Real-Time Measurement and Uncertainty Evaluation of Optical Path Difference in Fiber Optic Interferometer Based on Auxiliary Interferometer.

Optical interferometers are the main elements of interferometric sensing and measurement systems. Measuring their optical path difference (OPD) in real time and evaluating the measurement uncertainty are key to optimizing system noise and ensuring system consistency. With the continuous sinusoidal wavelength modulation of the laser, real-time OPD measurement of the main interferometer is achieved through phase comparison of the main and auxiliary interferometers. The measurement uncertainty of the main interferometer OPD is evaluated. It is the first evaluation of the impact of different auxiliary interferometer calibration methods on OPD measurements. A homodyne quadrature laser interferometer (HQLI) is used as the main interferometer, and a 3 × 3 interferometer is used as the auxiliary interferometer. The calibration of the auxiliary interferometer using optical spectrum analyzer scanning and ruler measurement is compared. The evaluation shows that the auxiliary interferometer is the most significant source of uncertainty and causes the total uncertainty to increase linearly with increasing OPD. It is proven that a high-precision calibration and large-OPD auxiliary interferometer can improve the real-time accuracy of OPD measurements based on the auxiliary interferometer. The scheme can determine the minimum uncertainty to optimize the system noise and consistency for vibration, hydroacoustic, and magnetic field measurements with OPDs of the ~m level.

Strain and Temperature Sensing Based on Different Temperature Coefficients fs-FBG Arrays for Intelligent Buoyancy Materials.

The temperature and strain fields monitoring during the preparation process of buoyancy materials, as well as the health status after molding, are important for mastering the mechanical properties of buoyancy materials and ensuring the safety of operators and equipment. This paper proposes a short and high-density femtosecond fiber Bragg grating (fs-FBG) array based on different temperature coefficients fibers. By optimizing the parameters of femtosecond laser point-by-point writing technology, high-performance fs-FBG arrays with millimeter level gating length and millimeter level spatial resolution were prepared on two types of fibers. These were successfully embedded in buoyancy materials to achieve in-situ online monitoring of the curing process and after molding. The experimental results show that the fs-FBG array sensor has good anti-chirp performance and achieves online monitoring of millimeter-level spatial resolution. Intelligent buoyancy materials can provide real-time feedback on the health status of equipment in harsh underwater environments. The system can achieve temperature monitoring with an accuracy of 0.56 °C and deformation monitoring with sub-millimeter accuracy; the error is in the order of micrometers, which is of great significance in the field of deep-sea exploration.

Review of Random Fiber Lasers for Optical Fiber Sensors.

A random fiber laser does not need a traditional resonant cavity and only uses the multiple scattering of disordered media to provide feedback to achieve laser output. Therefore, it has the advantages of a simple structure, narrow linewidth, and low noise and is particularly suitable for fiber optic sensors. This paper provides an introduction to the categories and corresponding principles of random fiber lasers. The research progress of random fiber lasers in the sensing field in recent years, including various aspects of random fiber lasers as low-noise light sources or sensitive elements for fiber sensing systems, is the main focus. Finally, the future development trend of random fiber lasers for optical fiber sensors is explored.

Optical Fiber Magnetic Field Sensors Based on 3 × 3 Coupler and Iron-Based Amorphous Nanocrystalline Ribbons.

Optical fiber interferometric magnetic field sensors based on magnetostrictive effects have several advantages, e.g., high sensitivity, strong adaptability to harsh environments, long distance transmission, etc. They also have great application prospects in deep wells, oceans, and other extreme environments. In this paper, two optical fiber magnetic field sensors based on iron-based amorphous nanocrystalline ribbons and a passive 3 × 3 coupler demodulation system were proposed and experimentally tested. The sensor structure and the equal-arm Mach-Zehnder fiber interferometer were designed, and the experimental results showed that the magnetic field resolutions of the optical fiber magnetic field sensors with sensing length of 0.25 m and 1 m were 15.4 nT/√Hz @ 10 Hz and 4.2 nT/√Hz @ 10 Hz, respectively. This confirmed the sensitivity multiplication relationship between the two sensors and the feasibility of improving the magnetic field resolution to the pT level by increasing the sensing length.

Improved multi-channel interferometric fiber-optic sensor demodulation based on the Goertzel algorithm.

In a multi-channel interferometric fiber-optic sensor system using space-division multiplexing (SDM) and phase-generated-carrier (PGC) demodulation, the phase delay and phase modulation depth fluctuation of each channel will affect the amplitude consistency and harmonic distortion of the demodulation results. In this paper, an improved demodulation scheme based on the Goertzel algorithm is proposed to calculate the multi-channel phase delay and phase modulation depth and to compensate for their fluctuations simultaneously. First, the carrier's 1st to 6th harmonic amplitudes in the interference fringe are extracted using the Goertzel algorithm. Then, the phase delay is calculated using the real and imaginary components of the 1st harmonic amplitude. The phase modulation depth is calculated with a combinatorial operation of the 1st to 6th harmonic amplitudes. In addition, a reference channel is introduced to implement phase delay and modulation depth feedback control. The experimental results demonstrate that the improved scheme can effectively suppress the harmonic distortion and improve the amplitude consistency of multi-channel interferometric fiber-optic sensors with low resource consumption.

Fiber optic jerk sensor.

Jerk is directly related to a physical mutation process of structural damage and human comfort. A fiber optic jerk sensor (FOJS) based on a fiber optic differentiating Mach-Zehnder interferometer is proposed. It can directly measure jerk by demodulating the phase of interference light, which avoids the high-frequency noise interference caused by differentiating the acceleration. The sensing theory and sensor design are given in detail. The experimental and theoretical results agree, demonstrating that the FOJS has a high sensitivity, an ultralow phase noise floor, a wide measuring range, and good linearity. The impact test shows that the FOJS can directly measure jerk and has good consistency with a standard piezoelectric accelerometer. The FOJS has potential applications in earthquake engineering, comfort evaluations, and railway design. This is the first time that directly measuring jerk with an optical sensor is reported.

Narrow Linewidth Half-Open-Cavity Random Laser Assisted by a Three-Grating Ring Resonator for Strain Detection.

A stabilized narrow-linewidth random fiber laser for strain detection, based on a three-grating ring (TGR) resonator and half-open-cavity structure, is proposed and investigated experimentally. The half-open-cavity structure proved to provide double optical gain of erbium-doped fiber, which was beneficial to increase the photon lifetime as well as further narrow the linewidth. Meanwhile, the stability and frequency noise of narrow lasing output was improved by suppressing the competition-induced undesired residual random lasing modes with the TGR resonator. The TGR resonator is composed of a double-cavity fiber Bragg grating Fabry-Perot (FBG-FP) interferometer, a section of single-mode fiber, and a circulator. The specially designed double-cavity FBG-FP interferometer embedded in the TGR resonator acted as the strain-sensing element and improved the resolution of the dynamic strain. A stable ultra-narrow linewidth of about 205 Hz was obtained. The frequency noise was reduced to about 2 Hz/√Hz. A high dynamic strain measuring resolution of 35 femto-strain (fε)/√Hz was achieved.

Train-Induced Vibration Monitoring of Track Slab under Long-Term Temperature Load Using Fiber-Optic Accelerometers.

The train-induced vibration response provides a flexible solution for the real-time monitoring deformation of high-speed railway track slab in actual operation. This paper proposes a long-term real-time monitoring method for track slab deformation based on wavelet packet energy (WPE) using fiber optic accelerometers to record train-induced vibration. We found that the vibration response law of track slab deformation could be established by using the WPE of the frequency band covering the first- and second-order frequencies induced by the adjacent carriages. A field test was carried out for more than one year on the Beijing-Shanghai high-speed railway to investigate the train-induced vibration response law of track slab that was continuously deformed under a long-term temperature load. The maximum values of the WPE characteristic index appeared in winter and summer, and they were positively correlated with the temperature difference between the air environment and the track slab under the daily temperature load. These results were demonstrated to be consistent with the track slab deformation law for long-term and daily temperature loads. The novel method based on fiber optic accelerometers and WPE provides a new method for the long-term and real-time monitoring of track slab deformation.

Demonstration of multi-channel fiber optic interrogator based on time-division locking technique in subway intrusion detection.

Low noise, good consistency, and long-term stability are critical for a multi-channel fiber optic intrusion detection system. This paper proposes a low-noise 32-channel fiber optic interrogator based on phase generation carrier technique, which emphasizes on the analysis of key parameters related to system consistency and stability. A novel time-division locking technique of the carrier modulation depth and carrier phase delay is proposed. By locking the carrier modulation depth and carrier phase delay simultaneously, the consistency and stability of the multi-channel interferometric interrogator is significantly improved. The results show that in the multi-channel system, the noise level of each channel is below 12.6 ng/√Hz @ 5 Hz, and the noise level differences between each two channels are less than 2.6 dB, the noise level fluctuations of all channels are less than 1.4 dB over 24 hours. A subway intrusion detection is demonstrated in Shenzhen, China. Geological drilling events can be clearly recorded and identified.

High-resolution random fiber laser acoustic emission sensor.

A high resolution fiber-optic acoustic emission (AE) sensor using a random fiber laser (RFL) is proposed. The AE probe is undertaken by a random-gratings-based erbium-doped RFL. A narrow linewidth π-FBG is used as a wavelength locking and sensing element in the RFL. The random distributed feedback in RFL significantly extends the effective cavity length of the laser, thus reduces the thermal frequency noise of the laser and improves the resolution of AE signal. A narrow lasing operation with a 20 dB linewidth of ∼10.41 kHz and a frequency noise of ∼10 Hz/√Hz above 1 kHz is realized. The 3×3 coupler interrogation technique is used for signal demodulation. A high AE signal resolution of ∼280 fɛ/√Hz @ 1 kHz is obtained. To the best of our knowledge, this is the first time that RFL is used in the 3×3 coupler based AE demodulation scheme to improve the system resolution.

Broadband FBG resonator seismometer: principle, key technique, self-noise, and seismic response analysis.

A broadband optical fiber seismometer based on FBG resonator is proposed for earthquake monitoring. The principle and key technique, high-resolution ultralow-frequency wavelength interrogation by dual-laser swept frequency and beat frequency method, are discussed and analyzed. From the simulation and test results, the seismometer works at broadband range from 0.01 Hz to 10 Hz with a sensitivity of better than 330 pm/g and the wavelength resolution of the interrogation system is better than 0.001 pm/√Hz from 0.1 Hz to 10 Hz. A three-channel correlation method is used to measure the self-noise of the seismometer. It reaches a noise level of 2.7 × 10-7 ms-2/√Hz@0.1 Hz, which is lower than the earth's background noise (the new high noise model, NHNM). An earthquake monitoring experiment is conducted in a low noise seismic station. The recorded seismic waves are analyzed, which suggests that the proposed seismometer has the ability to record the close microearthquake and distant great earthquake with a high signal-noise ratio (SNR). This is the first time that a FBG-based middle-long period seismometer with lower self-noise than NHNM and large dynamic range (100 dB) is reported.

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 20 Hz/√Hz at 1 kHz. An ultrahigh dynamic strain resolution of 140 fε/√Hz at 1 kHz is achieved.

[A numerical simulation of dendritic cells migration and induction of T cell specific proliferation during the initiation of skin inflammatory].

Dendritic cells (DCs) are the most potent and specialized antigen-presenting cells (APCs) currently known, which play a crucial role in initiating and amplifying both the innate and adaptive immune responses. During the process of immune function, migration ability of DCs and the number of effector T cells which activated by DCs are closely related to the efficiency of immune function. However, because of the complexity of immune system, in the immune response process caused by the skin chronic inflammatory, much is still unknown about the dynamic changes of cell count with time. Therefore, we created a differential equations model to reflect the initial stages of the immune response process caused by the skin chronic inflammatory via setting the function and initial conditions of parameters. The results showed that the model was able to simulate migration and proliferation of cells in vivo within realistic time scales in accordance with the proliferation and migration efficiency in real terms. In addition, the preliminary model can biologically predict the realistic dynamics of DCs and T cells at different time points. All these results may provide a theoretical reference for studying the immune function of DCs as well as guiding the clinical treatment for immune related diseases further.

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.

A Suitable Option for Gustilo and Anderson Grade III Injury.

BACKGROUND The management of Gustilo and Anderson grade III injury remains difficult, particularly due to the incidence of wound infections, delayed fracture union, and traumatic extremity amputation. However, little data is available on delayed skin graft or flap reconstructions of Gustilo grade III injury, especially using new technologies of wound coverage, such as vacuum sealing drainage (VSD) combined with limited internal and/or external fixation. MATERIAL AND METHODS Between June 2008 and May 2013, we performed the VSD technique combined with limited internal and/or external fixation on 38 patients (22 males and 16 females, with a mean age of 36.5 years) with Gustilo and Anderson grade III injury. VSD was regularly changed and delayed skin grafts or flaps were used to cover the defect. Two patients were lost to follow-up, and the remaining 36 were available for evaluation. The complications, wound healing, infections, and bony union were assessed for a mean duration of 2.5 years (range, 1-4 years). RESULTS Complications were seen in 5 of the 36 cases: 2 cases had infection alone, 1 case had delayed union or nonunion, 1 case had infection and delayed union, and 1 case had wound necrosis, infection, and nonunion. VSD was regularly changed 2-6 times. Morphological appearance and functional recovery were satisfactory in all cases. CONCLUSIONS Using VSD before skin grafts or flaps coverage, combined with limited internal and/or external fixation, is a suitable option for Gustilo and Anderson grade III injury.

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.

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.

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.

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.

Gradient boosting DD-MLP Net: An ensemble learning model using near-infrared spectroscopy to classify after-stroke dyskinesia degree during exercise.

This study aims to develop an automatic assessment of after-stroke dyskinesias degree by combining machine learning and near-infrared spectroscopy (NIRS). Thirty-five subjects were divided into five stages (healthy, patient: Brunnstrom stages 3, 4, 5, 6). NIRS was used to record the muscular hemodynamic responses from bilateral femoris (biceps brachii) muscles during passive and active upper (lower) limbs circular exercise. We used the D-S evidence theory to conduct feature information fusion and established a Gradient Boosting DD-MLP Net model, combining the dendrite network and multilayer perceptron, to realize automatic dyskinesias degree evaluation. Our model classified the upper limb dyskinesias with high accuracy: 98.91% under the passive mode and 98.69% under the active mode, and classified the lower limb dyskinesias with high accuracy: 99.45% and 99.63% under the passive and active modes, respectively. Our model combined with NIRS has great potential in monitoring the after-stroke dyskinesias degree and guiding rehabilitation training.