JP4742336B2 - Optical fiber strain sensor and measuring instrument using it - Google Patents

Description

  The present invention relates to an optical fiber strain sensor and a measuring instrument using the same, and more particularly to a strain sensor using an optical fiber grating and a measuring instrument that measures water pressure, water level, water depth, flow velocity, flow direction, and the like using the same.

  As shown in FIG. 7 (Non-Patent Document 1), the optical fiber grating is formed by changing the refractive index in the core of the optical fiber in the axial direction at a constant period to form a Bragg diffraction grating. Among them, light with a wavelength corresponding to the period is Bragg-reflected with high efficiency and returned to the incident side as reflected light, and the components not Bragg-reflected are transmitted as transmitted light. Hereinafter, the optical fiber grating is referred to as “FBG”.

  FBG is attracting attention as a sensor for disaster prevention monitoring such as prevention of landslides and building collapse as a strain sensor that is highly sensitive, highly reliable, and operates stably for a long period of time, but measures changes in the wavelength of light. Therefore, since an expensive device such as a wavelength meter or a spectrum analyzer is required, the system price is expensive, which is an obstacle to practical use.

However, the strain sensor is an element necessary for measuring landslides, tunnel wall collapses, bridge vibrations, building vibrations, etc., preventing disasters in modern societies that require advanced safety and eliminating social unrest. Therefore, it is essential that such an element is inexpensive and can be easily measured.
Junpei Uchiuchi et al. “Latest Optical Technology Handbook” (issued September 20, 2002, Asakura Shoten Co., Ltd.) p. 274

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a high-sensitivity, high-reliability optical fiber strain sensor that operates stably over a long period of time without using a wavelength meter other than an optical fiber grating, and the like. It is to provide a measuring instrument using the.

  The optical fiber strain sensor of the present invention is a strain sensor using an optical fiber grating, in which separate optical fiber gratings are fixed on the front surface and the back surface of the elastic plate, respectively, and is incident on one optical fiber grating from the light source and reflected. The distortion of the elastic plate is detected by making the light component incident on the other optical fiber grating and detecting the intensity of the light transmitted through the other optical fiber grating or the intensity of the reflected light.

  In this case, it is desirable that the center reflection wavelengths of one optical fiber grating and the other optical fiber grating are slightly shifted from each other and the reflection spectra partially overlap each other.

  In addition, by configuring the measurement plate so that a pressure to be measured is applied to one surface of the elastic plate of the optical fiber strain sensor, the measurement device using the optical fiber strain sensor of the present invention is configured to detect the pressure. Can do.

  In that case, you may make it arrange | position an elastic member in the other surface facing one surface where the to-be-measured pressure of the elastic board of an optical fiber strain sensor applies.

  Such a measuring instrument using the optical fiber strain sensor of the present invention can be used for any of a water pressure gauge, a water level gauge, a water depth gauge, a velocimeter, and a flow direction meter, for example.

  In the optical fiber strain sensor of the present invention, separate optical fiber gratings are fixed to the front and back surfaces of the elastic plate, respectively, and the light component incident on one optical fiber grating from the light source is incident on the other optical fiber grating. The strain of the elastic plate is detected by detecting the intensity of the light transmitted through or reflected by the other optical fiber grating. Therefore, high sensitivity and high reliability can be obtained without using a wavelength meter other than the optical fiber grating. Optical fiber strain sensors that operate stably for long periods of time can be provided at a very low cost, and measurement devices that measure water pressure, water level, water depth, flow velocity, flow direction, etc. can be constructed at low cost using such fiber optic strain sensors. Was not popular because it was expensive so far It can be spread strain sensors or the like using the fiber grating.

  Further, in the optical fiber strain sensor of the present invention, the wavelength information representing the strain is converted into a light intensity signal and becomes a differential type, so that the two optical fiber gratings are in exactly the same environment. There is an advantage that the influence of the is automatically compensated.

  Hereinafter, the principle and examples of the optical fiber strain sensor of the present invention and the measuring instrument using the same will be described.

  FIG. 1 is a diagram for explaining the basic structure of an optical fiber strain sensor according to the present invention. The first FBG 2 is on the front surface of the diaphragm 1 made of an elastic material such as metal or plastic, and the second FBG 3 is substantially parallel to the back surface. It is fixed and configured.

  Because of such a configuration, when the diaphragm 1 turns to the right in the drawing, the first FBG 2 extends and the second FBG 3 contracts. Conversely, when turning to the left in the figure, the reverse is true.

  Now, light is incident on the first FBG 2 from the light source 5 via the 3 dB coupler 4, and reflected light from the first FBG 2 is extracted in a direction different from the direction of the light source 5 via the 3 dB coupler 4 and is incident on the second FBG 3. Part of the reflected light from the first FBG 2 passes through the second FBG 3. This transmitted light is extracted as a signal output 7 by the photodetector 6.

  In such a configuration, the wavelength component detected by the photodetector 6 is a portion where the reflected wavelength component of the first FBG 2 and the transmitted light wavelength component of the second FBG 3 coincide.

  FIG. 2 shows an example of the FBG reflection spectrum. As shown in FIG. 2, the reflection wavelength from the FBG has a reflection characteristic that abruptly attenuates on both sides in a narrow wavelength range centering on the peak wavelength.

  In the configuration of FIG. 1, the center reflection wavelengths of the first FBG 2 and the second FBG 3 may be matched in a state where there is no expansion / contraction, but as shown in FIG. 3, for example, the first FBG 2 is relatively slightly moved to the longer wavelength side. When the deviation is set and the reflection spectra are partially overlapped, the signal output 7 is as shown in FIG. Here, the direction in which the distortion rate is positive corresponds to when the diaphragm 1 turns to the right.

  As described above, the optical fiber strain sensor according to the present invention fixes two FBGs 2 and 3 on the front and back of one elastic plate 1 such as metal or plastic, and after the light from the light source 5 is incident on one FBG 2, Reflected light from one FBG 2 is taken out via the coupler 4 and further this reflected light is incident on the other FBG 3. At this time, when there is a slight difference between the central reflection wavelengths of the two FBGs 2 and 3 and there is an overlap in the reflection spectrum, the reflected light from the other FBG 3 becomes a component of the overlapping portion of this spectrum. When the elastic plate 1 is distorted and one of the FBGs 2 and 3 expands and the other contracts, if the overlapping portion of the reflection spectrum moves in a large direction, the light intensity reflected by one FBG 2 and transmitted through the other FBG 3 decreases. When the overlapping portion moves in a small direction, the intensity of light reflected by one FBG 2 and transmitted through the other FBG 3 increases.

  Therefore, by observing the light intensity reflected by one FBG 2 and passing through the other FBG 3, the strain of the elastic plate 1 to which the FBGs 2 and 3 are fixed can be detected.

  The intensity of the light reflected by one FBG 2 and reflected by the other FBG 3 may be used, but the same light as the 3 dB coupler 4 is used to extract the light reflected by the other FBG 3. One additional coverr is required. FIG. 5 shows the output when the reflected light from the FBG 2 is the signal output 7. Although the polarity is different from the case of using the transmitted light in FIG. 4, it can be seen that in either case, an output proportional to the distortion rate can be obtained.

  In the optical fiber strain sensor of the present invention, when the temperature changes, both FBGs 2 and 3 simultaneously change by the same amount in the same direction, so the fluctuations cancel each other and do not appear in the differential output.

  Here, the 3 dB coupler 4 transmits the light from the light source 5 to the first FBG 2 side and guides the reflected light from the first FBG 2 in a direction different from the direction of the light source 5, and is an optical device such as a half mirror or a polarization beam splitter. Although an element can be used, a Y-shaped branching waveguide, a directional coupler, or the like can be used to configure the whole as an optical waveguide type. The optical path connecting the light source 5, the 3 dB coupler 4, the 3 dB coupler 4, the first FBG 2, the 3 dB coupler 4, the second FBG 3, the second FBG 3, and the photodetector 6 may be configured as an integrated type using an optical waveguide such as an optical fiber. desirable.

  The optical fiber strain sensor of the present invention as described above is used for various measuring devices that detect pressure depending on the degree of strain of the elastic plate 1, for example, various measuring devices such as a water pressure gauge, a water level gauge, a water depth gauge, a current meter, and a flow direction meter. be able to. An application example of a water pressure gauge is shown in FIG. FIG. 6A is a sectional view of the water pressure gauge, and FIG. 6B is a front view. A diaphragm 10 with two FBGs attached to the front and back is mounted in a metal tube 11 of a protective jacket, one surface of the diaphragm 10 receives water pressure 15 from the window 12, and the other surface is in contact with the airbag 13. Yes. Assuming that the air pressure of the air bag 13 is 1 atm, when the water pressure 15 is higher than this, the air bag 13 contracts and the diaphragm 10 is bent. This bending amount is detected as the strain rate of the diaphragm 10 as described above, and the pressure is calculated from the strain rate. Furthermore, if this water pressure gauge is converted into water depth, it becomes a water level gauge. Reference numeral 14 in FIG. 6 denotes an inlet of an optical fiber cable that sends light from the light source to one FBG fixed to the diaphragm 10 and extracts transmitted light or reflected light from the other FBG.

  In order to detect the pressure, a 1 atm airbag 13 is used as a pressure against the pressure to be measured on the back surface of the diaphragm 10, but an object that deforms the diaphragm 10 itself by pressure, such as a Breton tube, is used. May be.

  As mentioned above, although the optical fiber distortion sensor of this invention and the measuring instrument using the same have been demonstrated based on the principle and Example, this invention is not limited to these Examples, A various deformation | transformation is possible. In the optical fiber strain sensor of the present invention, in order to fix the FBG to the front and back surfaces of the diaphragm, a groove is cut in the surface of the diaphragm rather than using an adhesive in order to accurately transmit the expansion and contraction of the diaphragm to the FBG. It is desirable to embed the FBG in it.

It is a figure for demonstrating the basic structure of the optical fiber distortion sensor of this invention. It is a figure which shows one example of the reflection spectrum of FBG. It is a figure which shows the example of a reflection spectrum when the center reflection wavelength of 1st FBG and 2nd FBG is shifted. It is a figure which shows the relationship between the distortion rate of an example in the case of utilizing the transmitted light from 2nd FBG, and an output. It is a figure which shows the relationship between the distortion rate of an example in the case of utilizing the reflected light from 2nd FBG, and an output. It is sectional drawing and the front view which show the application example of a water pressure gauge. It is a figure for demonstrating the structure and effect | action of an optical fiber grating.

Explanation of symbols

1 ... diaphragm 2 ... first FBG
3 ... 2nd FBG
4 ... 3 dB coupler 5 ... light source 6 ... photo detector 7 ... signal output 10 ... diaphragm with two FBGs attached to the front and back (the present invention)
11 ... Protective jacket (metal tube)
12 ... Window 13 ... Airbag 14 ... Optical fiber cable inlet 15 ... Water pressure

Claims (2)

A strain sensor using an optical fiber grating, with separate optical fiber gratings fixed on the front and back surfaces of the elastic plate, and the light component incident on one optical fiber grating from the light source and reflected to the other optical fiber grating is incident, when detecting the distortion of the elastic plate by detecting the light intensity transmitted through the other side of the optical fiber grating, slightly mutually have name state of the central reflection wavelength of one optical fiber grating and other optical fiber grating stretch When the reflection spectra are partly overlapped with each other, the reflected light of one FBG is transmitted through the other FBG. Optical fiber characterized by detection Bar strain sensor. An optical fiber strain sensor using the two optical fiber gratings according to claim 1, wherein a physical quantity such as a temperature at a place where the two optical fiber gratings are fixed acts on the two optical fiber gratings in common. The differential optical fiber strain sensor is characterized in that since the reflection spectrum is shifted by the same amount at the same time, the shift of the spectrum does not increase or decrease, so that only the amount of distortion can be detected without appearing in the detected signal.

Images