KR20110086214A - A laser wavelength stabilized simultaneous multipoint fiber bragg grating acousto-ultrasonic sensing system - Google Patents

Abstract

PURPOSE: A laser wavelength stabilized simultaneous multipoint fiber bragg grating acousto-ultrasonic sensing system is provided to demodulate a plurality of strain-free fiber bragg grating acousto-ultrasonic sensors having identical spectrums using a single short wavelength variable laser. CONSTITUTION: A laser wavelength stabilized simultaneous multipoint fiber bragg grating acousto-ultrasonic sensing system comprises a short wavelength variable laser source(110), a 1xn beam splitter(120), n strain-free fiber bragg grating acousto-ultrasonic sensors(130), n photo-circulators(140), an asymmetric beam splitter(150), a power sensor(160), n photo-detectors(170), and an n+1 channel measurement and control unit(180). The short wavelength variable laser source irradiates a laser beam with variable wavelength. The 1xn beam splitter branches off the short wavelength variable laser beam provided from the short wavelength variable laser source. The n strain-free fiber bragg grating acousto-ultrasonic sensors demodulate and reflect the optical signals provided from the 1xn beam splitter. The n photo-circulators transmit the optical signals that are demodulated and reflected by the n strain-free fiber bragg grating acousto-ultrasonic sensors. The asymmetric beam splitter is connected to the n strain-free fiber bragg grating acousto-ultrasonic sensors via the photo-circulators. The power sensor is connected to the asymmetrical beam splitter. The n photo-detectors receive and detect the optical signals from the n photo-circulators. The n+1 channel measurement and control unit is connected to the power sensor and the n photo-detectors and receive the optical signals.

Description

Laser wavelength stabilized simultaneous multipoint fiber Bragg grating acousto-ultrasonic sensing system

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber Bragg grating acoustic ultrasonic sensing system. In particular, an ultrasonic wave sensing is performed even in a situation where strain is accompanied by using strain-free fiber Bragg grating sensors having the same spectrum without using a conventional optical fiber Bragg grating acoustic ultrasonic sensor. It is possible to enable multiple points at the same time, and also to solve the problem that the temperature of the fiber Bragg grating moves due to temperature change, so that the steep slope region of the main narrow of the fiber Bragg grating does not deviate from the initial wavelength setting of the short-wavelength tunable laser and the short-width demodulation condition is not established. By monitoring a part of the reflected light of one of the optical fiber Bragg gratings connected with the power sensor, the new spectrum scan area and scan interval can be determined by considering whether the optical fiber Bragg grating is out of the laser irradiation wavelength, moving direction, separation speed, and fiber bending loss. Automatically The present invention relates to a laser wavelength stabilized multi-point simultaneous optical fiber Bragg grating acoustic ultrasonic sensing system for deriving and reacquiring the short width demodulation condition automatically after reacquiring the spectrum.

Referring to FIGS. 1A and 1B, the principle of acoustic ultrasonic sensing using the optical fiber Bragg grating, which has been clarified, is irradiated to the laser beam of the short-wavelength variable laser into the optical fiber including the optical fiber Bragg grating and the wavelength of the laser beam Demodulates a large amount of reflected light that occurs when the acoustic ultrasonic waves modulate the optical fiber Bragg grating to the steep slope portion of the main lobe of the grating spectrum, ie the dynamic range, and demodulates it with the photodetector. . The short-wavelength tunable laser can be replaced by a combination of a broadband light source and a narrowband optical filter, but the sensitivity is greatly reduced. This sensing technique is called the short width demodulation method of the optical fiber Bragg grating. In FIG. 1B, λ ₁ is the lower limit of the dynamic region, λ ₂ is the upper limit of the dynamic region, and λ _M is the wavelength at half the height of the optical fiber Bragg grating spectrum.

Referring to FIG. 2, an acoustic ultrasonic sensing system using an optical fiber Bragg grating sensor developed in 2003 by DaimlerChrysler AG, Germany, based on the principle described with reference to FIGS. 1A and 1B will be described. Domain multiplexing is possible, but since the laser only outputs a single wavelength, only sequential demodulation is possible, not a method of simultaneously demodulating all the fiber Bragg gratings multiplexed in one line. Therefore, only a single sensor can operate when detecting acoustic emission waves caused by structural damage and impact in real time. In addition, acoustic ultrasonic sensing is not possible in situations involving structural deformation and temperature change. In FIG. 2, a tunable laser is a short-wavelength tunable laser, a fiber coupler is an optical splitter, an FBG is an optical fiber Bragg grating, a photo receiver is an optical detector, and data acquisition is a data measuring device.

Referring to FIGS. 3A and 3B, an ultrasonic ultrasonic sensing system using a fiber Bragg grating sensor developed in 2005 by a research team of the Korea Institute of Science and Technology [Document 2] will be described. While the variable laser has a single peak in the wavelength range, this configuration can produce multiple peaks. However, the peak width is wide and the output of the broadband light source has a disadvantage that the sensitivity is significantly lowered. In addition, we have constructed a system that stabilizes the peaks of Fabry-Parrot filters in the dynamic range through quasi-static strain and slow temperature changes through the Peltier temperature compensation circuit.However, each of the peaks produced by the Fabry-Parrot filters cannot be controlled independently. Wavelength multiplexing is only possible when Bragg lattice experiences the same quasi-static strain and temperature change. In the case of real structures, the distributed sensors are subject to different strains and temperature variations, making them inadequate as sensing systems for integrated structural health monitoring. In FIG. 3A, BBS is a broadband source, Tunable F-P filter is a tunable Fabry parot filter, WDM coupler is a wavelength domain multiplexed optical splitter, PD is a photodetector, and DAQ & DSP is a signal measurement and processor.

Referring to FIGS. 4A and 4B, an ultrasonic ultrasonic sensing system using a short-length optical fiber Bragg grating sensor developed in 2006 by a research team of the Tokyo University of Technology, Japan [Document 3] will be described in the optical fiber using a broadband light source and a multi-channel wavelength domain multiplexing filter. Both dynamic ranges of Bragg gratings are used for demodulation, but the increased sensitivity cannot be obtained because the spectral line width of the multichannel wavelength domain multiplexing filter is wider than the Fabry-Parlot filter. In addition, similar to the disadvantages of the research team of the Korea Institute of Science and Technology, simultaneous multi-point measurement is impossible in case of rapid temperature change or dynamic strain. In FIG. 4A, a super luminescent diode (SLD) refers to a broadband source, and an arrayed waveguide grating (AWG) refers to a multichannel wavelength domain multiplexed optical filter.

Acoustic ultrasonic waves using a pair of apodized fiber Bragg grating sensors and a dense Fabry Parrot filter developed in 2007 by a research team of the Japan Institute of Industrial Technology and transferred to Ishikawagima, Japan, with reference to FIGS. 5A and 5B. In the sensing system [4], it is possible to always demodulate the ultrasonic waves, even in the presence of large dynamic strain and temperature change. However, the use of a broadband light source and Fabry-Parlot filter results in low sensitivity, which inevitably reduces the sensitivity for multi-point simultaneous measurement, and increases the system price due to channel expansion. In FIG. 5A, FFP and FP are Fabry parot filters, ASE (Amplified spontaneous emission is amplified natural emission, AFBG (Apodized FBG) is an unpaired fiber Bragg grating, and OC (Optical circulator) means an optical circulator).

In summary, in order to perform structural integrity monitoring using the integrated structured fiber optic sensor, the acoustic emission wave generated by external shock or material damage in real time even during dynamic strain situation and environmental temperature change during structure operation is It should be possible to detect at the same time at the point. However, the above-mentioned conventional systems have a level of detecting ultrasonic waves generated by an ultrasonic generator in a static state when not in operation, or have a stabilization technique capable of maintaining demodulation conditions against quasi-static strain or slow temperature change, or have low sensitivity. There is a problem that simultaneous multi-point measurement is impossible. Comparison of the above-described prior arts is shown in Table 1 below.

Agency system sensor Intended
System function Sensitivity Strain and temperature change Single point Multipoint
Simultaneous measurement Daimler
Chrysler AG Short Width Wavelength Lasers Fiber optic
Bragg
grid Ultrasonic Measurement Using Multiplexing Sensor in Static State Great impossible impossible Korea Institute of Science and Technology Broadband and Fabry Parrot Filters Fiber optic
Bragg
grid Acoustic Emission Detection Using Multiplexing Sensor in Static State usually Possible for quasi-static strain and slow temperature changes impossible Tokyo Institute of Technology Broadband Light Source and Multi-Channel Wavelength Multiplexing Filter leader
Fiber optic
Bragg
grid Ultrasonic Measurement Using Multiplexing Sensor in Static State lowness Possible for quasi-static strain and slow temperature changes impossible Japan Institute of Industrial Technology Broadband and Dense Dual Fabry Parot Filters Untapped fiber
Bragg
grid Acoustic Emission Detection with Dynamic Strain and Rapid Temperature Change usually Possible for dynamic strain and fast temperature changes incongruity

Prior literature information

D C Betz, G Thursby, B Culshaw, W J Staszewski, Acousto-ultrasonic sensing using fiber Bragg grating, Smart Materials and Structures, 12 (2003) 122-128.

H J Bang, S M Jun, C G Kim, Stabilized interrogation and multiplexing techniques for fiber Bragg grating vibration sensors, Measurement Science and Technology, 16 (2005) 813-820.

T Fujisue, K Nakamura, S Ueha, Demodulation of Acoustic Signals in Fiber Bragg Grating Ultrasonic Sensors Using Arrayed Waveguide Gratings, Japanese Journal of Applied Physics, 45 (2006) 4577-4579.

J R Lee, H Tsuda, Y Akimune, Apodized fiber Bragg grating acousto-ultrasonic sensor under arbitrary strain using Fabry-Perot filters, Journal of Optics A: Pure and Applied Optics, 9 (2007) 95-100.

Accordingly, the present invention is to solve the above problems, by using a short-wavelength tunable laser that can maintain a high sensitivity, and when the temperature difference by the installation location of the sensor is large, by utilizing the short and unpaired fiber Bragg grating It further expands the dynamic range and also prevents the spectra of the fiber Bragg grating from shifting even when the structure is accompanied by a large dynamic strain. In addition, the spectral shift of the optical fiber Bragg grating due to temperature change can automatically reacquire the spectrum and maintain the short-width demodulation condition by intelligently determining the optical loss due to the moving direction, the moving speed, and the bending of the spectrum of the optical fiber Bragg grating. The laser wavelength is automatically controlled so that the fiber Bragg grating with the same spectrum can be branched and demodulated by using the high output of the short-wavelength variable laser to achieve acoustic ultrasonic waves even in the presence of dynamic strain and rapid temperature change at multiple points. An object of the present invention is to provide a laser wavelength stabilized multi-point simultaneous optical fiber Bragg grating acoustic ultrasonic sensing system that can be detected.

In order to achieve the above object, the laser wavelength stabilizing multi-point simultaneous optical fiber Bragg grating acoustic ultrasonic sensing system according to the present invention comprises: a short-wavelength variable wavelength laser beam irradiation unit for irradiating a laser beam having a variable wavelength; A 1 × n beam splitter for branching the short width variable wavelength laser beam incident from the short width variable wavelength laser beam irradiation unit into n channels; N strain-sensitive acoustic ultrasonic sensors based on optical fiber Bragg grating for modulating and reflecting an optical signal incident upon branching from a 1 × n beam splitter; N optical cycles for transmitting an optical signal modulated and reflected by the optical fiber Bragg grating-based n strain-sensitive acoustic ultrasonic sensor unit; An asymmetric beam splitter connected to an optical fiber Bragg grating based strain-free acoustic ultrasonic sensor unit having an average wavelength characteristic among n optical fiber Bragg grating spectra of the n-strain-sensitive acoustic ultrasonic sensor unit based on the optical fiber Bragg grating; A power sensor connected to the asymmetric beam splitter; n photodetectors for receiving and detecting optical signals from the n optical cyclers; And an n + 1 channel measurement and control unit connected to a power sensor and n photodetectors for receiving an optical signal and stabilizing a wavelength of the laser beam irradiated from the short-wavelength wavelength variable laser beam irradiation unit.

Here, the short-wavelength variable wavelength laser beam irradiation unit has a narrow line width, the wavelength variable region includes the wavelength shift region of the optical fiber Bragg grating due to the temperature change of the n strain-sensitized acoustic ultrasonic sensor unit based on the optical fiber Bragg grating, the maximum CW output The power can be set high at the level where there is no damage to the optical fiber. The line width is below 1 MHz, the wavelength variable region is above 0.05 nm, the intermediate wavelength is between 1465 nm and 1630 nm, and the maximum CW output power can be above 0.5 mW. have.

In addition, the 1 × n beam splitter determines the number of channels (n) of the sensing system, the asymmetric beam splitter is capable of branching less than 50%, less than 50% of the two ports can be connected to the power sensor have.

In addition, the optical fiber Bragg grating-based n strain-sensitized acoustic ultrasonic sensor unit is the optical fiber core by the acoustic ultrasonic wave generated by the acoustic emission wave and the excitation caused by the impact and damage of the optical signal branched from the 1 × n beam splitter It can be modulated and reflected from the refractive index grating engraved therein.

In addition, the n-strain-based acoustic ultrasonic sensor unit based on the optical fiber Bragg grating has the same spectrum, the strain of the structure is not transmitted to the optical fiber Bragg grating, the larger the temperature difference between the n sensors, the greater the dynamic range of the optical fiber Bragg grating It is possible to have a wide optical fiber Bragg grating, the sensor in which the strain of the structure is not transmitted to the optical fiber Bragg grating, the fiber Bragg grating acoustic ultrasonic sensor with one-end-free condition or the fiber Bragg grating acoustic ultrasonic with pressure coupling condition It may be a sensor.

The n + 1 channel measurement and control unit may convert the optical signal received from the photodetector into a digital signal after passing through a band pass filter.

In addition, the n + 1 channel measurement and control unit uses the information on the wavelength shift of the optical fiber Bragg grating of the short-wavelength variable laser beam irradiation unit obtained from the power sensor to measure the wavelength of the laser beam irradiated from the short-wavelength wavelength variable laser beam irradiation unit It is possible to stabilize, but analyzes the power reflected from the optical fiber Bragg grating of the short-wavelength variable laser beam irradiator obtained from the power sensor in the current laser beam wavelength in real time, and analyzes the power of the dynamic region of the Automatically determine the new scan area and scan interval by distinguishing the moving direction, the deviation from the laser beam wavelength, the wavelength moving speed at the time of departure, and the bending loss event of the optical fiber, and then converting the new laser beam wavelength into the dynamic region of the fiber Bragg grating Of the laser beam irradiated from the short-wavelength wavelength variable laser beam The wavelength can be stabilized.

According to the present invention, a single short-wavelength tunable laser can demodulate a plurality of same-spectrum strain-sensitive fiber Bragg grating acoustic ultrasonic sensors as well as temperature because the strain of the structure is not transmitted to the fiber Bragg grating. Only the phenomenon in which the dynamic region of the optical fiber Bragg grating deviates from the laser irradiation wavelength can be monitored by using the signal obtained from the power sensor, and the temperature change rate when leaving the dynamic region can be measured, thereby obtaining a new spectrum acquisition. Therefore, the new optimum laser irradiation wavelength can be determined.

In addition, since the sensors all have the same spectrum, it is possible to demodulate several sensors simultaneously with a single laser irradiation wavelength. Even if there is a temperature difference between the points where the sensors are installed, if a sufficient dynamic range is obtained by adjusting the length and refractive index profile of the fiber Bragg grating, Since the laser irradiation wavelength is located in the dynamic regions of all sensors, this case also has the effect of demodulating several sensors simultaneously.

1A and 1B illustrate the principle of operation of a conventional optical fiber Bragg grating acoustic ultrasonic sensor and system;
2 is a view illustrating an acoustic ultrasonic sensing system using an optical fiber Bragg grating and a short-wavelength variable laser developed by DaimlerChrysler AG, Germany.
3A and 3B are views illustrating an acoustic ultrasonic sensing system using an optical fiber Bragg grating and a Fabry-Parrot filter developed by a research team of the Korea Advanced Institute of Science and Technology.
4A and 4B are views illustrating an acoustic ultrasonic sensing system using a fiber Bragg grating and a multi-channel wavelength domain multiplexed beam splitter developed by a Japanese research team at the Tokyo University of Technology.
5A and 5B are views illustrating an acoustic ultrasonic sensing system using a pair of unpaired optical fiber Bragg gratings and dense narrow fabric parrot filters developed by a research team of the Japan Institute of Industrial Technology.
6 illustrates a laser wavelength stabilized multi-point simultaneous acoustic ultrasonic sensing system in accordance with the present invention.
7 is a view for explaining characteristics of optical fiber Bragg gratings having the same spectrum according to the present invention.
8 illustrates an optical fiber acoustic wave grating sensor according to the present invention.
9 is a view illustrating a first resonant frequency ( f ₁ ) determination technique by varying the length ( l ) of the sensing optical fiber in the optical fiber acoustic ultrasonic grating sensor according to the present invention.
10 is a diagram illustrating a resonant frequency ( f ₁ ) determination technique through material change of an optical fiber and a coating according to the present invention.
11a, 11b, 11c illustrate a pressure-coupled fiber Bragg grating acoustic sensor in accordance with the present invention;
12 is a view illustrating a dynamic region determination technique by controlling the length of an optical fiber Bragg grating according to the present invention.
13A and 13B show an algorithm for stabilizing a wavelength of a laser beam according to the present invention.
14A, 14B and 14C illustrate the main events that occur during stabilizing the wavelength of a laser beam in accordance with the present invention.
15A, 15B, and 15C are diagrams illustrating shock induced acoustic emission waves measured in a situation where strain and temperature change are accompanied by a laser wavelength stabilized multi-point simultaneous optical fiber Bragg grating acoustic ultrasonic sensing system according to the present invention;

Hereinafter, with reference to the accompanying drawings will be described in detail the present invention.

Specific features and advantages of the present invention will become more apparent from the following detailed description based on the accompanying drawings. Prior to this, terms and words used in the present specification and claims are to be interpreted in accordance with the technical idea of the present invention based on the principle that the inventor can properly define the concept of the term in order to explain his invention in the best way. It should be interpreted in terms of meaning and concept. It is to be noted that the detailed description of known functions and constructions related to the present invention is omitted when it is determined that the gist of the present invention may be unnecessarily blurred.

As shown in FIG. 6, the laser wavelength stabilizing multi-point simultaneous optical fiber Bragg grating acoustic ultrasonic sensing system according to the present invention has a short-wavelength variable laser beam irradiator 110, a 1 × n beam splitter 120, and n strain-free acoustics. Ultrasonic sensor unit 130, n optical circulator 140, asymmetric beam splitter 150, power sensor 160, n photo detector 170 and n + 1 channel measurement and control unit 180 is made of . In this case, the beam splitter 120, the optical circulator 140, the asymmetric beam splitter 150, and the photodetector 170 should include the wavelength of the laser beam used.

The short wavelength tunable laser beam irradiator 110 irradiates a laser beam having a variable wavelength, and in particular, has a narrow line width, and the wavelength variable region has a temperature variation of the n strain-sensitive acoustic ultrasonic sensor unit 130 based on the optical fiber Bragg grating. By including the wavelength moving region of the optical fiber Bragg grating, the maximum CW output power can be set high at a level that is not damaged to the optical fiber. Specifically, the line width is 1 MHz or less, the wavelength variable region is 0.05 nm or more, the intermediate wavelength is 1465 nm to 1630 nm, and the maximum CW output power is preferably 0.5 mW or more.

The 1 × n beam splitter 120 branches the short-wavelength variable laser beam incident from the short-wavelength variable laser beam irradiation unit 110 into n channels, where n determines the number of channels n of the sensing system.

The n strain-sensitive acoustic ultrasonic sensor unit 130 modulates and reflects an optical signal incident upon branching from the 1 × n beam splitter 120, specifically, the optical beam branched from the 1 × n beam splitter 120. The arc is modulated and reflected from the refractive index grating engraved in the optical fiber core by acoustic emission waves generated by impact and damage and acoustic ultrasonic waves generated by the exciter.

In particular, the n-strain-sensitized acoustic ultrasonic sensor unit 130 based on the optical fiber Bragg grating connected to each channel may have the same spectrum, that is, the same reflectance and the same spectral shape and the same wavelength. In general, when the optical fiber Bragg grating is manufactured by the same temperature and fiber tension in the same manufacturing equipment has the same spectrum. 7 is a spectrum of three optical fiber Bragg gratings, in which case it is advantageous to use the right dynamic region of the main lobe where the wavelengths of the dynamic regions coincide with each other. It is advantageous to choose.

The n strain-sensitive acoustic ultrasonic sensor unit 130 based on the optical fiber Bragg grating is a sensor in which the strain of the structure is not transmitted to the optical fiber Bragg grating. As the temperature difference between the n sensors increases, the dynamic range of the optical fiber Bragg grating increases. It can have a wide optical fiber Bragg grating.

The sensor in which the strain of the structure is not transmitted to the optical fiber Bragg grating may be an optical fiber Bragg grating acoustic ultrasonic sensor having a one-end-free condition or an optical fiber Bragg grating acoustic ultrasonic sensor having a pressure coupling condition. That is, for the strain-insensitive structure, as shown in FIG. 8, it may be a fiber acoustic wave grating sensor having a one-end-free condition, and the fiber acoustic wave grating sensor is a resonance type sensor. along the optical fiber and coating material as in the one-end-free by the change of the sensing optical fiber length (l) in a condition to determine the primary resonant frequency (f ₁₎ as shown in Fig. 9 and Fig. 10a, 10b, 10c By appropriate selection, the resonant frequency f ₁ can be determined by controlling the ultrasonic wave propagation speed c _f , which is a mechanical property.

It may also be a pressure-coupled fiber Bragg grating acoustic sensor as shown in Figure 11a, 11b, 11c. Specifically, the thickness of the silicon and the thickness of the sensor head is tens of micrometers larger than the depth of the center groove of the sensor cover, and the two sides of the sensor cover are bonded to the structure under pressure, thereby the elasticity of the silicon The sensor head containing the fiber Bragg grating is in close contact with the structure. Acoustic ultrasonic waves are also transmitted to the optical fiber Bragg grating without significant loss even in the pressure coupling.

In addition, the length of the optical fiber Bragg grating is determined in consideration of the temperature difference between each other at the position of the plurality of sensors, and the larger the temperature difference between each other, the shorter the length of the optical fiber Bragg grating, that is, the wider the dynamic region of the Bragg grating. As shown in FIG. 12, since a 1 mm optical fiber provides a dynamic range of 280 pm, even when a difference of about ± 14 ° C. is generated compared to a temperature of a sensor including an optical fiber Bragg grating to be monitored, it may be simultaneously demodulated. 5mm and 10mm fiber Bragg gratings do not allow temperature differences of more than about ± 6 ° C and ± 3 ° C, respectively. In the case of the optical fiber acoustic grating sensor, as another condition, the higher the resonant frequency to be set, the shorter the length of the sensing fiber should be taken into account.

The n optical circulators 140 transmit optical signals that are modulated and reflected by the n strain-sensitive acoustic ultrasonic sensor unit 130 based on the optical fiber Bragg grating.

The asymmetric beam splitter 150 is an optical fiber Bragg grating based strain-free acoustic ultrasonic sensor unit 130 having an average wavelength characteristic of the n optical fiber Bragg grating spectrum of the n optical fiber Bragg grating based on the optical fiber Bragg grating Is connected via an optical circulator 140. The asymmetric beam splitter 150 may be branched less than 50%, and less than 50% of the two ports may be connected to the power sensor 160.

The power sensor 160 is connected to the asymmetric beam splitter 150, and the n photodetectors 170 receive and detect the optical signals from the n optical cyclers 140.

The n + 1 channel measurement and control unit 180 is connected to the power sensor 160 and n photodetectors 170 to receive an optical signal, and the laser beam irradiated from the short-wavelength wavelength variable laser beam irradiation unit 110. Stabilize the wavelength. In this case, the n + 1 channel measurement and control unit 180 may convert the optical signal received from the photodetector 170, that is, the acoustic ultrasonic signal, after passing through the band pass filter, into a digital signal.

In addition, the n + 1 channel measurement and control unit 180 is a short-wavelength wavelength using information about the wavelength shift of the optical fiber Bragg grating of the short-wavelength variable laser beam irradiation unit 110 obtained from the power sensor 160 in detail. Stabilizing the wavelength of the laser beam irradiated from the variable laser beam irradiation unit 110, more specifically, the strain rate at the current laser beam wavelength of the short-wavelength variable wavelength laser beam irradiation unit 110 obtained from the power sensor 160 Analyze the power reflected from the optical fiber Bragg grating of the unsensitized acoustic ultrasonic sensor unit 130 in real time, and the moving direction of the optical fiber Bragg grating, the deviation from the laser beam wavelength, the wavelength moving speed at the time of departure, the bending of the optical fiber By identifying the lost events and automatically determining the new scan area and scan interval, the new laser beam wavelength is moved back into the dynamic range of the fiber Bragg grating. Chapter blasting it is possible to stabilize the wavelength of the laser beam emitted from a tunable laser beam irradiation section 110.

An algorithm for stabilizing the wavelength of the laser beam according to the optical fiber Bragg grating wavelength shift is shown in FIGS. 13A and 13B. As shown in Figs. 13A and 13B, the wavelength stabilization control algorithm of the laser beam is obtained after the initial spectrum is acquired, as shown in Fig. 14A through scanning to shift the wavelength of the laser according to the pre-process of Fig. 13A. In terms of the left dynamic region of the spectrum, the lower limit ( λ ₁ ) and the upper limit ( λ ₂ ) of the dynamic range are determined and the powers are determined at points with sample power closest to 0.1 and 0.8 due to the discrete sampling nature. It is represented by P ₁ and P ₂ . The laser irradiation wavelength λ _L is located at the midpoint (( λ ₁ + λ ₂ ) / 2) so as to have a symmetric dynamic region.

Since the laser wavelength stabilizing multi-point simultaneous optical fiber Bragg grating acoustic ultrasonic sensing system according to the present invention is based on a strain-insensitive sensor, this dynamic region can be used purely for temperature change management. The next step goes to the feedback control algorithm of FIG. 13B. The feedback control algorithm consists of three processes: a logging process, a monitoring process, and an updating process. During the recording process, the system records the normalized output to the maximum value of the initial spectrum at the laser irradiation wavelength reflected from the fiber Bragg grating through the power meter. The time interval of recording is set to 0.1 s or less and the recorded output is also normalized and denoted by P (λ _L ). At the end of the recording process, the system starts a monitoring process to determine the direction of movement of the fiber Bragg grating spectrum and to determine if it is out of the dynamic range. The lower limit of the threshold power (threshold power) in the pre-treatment process (P _TH1) and upper (P _TH2) is right of the λ ₁ as shown in Fig. Jyeotneunde is set with 14a defined in the following sampling point and λ ₂ directly power of sample points before. The threshold power wavelength can be expressed as follows.

Where λ _S represents the wavelength scanning interval. Then, the system compares the normal reflection power and the threshold power based on the inequality P (λ _L ) < P _TH1 and P (λ _L )> P _TH2 in FIG. 13B, so that the optical fiber Bragg grating spectrum is λ _TH1 < λ _L < It is determined whether λ _TH2 is satisfied. Through this, the moving direction of the optical fiber Bragg grating spectrum can be confirmed. If the normal reflected power satisfies P (λ _L ) < P _TH1 , the direction of the fiber Bragg grating spectrum is shifted to the right, indicating an increase in temperature. Conversely, if P (λ _L )> P _TH2 is satisfied, it is shifted to the left, which means a decrease in temperature. If the two inequalities are not satisfied, the demodulation condition is maintained in the dynamic range, and the process returns to the recording process. If one of the two inequalities is satisfied, the sensitivity of the ultrasonic wave to be measured is greatly reduced or demodulated into a malformed waveform. Thus, the system needs an update process to determine the new laser irradiation wavelength.

For this purpose, two counters, C _TH1 and C _TH2, are first prepared for the monitoring process. This counter is used to determine the total monitoring time ( T _m ) spent during the course of the normal reflected power from P _TH1 ( P _TH2 ) to P ₁ ( P ₂ ). In other words, the time taken to shift one wavelength scan interval. As shown in Fig. 13B, the total monitoring time is determined by multiplying the recording time interval [Delta] t of the recording process by the total count number of the counter. During the monitoring process, the counters are reset to zero if the normal reflected power does not satisfy two inequalities P (λ _L ) < P _TH1 , P (λ _L )> P _TH2 . On the other hand, if P (λ _L ) < P _TH1 ( P (λ _L )> P _TH2 ) is met for temperature increase (decrease), then C _TH2 ( C _TH1 ) is reset to zero and the other counter is increased by one. do.

If the normal reflected power in the monitoring process then meets the inequality P < P ( λ ₁ ) or P > P ( λ ₂ ), the system proceeds to an update process to determine the new laser irradiation wavelength. At this time, only one slope of the optical fiber Bragg grating spectrum is scanned to reduce the scan time. The scan area and scan interval must be determined automatically to maintain temperature compensation feedback control. Once the normal reflected power satisfies P < P ( λ ₁ ) or P > P ( λ ₂ ), the feedback control algorithm indicates that the fiber Bragg grating spectrum has shifted by half of the dynamic range (( λ ₂ - λ ₁ ) / 2). To judge. But the important thing is that the spectrum keeps moving for the time required for scanning. In other words, the actual wavelength shift after scanning is very likely greater than ( λ ₂ -λ ₁ ) / 2. In order to take into account the spectrum moving continuously during scanning, the average wavelength travel velocity is introduced as follows using the total monitoring time and the current scan interval.

Next, the wavelength shift expected during scanning may be expressed as follows using the above equation.

Where N is the total number of samples included in the new scan area and Δ t _s is the scan sampling time. Thus, the end wavelengths ( λ _{stop , new} ) of the new scan region are determined as follows.

Where λ _max is the wavelength at the maximum of reflectivity of the fiber Bragg grating spectrum. The scan area start wavelength (λ _{start, new)} and the predicted full-related wavelength shift amount _{((λ 2 - λ 1)} / 2 + △ λ e) prior to the start without further a wavelength of λ ₁ is still used. This is because, as illustrated in Figure 14b, the system can misunderstand the fiber bending loss that can occur in a fiber optic network as an event of increased temperature. Thus, the size of the scan area in Equation 4 does not miss the optical fiber Bragg grating spectrum even in fiber bending loss. Then the new wavelength spacing for scanning is determined as follows.

After the size of the new scan area and the scan interval are automatically determined, the system starts the scan and scans right from point O. The related event at this time is shown in FIG. 14C. The first graph on the left side of FIG. 14C is the spectrum at P ( λ _L ) = P ₁ instant but the system does not need to actually acquire this spectrum. In practice, the system acquires a second graph with N sample points. This spectrum is different from the actual fiber Bragg grating spectrum because the spectrum continues to move during the scan. Therefore, the laser irradiation wavelength at point A is not optimal. This is because the optimal laser irradiation wavelength has already shifted to point B when scanning has been completed. In the end, the wavelength shift compensation is required and is determined as follows using the average wavelength shift speed.

Where T _AB is the total time taken for the laser wavelength shift from point A to point B in FIG. 14C and is calculated as follows.

Where N _OA is the total number of samples from point O to A and T _N is the time taken to complete the scan of N samples. The new laser wavelength at point B is expressed as

In the same way, the limits of the new dynamic range for the new recording process are determined as follows.

In the case of temperature reduction, the algorithm and derivation for determining the new scan range and scan interval are similar and Equation 4 changes as follows.

Here, the scan end wavelength may not be able to sample the maximum reflected power wavelength of the shifted spectrum if the actual average wavelength shift during the update process is smaller than the average wavelength shift calculated in the monitoring process, thus maintaining the previous scan termination wavelength λ _max . do. Then, compensation of the wavelength shift amount is made by Equation (8).

And the corresponding new wavelength range is given by

After the source of the short-wavelength tunable laser is transferred to the new laser irradiation wavelength, the system restarts the recording process.

The overall system equipped with the feedback control algorithm in the present invention is characterized by the effect of the strain-insensitive sensor configuration and the high-frequency pass filter and the wavelength stabilization and the laser wavelength feedback control algorithm described above even when the structure is accompanied by a dynamic strain of several kHz. By the effect of the system, it is possible to simultaneously sense acoustic ultrasonic waves at multiple points while automatically recovering the demodulation condition deviated by a rapid high temperature change rate of 1 DEG C / s in a few seconds. 15 is a demodulation condition automatically as shown in FIG. 15B in a situation in which a dynamic strain as shown in FIG. 15A and a rapid temperature change as shown in FIG. 15B are accompanied by using a laser wavelength stabilized multi-point simultaneous optical fiber Bragg grating acoustic ultrasonic sensing system according to the present invention. While recovering, it is shown that the shock induced acoustic emission wave generated at a point 150 mm away from two sensors mounted on the composite plate is simultaneously detected as shown in FIG. 15C.

As described above and described with reference to a preferred embodiment for illustrating the technical idea of the laser wavelength stabilizing multi-point simultaneous optical fiber Bragg grating acoustic ultrasonic sensing system according to the present invention, the present invention is the configuration and operation as described and described as It will be appreciated by those skilled in the art that many modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Accordingly, all such suitable changes and modifications and equivalents should be considered to be within the scope of the present invention.

110: short-wavelength variable laser beam irradiation unit 120: 1 × n beam splitter
130: strain-free acoustic ultrasonic sensor unit 140: optical circulator
150: asymmetric beam splitter 160: power sensor
170: photodetector 180: n + 1 channel measurement and control

Claims (10)

A short-wavelength tunable laser beam irradiator 110 for irradiating a laser beam of which wavelength is variable;
A 1 × n beam splitter 120 for splitting the short width variable wavelength laser beam incident from the short width variable wavelength laser beam irradiation unit 110 into n channels;
N strain-sensitive acoustic ultrasonic sensors 130 based on optical fiber Bragg gratings for modulating and reflecting an optical signal incident by branching from the 1 × n beam splitter 120;
N optical cyclers 140 for transmitting optical signals modulated and reflected by the optical fiber Bragg grating-based n strain-sensitized acoustic ultrasonic sensor unit 130;
The optical fiber Bragg grating-based strain-free acoustic ultrasonic sensor unit 130 of the n optical fiber Bragg grating spectrum of the optical fiber Bragg grating based strain-free acoustic ultrasonic sensor unit 130 through the optical circulator 140 An asymmetric beam splitter 150 connected;
A power sensor 160 connected to the asymmetric beam splitter 150;
n photodetectors 170 for receiving and detecting optical signals from the n optical cycles 140;
N + 1 channel measurement and control unit 180 connected to the power sensor 160 and the n photodetectors 170 to receive an optical signal and to stabilize the wavelength of the laser beam irradiated from the short-wavelength variable laser beam irradiation unit 110. Laser wavelength stabilization multi-point simultaneous optical fiber Bragg grating acoustic ultrasonic sensing system comprising a). The method according to claim 1,
The short-wavelength wavelength variable laser beam irradiation unit 110 has a narrow line width, and the wavelength-variable region includes the wavelength shift region of the optical fiber Bragg grating due to the temperature change of the n strain-sensitive acoustic ultrasonic sensor unit 130 based on the optical fiber Bragg grating. The maximum CW output power is set to a high level at the level of damage to the optical fiber laser wavelength stabilizing multi-point simultaneous optical fiber Bragg grating acoustic ultrasonic sensing system. The method according to claim 2,
The short-wavelength tunable laser beam irradiation unit 110 has a line width of 1 MHz or less, a wavelength variable region of 0.05 nm or more, and an intermediate wavelength of 1465 nm to 1630 nm, and a maximum CW output power of 0.5 mW or more. Wavelength stabilized multi-point simultaneous fiber Bragg grating acoustic ultrasonic sensing system. The method according to claim 1,
The 1 × n beam splitter 120 determines the number of channels n of the sensing system,
The asymmetric beam splitter 150 is capable of branching less than 50%, less than 50% of the two ports are connected to the power sensor 160, laser wavelength stabilization multi-point simultaneous optical fiber Bragg grating acoustic ultrasonic sensing system. The method according to claim 1,
The n-strain-sensitized acoustic ultrasonic sensor unit 130 based on the optical fiber Bragg grating is a sound generated by an acoustic emission wave generated by an impact and damage and an excitation of an optical signal incident by branching from the 1 × n beam splitter 120. A laser wavelength stabilized multi-point simultaneous optical fiber Bragg grating acoustic ultrasonic sensing system characterized by modulated reflection from a refractive index grating engraved in an optical fiber core by ultrasonic waves. The method according to claim 1,
The n strain-free acoustic ultrasonic sensor unit 130 based on the optical fiber Bragg grating has the same spectrum, and the strain of the structure is not transmitted to the optical fiber Bragg grating, and as the temperature difference between the n sensors increases, Laser wavelength stabilization multi-point simultaneous optical fiber Bragg grating acoustic ultrasonic sensing system, characterized in that the dynamic region has a wide optical fiber Bragg grating. The method of claim 6,
The sensor for which the strain of the structure is not transmitted to the optical fiber Bragg grating is an optical fiber Bragg grating acoustic ultrasonic sensor having a one-end-free condition or an optical fiber Bragg grating acoustic ultrasonic sensor having a pressure coupling condition. Simultaneous Fiber Bragg Grating Acoustic Ultrasonic Sensing System. The method according to claim 1,
The n + 1 channel measurement and control unit 180 laser wavelength stabilizing multi-point simultaneous optical fiber Bragg grating acoustic ultrasonic sensing system, characterized in that for converting the optical signal received from the photodetector 170 through a band pass filter to a digital signal . The method according to claim 1,
The n + 1 channel measurement and control unit 180 uses the information about the wavelength shift of the optical fiber Bragg grating of the short-wavelength variable laser beam irradiation unit 110 obtained from the power sensor 160 by using the short-wavelength variable-wavelength laser beam irradiation unit ( Laser wavelength stabilization multi-point simultaneous optical fiber Bragg grating acoustic ultrasonic sensing system, characterized in that to stabilize the wavelength of the laser beam irradiated from 110). The method according to claim 9,
The n + 1 channel measurement and control unit 180 is an optical fiber Bragg grating of the strain-sensitive acoustic ultrasonic sensor unit 130 at the current laser beam wavelength of the short-wavelength variable laser beam irradiation unit 110 obtained from the power sensor 160. Analyze the power reflected from the sensor in real time, and identify the new scan area and scan interval by distinguishing the moving direction of the optical fiber Bragg grating, whether it deviates from the laser beam wavelength, the wavelength moving speed at the time of departure, and the bending loss event of the optical fiber. After automatically determining, the laser wavelength stabilization multi-point simultaneous optical fiber, characterized in that to stabilize the wavelength of the laser beam irradiated from the short-wavelength variable wavelength laser beam irradiation unit 110 by moving the new laser beam wavelength in the dynamic region of the optical fiber Bragg grating Bragg Grating Acoustic Ultrasonic Sensing System.

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