Liquan Dong

An event recognition method for fiber distributed acoustic sensing systems based on the combination of MFCC and CNN

Fiber distributed acoustic sensing (FDAS) systems have been widely used in many fields such as oil and gas pipeline monitoring, urban safety monitoring, and perimeter security. An event recognition method for fiber distributed acoustic sensing (FDAS) systems is proposed in this paper. The Mel-frequency cepstrum coefficients (MFCC) of the acoustic signals collected by the FDAS system are computed as the features of the events, which are inputted into a convolutional neural network (CNN) to determine the type of the events. Experimental results based on 2300 training samples and 946 test samples show that the precision, recall, and f1-score of the classification model reach as high as 98.02%, 97.99%, and 97.98% respectively, which means that the combination of MFCC and CNN may be a promising event recognition method for FDAS systems.

Environmental parameters monitoring using a single mode-bare core multimode-single mode optical fiber sensor

We report a single mode-bare core multimode-single mode (SBMS) optical fiber sensor, for environmental parameters in-situ monitoring. Compared with the reflection structure and the transmission structure of the SBMS optical fiber sensors, we found that the repeatability and stability results of the reflection structure were much better than that of the transmission structure’s. The principle of this experimental design is based on the optical fiber which can be transmitted by the outside temperature modulation. Because the parameters of the light transmitted in the single mode-multimode single mode (SMS) fiber structure are subject to the change of the external physical factors, this type of fiber optic temperature sensor is made by multiple modes of transmission in a multimode fiber. As the light source transmits a distance in the multimode fiber, it will change greatly, making the input light and the final output light produce a great difference. While the length of the bare core multimode fiber was around 8.1cm, the temperature cross-sensitivity of the device had good linearity and was around 0.009075nm/°C , from 50 °C to 350 °C. Furthermore, the increased temperature curve was almost coincided with the decreased temperature curve.

Precise sample preconcentration based on plasmon-assisted optical manipulation for a bead-based Raman biosensor

We developed a novel lab-on-a-chip device with the capability of rapidly pre-concentrating for Raman detection that use gold bead as the solid carrier of biomolecules. The device combines an array of patterned plasmonic surface (i.e. gold nano-ellipses), as the bead manipulation element. The purpose of gold bead manipulation is to provide sample pre-concentration in close proximity of the Raman detecting region. In the presence of an external uniform electric field, the gold ellipses create local electric field gradients (which is usually called hot spots) that capture the gold beads. The location of hot spots within a plasmonic nanostructure is polarization dependent, and inhomogeneous electric field between two adjacent nano-ellipses perpendicular to each other leads to highly unbalanced trap potential that give the chance of transferring trapped particles in a given direction through rotating the polarization. Nano-optical conveyor belts with staircase pattern of nano-ellipses were arranged with their terminus collected at detection area to gather biomolecules. With the capacity to transfer biomolecules precisely, our design offers an attractive scheme for rapid, high throughput and highly sensitive sensing of low abundance analytes.

Radiation resilient fiber Bragg grating sensors for sensing applications in nuclear reactor cores

This paper reports testing results of radiation resilient fiber Bragg grating (FBG) in radiation resistant fibers in the nuclear reactor core at MIT Research Reactor Lab. FBGs were fabricated by 140-fs ultrafast laser pulse using a phase mask approach. In-core test of fiber Bragg gratings was carried out in the core region of a 6-MW research reactor at temperature > 600°C and an average fast neutron (>1 MeV) flux >1×1014 n/s/cm2. First 100-day tests of FBG sensors shows less than 5 dB reduction in FBG peak strength after over 1×1020 n/cm2 of accumulated fast neutron dosage. To test temporal responses of FBG sensors, a number of reactor anomaly events were artificially created to abruptly change reactor power, temperature, and neutron flux over short periods of time. The thermal optical coefficients and temporal responses of FBG sensors are determined at different accumulated dosages of neutron flux. Results presented in this paper reveals that temperature-stable Type-II FBGs fabricated in radiation-hardened fibers could be used as sensors to perform in-pile measurements to improve safety and efficiency of existing and next generation nuclear reactors.

Monitoring and warning system of slope based on distributed fiber optic sensor technology

This paper illustrates the principle of several common distributed fiber sensing techniques, especially Brillouin optical time domain reflectometry (BOTDR) and optical time domain reflectometry (OTDR). By measuring the frequency shift of spontaneous Brillouin scattering light in fiber, BOTDR could simultaneously monitor both strain and temperature with a high spatial resolution. But the spontaneous Brillouin scattering signal is so weak that it has a high demand of the laser generator and the signal-to-noise ratio of the whole system. Therefore, the BOTDR system is usually too complex, expensive and difficult to be widely used. Unlike BOTDR, OTDR utilizes Rayleigh scattering to measure the loss of fiber. Rayleigh scattering signal is much stronger than spontaneous Brillouin scattering signal, thus OTDR system has the advantages of high sensitivity, long distance and relatively low price. These advantages make OTDR very suitable for wide application in the field of slope monitoring, especially in remote areas where the geographical environment is complex and are difficult for staff to stay. This paper designed and implemented a slope monitoring and warning system based on the technology of optical time domain reflectometry(OTDR). The test result shows that the system has high sensitivity, strong real-time and provides user friendly interface.

Study on the characteristics of microfluidic oscillator based on a new type of normally closed valve structure

This article introduced a new type of normally closed micro-valve (NCV) equipped with curved channels and trapezia-shaped valve seat, which could solve the issues of large area of dead zone, high threshold pressure and slow response speed of NCV in the current microfluidic oscillators. The new type of NCV has a three-layered structure with a top controlling layer, medium membrane layer and the bottom feedback layer. The membrane channel in the feedback layer is specifically deigned as curved to decrease the dead zone area. The valve seat is designed as a trapezoid to reduce the adhesion between the membrane and the valve seat and the threshold pressure of the NCV , and to improve the response speed of the system .The results of simulation study on COMSOL shows that this micro-valve structure reduces the NCV threshold pressure by 45% and the oscillation period of the micro-oscillator by 36%. This article conducted further simulation studies on the factors that influence the oscillation period of micro-oscillator. When enlarging inlet flow rate of micro-oscillator, lowering NCV threshold pressure and decreasing valve seat angle, the oscillation period would be reduced. Otherwise, the oscillation period will be increased.

Sensing with slow light in an active fiber Bragg grating

The phase-shifted sensitivity of an interferometer can be enhanced by increasing the group index. In this paper, we experimentally demonstrate a slow light sensor by placing an active fiber Bragg grating (FBG) in one arm of the Michelson’s interferometer. A 25 KHz AC voltage was applied to a piezoelectric (PZT) set nearby the active FBG. Once the wavelength is varied to near the FBG band edge, the maximum phase-shifted amplitude appears, which is about 1.8 rad and is 4 times greater than that when wavelength is near the center of the reflection band. The active FBG is pumped by a 980 nm laser diode, which can help us to stabilize the system works in the slow light regime to obtain the maximum phase shift. It provides a very simple approach to increase the phase-shifted sensitivity, which is likely to have important applications for strain and acoustic sensors.

Application of the probe pulse with ergodic SOPs in detecting multi-vibrations using POTDR

Polarization Optical Time Domain Reflectometer (POTDR) can be used to detect the external vibration information by measuring the change of state of polarization (SOP) of the Rayleigh backscattering along the sensing fiber. However, the traditional POTDR system is suffer from the false negative when the vibrations along the fiber are with the same frequency. In this article, we propose and numerically simulated a scheme of POTDR system which can be used to detect multi-vibration with the same frequency. By scanning the SOP of the probe pulse, a series of vibration spectra along the fiber are obtained. The sum of these vibration spectra whose amplitudes are theoretical immune to certain SOP of the probe pulse and birefringence of the sensing fiber is used to analyze the external vibration information. The proposed system with input SOPs of uniform distribution and random distribution are analyzed by numerical simulation, respectively. The misdiagnosis rate of detecting multi-vibrations with different frequencies are greatly reduced. In addition, multi-vibrations with same frequency detection system using POTDR is successfully achieved by analyzing the amplitude change of the add up spectra along the sensing fiber, which would greatly promote the development of POTDR.

Distributed dynamic stress sensor based on white light interferometer

In this paper, we proposed a distributed stress sensor based on white light interferometer. The measurement including two steps: firstly, the moveable mirror of Michelson interferometer scans to detect the interferogram, and the position of dynamic stress can be obtained from the interferogram. Secondly, the moveable mirror of Michelson interferometer adjusted to compensate the optical path difference generated in the polarization maintaining fiber, and the photodiode detect the interference intensity. By applying wavelet transform to the detect signal, the frequency of dynamic stress can be demodulate. In our experiments, the measurement errors for 100 Hz and 1 kHz sinusoidal stress are 0.26 Hz and 0.3 Hz, respectively. And 20 Hz - 60 Hz chirp signal is also measured successfully. In the end, the harmonics in the time frequency distribution image and the factors resulting in the measurement error are discussed in detail.

An MEMS optical fiber pressure sensor fabricated by Au-Au thermal-compression bonding

In this paper, an optical fiber Fabry–Perot (F-P) pressure sensor based on micro-electro-mechanical system (MEMS) techniques is presented. We use SOI wafer and Pyrex glass wafer with micro-circular shallow pit array to fabricate the sealed F-P cavity structure by employing Au-Au thermal-compression bonding technique which avoids the gas releasing due to chemical reaction during anodic bonding process. The loaded pressure on the silicon diaphragm is transferred to cavity length information and measured by using polarization low-coherence interference demodulator. The response range and sensitivity of this pressure sensor can be simply altered by adjusting the parameters of radius and thickness of silicon diaphragm. This batch fabrication process is helpful for keeping performance consistency of the sensors. Fabrication and experimental investigation of the sensors are described. Results show that the sensor exhibits a relatively linear response within the pressure variation range of 3-283kPa with a sensitivity of 23.63 nm/kPa and the repeatability of the sensor is about 0.119%F.S. Additionally, the temperature dependency is approximately linear with 1.7nm/°C from -20°C to 70°C.