JP2004205368A - Multipoint strain measuring system using ofdr method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To upgrade a real-time property and a speed of response of a measurement by improving a data analyzing method in a multipoint strain measuring system using an OFDR (optical frequency domain reflectometry) method in which a plurality of FBGs are arranged in an optical fiber. <P>SOLUTION: In the multipoint strain measuring system, the plurality of fiber Bragg gratings (FBGs) which make up a periodic structure of refractive index inside the core of the optical fiber, are arranged in one optical fiber, and the measuring method (OFDR) which uses periodic changes in interferential intensity of light, is conducted. The system also has such a function that signals from the FBG on a specific position are quickly separated by using a band-pass filter with a preset bandpass range which filters out detected light signals. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an OFDR multi-point strain measuring apparatus in which a plurality of fiber Bragg gratings (FBGs) are arranged in one optical fiber.
[0002]
[Prior art]
Optical fiber embedded sensors are expected to be applied to smart structures and materials due to their light weight, strength, small size, and flexibility. And excitation measurement of flexible structures such as large artificial satellites. As the first soundness evaluation, it is possible to evaluate the soundness of the airframe structure by measuring the distortion of the operating aerospace equipment. At that time, the necessary conditions for the sensor include high spatial resolution and real-time measurement. It is also conceivable to evaluate the soundness by monitoring the secular change in residual strain between operations. The condition required at this time is whether or not distortion can be measured.
Accelerometers are mainly used to measure the vibration of artificial satellites, but the number of channels is limited due to the limitations of the entire satellite system. However, if a distributed sensor having a simple configuration such as an optical fiber sensor becomes possible, the contribution to understanding the vibration characteristics of the artificial satellite will be significant. In that case, of course, responsiveness corresponding to the measurement frequency is required.
[0003]
Various sensor systems using an optical fiber have been proposed so far. Fiber Bragg Grating (FBG), which has a periodic refractive index structure in the core of an optical fiber, is placed on a single optical fiber, and the strain and temperature at the position where the FBG exists are measured. There is a system to do. In this method, incident light having a broadband wavelength is used, the amount of change in the wavelength of reflected light is observed, distortion is measured, and temperature is measured. The FBG has a size of about 10 mm and can perform local measurement like a general strain gauge. Further, unlike a strain gauge, it can also measure an absolute strain. Further, currently commercialized systems have a response speed of about several tens of Hz. Incidentally, there is Patent Document 1 “Multipoint strain and temperature sensor” as this kind. This is intended to provide a multi-point strain and temperature sensor that is suitable for measurement at multiple points and that can accurately measure distortion along with temperature. As shown in FIG. A plurality of FBGas whose wavelengths change are inserted in series into the optical fiber b, and the amount of reflected light of a specific wavelength 1 from each FBGa from one end of the optical fiber b is measured by OTDR (Optica 1 Time Domain Ref1ectometory), and the optical fiber b The temperature distribution along the longitudinal direction of the optical fiber b is obtained by measuring the backscattered light from the BOTDR (Brillouin Optica1 Time Domain Ref1ectometory), and the amount of reflected light change due to the temperature dependence of each FBGa is corrected based on this temperature distribution. Then, an accurate distortion amount in each FBGa is obtained from the reflected light amount corrected for temperature. Using a pulsed input light, the position is specified by the arrival time of the reflected light. However, since this measurement method detects the shift amount of the wavelength of the reflected light, the number of measurement points (the number of FBG sensors) per optical fiber is limited by the measurement range and the width of the light source band. Several multiplexing methods have been proposed, but all have problems in stability, cost, and the like due to the complexity of the system.
[0004]
Based on these facts, the present inventors have paid attention to a system using Optical Frequency Domain Reflectometry (OFDR). This system can be classified as one of the multiplexing technologies of the FBG type sensor, and a fiber Bragg grating (FBG) with a periodic refractive index structure in the core of the optical fiber is integrated into one optical fiber. A plurality of FBG sensors are used to measure and specify the position of the FBG sensor by using a wavelength-variable narrow-band light source and observing the change in light intensity caused by the interference between the reflected light from the measuring unit and the reflected light from the reference reflecting surface. is there. Until now, there have been reports by Childers et al. That 800 FBGs are arranged in an 8 m optical fiber and simultaneous strain measurement of a total of 3000 points or more is performed using four optical fibers (Non-Patent Documents 1 and 2). However, measurement and offline analysis of data take several minutes.
That is, in the conventional analysis method, the reflection characteristic is obtained for each FBG using the fast Fourier transform and its inverse transform. In the prior art, only static measurement was performed. However, if dynamic measurement of a structure or the like is considered in the future, it is considered that it is difficult to speed up the measurement by the conventional analysis method. Further, the conventional analysis method does not sufficiently describe a method for obtaining the reflection characteristics of each FBG. Further, in the conventional analysis method, the amount of distortion in the FBG sensor unit is obtained based on the wavelength output value of the light source. In this case, there is a problem that the measurement accuracy is largely affected by the wavelength output accuracy of the light source.
[0005]
[Patent Document 1] JP-A-10-141922 [Non-Patent Document 1] Brook A. Childers aand etc., "Use of 3000 Bragg Grating Strain Sensor Distributed on Four Eight-Meter Optical Fibers During Static Load Tests of a Composite Structure , ”SPIE's 8th International Symposium on Smart Structure and Materials, Newport Beach, Califorornia, March4-8, 2001.
[Non-Patent Document 2] Mark Frogatt and Jason Moore “Distributed measurement of static strain in an optical fober with multiple Bragg gratingsat nominally equal wavelengths” APPLIDE OPTICS Vol.37 No.10, 1 April 1998.
[0006]
[Problems to be solved by the invention]
In view of the above problems, an object of the present invention is to arrange a plurality of FBGs in an optical fiber, improve a data analysis method in a multi-point strain measurement system using OFDR, and improve real-time measurement and response speed. The purpose is to aim for improvement.
[0007]
[Means for Solving the Problems]
The proposed OFDR optical fiber multi-point strain measuring device is composed of a plurality of fiber Bragg gratings (FBGs) having a periodic refractive index structure in the core of the optical fiber, which are arranged in a single optical fiber to provide optical interference. This system applies a system that specifies the position of the FBG sensor using the periodic change in intensity, and measures the distortion and temperature of the position where the FBG sensor is located based on the change amount of the center light wave number of the reflected light of each FBG. As a technical idea of (1), in the conventional analysis method, the fast Fourier transform and the inverse transform were used for each individual FBG, but in the present method, the processing speed is reduced by using a digital bandpass filter. Can be speeded up.
(2) The obtained data after the band-pass filter is further subjected to a low-pass filter on the absolute value to obtain the envelope of the light intensity and analyze the amount of distortion, thereby stabilizing the measurement. It is possible to secure the property.
{Circle around (3)} As an application of the FBG array, by using one of the FBGs as a reference for light source wavelength correction and temperature correction, it is possible to dramatically improve strain measurement accuracy.
{Circle around (4)} A reference interferometer having a total reflection terminal having the same optical path difference as the interval at which the FBGs are arranged on the fiber is provided, and the reflection from each FBG is triggered by the light intensity measured by the detector of the reference interferometer. The light intensity is measured with a detector.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
The principle of the strain measurement method using OFDR of the present invention will be described. The problem in the optical fiber multi-point strain measurement is how to specify the position and strain of the measurement point. Here, an example of an OFDR optical fiber sensor system having one simple FBG will be described as an example, and the influence of the sensor position and distortion on its reflection characteristics will be described.
First, regarding the reflection characteristics of the optical fiber sensor system, the wavelength tunable laser light source (VS), light intensity detector (D), total reflection end (R), and FBG sensor (FBG) are shown in FIG. Place and connect with optical fiber. A laser beam having a certain wavelength incident from a wavelength variable light source is branched by a coupler (C), reflected by a reflection end and an FBG sensor section, then combined again by a coupler, and the intensity is detected by a detector. . Since the reflected light from the FBG strongly reflects only light of a certain wavelength, the relationship between the light wave number k on the horizontal axis and the reflected light intensity is as shown in the lower right of FIG. Further, the value of the light wave number k indicating the peak changes depending on the magnitude of the distortion in the FBG section based on the characteristics of the FBG. Here, the light wave number k and the value of the wavelength λ have the following relationship.
k = 2π / λ (1)
On the other hand, since the reflected light from the FBG and the reflected light from the total reflection end R have an optical path difference of 2 L, if the refractive index of the optical fiber is n, the light has a phase difference of 2 nL, and the combined optical signal is As shown below, it varies sinusoidally depending on the light wave number k. Assuming that the light intensity of the light reflected from the total reflection terminal R and the light reflected from the FBG are equal, Amp_ITF = ACOS (2nLk) as shown in the following equation.
That is, this relationship is as shown in the lower center of FIG.
Due to the two actions described above, the light intensity detected by the detector changes with a certain period and a peak with respect to the light wave number k, as shown in the lower left part of FIG. From this cycle, the path difference 2L, that is, the position of the FBG sensor, and the value of the distortion can be estimated from the value of the light wave number k indicating the peak.
Even when there are a plurality of FBGs, the optical path differences are different corresponding to the respective FBGs. Therefore, the positions of the respective FBGs are determined by paying attention to the frequency depending on the optical path differences, and the lightwaves showing the respective peaks are determined. By determining the number, the value of the no at each FBG can be determined.
[0009]
Next, a data analysis method will be described. The general flow of data analysis is shown in FIG. In the figure, the flow of analysis (Non-Patent Documents 1 and 2) performed by Childrens et al. Is shown on the left side, and the flow of analysis of the present invention is shown on the right side. Childers et al. First analyzed the light intensity observed by the detector as a function of the light wave number k, after taking it into the PC. As described above, when the observed light intensity is expressed as a function of the light wave number, the path difference 2L acts like a frequency. Therefore, when a Fast Fourier Transform (FFT) is performed on the function of the light wave number, when a plurality of FBGs exist on the optical fiber, the intensity of the reflected light from each FBG becomes as shown in the upper graph in the figure. It is represented in a separated form corresponding to each position. Focusing on one FBG, extracting only the component at that position through a filter and performing inverse fast Fourier transform, only the intensity of the reflected light from the focused FBG becomes the light wave number as shown in the lower graph in the figure. Obtained as a function. Here, since the light wave number indicating the peak changes in proportion to the value of the distortion in the FBG, the amount of distortion in the focused FBG can be detected by observing the amount of change. Childers et al. Perform the above data processing offline.
[0010]
By the way, even if a plurality of FBGs are arranged on the optical fiber, it is generally possible to grasp and fix the position of each sensor on the optical fiber in advance. That is, if the distance L between the FBGs is known, the phase difference 2nL based on the optical path difference 2L acts like a frequency as described above, so that a bandpass filter is designed in advance for each FBG, If the filtering is performed on the light intensity measured by the detector, the reflected light from the focused FBG can be selectively observed. This is the basic technical idea of the present invention. That is, the signal from the graph directly below is extracted without the process from the FFT processing to the inverse FFT processing of the conventional method. This signal extraction method can significantly reduce the processing time as compared with conventional signal processing that performs fast Fourier transform. After that, the amount of transition of the peak value of the light wave number is calculated by the same procedure as above, and the distortion value in the focused FBG is calculated. Although this signal processing method requires time and effort for designing a bandpass filter, once the system is finished, the amount of calculation at the time of measurement is greatly reduced, so that it is considered that online real-time measurement is possible.
[0011]
FIG. 3 shows a measurement result obtained by measuring the intensity of light obtained by combining the reflected light from a plurality of FBGs and the reflected light from the reference reflecting surface with a detector, and plots the light intensity on the vertical axis and the light wave number on the horizontal axis. It is displayed. The figure on the right shows the enlarged horizontal axis of the peak portion. It can be seen that the light intensity changes periodically with respect to the light wave number due to the interference due to the path difference.
Focusing on one FBG, a bandpass filter that passes only a frequency determined according to the path difference from the reference reflection surface is designed, and this filter processing is performed on the measurement result shown in FIG. Thereby, only the component of the reflected light from the focused FBG can be extracted. FIG. 4 shows the result, in which a wave having a single period is amplitude-modulated. As shown on the left side of FIG. 4, the FBG has a characteristic of strongly reflecting only light of a certain light wave number. The light wave number at which the intensity of the reflected light is maximized is referred to as a center light wave number, and the value of the center light wave number changes depending on distortion or temperature acting on the FBG. Although the light wave number showing the maximum value in FIG. 4 can be used as the center light wave number as it is, as can be seen from the enlarged view on the right side, the amplitude is slightly changed near the maximum value, so that the center light wave number is stable. It is difficult to get numbers. Therefore, the present invention proposes a method for extracting a stable center light wave number.
[0012]
In the present invention, an envelope signal having the waveform shown in FIG. 4 is created, and the central light wave number is obtained from the signal. That is, by taking the absolute value of the waveform shown in FIG. 4 and subjecting it to an appropriate digital low-pass filter processing, the reflection characteristic from the focused FBG can be obtained. The obtained signal is a waveform shown in FIG. The number of light waves is specified from the peak value of this waveform. However, since the vicinity of this peak value fluctuates finely, in the present invention, upper and lower light waves indicating a value of a predetermined ratio of the maximum amplitude value in the peak waveform of the envelope signal. A method of calculating the center value of the numerical value as the number of detected light waves was adopted. The inventor has set the predetermined ratio of the maximum amplitude value to の of the peak value, but is not necessarily specified to １ ／. By adopting the median value of the two light wave numbers having the same amplitude value as the center light wave number, it is possible to remove small fluctuations near the maximum value of the reflection characteristic and obtain a very stable value as the center light wave number. Becomes possible.
This processing is performed on a plurality of FBGs, and the central light wave number from each FBG is extracted. FIG. 6 summarizes the results obtained when five FBGs were used. The center light wave number of the reflected light of each FBG changes depending on the strain acting on each FBG and the temperature. Here, based on the center light wave number of one FBG (FBG 5 in FIG. 6) to which no distortion acts, the change of the center light wave number of the other FBG is obtained. As a result, instability of the light source can be removed, and if all the FBGs have the same temperature, temperature correction becomes possible, and accurate distortion of each FBG sensor can be obtained.
[0013]
Embodiment 1
Next, an outline of an embodiment of the optical fiber sensor system according to the present invention will be described. FIG. 7 is a diagram showing the basic configuration. A portion surrounded by a wavy line in the figure is housed in a housing (430 mm × 330 mm × 105 mm), and weighs 6.2 kg. The main components are as follows.
D1, D2, D3, D4 Light intensity detector C1 5% -95% Broadband coupler C2-C5 50% -50% Broadband coupler R1-R4 Termination FC / PC1-3 where total reflection occurs FC / PC for optical fiber coupling FIG. 8 is a schematic diagram of an optical fiber in which five FBGs, which are optical fiber measuring units in which five BNC connectors FBG1-2 FBGs are arranged, are shown. Although the FBG portion is shown thick in the schematic diagram, it is actually manufactured by periodically changing the refractive index of the fiber core portion using laser light, and there is no change in thickness as compared with the general portion of the optical fiber. . FIG. 9 shows the results of measuring the light transmission characteristics of the FBG manufactured this time. In this FBG, the transmission characteristic of light having a wavelength of 1550.2 mm is 0.18 dB. This means that light of that wavelength is reflected by about 4% at the FBG section. All other FBGs have approximately the same reflectivity characteristics.
For the measurement, a laser beam is supplied from a wavelength variable type light source, and the intensity of reflected light is measured by a detector while sweeping the wavelength. As described above, since the data processing uses the fast Fourier transform and the digital filter, it is necessary to perform measurement at every light wave number at a constant interval. Therefore, in this embodiment, a reference interferometer corresponding to the upper part of FIG. 7 is separately prepared. The light intensity measured by the detector D1 periodically changes at a light wave number interval Δk represented by the following equation.
Δk = π / nL (3)
Using the light intensity at the detector D1, which periodically changes with the light wave number, as a trigger, the intensity of the reflected light from each FBG, which is the sensor unit, is measured by the detectors D2, D3. A general A / D converter was used for data acquisition, and analysis was performed on a PC.
[0014]
【The invention's effect】
The multi-point strain measuring apparatus according to the present invention is arranged such that a plurality of FBGs having a periodic refractive index structure in the core of an optical fiber are arranged in one optical fiber, and OFDR utilizing a periodic change in the interference intensity of light is used. In the above, the detection light signal is passed through a band-pass filter in which a band is set in advance to separate and extract a signal from the FBG at a specific position. Therefore, the conventional fast Fourier transform and its inverse transform are performed for each individual FBG. The analysis processing can be performed much more than the signal processing that had been performed.
A means for obtaining an envelope signal of the light intensity with respect to the frequency by further passing the absolute value of the data signal after the band-pass filter to the absolute value thereof, and a maximum in the peak waveform of the envelope signal. A book comprising: means for determining the center value of the values of the two-sided light wave numbers at the position of a predetermined ratio such as の of the amplitude value as the center light wave number; and means for analyzing the amount of distortion based on the center light wave number. In the multipoint strain measuring apparatus according to the present invention, a stable center light wave number can be obtained, so that accurate analysis of the strain amount is possible.
[0015]
Further, among a plurality of FBGs, the multi-point strain measuring apparatus of the present invention, which uses the data of one undistorted FBG as a reference for light source wavelength correction and temperature correction, has a dramatic increase in strain measurement accuracy. Improvement is possible.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining the principle of OFDR as a basis of the present invention.
FIG. 2 is a comparison diagram of a conventional technique and a detection data analysis method of the present invention.
FIG. 3 is a diagram illustrating the intensity of combined reflected light from a plurality of FBGs and a reference reflection surface with respect to wavelength.
FIG. 4 is a diagram illustrating that a synthetic reflected light corresponding to a specific FBG is extracted through a band-pass filter, and the intensity of the light is shown with respect to a wavelength.
FIG. 5 is a diagram illustrating a method of determining a center wavelength from an envelope waveform of an absolute value of the waveform of FIG. 4;
FIG. 6 is a diagram showing reflection characteristics of five FBGs.
FIG. 7 is a diagram showing a configuration of an optical fiber sensor system according to one embodiment of the present invention.
FIG. 8 is a diagram schematically showing an arrangement in which a plurality of FBGs are arranged on one fiber.
FIG. 9 is a diagram showing light transmission characteristics of an FBG.
FIG. 10 is a diagram showing a basic configuration of a conventional multipoint strain and temperature sensor using FBG.
[Explanation of symbols]
VS Tunable laser light source D Detector R Total reflection termination C Coupler FBG FBG sensor FC / PC Optical fiber coupling connector BNC Light intensity voltage conversion output connector

Claims (4)

A measurement method (OFDR) in which a plurality of fiber Bragg gratings (FBGs) having a periodic refractive index structure in the core of an optical fiber are arranged in one optical fiber and a periodic change in the interference intensity of light is used. 3), a function of instantaneously separating a signal from an FBG at a specific position by passing a detection light signal through a band-pass filter whose band is set in advance. Means for obtaining an envelope signal of light intensity with respect to frequency by further passing a low-pass filter to the absolute value of the data signal after the band-pass filter, and a maximum amplitude value in a peak waveform of the envelope signal 2. The multi-point strain measurement according to claim 1, further comprising: means for calculating a center value of the upper and lower light wave numbers indicating the value of the predetermined ratio as a center light wave number, and means for analyzing a distortion amount based on the center light wave number. apparatus. 3. The multi-point strain measuring apparatus according to claim 2, wherein the predetermined ratio is set to approximately 1/2 of the maximum amplitude value. By arranging a plurality of FBGs, preparing one FBG that is not distorted, and providing a means for performing correction as a reference data for light source wavelength correction and temperature correction using a detection signal from the FBG, 4. The multi-point strain measuring device according to claim 1, wherein a dramatic improvement in strain measuring accuracy is obtained.

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