JP2005114702A - Fbg sensing system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an FBG sensing system without use of two reflection wavelength bands of a FBG, capable of measuring the quantity of distortion using one reflection wavelength band and very small temperature characteristic, and excellent in wavelength utilizing efficiency. <P>SOLUTION: An FBG sensing apparatus is provided with the FBG (a fiber Bragg grating) for reflecting a light having the predetermined wavelength band and shifting the reflection wavelength band due to a temperature change and the distortion, a light source for outputting the light having the reflection wavelength band of the FBG, and an apparatus for detecting the shift of the wavelength band of the light output from the light source and reflected from the FBG. Two FBGs are provided in the same reflection wavelength band. One of FBGs is installed so as to simultaneously receive the temperature change and the distortion. The other is installed so as to receive only temperature change. A difference between the shifts of the wavelength bands of two FBGs is detected as the quantity of the distortion by allowing the temperature changes to be compensated. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an FBG sensing system that performs strain measurement using an FBG made of an optical fiber.

  In recent years, the structure that has been built rapidly during the high growth period has rapidly deteriorated, and research on the method for evaluating the soundness of the structure has been actively conducted mainly by the government. Conventionally, electrical methods using resistance strain gauges have been the mainstream method for measuring strain in structures, but there are problems such as reliability, lightning strikes, and electromagnetic noise. Measurement (sensing) is drawing attention.

  In the field of optical fiber sensing, FBG (fiber Bragg grating) is used in combination with a broadband light source or a wavelength tunable light source to measure the strain of a structure.

  FIG. 6 is a diagram showing an outline of the FBG.

  As shown in FIG. 6, the FBG 101 is obtained by changing the refractive index of the core 103 of the optical fiber 102 at a constant interval D. The light incident on the optical fiber 102 reflects light having a wavelength determined by the interval D and the refractive index of the core 103, and transmits other wavelengths. For this reason, the reflection wavelength has a feature that it is displaced by a change in the distance D due to strain applied to the FBG 101 or a change in the refractive index due to temperature. The temperature characteristics of the FBG 101 are about 0.01 nm / ° C., and the strain characteristics are about 0.0012 nm / με. The characteristics are used for temperature measurement and strain measurement.

  Consider the case where it is used for strain measurement. When performing strain measurement, it is necessary to consider not only the strain characteristic but also the temperature characteristic, and it is necessary to subtract the displacement of the reflected wavelength due to the strain change from the reflected wavelength displacement of the temperature change. Therefore, when performing strain measurement, it is necessary to use two FBGs having different reflection wavelengths.

  Non-Patent Document 1 describes an FBG sensing system that uses a broadband light source and performs FBGs having a plurality of different reflection wavelengths with a single optical fiber.

  FIG. 12 is a diagram showing the configuration of the FBG sensing system described in Non-Patent Document 1.

  The light derived from the broadband light source 201 passes through the optical splitter 202, passes through the optical fiber 205, and reaches the FBG 203. A plurality of different wavelengths of light are reflected by the FBG 203, pass through the optical fiber 205, pass through the optical splitter 202, reach the wavelength detector 204, and detect the reflected wavelength.

  By using the broadband light source 201 in this way, an FBG having a plurality of different reflection wavelengths can be provided in one optical fiber, and a plurality of strains and temperature amounts can be measured simultaneously.

  In general, an SLD (Super Luminescent Diode) light source or an ASE (Amplified Spontaneous Emission) light source is used as a broadband light source, and the wavelength band thereof is about 30 nm to 50 nm.

  Here, the number of FBGs for the band of the broadband light source 201 will be described.

  As described above, the use band of one FBG 203 has a temperature characteristic of about 0.01 nm / ° C. and a strain characteristic of about 0.0012 nm / με. For example, ± 1000 με in the temperature range from 0 ° C. to 50 ° C. When measuring the amount of strain, it is necessary that 0.01 nm × 50 ° C. + 0.0012 nm × 2000 με = 2.9 nm. Further, the reflection wavelength band of the FBG 203 is about 0.2 nm in half width, and considering this, the use band of one FBG 203 needs to be about 3.1 nm. In general, a band having a margin of 4 nm or more is used. As a result, the number of FBGs 203 in different reflection wavelength bands is about ten.

  When strain measurement is performed, two FBGs 203 for strain measurement and temperature compensation are required as described above, and the number of strain measurements is five. The number of points required for strain measurement is 10 or less, but there are also several tens or more, which is not a sufficient score.

Increasing the bandwidth of a broadband light source as a means for increasing the measurement location is easily considered, and light sources having a bandwidth of about 100 nm are already on the market, but are not actively used because the types of FBG 203 increase. Currently, it is generally handled by increasing the number of optical fibers.
Shinji Yamashita et al., Latest Materials on Optical Communication Technology of Optronics, Application to Optical Measurement / Sensor, December 1995

  In the above prior art, when strain measurement is performed, it is necessary to use the strain measurement FBG and the temperature compensation FBG as a set, and there is a disadvantage that the use efficiency of the wavelength band is poor.

  Here, it is an object to provide an FBG sensing system with high wavelength utilization efficiency by using a strain measurement FBG and a temperature compensation FBG in the same wavelength band.

  The present invention is for solving these problems. An FBG (fiber Bragg grating) having a characteristic that a reflected wavelength band is changed by reflecting light in a specific wavelength band and receiving a temperature change or distortion. An FBG sensing device comprising: a light source that outputs light in the reflected wavelength band of the FBG; and a device that detects a change in the wavelength band of reflected light from the FBG that is output from the light source. Two FBGs are provided, one is installed so as to receive temperature change and strain at the same time, and the other is installed so as to receive only temperature change so that the temperature change can be compensated. The difference between the band shift amounts is detected as a distortion amount.

  Further, the reflection wavelength band of the two FBGs has a shared band.

  Further, the difference between the reflection wavelength band shift amounts of the two FBGs is detected as a shift in the bandwidth of the combined light.

  Further, the two FBGs are apodized.

  As described above, according to the present invention, there is provided an FBG (fiber Bragg grating) that reflects light in a specific wavelength band and has a characteristic that the reflected wavelength band changes due to temperature change or distortion, and the reflected wavelength of the FBG. A FBG sensing device comprising a light source that outputs light in a band and a device that detects a shift in the wavelength band of light reflected from the FBG that is output from the light source, and includes two FBGs in the same reflection wavelength band; One is installed to receive temperature change and strain at the same time, and the other is installed to receive only temperature change so that the temperature change can be compensated for, thereby distorting the difference between the two FBG wavelength band shifts. It is possible to provide an FBG sensing system characterized in that it is detected as a quantity.

  As an example, an FBG sensing system according to the present invention will be described below.

  FIG. 1 is a diagram showing the configuration of the FBG sensing system of the present invention.

  A broadband light source 1, a wavelength detector 4, an optical splitter 2, FBG 3 a, FBG 3 b, and an optical fiber 5 are configured, and each component is coupled by an optical fiber. It is desirable that the FBG 3a and the FBG 3b have reflection bands having substantially the same wavelength band, and share more reflection bands. The optical branching device 2 may be a two-branch coupler or an optical circulator.

The FBG 3b is distorted by pulling both ends, and is fixed to a solid having a linear thermal expansion coefficient equivalent to that of the FBG 3b. The linear thermal expansion of the FBG 3b is as small as 10 ⁻⁶ or less, and an invar having a low linear thermal expansion is preferable as the solid to be fixed. Here, the distortion amount is not directly measured, and the distortion amount is converted from the wavelength shift amount. In general, the strain amount of the FBG 3b changes linearly with respect to the shift amount of the reflection wavelength of the FBG 3b, and it is known that a wavelength displacement of 0.0012 nm occurs with respect to the strain of 1 με, and this relationship is used. On the other hand, the FBG 3a is not fixed at both ends.

  The light output from the broadband light source 1 passes through the optical splitter 2, passes through the optical fiber 5, and reaches the FBG 3a. The light having a specific wavelength is reflected by the FBG 3a. Wavelengths other than the specific wavelength of the FBG 3a are transmitted, reach the FBG 3b, and light having different specific wavelengths is reflected by the FBG 3b. The light reflected by the FBG 3b passes through the FBG 3a, is combined with the light reflected by the FBG 3a, passes through the optical fiber 5, passes through the optical branching device 2, reaches the wavelength detector 4, and is multiplexed by the FBG 3a and FBG 3b. Detected light.

  FIG. 2 is a diagram showing the reflection spectrum of each of the FBGs 3a and 3b at 25 ° C.

  It can be seen that both FBGs 3a and 3b have a central wavelength in the vicinity of 1553.9 nm, have a reflection band with a half width of about 0.2 nm, and share almost all bands. Clearly, the more shared areas, the higher the wavelength utilization efficiency.

  FIG. 3 is a diagram showing the relationship between the combined reflection spectrum of the FBGs 3a and 3b and the amount of distortion over the FBG 3b in the configuration of the present invention. It can be seen from FIG. 3 that it is difficult to detect the peak wavelengths of the combined reflection spectra of the FBGs 3a and 3b. However, since the FBG 3a is fixed and only the reflected light of the FBG 3b with distortion is wavelength-shifted, it is determined that the spread of the reflected wavelength band and the amount of distortion are related.

  FIG. 4 is a diagram showing the relationship between the amount of distortion and the reflection wavelength band (half-value width). It can be seen that the actually measured value indicated by the solid line almost coincides with the approximate straight line indicated by the dotted line. Thus, it can be seen that the half-value width of the combined reflection spectrum of the FBGs 3a and 3b is substantially proportional to the amount of distortion.

  FIG. 5 is a diagram showing the relationship between the temperature when the strain amount applied to the FBG 3b is constant and the half width of the reflection spectrum where the FBGs 3a and 3b overlap.

  As shown in FIG. 5, the full width at half maximum is confirmed to vary by 0.05 nm from 0 ° C. to 50 ° C. This is a fluctuation amount of about 5 ° C. of the temperature characteristic of the FBG alone (about 0.01 nm / ° C.). Which corresponds to about 1/10. From FIG. 4, the fluctuation amount of 0.05 nm is about ± 20 με in terms of strain, which is a value of an error in strain measurement of several hundred με, and it is confirmed that temperature compensation can be performed in this embodiment.

  Next, a second embodiment of the present invention will be described.

  In the present invention, in the FBG sensing system shown in FIG. 1, FBG3c is used instead of FBG3a, and FBG3d is used instead of FBG3b. FIG. 8 shows the reflection spectrum of the FBG 3c. It can be seen that the reflection spectrum of the FBG 3c has a very small ripple compared to the reflection spectra of the FBGs 3a and 3b shown in FIG.

  FIG. 7 is an image of the refractive index distribution of FBG. FIG. 7A shows the refractive indexes of the FBGs 3a and 3b, and the refractive indexes are made constant. In this case, resonance occurs between the gratings, and ripples occur at both ends of the peak wavelength. On the other hand, FBGs 3c and 3d have no ripple due to apodization. Apodization means that the refractive index is modulated so that the refractive index inclusion curve shows a Gaussian distribution as shown in FIG.

  In general, FBGs are produced by irradiating with ultraviolet rays and having a distribution in refractive index. Since apodization can be handled by changing the UV irradiation amount, it can be produced relatively easily.

  Similar to the first embodiment, the FBG 3d pulls at both ends, thereby generating distortion and fixing it to a solid having a linear thermal expansion coefficient equivalent to that of the FBG 3d. Here, the distortion amount is not directly measured, and the distortion amount is converted from the wavelength shift amount. In general, the strain amount of the FBG 3d changes linearly with respect to the shift amount of the reflection wavelength of the FBG 3d, and it is known that a wavelength displacement of 0.0012 nm occurs with respect to the strain of 1 με, and this relationship is used. On the other hand, the FBG 3c is not fixed at both ends.

  The light output from the broadband light source 1 passes through the optical splitter 2, passes through the optical fiber 5, and reaches the FBG 3a. The light having a specific wavelength is reflected by the FBG 3c. Wavelengths other than the specific wavelength of the FBG 3c are transmitted, reach the FBG 3d, and light having different specific wavelengths is reflected by the FBG 3d. The light reflected by the FBG 3b passes through the FBG 3c, is combined with the light reflected by the FBG 3c, passes through the optical fiber 5, passes through the optical branching unit 2, reaches the wavelength detector 4, and is combined with the FBG 3c and FBG 3d. Detected light.

  FIG. 9 is a diagram showing the relationship between the combined reflection spectrum of the FBGs 3c and 3d and the amount of distortion over the FBG 3d in the configuration of the present invention. Similar to the first embodiment, it is determined that the spread of the reflection wavelength band and the amount of distortion are related.

  FIG. 10 is a diagram showing the relationship between the amount of distortion and the reflection wavelength band (half-value width). It can be seen that the actually measured value indicated by the solid line almost coincides with the approximate straight line indicated by the dotted line. Thus, it can be seen that the half-value width of the combined reflection spectrum of the FBGs 3c and 3d is substantially proportional to the amount of distortion.

  FIG. 11 is a diagram showing the relationship between the temperature when the strain amount applied to the FBG 3b is constant and the half width of the reflection spectrum where the FBGs 3c and 3d overlap.

  As shown in FIG. 11, the full width at half maximum is a fluctuation of 0.01 nm or less from 0 ° C. to 50 ° C. FIG. 8 confirms that the fluctuation amount of 0.01 nm is 10 με or less in terms of the strain amount, and that temperature compensation can be performed with extremely high accuracy.

  In this way, one FBG 3a and FBG 3b having substantially the same reflection wavelength band is used for strain measurement, the other is used for temperature compensation, and the bandwidth of these combined reflection bands is measured, so that one FBG is measured. It is possible to measure the amount of strain in the reflection wavelength band with a very small temperature characteristic, and the wavelength utilization efficiency can be doubled compared to the conventional measurement method using two reflection wavelength bands. It becomes.

  Further, by using the apodized FBGs 3c and 3d instead of the FBGs 3a and 3b, the temperature characteristics are further reduced, and a more accurate distortion amount can be measured in the reflection wavelength band of one FBG.

  The FBG sensing system shown in the first embodiment of the present invention was created.

  As the broadband light source 1, an ASE light source that outputs light of −10 dBm / nm or more in a band from 1530 nm to 1560 nm was used. As the wavelength detector 4, an optical spectrum analyzer generally used in optical communication was used. As the optical branching device 2, a circulator having a transmission loss of about 0.5 dB and a wavelength characteristic of the transmission loss of less than 0.1 dB was used. As FBGs 3a and 3b, those having a reflectance of 95% or more, a full width at half maximum of the reflection spectrum of 0.25 nm or less, and a reflection peak wavelength at 25 ° C. of 1553.9 nm were used. The transmission loss at wavelengths other than the reflection wavelength is about 0.1 dB.

  A strain of 200 με was applied to the FBG 3b, and both the FBGs 3a and 3b were placed in a thermostatic bath, and the half-value width of the combined reflected light of the FBGs 3a and 3b from 0 ° C. to 50 ° C. was confirmed. It was confirmed that the temperature characteristics were almost the same as in FIG. The error of the strain amount is about ± 10% with 25 ° C. as a reference. Similarly, different strain amounts were given to the FBG 3b, and it was confirmed that the same results were obtained.

  Next, the FBG sensing system shown in the second embodiment of the present invention was created.

  FBG3c and 3d are apodized and have a small ripple. The reflectance is 95% or more, the full width at half maximum of the reflection spectrum is 0.2 nm or less, and the reflection peak wavelength at 25 ° C. is 1548.0 nm. The transmission loss at wavelengths other than the reflection wavelength is about 0.1 dB. Other components were the same as those in the first embodiment, and the same measurement was performed.

  It was confirmed that the temperature characteristics were almost the same as in FIG. The temperature error of the distortion amount is extremely high when it is 10 με or less.

It is a figure which shows the structure of the FBG sensing system of this invention. It is a figure which shows the reflection spectrum in 25 degreeC of FBG. It is a figure which shows the relationship between the reflection spectrum combined with FBG, and the amount of distortion over FBG. It is a figure which shows the relationship between distortion amount and a reflective wavelength band (half-value width). It is a figure which shows the relationship between the temperature at the time of making distortion amount constant, and the half value width of the reflection spectrum which FBG overlaps. It is a figure which shows the outline of FBG. It is the figure which imaged the refractive index distribution of FBG. It is a figure which shows the reflection spectrum of FBG3. It is a figure which shows the relationship between the reflected spectrum combined of FBG3c, 3d, and the amount of distortion over FBG. It is a figure which shows the relationship between a distortion amount and a reflective wavelength band (half-value width). It is a figure which shows the relationship between the temperature at the time of making distortion amount constant, and the half value width of the reflection spectrum which FBG overlaps. It is a figure which shows the structure of the FBG sensing system described in the nonpatent literature 1. FIG.

Explanation of symbols

1: Broadband light source 2: Optical splitter 3a, 3b: FBG
3c, 3d: Apodized FBG
4: Wavelength detector 5: Optical fiber 101: FBG
102: Optical fiber 103: Core 201: Broadband light source 202: Optical splitter 203: FBG
204: Wavelength detector 205: Optical fiber

Claims (4)

A light source for reflecting light in a specific wavelength band, and having an FBG (fiber Bragg grating) having a characteristic that the reflected wavelength band is changed by being subjected to temperature change or strain, and outputting light in the reflected wavelength band of the FBG; In the FBG sensing device provided with a device that detects a change in the wavelength band of the reflected light from the FBG output from the light source, the FBG sensing device includes two FBGs in the same reflection wavelength band, one of which is subject to temperature change and strain simultaneously. The difference between the reflection wavelength band shift amounts of the two FBGs is detected as a distortion amount by installing the other so that only the temperature change is received and allowing the temperature change to be compensated. FBG sensing system. The FBG sensing system according to claim 1, wherein a reflection wavelength band of the two FBGs has a shared band. The FBG sensing system according to claim 1, wherein a difference between reflection wavelength band shift amounts of the two FBGs is detected as a shift of the bandwidth of the combined light. The FBG sensing system according to claim 1, wherein the two FBGs are apodized.

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