Dynamic Strain Resolution Research Articles

Acoustic emission sensing in fiber Bragg grating based on phase demodulation for material health detection

Acoustic emission detection is widely employed in the field of material health monitoring as an important non-destructive testing method. An acoustic emission sensing system based on phase demodulation in fiber Bragg grating is demonstrated in experiment to solve the problems existing in acoustic emission sensor, such as low sensor sensitivity and poor anti-interference capabilities. According to experimental results, the system can effectively detect both continuous and burst acoustic emission signals from 100 kHz to 700 kHz. The signal-to-noise ratio can reach 77 dB and the sensor phase sensitivity is 0.012 rad/με. The dynamic strain resolution of the system is 3.77 nε/√Hz. The system is also able to detect weak burst acoustic emission signals emitted during concrete damage, and the signal-to-noise ratio is 21.5 dB in the acoustic emission sensing in corrosion of reinforcement.

Hybrid B-OTDR/Φ-OTDR for multi-parameter measurement from a single end of fiber.

The multi-parameter sensing is in great demand for comprehensive recognition in various application scenarios. We demonstrate a distributed optical fiber sensor (DOFS) for simultaneous vibration and temperature/strain sensing based on the integration of Brillouin optical time-domain reflectometry (BOTDR) and phase-sensitive optical time-domain reflectometry (Φ-OTDR). A double heterodyne detection configuration is set up to receive the Rayleigh scattering and Brillouin scattering simultaneously, which can enhance the signal-to-noise ratio (SNR) and reduce the polarization fading noise for the Brillouin signal at the same time. Most components in the setup are multiplexed for the Brillouin and Rayleigh signals. The overall cost is much lower than the summation of the BOTDR and the Φ-OTDR setup. In experiments, the setup is capable of measuring temperature change near the end of the 49.9 km long fiber with 0.381 MHz measurement uncertainty and restore different vibration patterns correctly by demodulating the phase variation of Rayleigh scattering signal, owning a 20 m spatial resolution. The sampling rate of the vibration measurement is 1.8 kHz, and the achieved dynamic strain resolution is 1.235 nɛ/√Hz with 100 Hz vibration.

Algebraic solutions for the Fourier transform interrogator.

A new method for fast, high resolution interrogation of an array of photonic sensors is proposed. The technique is based on the integrated Fourier transform (FT) interrogator previously introduced by the authors. Compared to other interferometric interrogators, the FT-interrogator is very compact and has an unprecedented tolerance to variations in the nominal values of the sensors' resonance wavelength. In this paper, the output voltages of the interrogator are written as a polynomial function of complex variables whose modulus is unitary and whose argument encodes the resonance wavelength modulation of the photonic sensors. Two different methods are proposed to solve the system of polynomial equations. In both cases, the Gröbner basis of the polynomial ideal is computed using lexicographical monomial ordering, resulting in a system of polynomials whose complex variable contributions can be decoupled. Using an NVidia graphics processing card, the processing time for 1 026 000 systems of algebraic equations takes around 9 ms, which is more than two orders of magnitude faster than the interrogation method previously introduced by the authors. Such a performance allows for real time interrogation of high-speed sensors. Multiple solutions satisfy the algebraic system of equations, but, in general, only one of the solutions gives the actual resonance wavelength modulation of the sensors. Other solutions have been used for optimization, leading to a reduction in the cross-talk among the sensors. The dynamic strain resolution is 1.66 n ε/H z.

Fiber Bragg Grating Sensors for Dynamic Strain Measurements in Gasoline Direct Injectors

The strain in a gasoline direct injector was measured during a single injection process by using a fiber Bragg grating sensor installed along the axis of the injector stem in different operating pressure conditions. A fast FBG interrogator, based on linear transmission filters and direct detection, permits to measure dynamic strain up to 240 MHz frequency with a detection limit of 1.1 με and a dynamic strain resolution of 20.2 nε/√Hz. The FBG sensor was first calibrated for an internal pressure ranging from 50 to 250 bar and an energizing duration of 5ms; the FBG sensor allows to measure the static internal pressure of the injector inlet, which is linearly dependent on the FBG static strain, as well as the dynamic strain during the opening/spray/closing processes induced by pressure fluctuations, mechanical vibrations and the impact forces related to the injector needle bounces. The fast FBG interrogator, has been first validated by comparison with commercial interrogators, and then used to detect fast transients. The measured FBG strain variation provides important information on the dynamic behavior of the injector during the spray, confirmed by the analysis of the spray injection performed through a High Speed Video camera. The static and dynamic measurements demonstrate the effectiveness of the FBG sensors technology for monitoring the behavior of the injector and controlling the engine performance and emissions.

Improvement of Strain Measurement Accuracy and Resolution by Dual-Slope-Assisted Chaotic Brillouin Optical Correlation Domain Analysis

A dual-slope-assisted chaotic Brillouin optical correlation domain analysis (DSA-CBOCDA) technique is proposed to improve the measurement accuracy and resolution for static and dynamic strain sensing. By using the gain ratio obtained from two slopes of the Brillouin gain spectrum (BGS), the influence of random fluctuation in chaotic pump power on the measurement result is alleviated. Compared with the previous single-slope-assisted CBOCDA, the measurement accuracy of the DSA-CBOCDA is improved from 25.0 μϵ to 3.9 μϵ for the static strain, and from 28.5 μϵ to 6.2 μϵ for the dynamic strain. Meanwhile, the static strain resolution of 10 μϵ and dynamic strain resolution of 15 μϵ are verified in the experiment. Moreover, the dynamic strain measurement range of 800 μϵ for the proposed DSA-CBOCDA system is also experimentally confirmed, which is approximately fivefold larger than the current DSA-based Brillouin sensing system.

Multiplexable intrinsic Fabry-Pérot interferometers inscribed by femtosecond laser for vibration measurement in high temperature environments

This paper reports fabrication technique and demodulation algorithm developments to use multiplexable intrinsic Fabry-Pérot interferometer (IFPI) fiber sensor array for distributed vibration measurements at high temperatures. Using femtosecond laser direct writing scheme, IFPI array were fabricated through laser-induced Type II scattering points in single mode fiber cores. Reflection spectra of IFPI array were demodulated in real-time using modified Bunemen frequency analysis. The demodulation algorithm, which was implemented using a photodetector-array spectrometer, achieves 64-nε dynamic strain resolution at 1-kHz spectral acquisition rate and 2-kHz maximum vibration bandwidth. Performance and stabilities of IFPI sensor array were characterized from room temperatures to 800 °C. The static strain resolution was 0.6 με and the minimum detectable dynamic strain amplitude was 23 nε/√Hz at 800 °C. The multiplex performance was also tested by measuring dynamic strain in time domain through six IFPI sensor array fabricated in one fiber. This paper presents an integrated and low-cost sensing solution to perform distributed vibration measurements in harsh environments.

Silicon-microring-based interrogator for TDM-FBG sensors enabled by pulse compression.

We propose a silicon microring resonator (MRR)-based high-resolution interrogator enabled by novel slope-assisted filtering and pulse compression technology for time division multiplexed fiber Bragg grating (TDM-FBG) sensors. Golay coded optical pulses are launched into an FBG sensor array to produce a series of reflected pulses with different time delays due to various transmission distances. A tunable MRR filter is used for interrogation of all FBG sensors by direct detection and subsequent signal processing. As a proof of concept, a strain sensing experiment is conducted using a 2×1 TDM-FBG sensor array. The performance of the proposed interrogation system is evaluated experimentally, and the dynamic strain resolution can be as high as 20nε/Hz.

Integrated Dynamic Wavelength Division Multiplexed FBG Sensor Interrogator on a Silicon Photonic Chip

A wavelength division multiplexed fiber Bragg grating (FBG) sensor interrogator using an integrated unbalanced Mach–Zehnder interferometer followed by an arrayed waveguide grating on silicon-on-insulator platform is presented. The Bragg wavelength shift is determined through the phase-generated carrier demodulation technique, allowing accurate dynamic FBG interrogation. The performance of the proposed device was validated by comparing the measurements with a commercial FBG readout unit based on a spectrometer, serving as a reference. Experimental results demonstrate a dynamic strain resolution of 4.56 nϵ/√Hz and minimum detectable strain down to ∼0.49 μ ϵ at 1-kHz bandwidth.

Miniature interrogator for multiplexed FBG strain sensors based on a thermally tunable microring resonator array.

We present a high-resolution and miniature multi-wavelength Fiber Bragg Grating (FBG) interrogator based on a thermally tunable microring resonator (MRR) array. A phase detection method using dithering signals is exploited to generate an antisymmetric error signal curve, which is utilized for the feedback locking of the MRR with the FBG sensor. Dynamic strain sensing of both single FBG and multiple FBGs are experimentally demonstrated, with a dynamic strain resolution of 30 nε/√Hz over 100 Hz to 1 kHz. The proposed interrogator shows the great improvements in both resolution and wavelength accuracy compared with the reported MRR-based interrogators and is promising for scalable multiplexed sensing applications.

Ultrahigh resolution optic fiber strain sensor with a frequency-locked random distributed feedback fiber laser.

This Letter presents an ultrahigh resolution optic fiber strain sensor. A random distributed feedback fiber laser (RDFL) is locked by the proposed frequency-shift Pound-Drever-Hall technique, which tracks the resonant frequency change of a π-phase-shifted fiber Bragg grating. The random distributed feedback for the RDFL gives a Lorentzian envelope over the original laser frequency noise, which can suppress the thermally induced frequency noise. The frequency noise of the laser is reduced from about 100 to 20Hz/√Hz at 1kHz. An ultrahigh dynamic strain resolution of 140 fε/√Hz at 1 kHz is achieved.

Wavelength-scanning coherent OTDR for dynamic high strain resolution sensing.

Distributed vibration sensing in optical fibers opened entirely new opportunities and penetrated various sectors from security to seismic monitoring. Here, we demonstrate a most simple and robust approach for dynamic strain measurement using wavelength-scanning coherent optical time domain reflectometry (C-OTDR). Our method is based on laser current modulation and Rayleigh backscatter shift correlation. As opposed to common single-wavelength phase demodulation techniques, also the algebraic sign of the strain change is retrieved. This is crucial for the intended applications in structural health monitoring and modal analysis. A linear strain response down to 47.5 pε and strain noise of 100 pε/√Hz is demonstrated for repetition rates in the kHz range. A field application of a vibrating bridge is presented. Our approach provides a cost-effective high-resolution method for structural vibration analysis and geophysical applications.

Distributed dynamic strain measurement using optical frequency-domain reflectometry.

Distributed dynamic strain measurement based on optical frequency-domain reflectometry is proposed. The technique makes use of the wide scanning range of a tunable laser source in a short sweeping time, and subdivides the overall spectrum into narrower frequency windows. The advantage of subdividing the laser spectral range is to improve the measurement uncertainty induced by the laser wavelength difference between repeated scans. The noise-limited dynamic strain resolution is investigated experimentally, indicating that a minimum detectable strain is less than 200 nε for a spatial resolution of 20 cm. By measuring the subdivided spectral shifts in the time sequence along the sensing fiber, the dynamic strain can be properly quantified over a 30 m measurement range for a highest sampling rate of up to 50 Hz.

Integrated FBG Sensors Interrogation Using Active Phase Demodulation on a Silicon Photonic Platform

The authors experimentally demonstrate the feasibility of interrogating Fiber Bragg Grating (FBG) sensors using an integrated unbalanced Mach–Zehnder Interferometer and active phase demodulation on silicon-on-insulator platform. The use of an external arrayed waveguide grating at the output of the circuit allows the interrogation of multiple FBGs through wavelength division multiplexing. Signal processing employing the phase-generated carrier demodulation technique is used to extract the wavelength shift from the signal patterns, allowing accurate dynamic FBG interrogation. The performance of the proposed integrated FBG interrogator is validated by comparing it with a commercial FBG readout unit based on a spectrometer and used as a reference. Experimental results demonstrate a dynamic strain resolution of 72.3 nϵ/√Hz.

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.

High Performance Time Domain FBG Dynamic Interrogation Scheme Based on Pulse Coding

A novel method to improve the measurement range, resolution, and multiplexing capability of time division multiplexed (TDM) fiber Bragg grating (FBM) sensors, based on advanced cyclic pulse coding, is proposed and experimentally demonstrated. The mechanism of noise reduction by quasi-periodic cyclic coding is quantitatively demonstrated, pointing out significant improvement in dynamic strain resolution with respect to single pulse TDM FBG interrogation. The use of cycling pulse coding allows for enhanced dynamic performance of real time strain measurement with respect to other coding techniques. The proposed technique can also enhance the sensing range of hybrid optical fiber sensor systems in which continuous static temperature/strain distributions and discrete dynamic strain in specific critical points are simultaneously measured over the same sensing fiber.

Integrated hybrid Raman/fiber Bragg grating interrogation scheme for distributed temperature and point dynamic strain measurements

We propose and experimentally demonstrate the feasibility of an integrated hybrid optical fiber sensing interrogation technique that efficiently combines distributed Raman-based temperature sensing with fiber Bragg grating (FBG)-based dynamic strain measurements. The proposed sensing system is highly integrated, making use of a common optical source/receiver block and exploiting the advantages of both (distributed and point) sensing technologies simultaneously. A multimode fiber is used for distributed temperature sensing, and a pair of FBGs in each discrete sensing point, partially overlapped in the spectral domain, allows for temperature-independent discrete strain measurements. Experimental results report a dynamic strain resolution of 7.8 nε/√Hz within a full range of 1700 με and a distributed temperature resolution of 1°C at 20 km distance with 2.7 m spatial resolution.

A FBG sensor system with cascaded LPFGs and Music algorithm for dynamic strain measurement

A Mach-Zehnder (M-Z) interferometer formed by two cascaded long-period fiber gratings (LPFGs) serving as ‘many’ multi-channel edge filters is used to detect the wavelength shift, induced by measurand of fiber Bragg grating (FBG) sensors with high optical SNR of >60dB due to the use of a fiber ring laser, for the first time to our knowledge. Advantages of such a demodulation device are low-cost, compact, easy fabrication and implementation, good immunity of vibration and temperature, etc. In addition, in order to improve the resolution for dynamic strain measurement, a spectral analyzing algorithm called Music modern spectrum estimation is adapted. The experimental results show that a dynamic strain resolution of 0.1μɛ/Hz1/2 at 700Hz is obtained, which is 10 times improvement in strain resolution when compared with conventional FFT method.

Interrogation of fibre Bragg grating sensors using an arrayed waveguide grating

We experimentally investigate the use of an arrayed waveguide grating (AWG) to interrogate fibre Bragg grating (FBG) sensors. A broadband light source is used to illuminate the FBG sensors. Reflected spectral information is directed to the AWG containing integral photodetectors providing 40 electrical outputs. Three methods are described to interrogate FBG sensors. The first technique makes use of the wavelength-dependent transmission profile of an AWG channel passband, giving a usable range of 500 με and a dynamic strain resolution of 96 nε Hz−1/2 at 13 Hz. The second approach utilizes wide gratings larger than the channel spacing of the AWG; by monitoring the intensity present in several neighbouring AWG channels an improved range of 1890 με was achieved. The third method improves the dynamic range by utilizing a heterodyne approach based on interferometric wavelength shift detection, providing an improved dynamic strain resolution of 17 nε Hz−1/2 at 30 Hz.

Simultaneous multi-parameter monitoring using a serial fibre-Fabry-Perot array with low-coherence and wavelength-domain detection

A sensor multiplexing scheme for the simultaneous monitoring of dynamic strain and temperature is presented. The scheme combines low-coherence and wavelength-division signal processing with a serial array of fibre-Fabry-Pérot (FFP) interferometric sensors defined by in-fibre grating mirrors of matched Bragg wavelength. Heterodyne measurements of signal phase from a 39 cm long FFP interferometer result in a dynamic strain resolution of about at acoustic frequencies. Temperature measurements via the induced changes in Bragg wavelength have a sensitivity of about . The cross-talk between the strain and temperature channels is within the system noise.

All-fibre interrogation technique for fibreBragg sensors using a biconical fibre filter

A passive self-referencing all-fibre technique for Bragg wavelength shift detection using a biconical fibre filter is described. This filter, basically a tapered depressed-cladding fibre, exhibits a periodic spectral power dependence which permits direct tracking of wavelength shifts in a 20 nm unambiguous range. Dynamic strain resolution of 1.5 µstrain/√Hz has been achieved.