This paper proposes a wavelength-division multiplexing fiber laser acoustic emission sensing technique based on 3×3 coupler type interrogation method and a FPGA parallel processing algorithm. Narrow-linewidth (about 3 kHz) singlefrequency distributed feedback fiber lasers are used for acoustic emission probes. A NI FlexRIO device is used to acquire the original signals from the photodetectors. A symmetric demodulation algorithm is executed in the FPGA using parallel data processing structure. The acoustic emission sensing system with four parallel channels and 2 MHz sampling rate achieves a wavelength resolution of 2 × 10-7 pm/√Hz @ 100 kHz.
A novel short-cavity-based narrow linewidth random fiber laser (RFL) is proposed. The random distributed feedback mechanism of RFL is assumed by a set of arbitrarily distributed weak reflection fiber Bragg grating arrays (FBGs). A π-phase-shifted FBG with a narrow transmission window is used in the RFL to further limit the number of random subcavity modes and suppress the output linewidth. High gain erbium-doped fibers and half-open cavity designs are used to maintain low lasing thresholds. A stable single-mode lasing operation with 3-dB linewidth of 211 Hz and 58 dB sidemode-suppression-ratio is established.
We proposed a novel structure of short-cavity random fiber laser (RFL) with a stable narrow linewidth output. A random-spaced Bragg grating array is used for random feedback. A ring optical path and the grating array form a short half-open cavity, and a high-precision π-phase-shifted grating (π-FBG) is placed in the ring path for filtering and modelocking to ensure a stable single-mode random laser operation. The linewidth of the laser is 257 Hz with 50 dB sidemode-suppression-ratio (SMSR). The laser wavelength drift measured in the laboratory is less than 1 pm within 20 min. The RFL has a simple structure and can achieve a stable narrow linewidth single mode output, which provides a new choice for high resolution optical fiber sensing.
In this manuscript, we propose a new technique to suppress the noise level in an interferometric optical fiber sensing system. Previous work shows that laser frequency noise is the main factor that limits the resolution in a phase generated carrier (PGC) demodulation system. Using a bidirectional pumped random distributed feedback fiber laser (RDFL), the laser frequency noise can be suppressed from 30 Hz/√Hz to 4 Hz/√Hz above 1 kHz. Therefore, the wavelength resolution is improved from 5×10-7 pm/√Hz@ 1 kHz to 5×10-8 pm/√Hz@ 1 kHz.
We present a simple method to measure the 3 dB linewidth of the sharp peak in the spectrum of the fiber Bragg gratingbased Fabry–Perot (FBG-FP) interferometer. A narrow linewidth tunable laser is used for sweeping the FBG-FP to achieve the reflection spectrum. A triangle wave is used to drive the narrow linewidth tunable laser. The linewidth of the sharp peak is measured with a resolution of 0.001 pm.
In the experiment, a fiber Bragg grating embedded in a hollow glass bead/epoxy resin composite was used to monitor the strain changes at different stages of curing, while two free fiber gratings were placed in the oven to realize temperature compensation. At the end of the cooling stage, the minimum strain monitored is - 6123 με. In addition, the study shows that the variation of internal strain in the sample is delayed relative to the temperature change in the oven. This test verifies the feasibility of using FBG to monitor the curing cycle of buoyancy materials.
We proposed an high-resolution broadband phase interrogation technique for an interferometric fiber optic sensor which is designed as an unbalance michelson interference structure. The frequency stabilization technique based on PoundDrever-Hall (PDH) method is used for reducing low-frequency phase noise of the laser. An (Piezoelectric Transition) PZT, acting on one arm of the interferometer, is used to generate additional phase modulation. A phase generation carrier (PGC) algorithm is proposed for phase demodulation. The experiment results show that the proposed system has a very wide wavelength detection capability (from 0.1 Hz to 500 Hz) with an ultrahigh resolution of 2 × 10-4 rad@0.1 Hz, and better than 10-5 rad in the frequency range from 1 Hz - 500 Hz.
An optical fiber ocean bottom vector magnetometer (OOBVM) array based on fiber laser sensing technology is proposed. The fiber laser is modulated at the natural frequency of the sensing structure by the Ampere force in the magnetic field. In this way, the sensitivity is enhanced and the effect of power fluctuation of fiber laser is eliminated. The test result shows that its magnetic field resolution is about 0.86 nT / √ Hz@100 mA. The design and assembly process of the OOBVM array are demonstrated. A field test in South China Sea demonstrated that the OOBVM array can detect the movement of ferromagnetic objects in water, which implies the potential for applications in underwater object detection and marine geophysics.
Proc. SPIE. 10323, 25th International Conference on Optical Fiber Sensors
KEYWORDS: Signal to noise ratio, Fiber optics, Sensors, Magnetism, Fiber lasers, Magnetic sensors, Electronic filtering, Detection theory, Magnetometers, Temperature metrology
A fiber optic accelerometer-magnetometer (FLAM), which can measure acceleration and magnetic field simultaneously, based on fiber laser sensing is proposed. The principle of the FLAM and the theory of the decoupling the signals of acceleration and magnetic field are presented. The FLAM was interrogated using phase-generated-carrier (PGC) method. A test of simultaneously measuring acceleration and magnetic field was performed. The results show that the responding mixed signal achieves a good SNR and prove that the function of the sensor has been realized and the theory of decoupling signal is reliable.
Proc. SPIE. 10026, Real-time Photonic Measurements, Data Management, and Processing II
KEYWORDS: Fabrication, Cladding, Single mode fibers, Fiber lasers, Gas lasers, Optical alignment, Carbon monoxide, Camera shutters, Temperature metrology, Laser systems engineering
In the paper, the Long period fiber gratings (LPFG) were fabricated in a single-mode fiber using a high frequency CO2
laser system with the point-to-point technique. The experimental setup consists of a CO2 laser controlling system, a
focusing system located at a motorized linear stage, a fiber alignment stage, and an optical spectrum analyzer to monitor
the transmission spectrum of the LPFG. The period of the LPFG is precisely inscribed by periodically turning on/off the
laser shutter while the motorized linear stage is driven to move at a constant speed. The efficiency of fiber writing
process is improved.
Compared with Fiber Bragg grating (FBG), Distributed Feedback fiber laser (DFB-FL) sensors has the advantages of ultra-narrow line-width, high output power, and low noise level, which will result in a better performance in ultra-slight acoustic emission (AE) detection. In this paper, we demonstrate a DFB fiber laser acoustic sensor. The intensity response of DFB-FL to external acoustic waves has been investigated. The frequency response of the DFB fiber laser based AE sensor is measured in aluminum plate. The experiment results show that the intensity modulated DFB fiber laser acoustic sensor can accurately record the continuous acoustic emission signal and the pencil lead-broken acoustic emission waves.
In this paper we report the field test of fiber optic ocean bottom seismograph (OOBS) which can be used in the active source seismic research. There are three fiber laser accelerometers (FLAs) and one fiber laser hydrophone (FLH), which is wavelength division multiplexed, in the OOBS. The interrogation system is put on shore and is connected with the OOBS with optical fiber cable. The field test of using an air gun is carried out under water with a depth of 30 m. The results show that the OOBS has similar performance as conventional electric OBS.
We propose a high-resolution static-strain sensor based on a FBG Fabry-Perot interferometer (FBG-FP) and a wavelet domain cross-correlation algorithm. This sensor is used for crust deformation measurement, which plays an important role in earthquake precursor observation. The Pound-Drever-Hall (PDH) technique based on a narrow-linewidth tunable fiber laser is used to interrogate the FBG-FPs. A demodulation algorithm based on wavelet domain cross-correlation is used to calculate the wavelength difference. The FBG-FP sensor head is fixed on the two steel alloy rods which are installed in the bedrock. The reference FBG-FP is placed in a strain-free state closely to compensate the environment temperature fluctuation. A static-strain resolution of 1.6 n(epsilon) can be achieved. As a result, clear solid tide signals and seismic signals can be recorded, which suggests that the proposed strain sensor can be applied to earthquake precursor observation and earthquake monitoring.
In this paper we present the field test of the fiber optic ocean bottom cable (FOOBC). The FOOBC are several ocean bottom seismic stations connected by optical fiber cables. In the ocean bottom seismic station, there are three orthogonal fiber optic accelerometers and one fiber optic hydrophone. The design of the sensors and stations are introduced. The field demonstration of an ocean bottom seismic station is carried out in Yunnan Province, China. The test results show that the three accelerometers has similar response to the seismic signal with traditional MEMS accelerometers. We believe that the fiber optic seismic cable is promising in the field of oil and gas exploration and earthquake monitoring.
Liquid damped fiber laser accelerometer (LD-FLA) was proposed and a kind of LD-FLA structure, of which damping liquid could be easily change, was designed to study its damping characteristic. The principles of a LD-FLA were analyzed comparing with a common FLA, which found that liquid damper could produce additional liquid pressures in vibration so that not only damping responses but sensitivities and frequency responses could be changed at the same time. Four double diaphragm-based LD-FLAs were manufactured and experiments under several kinds of damping liquids were carried out. The experimental results are close to the calculated values and verify the theoretical analysis on LDFLA. The theoretical model provides guidance for further research on damping characteristic.
This paper presents a novel fiber flowmeter based on distributed feedback fiber laser (DFB-FL). The DFB-FL is encapsulated in a PU tube which is put in the center of the flow pipe. When the flow speed changes, the flow induced pressure turbulence on the surface of the PU tube will change correspondingly. This pressure fluctuation is demodulated using phase-generated-carrier (PGC) method. The test result shows that the sensitivity of the fiber laser flowmeter is about 8×10-3 pm/(m3/h). A resolution of 1.25×10-4 m3/h is achieved.
Fiber laser has the advantages of ultra-narrow linewidth, low phase and intensity noise, which is beneficial for ultra-high-resolution strain sensing. This paper presents a novel demodulation technique for sub-nanostrain-resolution static strain measurement based on two distributed feedback fiber lasers (DFB FLs). A commercial PZT-tunable laser is used to interrogate the DFB FLs and get the periodic frequency-difference characteristics (two linear chrip signals) by swept beat-frequency principle. Two polarization controllers are used for adjusting the polarization direction of DFB FLs. And one of the two DFB FLs is used for temperature compensation and eliminating the frequency shift influence of the commercial laser. Static strain is demodulated by calculating the difference of the direct current (DC) components of the two swept beat-frequency signals. A static-strain resolution of 0.88 nε is obtained in the laboratory test.
A fiber optic Fabry-Perot accelerometer (FOFPA) with diaphragm-mass-collimator (DMC) gathered structure is presented. This design makes the structure more compacts and the manufacturing process more controllable. The operation principle based on Fabry-Perot interference is described. Several tests using intensity demodulation scheme which can control the working point of FOFPA were carried out. Experimental results show that: axis sensitivity of the proposed FOFPA is 36.07 dB (re: 0 dB=1 V/g) with a fluctuation less than 0.9 dB in a frequency bandwidth of 10-125 Hz, the resonant frequency is about 350 Hz, measurement range is about 70 dB@100 Hz. which are much close to theoretical values
The paper proposes an accelerometer construction based on 45-degrees Fabry-Perot (F-P) interferometer cavity. The uniform intensity cantilever consists of a mass block in the middle and a 45-degrees F-P cavity fixed inside the mass. The mass block can oscillate freely when the vibrating sensor is subject to the vibration and the F-P cavity length is changing. The G-lens end face and total reflective film make up the two reflective films of the F-P cavity, and the reflectivity are 4% and 90% respectively. In the F-P cavity, a 45-degrees mirror fixed in the middle of the G-lens and total reflective film. The mirror can change the transmission of the light and increase the optical path difference. The total reflective film fixed in the steel tube and the G-lens fixed in the fine tuning bolt. The bolt can fine adjust the F-P cavity in sensor encapsulating. The sensor structure lead to the optical loss in the airborne and tilted mirror, besides the distance of F-P gap in steel tube and the optical coupling efficiency can’t work out accurately, so we did a series deterministic test before encapsulating, for example the selection of the structures, the diameter of the optical fibers and the diameter of the reflective films. At last, 9/125 μm optical fiber, 1.4 mm total reflective film and the structure of total reflective film out of steel tube were used for the accelerometer. The sensitivity can reach 0.042 rad/g and the resonant frequency of the accelerometer is 400 Hz.
We present an improved phase generated carrier (PGC) demodulation method named PGC differential-self-multiplying-integrate (PGC-DSMI) algorithm in this paper. The performances of the new demodulation method are ameliorated via the elimination of the coefficients in the result of conventional PGC-differential-and-cross-multiplying (PGC-DCM) method. The advantages of the improved method are demonstrated theoretically and proved in the simulation. The results show that PGC-DSMI has a better linearity than PGC-DCM.
In this paper, π-phase-shifted FBG (π-FBG) is used for high-resolution static-strain sensing. A novel laser tuning technique based on triangle-wave voltage driver is proposed to improve wavelength scanning stability of tunable laser. And a static-strain demodulation algorithm based Gauss curve fitting and peak detection is tested. As the reflectance spectrum of π-FBG have high signal-to-noise ratio, a static-strain resolution of 0.83 nε is obtained in a vacuum environment, which shows that the proposed system have a good application prospect for geophysics applications.
This paper introduces a highly-sensitive fiber optical acoustic emission (AE) sensor and a parameter analysis method aiming at concrete structure health monitoring. Distributed feedback fiber-laser (DFB-FL), which is encapsulated to have a high acoustic sensitivity, is used for sensor unit of the AE sensor. The AE signal of concrete beam in different work stages, based on the four-point bending experiment of the concrete beam, is picked up, and the relationship between the concrete beam work stages and the AE parameter is found. The results indicate that DFB-FLAES can be used as sensitive transducers for recording acoustic events and forecasting the imminent failure of the concrete beam.
Fiber laser hydrophone has attracted more and more attention because of its potential application in novel sonar system. For a towed fiber optic hydrophone array, the flow noise is an important source of the system noise. This paper presents the field test result of the flow noise of an eight-element fiber laser hydrophone array. The structure of the fiber laser hydrophone and the array is also introduced. The field test shows that obvious flow noise can be measured when the array is towed at the speed of 3 kn.
The fluctuation of polarization may cause wrong demodulation results in high-resolution static-strain sensing based on FBG-FP. This paper presents a polarization-insensitive demodulation algorithm using time-wavelet energy spectrum for single mode fiber (SMF) based FBG-FP sensor. This method can eliminate the influence of polarization instability effectively. Time-wavelet energy spectrum is calculated to get the energy distribution of the reflectance spectrums of FBG-FP on the timeline. The maximum value of the energy distribution is used to obtain the strain signal. In the laboratory, a high-resolution strain demodulation result without polarization controller is obtained.
Three-component seismometer is widely used in oil or gas exploration, earth quake monitoring. In this paper, an allmetal 3-component optical fiber seismometer is proposed and experimentally demonstrated. The theoretical analysis is given based on electro-mechanical theory. Calibration results show that axis sensitivity is 41 dB (re: 0 dB=1 rad/g) with a fluctuation ±2 dB in frequency bandwidth of 5~400 Hz. A transverse sensitivity of about -40 dB is achieved. The fluctuation of the acceleration sensitivity for the three accelerometers in the seismometer is within ±2.5 dB. The minimum phase demodulation detection accuracy of the phase-generated carrier (PGC) is 10-5 rad/√Hz, and the minimum detectable acceleration is calculated to be 90 ng/√Hz. With an all-metal structure, the proposed seismometer is expected to improve reliability of long-term use in harsh environment such as ocean bottom seismic wave monitoring in oil or gas exploration.
In order to measure the distance between switch point and stock rail in railway system high precision, a wide range and high precision fiber Bragg grating (FBG) displacement sensor is designed. The sensitivity as well as the measurement range of the senor is increased through the effective cooperation of the spring and equal strength beam. At the same time, differential compensation method is exploited for eliminating temperature effects, which brings further improvement of the sensitivity as well. Test results show that, in the range of 160 mm, the precision of the sensor is 0.017 mm.
A fiber optic Fabry-Perot (FP) hydrophone with hydrostatic pressure compensation was demonstrated. A polyimide (PI) diaphragm attached on the end of an Acrylonitrile Butadiene Styrene (ABS) tube was used as the sensing element. A pair of grooves was designed in an inner ABS tube to connect the Fabry-Perot cavity with the outside environment, which made the hydrophone hydrostatic pressure compensated. The operation principle, design and testing of polyimide diaphragm-based sensor were described. Experiment results show that it has not only high stability in different hydrostatic pressures, but also flat frequency response of about 158 ±3 dB within 300-3000 Hz.
A novel Cantilever-based two-axis FBG accelerometer (FBGA) is presented. By installing two fiber Bragg gratings on the cantilever, the proposed FBGA can detect the acceleration, both its magnitude and direction. Theoretical analyses including acceleration response and natural frequency are given. The experimental results show an acceleration sensitivity of 40 pm/g in the X-axis, 18 pm/g in the Y-axis, a directivity of better than 20 dB, and a natural frequency of about 100 Hz. These results agree well with theoretical expectations.
Landslide is typical geo-hazards which causes serious threat to the human lives and infrastructures. It's necessary to develop a monitoring system to avoid landslide disasters. In this paper, a real time monitoring system based on optical fiber sensing technology was designed to monitor the health status of slope. Three types of optical fiber sensors, including FBG strain gauge embedded in anchor, FBG inclinometer, and FBG soil-pressure sensor, were demonstrated. A light switch was introduced to expand the demodulation equipment to 16 channels, which enlarges the capacity of more than 200 FBG sensors. This remote and real-time monitoring system can be used not only for landslide monitoring but also for large scale structure health monitoring.
In this paper a fiber optic accelerometer (FOA) based on camped beam is proposed. The clamped beam is used as the elastic element and a mass installed on the clamped beam is used as the inertial element. The accelerometer is based on a fiber optic Michelson interferometer and has a sensing arm and a reference arm. The optical fiber of the sensing arm is wrapped on the clamped beam and the mass, which are both cylinder shaped. The sensitivity of the FOA is analyzed based on the theory of elasticity; the frequency response is analyzed based on the theory of vibration. Experiment is carried out to test the performance of the fiber optic accelerometer. The experiment results show a high sensitivity and a flat frequency response within the low frequency range of 5-250 Hz, which agrees well with the theoretical result.
A fiber Bragg grating (FBG) soil-pressure sensor based on dual L-shaped levers is proposed and demonstrated. Using dual L-shaped levers, the deformation of the diaphragm, which experiences the soil pressure, will be transferred to the longitudinal strain of the FBG. The theoretical analysis on the sensitivity of the proposed sensor is given. The sensor has been calibrated in the laboratory. Experimental results show a good agreement with theoretical expectations. Furthermore, this soil-pressure sensor has been installed in an earth dam in Beijing, China. Drifting of the FBG wavelength has been collected in rainy days from May to July 2012. It has been found that the earth dam has an obvious change of internal stress in the rainstorm.
A systematic study of rock mass acoustic emission (AE) detection using distributed feedback (DFB) fiber grating lasers is presented. In this, a minimum detectable strain of 10-12 is achieved in the experiment when using a fiber optic interferometric demodulation. The dynamic strain sensitivity of DFB fiber grating laser in different coupled modes is calibrated by a comparison with PZT AE sensor.. The acoustic emission directional characteristics of the DFB fiber grating laser are investigated by two kinds of analyzing method. In the first experiment, the relationship between the wavelength drift and the angle of a continuous acoustic emission source is determined. In the second experiment, the relationship between the percentage of wavelet packet energy and the angle of the impact response is established. Their acoustic emission directional characteristics make them suitable for determining principal strains and the direction of propagation of acoustic waves. Because of these advantages, the DFB fiber lasers will provide a practical and advanced measurement means in large civil structural health monitoring.
This paper presents a rail corrugation measurement system based on fiber laser accelerometers (FLAs) which are
installed on the bogie of the train. When the train passes the rail with corrugation, the corrugation induced acceleration of
the bogie will be detected by the FLAs. An inertial algorithm based on double integration and wavelet denoising method
is proposed to accurately estimate the rail corrugation. Field test is carried out on the Datong-Qinhuangdao Railway in
north China. The test results are compared with the results of a rail inspection car, which shows that the fiber laser
sensing system have a good performance in rail corrugation monitoring.
In this paper we repot the field test of an eight-element fiber laser hydrophone (FLH) array. The design of the fiber laser
hydrophone, the schematic of the array, the WDM based interrogator system, and the field test results are given. The
field test results show that the eight-element FLH array have equivalent performance as piezoelectric hydrophone array.
In this paper we report the earthquake monitoring using a fiber laser borehole seismometer. A fiber laser based
accelerometer is used in the seismometer to detect the earthquake induced acceleration. Due to the ultra-narrow line
width of the fiber laser (up to several kHz), an equivalent noise level of 50 ng can be achieved when using a fiber optic
interferometric method. The fiber laser borehole seismometer is installed in a 400 m depth hole in Yunnan Province. A
micro-earthquake (M=1.2) in Puer area is detected.
In this paper, we report the field test of a fiber laser geophone array. The field test is performed in a 3000 m depth well in
Liaohe Oil Field in northeast China. There are eight fiber laser geophones in the array with a space of ten meters. The
test results show good performance and reliability of the fiber laser geophone. This fiber laser seismic system is
promising in the in-well VSP system.
In this paper we repot a novel acoustic emission detection method using a fiber laser array. The ultra-narrow line width
of the DFB fiber laser will result in a low equivalent noise level when using a fiber optic interferometric demodulation.
A minimum detectable strain of 10-12 is achieved in the experiment. Three fiber lasers are used in the array and the
acoustic emission is detected. The result is compared with that of a piezoelectric acoustic emission sensor.
In this paper a push-pull fiber optic accelerometer (FOA) is proposed. Using two cantilevers, the accelerometer operates
in a push-pull configuration. The sensitivity is analyzed based on the theory of elasticity; the frequency response is
analyzed based on the theory of vibration. Experiment is carried out to test the performance of the fiber optic
accelerometer. The experiment results agree well with the theoretical result.
In this paper we repot the field demonstration of fiber laser seismic system. The structure of the fiber laser geophone, the
seismic towed array, the interrogator system, and the test results are given. The results show that the four-element fiber
laser seismic array have good performance.
A fiber laser hydrophone (FLH) based on a flat diaphragm and an L shaped lever is presented. This hydrophone uses an
L shaped lever to transfer the acoustic pressure induced defection of the flat diaphragm to the axial elongation of the
fiber laser. The curve where the L shaped lever contacts the diaphragm is a segment of an Archimedes spiral, which is
used to enhance the responsivity. To compensate the hydrostatic pressure, a capillary tube is fixed at the end of the
hydrophone. Both theoretical and experimental investigations are presented in this paper. The result shows that the
proposed design has a high sensitivity of a flat frequency response below 1 kHz.
A novel fiber laser vector hydrophone (FLVH) based on accelerometers is presented. Three fiber laser accelerometers
(FLA), perpendicular to each other, are used to detect the acoustic induced particle acceleration. Theoretical analyses of
the acoustic sensitivity and the natural frequency are given. Experiment shows a sensitivity of 0.1 pm/Pa@100 Hz is
achieved, which results in a minimum detectable acoustic signal of 100 μPa/@Hz@100 Hz. Field demonstration shows
that the proposed vector hydrophone has good directivity.
We demonstrate a high performance wavelength demodulator for DFB FL sensor using novel PGC algorithm and
reference compensation method (RCM). An unbalanced fiber Michelson interferometer is employed and the phase
demodulation is accomplished by a digital PGC-Arctan-DSM algorithm. The RCM is adopted to eliminate the
environmental noise using a shielded and multiplexed DFB FL as a reference. Experimental results have demonstrated a
resolution of 3.5×10-7 pm/√Hz, a dynamic range of 120 dB @ 100 Hz, a THD of below 0.1 %, and a linearity of 99.99 %.
More than 50 dB reduction of the environmental noise has been experimentally achieved within 10-1000 Hz.
This paper presents a two-axis fiber laser vector hydrophone which uses a V-shaped flexed beam to enhance the
sensitivity and reduce the dimensions. Theoretical analyses of the sensitivity and frequent response are given. The key
parameters that determine the sensitivity and resonant frequency are discussed. The experimental results show an
acceleration sensitivity of 39.2 pm/g and 53.2 pm/g at the x, y axis respectively, a resonant frequency of about 310 Hz,
and a directivity resolution larger than 20 dB.
A novel underwater fiber laser vector hydrophone is presented. Theoretical and experimental analyses are carried out to
test the performance of the hydrophone, which shows a sensitivity of 25 pm/g and a flat frequency response in the range
of 5 Hz~200 Hz are achieved. Field demonstration shows that the vector hydrophone has good directivity.
A novel underwater fiber laser geophone is presented. Theoretical and experimental analyses are carried out to test the
performance of the geophone, which shows a sensitivity of more than 30 pm/g and a flat frequency response in the range
of 5 Hz~200 Hz are achieved.ati
Accelerometers are often used in structural health monitoring, smart structures, and aircraft damage detection. In this
paper, an ultra thin fiber laser accelerometer (FLA) for structure vibration detection based on a flat diaphragm is
presented. This accelerometer uses an flat diaphragm to transfer the acceleration-induced displacement of the mass to the
axial elongation of the fiber laser. The flat diaphragm is clamped by the sensor shell to reduce the transverse sensitivity.
The interrogation of the fiber laser accelerometer is achieved by using phase generated carrier (PGC) demodulation. A
piezo-electric fiber stretcher in one of the unbalanced Mach-Zehnder interferometer (MZI) arms in the demodulator
induces a phase-shift carrier signal on the sensor output signals that enables passive recovery of dynamic phase-shift
information. This set-up uses a commercially available Dense Wavelength Division Multiplexer (DWDM) as a
wavelength filter at the output of the MZI to interrogate multiple sensors. Both theoretical and experimental
investigations are presented in this paper. The result shows that the proposed accelerometer has a high sensitivity and a
flat frequency response. Owing to the greater deformation of the diaphragm with a mass at its center, ultra thin
dimensions have been achieved.
A pressure-gradient fiber laser hydrophone (FLH) is demonstrated. Two brass diaphragms are installed at the end of a
metal cylinder as the sensing element. There are two orifices at the middle of the cylinder. This structure can work as a
pressure-gradient microphone in the acoustic field. Thus the DFB fiber laser fixed at the center of the two diaphragms is
elongated or shortened due to the acoustic wave. Theoretical analysis is given based on the electro-acoustic theory.
Experiments are carried out to test the performance of the hydrophone. A sensitivity of 100 nm/MPa has been achieved.
Furthermore, the hydrostatic pressure is self-compensated and a ultra-thin dimension is achieved based on the proposed
structure.
A fiber Bragg grating (FBG) pressure sensing scheme based on a flat diaphragm and an L-shaped lever is presented. An L-shaped lever transfers the pressure-induced defection of the flat diaphragm to the axial elongation of the FBG. The curve where the L-shaped lever contacts the diaphragm is a segment of an Archimedes spiral, which is used to enhance the responsivity. Because the thermal expansion coefficient of the quartz-glass L-shaped lever and the steel sensor shell is different, the temperature effect is compensated for by optimizing the dimension parameters. Theoretical analysis is presented, and the experimental results show that an ultrahigh pressure responsivity of 244 pm/kPa and a low temperature responsivity of 2.8 pm/°C are achieved.
A fiber Bragg grating (FBG) temperature insensitive packaging scheme based on a flat diaphragm and an L shaped lever
is presented. The curve where the L shaped lever contacts the diaphragm is a segment of an Archimedes spiral, which is
used to reduce the temperature responsivity. Because the thermal expansion coefficient of the quartz-glass L shaped
lever and the steel sensor shell is different, the temperature effect is compensated for by optimizing the dimension
parameters. Theoretical analysis is presented, and the experimental results show that a low temperature responsivity of
2.8pm/°C are achieved.
Polarization-induced phase noise in Michelson interferometer with imperfect Faraday rotator mirrors was investigated.
This kind of noise generates from the rotation angle errors of Faraday rotator mirrors and external polarization
perturbation. The conversion factor κ, representing the magnitude conversion ability from polarization-noise to
polarization induced phase-noise, have been theoretically evaluated and experimentally investigated.
A fiber Bragg grating (FBG) sensors network in the roadbed temperature monitoring system in Qinghai-Tibet railway is
presented. This sensor network is composed of FBG sensor chains embedded in the roadbed, slave optical cables, work
stations which include the FBG sensor interrogator and the optical router, master optical cables, and center workstation.
We give the primary experimental results of this sensor network in monitoring the roadbed temperature of the railway at
DK0985+000 near Chumar River. The temperature data was collected after a month when the temperature field in the
monitoring hole became stable. This experiment had been carried out for more than one year. The FBG sensor network
has successfully measured the temperature in the roadbed with an accuracy of 0.1 centigrade. Furthermore, the
experimental result shows the stability and reliability of the FBG sensor network in the harsh environment of Qinghai-Tibet plateau. The prospect of the FBG sensor network in monitoring other parameters in the Qinghai-Tibet plateau is
also presented.
In this paper a novel distributed fiber optic crack sensor with polymer encapsulation based on optical time domain
reflectometer (OTDR) is presented. The sensor head includes two layers of polymer and a bare fiber embedded in them.
The crack induced deformation in the sensor head will result in the bend loss in the fiber. Utilizing the OTDR
measurement, the position and the size of the crack can be determined. Two types of fiber arrangement are introduced in
this paper. Experiment was carried out to evaluate the performance of the distributed fiber optic crack sensor. The
primary results show that the fiber optic crack sensor has good linearity between the bend loss and the crack open size.
The crack size resolution is better than 1mm.
This paper aims at proposing a modified algorithm based on spectrum adjusting. The theoretical analysis of the error
between the real light source spectrum and the Gaussian spectrum is presented. The hardware of the demodulation
system is introduced, including the coupler, collimation lens, volume phase grating, focus lens, CCD array, A/D card,
and PC. The modified algorithm will adjust the interferometric spectrum of the sensor by getting rid of the effect of the
light source spectrum. Experiment was carried out to test the performance of the demodulation system. It can be found
from the result that the algorithm has improved the accuracy of the demodulation system significantly. The demodulation
accuracy for the strain sensor is better than 0.5 microstrain.
The research of fiber optic extrinsic Fabry-Perot interferometer (EFPI) sensors and their applications in concrete
structural health monitoring are presented in this paper. Different types of fiber optic EFPI sensors are designed and
fabricated. Experiments are carried out to test the performance of the sensors. The results show that the sensors have
good linearity and stability. The applications of the fiber optic EFPI sensors in concrete structural health monitoring are
also introduced. Ten fiber optic sensors are embedded into one section of the Liaohe Bridge in Qinghuangdao-Shenyang
Railway. Field test demonstrates that the results of fiber optic sensors agree well with conventional strain gauges.
In this paper, fiber Bragg grating (FBG) sensors network in the subgrade temperature monitoring system in Qinghai-
Tibet railway is presented. The principle of the FBG temperature sensor is introduced and the feasibility of the FBG
sensors in the permafrost regions of Qinghai-Tibet plateau is analyzed. The FBG sensor network architecture is
introduced. This sensor network includes the sensor chains embedded in the roadbed, slave optical cables, work stations
including the FBG sensor interrogator and the optical router, master optical cables, and center workstation. We give the
primary experimental result of this sensor network in monitoring the roadbed temperature of the railway at DK0985+000
near Chumar River. Fifteen FBG temperature sensors were embedded into the roadbed monitoring hole. The temperature
data was collected after a month when the temperature field in the monitoring hole is stable. The result of the experiment
shows that the FBG temperature sensors have an accuracy of 0.1 centigrade. Furthermore, the experimental result shows
the stability and reliability of the FBG sensor network in the harsh environment of Qinghai-Tibet plateau. In the end, the
conclusion is drawn and the prospect of the FBG sensor network in monitoring other parameters in the Qinghai-Tibet
plateau is also presented.
In this paper, a novel fiber optic concrete sensor based on extrinsic fiber Fabry-Perot interferometer (EFPI) is designed
and analyzed. Two fiber ends are inserted into a glass capillary and encapsulated into a cement cylinder to act as the
sensor head. In this way, the cement cylinder itself is the sensor head instead of the traditional steel tube, which makes it
very convenient to embed the sensor head into the concrete, because the cement consists with the concrete well. Based
on the theory of white light interferometry and the theory of elasticity, the wavelength modulation method and the strain
transfer are analyzed theoretically. The demodulation system is also introduced in this paper. The experiment being made
by our research group is aimed at testing the consistency, stability, reliability and the sensitivity of the fiber optic sensor.
The sensor head of the cement cylinder is embedded into a model ferroconcrete beam together with traditional strain
gauges. The experiment is carried out using the PEM-500A hydraulic pulsation fatigue test machine after 2 million stress
circles. The readout of the fibre optic sensor and the strain gauges is recorded and made a contrast. It can be found from
the result that the fibre optic sensors have good stability and reliability, the accuracy for the fibre optic sensor is better
than 0.5 micro-strain, which shows that the sensor can meet the demand of the long-term monitoring of large-size
concrete structure.
In this paper a novel fiber Bragg grating and extrinsic fiber Fabry-Perot interferometer (FBG/EFPI) sensor which can
measure the strain and temperature simultaneously in the smart materials is designed and analyzed. The FBG and EFPI
are encapsulated into a special stainless steel structure. The structure makes the FBG strain-sensitive. On the other hand,
the EFPI is insensitive to the strain but sensitive to the temperature. The strain sensitivity and the temperature sensitivity
are derived in details. Experiments show that the sensor has good performance and high accuracy (0.1°C for temperature
and 1.3pm/με for strain). In conclusion, the FBG/EFPI sensor is applicable for simultaneous measurement of strain and
temperature in smart materials and structure health monitoring.
Fiber optic Fabry-Perot sensor has been shown to possess high sensitivity among phase-modulated fiber optic sensors.
In the last decade, they have found numerous applications in industry, military, and civil engineering. But they are not
easy to manufacture. To avoid the difficult in fabricating the temperature sensor by embedding the gauge in to
temperature-sensitive materials, a novel Fabry-Perot sensor is designed and fabricated. The most influential factors of
the thermal sensitivity of the sensor are analyzed, which gives the guidelines in design of different sensitivity sensors.
Experiments were carried out to verify the theoretical analysis. And the results show that the sensors have good
performance and by changing the gauge length the sensitivity of the sensor can be easily controlled.
Pressure sensitivity of the fiber optic mandrel hydrophone is analyzed in this paper. Based on the theory of elasticity, the
mechanism of the pressure response is studied. The influence of the optical fiber on the compliant mandrel on the
pressure response is taken into consideration for the first time. The radial deformation of the mandrel under the pressure
of the fiber optic and the underwater pressure is analyzed in details. Based on the theory of photo-elasticity, the phase
shift of the Mach-Zehnder interferometer is given. The pressure sensitivity is evaluated both theoretically and
experimentally, and the results show a good correlation between the theoretical and experimental results.
Fiber optic sensors are widely used in structure monitoring and smart materials to measure the temperature. In this
paper, a novel fiber-mercury temperature sensor (FMTS) is designed and tested. The sensor is made using the surface of
the mercury to act as the reflector. The intensity of the reflective light is the function of the distance between the surface
of the mercury and the fiber end, which is determined by the thermal extension of the mercury. And different length of
the mercury in the capillary defines different sensitivity of the sensor. The modulation function of the sensor is analyzed.
Experiment was carried out to evaluate the performance of the sensor, and the results present a good correlation between
the theoretical and experimental result.
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