Wenzhu Huang

Swept optical SSB-SC modulation technique for high-resolution large-dynamic-range static strain measurement using FBG-FP sensors

This Letter presents a static strain demodulation technique for FBG-FP sensors using a suppressed carrier LiNbO(3) (LN) optical single sideband (SSB-SC) modulator. A narrow-linewidth tunable laser source is generated by driving the modulator using a linear chirp signal. Then this tunable single-frequency laser is used to interrogate the FBG-FP sensors with the Pound-Drever-Hall (PDH) technique, which is beneficial to eliminate the influence of light intensity fluctuation of the modulator at different tuning frequencies. The static strain is demodulated by calculating the wavelength difference of the PDH signals between the sensing FBG-FP sensor and the reference FBG-FP sensor. As an experimental result using the modulator, the linearity (R2) of the time-frequency response increases from 0.989 to 0.997, and the frequency-swept range (dynamic range) increases from hundreds of MHz to several GHz compared with commercial PZT-tunable lasers. The high-linearity time-wavelength relationship of the modulator is beneficial for improving the strain measurement resolution, as it can solve the problem of the frequency-swept nonlinearity effectively. In the laboratory test, a 0.67 nanostrain static strain resolution, with a 6 GHz dynamic range, is demonstrated.

pi-Phase-Shifted FBG for High-Resolution Static-Strain Measurement Based on Wavelet Threshold Denoising Algorithm

We propose a high-resolution static-strain sensor based on a π-phase-shifted Bragg grating (π-FBG) combined with a wavelet threshold denoising algorithm. The Pound-Drever-Hall (PDH) technique based on a narrow-linewidth fiber laser is used to interrogate the sensor. A reference FBG-based Fabry-Perot interferometer (FBG-FP) is used to compensate the temperature and laser source frequency fluctuation. As the reflect spectrums of π-FBG have a higher SNR than FBG-FP, the SNR of PDH signals from FBG-FP and π-FBG is increased from 18.1 to 35.6 dB. Wavelet threshold denoising algorithm is used to further improve the SNR of the PDH signals (about 8 dB). As a result, a static-strain resolution of 1.0 nε is obtained in the laboratory test.

A Cross-Correlation Method in Wavelet Domain for Demodulation of FBG-FP Static-Strain Sensors

Static-strain can be detected by measuring a cross-correlation of two FBG-based Fabry-Pérot interferometers (FBG-FPs). This letter addresses applying the wavelet transform to cross-correlation processing in noise-contaminated FBG-FP reflection spectrums for crust deformation measurement. Since the wavelet transform has a unique feature of varying time-frequency resolution, cross-correlation processing in the wavelet domain is more robust with correlated noise and window size and is less sensitive to nonstationary data. We discuss the experimental results in respect of the factors that could influence the sensor resolution. In addition, a staticstrain resolution of 1.3 n\(\varepsilon \) , higher than conventional cross-correlation method is obtained in the laboratory test by using this technique.

Acoustic Emission Source Location Using a Distributed Feedback Fiber Laser Rosette

This paper proposes an approach for acoustic emission (AE) source localization in a large marble stone using distributed feedback (DFB) fiber lasers. The aim of this study is to detect damage in structures such as those found in civil applications. The directional sensitivity of DFB fiber laser is investigated by calculating location coefficient using a method of digital signal analysis. In this, autocorrelation is used to extract the location coefficient from the periodic AE signal and wavelet packet energy is calculated to get the location coefficient of a burst AE source. Normalization is processed to eliminate the influence of distance and intensity of AE source. Then a new location algorithm based on the location coefficient is presented and tested to determine the location of AE source using a Delta (Δ) DFB fiber laser rosette configuration. The advantage of the proposed algorithm over the traditional methods based on fiber Bragg Grating (FBG) include the capability of: having higher strain resolution for AE detection and taking into account two different types of AE source for location.

DFB fiber laser static strain sensor based on beat frequency interrogation with a reference fiber laser locked to a FBG resonator

We report on a high-resolution static strain sensor developed with distributed feedback (DFB) fiber laser. A reference FBG resonator is used for temperature compensation. Locking another independent fiber laser to the resonator using the Pound-Drever-Hall technique results in a strain power spectral density better than Sε(f) = (4.6 × 10-21) ε2/Hz in the frequency range from 1 Hz to 1 kHz, corresponding to a minimum dynamic strain resolution of 67.8 pε/√Hz. This frequency stabilized fiber laser is proposed to interrogate the sensing DFB fiber laser by the beat frequency principle. As a reasonable DFB fiber laser setup is realized, a narrow beat frequency line-width of 3.23 kHz and a high beat frequency stability of 0.036 MHz in 15 minutes are obtained in the laboratory test, corresponding to a minimum static strain resolution of 270 pε. This is the first time that a sub-0.5 nε level for static strain measurement using DFB fiber laser is demonstrated.

Diaphragm-Based Fiber Optic Fabry-Perot Accelerometer With High Consistency

Consistency is one of the most important parameters influenced on the practicability of sensors. A fiber optic Fabry-Perot accelerometer (FPA) with a diaphragm-mass-collimator (DMC) integrated structure to achieve high consistency is presented. This design makes the structure more compacts and the manufacturing process more controllable. Theoretical analysis is used to evaluate the performance of sensitivity, resonant frequency, resolution, and dynamic range. Four DMC fiber optic FPAs were manufactured, and experiments on several technical indicators were carried out. The experimental results are close to the calculated values and show that the maximum sensitivity difference of the DMC-FPAs is less than 0.38 dB (re: 0 dB = 1 V/g) in a frequency band of 10-125 Hz, and the maximum sensitivity fluctuation is less ±0.36 dB. The results verify the improvements on consistency, and the reasons for the good performances are discussed.

Detection of rail corrugation based on fiber laser accelerometers

Efficient inspection methods are necessary for detection of rail corrugation to improve the safety and ride quality of railway operations. This paper presents a novel fiber optic technology for detection of rail corrugation based on fiber laser accelerometers (FLAs), tailored to the measurement of surface damage on rail structures. The principle of detection of rail corrugation using double integration of axle-box acceleration is presented. Then we present the theoretical model and test results of FLAs which are installed on the bogie to detect the vertical axle-box acceleration of the train. Characteristics of high sensitivity and large dynamic range are achieved when using fiber optic interferometric demodulation. A flexible inertial algorithm based on double integration and the wavelet denoising method is proposed to accurately estimate the rail corrugation. A 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 has a good performance in monitoring rail corrugation.

A High-Resolution Demodulation Algorithm for FBG-FP Static-Strain Sensors Based on the Hilbert Transform and Cross Third-Order Cumulant

Static strain can be detected by measuring a cross-correlation of reflection spectra from two fiber Bragg gratings (FBGs). However, the static-strain measurement resolution is limited by the dominant Gaussian noise source when using this traditional method. This paper presents a novel static-strain demodulation algorithm for FBG-based Fabry-Perot interferometers (FBG-FPs). The Hilbert transform is proposed for changing the Gaussian distribution of the two FBG-FPs’ reflection spectra, and a cross third-order cumulant is used to use the results of the Hilbert transform and get a group of noise-vanished signals which can be used to accurately calculate the wavelength difference of the two FBG-FPs. The benefit by these processes is that Gaussian noise in the spectra can be suppressed completely in theory and a higher resolution can be reached. In order to verify the precision and flexibility of this algorithm, a detailed theory model and a simulation analysis are given, and an experiment is implemented. As a result, a static-strain resolution of 0.9 nε under laboratory environment condition is achieved, showing a higher resolution than the traditional cross-correlation method.

Rock mass acoustic emission detection using DFB fiber lasers

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.

Optoelectronic hybrid fiber laser sensor for simultaneous acoustic and magnetic measurement.

An optoelectronic hybrid fiber optic acoustic and magnetic sensor (FOAMS) based on fiber laser sensing is proposed, which can measure acoustic and magnetic field simultaneously. A static magnetic field signal can be carried by an AC Lorentz force, and demodulated in frequency domain together with acoustic signals. Some experiments of acoustic pressure sensitivity, magnetic field sensitivity, and simultaneous acoustic and magnetic measurement on a fabricated FOAMS were carried out. The acoustic pressure sensitivity was about -164.7 dB (0 dB re 1 pm/μPa) and the magnetic field sensitivity was 0.6 dB (0 dB re 1 pm/ (T•A)). The experiment of simultaneous acoustic and magnetic measurement shows that the detections of acoustic and magnetic field have little effect on each other in dynamic range and simultaneously measuring acoustic and magnetic field is feasible.