Research On Ultrasonic Signal Sensing Technology Based On Micro - Nano Fiber Grating

With rapidly development of the technology in the area of marine resources exploration, the modern anti-submarine warfare, underwater weapon test and underwater acoustic communications, the underwater ultrasonic detection technology plays a more and more important role in these areas. Piezoelectric ceramic is mainly used as the sensing element in traditional underwater acoustic detection, but there are some inherent defects for piezoelectric ceramics, such as poor resistance to electromagnetic interference, with large volume and heavy weight, low dynamic measurement range, not suitable for long distance transmission and being mass arrays. The most important thing is that it is not suitable for the detection of weak acoustic signal because of its low sensitivity due to its noise. But optical fiber sensor has some advantages, such as high sensitivity, wide response frequency band and easy to be arrays and so on. Therefore, in the past 30 years, it made great progress of the optical fiber hydrophone technology. With the development of modern technology, the sensitivity of the sensors is becoming higher and higher to adapt to the complex detection environment.Micro-nano optical fiber grating is suitable for the detection of underwater acoustic signal because of its high sensitivity and its miniaturization structure. This thesis begin with the analysis of ultrasonic propagation characteristics and spectral characteristics of micro-nano optical fiber grating and the response characteristics for strain and temperature, and then analyzes the micro-nano fiber grating response characteristics to ultrasonic systematically. According to the high frequency characteristics of the ultrasonic, we demodulate the ultrasonic with laser matched filtering. Micro-nano optical fiber grating sensor with high sensitivity should possess the following factors:1) high reflectivity; 2) narrow 3-dB width; 3) large spectroscopy slope; 4) modest diameter of the micro-nano grating. In order to get the best sensitivity of micro-nano fiber Bragg grating to ultrasonic, we have experimentally obtained the best parameters of micro-nano optical fiber grating with the maximum sensitivity to ultrasound:the reflectivity is 97%, the 3-dB bandwidth is 0.295 nm, the diameter of the micro-nano grating is 40μm.