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The quick and accurate picking of the first arrival on microseismic signals is one of the critical processing steps of microseismic monitoring. This study proposed a first arrival picking method for application to microseismic data with a low signal-to-noise ratio (SNR). This approach consisted of two steps: feature selection and clustering. First of all, the optimal feature was searched automatically using the ReliefF algorithm according to the weight distribution of the signal features, and without manual design. On that basis, a k-means clustering method was adopted to classify the microseismic data with symmetry (0–1), and the first arrival times were accurately picked. The proposed method was validated using the synthetic data with different noise levels and real microseismic data. The comparative study results indicated that the proposed method had obviously outperformed the classical STA/LTA and the k-means without feature selection. Finally, the microseismic localization of the first arrivals picked using the various methods were compared. The positioning errors were analyzed using box plots with symmetric effect, and those of the proposed method were the smallest, and stable (all of which were less than 0.5 m), which further verified the superiority of this study’s proposed method and its potential in processing complicated microseismic datasets.

A ring cavity fiber laser was proposed by using graphene as a saturable absorber for fiber optic communication and material processing.The principle how the grapheme could generate pulse trains and the laser output characteristics were studied experimentally.Then the laser induced deposition method was used to transfer the graphene to the fiber end face and place it in the ring laser cavity.By taking a 974 nm semiconductor laser as the pumping source and the Er3+-doped fiber as the gain medium,the laser generated stable pulse trains by changing the orientations of a polarizer above the threshold pump power.The experiment shows that the center wavelength of laser is 1 560.1 nm with a full width at half maximum spectrum of 0.27 nm and the pulse repetition rate is 7.89 MHz with a pulse width of 14.7 ps.It means that the graphene is expected to replace single-walled carbon nanotubes in pulsed laser applications as a novel mode-locked material because of its good saturable absorption properties and a higher damage threshold.

Long Period Fiber Gratings(LPFGs) were inscribed by using a 800 nm femtosecond laser and their high-temperature properties were verified in this paper.Firstly,an inscription system for the laser into fiber cores was designed using a camera and an electric moving platform.With controlling a pulse laser of 1.3 mW,a LPFG was inscribed in a non-hydrogen-loaded bare fiber using point by point technique,and the resonant peak of the LPFG is about-17 dB in the 1 200~1 700 nm band.Then,high temperature sensing experiments were performed on the LPFG by a high temperature box,obtained results show that the linearity of the resonance peak changes is about 0.992 and temperature sensitivity is 0.056 nm/℃ at 300-800 ℃.It demonstrates that the inscription technology for LPFGs by using the 800 nm femtosecond laser is feasible and obtained LPFGs have good response characteristics and stability,and are suitable for the high-temperature measurement.

Based on a wavelength scanning technique and the characteristics of ultra-narrow-linewidth laser,a kind of cavity enhanced absorption gas detection system with high sensitivity is designed.A semiconductor laser with ultra-narrow-linewidth and tunable wavelength is used as a light source,and an optical cavity which consisted of two mirrors with high reflectivity is also used as an absorption cell in this system.The laser frequency overlaps with one of cavity modes via scanning the cavity length and the input laser is switched off by using the laser detuning technique.Then,the ring-down detection of acetylene with low concentration is achieved.With the cavity enhanced absorption technology,spectra of acetylene near the region of 6 518.824 cm-1 and laser ring-down time at different concentrations are measured and also analyzed.The experiments for concentration measurement indicate that the relative error is less than 2.5%,relevance coefficient is 0.999 and the lowest detection limit is 2×10-6.By filling a volume of acetylene into the gas cell gradually,the system dynamic response time is less than 10 s.Obtained results show that the system has advantages of good accuracy,high sensitivity and quick response,which can be used in the fault gas on-line monitoring for power transformers in real time.

On the basis of the wavelength scanning and spectrum modulation techniques of an ultra-narrow-linewidth semiconductor laser,a kind of multi-component gas online detection system with high precision was designed.A semiconductor laser with ultra-narrow-bandwidth and tunable wavelength was utilized as a light source and a novel series gas cell was also used to compress the horizontal space and increase the optical path of the system.By taking the sawtooth signal to modulate the spectrum of laser and using space division multiplexing(SDM) and time division multiplexing(TDM) techniques,the detection of multi-component gases(CO,CO2,CH4) based on ultra-narrow-linewidth laser characteristics was achieved.The experiments indicate that the relative measurement error is less than 2% and linearity relevance coefficient is 0.99 for the three kinds of gases.Furthermore,the system dynamic response time is less than 10 s by filling a volume of multi-component gas into the gas cell gradually.Finally,the result of long time stability monitoring shows that the maximum concentration fluctuation is less than 0.02%.The system has advantages of high accuracy,good stability and quick response,and is suitable for the fault gas monitoring of power transformers in real time.

This paper studies the relationship between the fusing characteristics of copper wire and the overload current. Based on a physical model, the fusing curve for the copper wire is obtained. At the same time, fiber Bragg grating (FBG) is used to monitor the temperature change of aerospace conductor. According to the experimental result, FBG can detect temperature in time, and current and fusing state can be predicted by the temperature and the rate of temperature rise. So FBG is suitable for the aerospace wire fault monitoring.

Optical fiber grating (FBG) has been widely used in the measurement of parameters such as temperature and strain. However, FBG is too slim to broken, whose outside protective layer tends to shedding easily, and it is also hard to change the temperature and strain sensitivity. In order to overcome the above disadvantages and to further expand the application range of FBG, this paper improves the technology of fiber grating metal film plating process firstly. It adopts a compositive method including chemical plating and electroplating to gild FBG, copper FBG and galvanize FBG, which all get good metal coating. Then, the temperature and strain sensing properties of metalized FBG is studied in detail. Multiple metal coating FBGs were put in high-low temperature test-box together, and then the test-box worked continuously at the temperature range of 0°C～95°C. After several experiments, it concludes that metal plating enhances the temperature sensitivity of fiber grating, and the one with galvanization has the highest temperature sensitivity of 0.0235. At last, FBGs with various cladding were pasted on carbon fiber cantilever beam respectively and the pressure on the top of the cantilever increased gradually. The experimental results show that wavelength of fiber grating shift toward the long wavelength with the increase of the pressure, and the one with galvanization has the maximum strain sensitivity which has minimal impact on fiber properties.

This paper presents a novel fiber-Bragg-grating- (FBG-) based system which can monitor and analyze multiple parameters such as temperature, strain, displacement, and seepage pressure simultaneously for forecasting coalmine water inrush disaster. The sensors have minimum perturbation on the strain field. And the seepage pressure sensors adopt a drawbar structure and employ a corrugated diaphragm to transmit seepage pressure to the axial strain of FBG. The pressure sensitivity is 20.20 pm/KPa, which is 6E3 times higher than that of ordinary bare FBG. The FBG sensors are all preembedded on the roof of mining area in coalmine water inrush model test. Then FBG sensing network is set up applying wavelength-division multiplexing (WDM) technology. The experiment is carried out by twelve steps, while the system acquires temperature, strain, displacement, and seepage pressure signals in real time. The results show that strain, displacement, and seepage pressure monitored by the system change significantly before water inrush occurs, and the strain changes firstly. Through signal fusion analyzed it can be concluded that the system provides a novel way to forecast water inrush disaster successfully.