed for measuring pressure using an optical fiber as a sensor. Most of them, as shown in FIG. 2, are half mirrors of the light from the laser light source 41.
It has an interferometer configuration in which it is divided into two by 46, and they are multiplexed again to cause interference and detected by the detector 44. In FIG. 2, it is read that the interference between the light propagating through the reference fiber 43, which is one optical path, and the light propagating through the sensor fiber 42, which is the other optical path, changes under the influence of the pressure P of the measurement object and changes in pressure. It is converted into. Here, in order to change the interference state, it is effective to give a displacement in the length direction to the optical fiber on the sensor side, and a conversion means for that purpose is required. FIG. 3 shows a structure for converting a pressure change in the pressure vessel 47 into a change in the length of the sensor fiber 42 tightly wound around the pressure container 47.

As an example of not forming an interferometer, a method using microbend loss of an optical fiber has been proposed. This utilizes the fact that when a slight bend is induced in the optical fiber, a part of the propagation mode becomes a cladding mode and is lost from the optical fiber. FIG. 4 shows an example of the configuration of a pressure sensor utilizing microbend loss. (a) is the overall configuration, and (b) is the configuration of the converter part. 51 is a laser light source, 55 is a sensor fiber, 52 and 52 'are mode separators, 54 is a light receiver, and 53 is a converter for giving a microbend. The mode separators 52 and 52 'are configured to remove the light propagating through the clad, and the pressure to be measured is the pressure plate.
Converted into 56 displacements to induce microbends in the sensor fiber. With a configuration in which the displacement amount of the pressure plate and the pressure have a constant relationship, the pressure can be measured by measuring the intensity of the output light. Even in this case, the structure for giving the displacement to the sensor fiber requires the second
The configuration is the same as that shown in FIGS.

[Problems to be solved by the invention]

In a conventional pressure sensor using an optical fiber, it is difficult to measure hydrostatic pressure stably without using a structure that accurately converts pressure into mechanical displacement. Has a complicated optical configuration and means for detecting the amount of change.

The present invention solves these problems and enables pressure measurement using a single sensor fiber without the need to configure an interferometer and without using a mechanical structure that converts pressure into displacement. And an optical fiber pressure sensor.

[Means for solving problems]

In the present invention, as the sensor fiber, a side hole fiber in which the clad portion around the core that propagates only a single mode has a continuous hole in the longitudinal direction is used. In this fiber, if a pressure to be measured is applied to the inner hole, an anisotropic displacement is generated in the core, and birefringence is induced elastically and optically. Therefore, the polarization state of the propagating light changes. The same phenomenon occurs when the internal pressure of the hole inside the fiber is kept constant and the external pressure applied to the fiber changes. The pressure and the magnitude of the induced birefringence are mainly determined by the structure of the sidehole fiber. Changes in the polarization state can be detected by known optical means. Since the displacement of the fiber at a constant pressure is constant, hydrostatic pressure and dynamic pressure can be measured with good reproducibility.

[Work]

FIG. 5 is a cross section of a sidehole fiber used in the present invention.

1 is a core, 2 is a clad, and 3a and 3b are holes.

Now, assuming that the length of the portion to which the pressure is applied is l and the ratio of the change of the phase φ by the pressure P is dφ / dp, the sensitivity coefficient with respect to the pressure is s = l ₋₁ dφ / dp. The birefringence change of the fiber due to the pressure change in the side hole is expressed by S _i P _i where the pressure inside the side hole is P _i and the sensitivity coefficient is Si, and the birefringence change due to the change in external pressure applied to the fiber. it is if the external pressure _{P o,} the phase sensitivity and _{S o,} it is S o _{P o.} At this time, the pressure change P _i in the side hole and the change P _o of the external pressure applied to the fiber give a displacement in the opposite direction to the fiber core, so that S _o = −S _i .

Therefore, the birefringence B of the sensor fiber when the internal pressure P _{i of} the side hole or the external pressure P _o changes is as follows.

B ₌ in _{B o -S i P i + S} o P o (1) where, Bo is the residual birefringence side holes fibers have unique.

The coefficient S and the parameters of the sidehole fiber are as follows: the sidehole diameter is 2r, the fiber diameter is 2b, and the core refractive index is
n ₀ , the photoelastic tensor of silica is π ₁₁ , π ₁₂ The wavelength of light is λ
Then, Have a relationship. If λ = 0.633 μm is selected as the wavelength of light, when a sidehole fiber with Δ = 0.24%, 2b = 200 μm 2r ₀ = 65 μm is used, silica has
₁₁ −π ₁₂ ) −2-4 × 10 ⁻¹² Pa ⁻¹ , so Si9.6 × 1
It is predicted to be 0 ^-5 radm ^-1 Pa ^-1 .

〔Example〕

The present invention is based on the fact that an optical fiber having a side hole is used as a sensor to measure a change in polarization state of light propagating inside. With regard to the installation of the sensor fiber, the configuration allows a mode in which the internal pressure P _{i of the} side hole changes or a mode in which the external pressure P _o changes while P _i is constant. Further, as a measuring system of the change in the polarization state of the propagating light, a mode of measuring the light passing through the sensor fiber or a mode of measuring the light returning from the far end of the sensor fiber is possible. These are the objects to be measured,
They can be used in any combination depending on the measurement conditions. The change in polarization state is measured by a conventional measuring method.
In order to convert a change in polarization state into a change in pressure, it is necessary to measure a known pressure and change in polarization state and determine a specific sensitivity coefficient.

Embodiment 1. FIG. 1 is a view showing an embodiment of the optical fiber pressure sensor of the present invention, in which the change in the external pressure P _o is detected from the transmitted light of the sensor fiber at a constant pressure (atmospheric pressure) in the side hole. Have a configuration. In FIG. 1, 4 is a laser light source, 5 is a polarizer, 6 and 13 are quarter-wave plates, 7a and 7b are lenses, 8 is a sensor fiber, 9 is a pressure vessel, 10a and 10b are seal parts, 11
Is a half mirror, 12a and 12b are Wollaston prisms, 14
Is a mirror, 15a, 15b, 15c and 15d are photodetectors, and 16 is an arithmetic unit including an electronic circuit and a display unit. The light from the laser light source 4 is completely circularly polarized by passing through the polarizer 5 and the quarter-wave plate 6 and is coupled to the core of the sensor fiber 8 via the lens 7a.

When P _i = P _o (= 1 atm), the incident light is emitted as circularly polarized light when the residual birefringence B _o of the sensor fiber 8 is zero or small. In the light receiving system, the emitted light is collimated by the lens 7b and goes straight to the Wollaston prism 12
The light incident on a is adjusted by adjusting the principal axis of 12a to detect photodetectors 15a, 15a.
The light is evenly divided into b. Further, the emitted light reflected by the half mirror 11 and the mirror 14 is returned to linearly polarized light by the 1/4 wavelength plate 13, and the main axis of the Wollaston prism 12b is made to coincide with this polarization direction, so that 15c or 15d.
Is detected by only one of the photodetectors. This state is the reference state. By P _o is applied to the pressure vessel, the difference in P _o and P _i is generated, the (1) light emitted from the sensing fiber birefringence occurs along the equation elliptically polarized. At this time, the inclination of the ellipsoid of the elliptically polarized light can be known from the ratio of the light emitted from the Wollaston prism 12a and received by the photodetectors 15a and 15b. Light passing through the quarter-wave plate 13 and Wollaston prism 12b is received by the light receiver 1
The ellipticity of elliptically polarized light can be known from the ratio of light received by 5c and 15d. A structure for obtaining the change in the birefringence index B of the sensor fiber from the polarization state obtained by obtaining the light amount ratios received by 15a and 15b and 15c and 15d is included in the 16 electronic circuits and the display section. FIG. 6 shows the measurement results. A birefringence of about 300 rad / m was observed even when P _o and P _i were equal. It is understood that this is mainly due to the deformation of the core that occurs during the production of the sidehole fiber. A sidehole fiber is generally formed by forming a hole along the core in the longitudinal direction with an ultrasonic drill or the like in an optical fiber preform having a core / clad structure, and drawing this preform with a hole to form a sidehole fiber. Is. At this time, the core after drawing may have an elliptical cross section because the periphery of the core is not uniform. The change in birefringence B when P _o is increased is obtained as a plot on the right side from the vertical axis in FIG. The coefficient obtained from the slope of this straight line is S
_o = 11.0 × 10 ^-5 radm ^-1 Pa ^-1 . The difference from the theoretical value is considered to be due to other complex factors.

Second Embodiment FIG. 7 shows the configuration of the second embodiment. In this embodiment, the pressure inside the reaction vessel is detected by receiving the reflected light from the far end of the sensor fiber. Reference numeral 4 is a laser light source, and 19 is a half mirror for guiding the reflected light to the light receiving system. The principle of the light receiving system beyond the half mirror 11 is the same as that of the first embodiment.

In the present embodiment, the pressure change in the container 9 is caused by the sensor fiber 8
The side hole is closed at the incident end 8a of the sensor fiber 8 so that the internal pressure P _i in the side hole changes, and the side hole is opened inside the container at the fast end 8b.

In this configuration, the length of the sensor fiber 8 inserted into the container 9 is meaningless, and the entire length of the sensor fiber 8 acts to give a phase change to the propagating light. Since the propagating light is affected by a round trip, the effective sensor length is twice the sensor fiber length. Far end of sensor fiber
In order to improve the reflection efficiency while maintaining the condition that the side hole is opened in the container 9, 8b is effective in improving the measurement accuracy by performing a process such as vapor deposition and forming a mirror.

The left side of the vertical axis in FIG. 6 shows the result of measurement in this example using the sensor fiber having the same effective length as that in the first example. The coefficient obtained from the measurement result is S _i = 10.2 × 10 ^-5 radm
^-1 Pa ^-1 , which shows a value slightly different from S _o . This is considered to be due to the difference in strain depending on the direction of the sensor fiber.

Third Embodiment FIG. 8 shows a third embodiment of the present invention. In the present embodiment, the sensor fiber 8 is measured by measuring the backward Rayleigh scattered light of the light incident in a pulse form from the pulse light source 4 '.
It is intended to detect the pressure in each of the pressure vessels 22a, 22b, 22c, ... The configuration of the light receiving unit is the same as that of the second embodiment except that the received light is backward Rayleigh scattered light from the sensor fiber 8. Since the backward Rayleigh scattered light is generated over the entire length of the sensor fiber 8, the scattered light emitted from the rear end of the pressure container 22a is subjected to the phase change by P ₀₁ over twice the length of the sensor fiber in the container 22a. Become. Similarly, the scattered light emitted from the rear end of the pressure vessel 22b receives the phase change due to P ₀₂ over the length of twice the sensor fiber length in the pressure vessel 22b in addition to the phase change. Internal unit
The scattered light returning from 22a and the returning light from 22b are received by the receiver 15
Since it arrives at a to 15d with a time difference of only the optical path length difference,
If the signal from the photodetector is temporally decomposed and processed, 22a
Information about pressures in and 22b can be obtained separately.
The same applies when a container of 22c is installed.

In the above embodiments, the light receiving system has a configuration using a Wollaston prism and a quarter-wave plate, but can be replaced with another configuration as long as it can measure the polarization state of reflected light or transmitted light. .

The sidehole fiber used as the sensor fiber is
Although the fiber having holes on both sides of the core shown in FIG. 5 is used, the number of holes may be one or two or more as shown in FIG. 6, and uneven strain may be generated in the fiber core due to changes in internal pressure and external pressure. Anything that gives

〔The invention's effect〕

As described above, according to the present invention, it is not necessary to configure an interferometer like the conventional optical fiber sensor or a configuration for converting pressure into displacement, and it is possible to measure pressure only with the sensor fiber. .

In the optical fiber pressure sensor of the present invention, if the fiber internal pressure P _i is set to be constant, the sensitivity is determined by the length of the fiber existing in the portion to which the pressure is applied, so that the pressure can be measured at any sensitivity. Is possible. Further, if the internal pressure P _i is changed, the entire length of the sensor fiber acts as an effective length, so that a minute change in pressure can be measured. According to the measurement principle of the present invention, the pressure to be measured may be hydrostatic pressure or dynamic pressure. In addition, if the backscattered light is measured, the pressure distribution can be measured.

Since the sensor fiber of the present invention is mainly formed of silica-based fiber, it is stable against strong acids other than hydrofluoric acid,
It can also be used at high temperatures up to about 1000 ° C.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of an optical fiber pressure sensor of the present invention, FIG. 2 is a diagram showing a general configuration of a conventional optical fiber pressure sensor, and FIG. 3 is a diagram showing pressure P as a sensor fiber length. FIG. 4 is a diagram showing a configuration for converting into a change in the height, FIG. 4 is a configuration diagram of a pressure sensor using a microbend loss, FIG. 5 is a sectional view of a sidehole fiber as a sensor fiber, and FIG.
The figure shows an example of pressure measurement, FIG. 7 shows an example using reflection, FIG. 8 shows an example of a pressure sensor using backward Rayleigh scattering, and FIG. 9 shows another example of a sidehole fiber. FIG. 1 ... Core, 2 ... Clad, 3a, 3b ... Hole, 4 ... Laser light source, 4 '... Pulse light source, 5 ... Polarizer, 6,13 ...
… 1/4 wavelength plate, 7a, 7b …… Lens, 8 …… Sensor fiber, 9 …… Pressure vessel, 10a, 10b …… Seal part, 11,19 ……
Half mirror, 12a, 12b ... Wollaston prism, 14
...... Mirror, 15a, 15b, 15c, 15d ...... Photodetector, 16 ...... Electronic circuit and display, 43 ...... Reference fiber, 53 ...... Transducer, 56 ...... Pressure plate.

Claims (1)

[Claims] 1. A sidehole fiber having a fixed length, which has one or a plurality of holes continuous in the longitudinal direction in a clad portion, is used as a sensor portion, and light of a specific polarization state is incident from one end of the fiber. A light source and an optical system, and an optical system that detects the light emitted from the other end of the side hole fiber or the light returning to the incident end, including its polarization state, after being incident from one end of the side hole fiber and propagating through the fiber, An optical fiber pressure sensor having a detector and an arithmetic unit.