Research on the Surface Subsidence Monitoring Technology Based on Fiber Bragg Grating Sensing

In order to monitor the process of surface subsidence caused by mining in real time, we reported two types of fiber Bragg grating （FBG） based sensors. The principles of the FBG-based displacement sensor and the FBG-based micro-seismic sensor were described. The surface subsidence monitoring system based on the FBG sensing technology was designed. Some factual application of using these FBG-based sensors for subsidence monitoring in iron mines was presented.

Fiber Laser Sensing System and Its Applications

An overview of fiber laser sensing is presented.The design and the characteristics of distributed feedback(DFB)fiber lasers for high performance sensing applications are described.Demodulation techniques based on unbalanced fiber interferometer are discussed,especially for the noise level,the dynamic range,and the crosstalk in dense-wavelength-division multiplexing.Finally,the fiber laser sensing system configurations and field demonstrations for different applications are illustrated.

Short Cavity DFB Fiber Laser Based Vector Hydrophone for Low Frequency Signal Detection

A short cavity distributed feedback （DFB） fiber laser is used for low frequency acoustic signal detection. Three DFB fiber lasers with different central wavelengths are chained together to make three-element vector hydrophone with proper sensitivity enhancement design, which has extensive and significant applications to underwater acoustic monitoring for the national defense, oil, gas exploration, and so on. By wavelength-phase demodulation, the lasing wavelength changes under different frequency signals can be interpreted, and the sensitivity is tested about 33 d13 re pm/g. The frequency response range is rather fiat from 5 Hz to 300Hz.

High-sensitivity FBG microphone with static pressure equalization and optimized response

We present a high-sensitivity fiber Bragg grating(FBG)-based microphone with a flat response and static pressure equalization. A very high dynamic sensitivity of the microphone is achieved by a pressure sensing structure based on a carbon fiber diaphragm. Static pressure equalization is realized by a balance structure with a capillary glass tube. The resonances of the sensor are suppressed by air damping structures, and a broadened flat response is achieved. An acoustic-solid interaction model was used to analyze the response characteristics of the microphone. An experimental prototype was produced and tested, and the results are well consistent with the design.The tested sensitivity was-10 d B re 1 pm/Pa from 10 Hz to 2.5 kHz with a fluctuation of less than ±1.5 dB.Combined with the phase-generated-carrier-based coherent detection scheme, the microphone can achieve a sound resolution of the milli-pascal level. The static pressure sensitivity is measured to be-0.27 pm/atm, which is 100 dB lower than the dynamic sensitivity.

Study on the Characters of Phase-Shifted Fiber Bragg Grating in Asymmetric Perturbation and Its Application in Fiber Laser Acoustic Sensor

A n-phase-shifted fiber Bragg grating theoretical model is established, and the effects of an asymmetric and symmetrical perturbation field on a phase-shifted fiber Bragg grating are investigated in this paper. The trends of wavelength shifting caused by effective refraction index of phase shift grating in symmetric and asymmetric acoustic field are investigated in detail. Then, the fiber laser acoustic sensors packaged in asymmetric and symmetrical structures are designed and tested, respectively. The results show that the acoustic response of the wavelength of the distributed feedback （DFB） fiber laser （FL） in an asymmetric packaging structure is much more sensitive than in that in the symmetrical structure. The sensor packaged in the asymmetrical structure has a better low frequency （0Hz-500Hz） performance and a higher sensitivity than that in the symmetrical structure, and the sensitivity is improved about 15dB in average and 32.7dB in maximum. It provides a new method to improve the sensitivity of the fiber acoustic sensor.