Study On Ultrasound Sensor Base On Hollow Core Photonic Crystal Fiber

Compared with traditional ultrasound sensors, optical fiber ultrasound sensors have attracted much attention internationally due to their advantages, including immunity to electromagnetic interference, corrosion resistance, light weight, easily multiplexing, high temperature resistance, etc. All fiber, miniature and intelligent ultrasound sensors will be the important ones in the ultrasound detection. As an important member of optical fiber ultrasound sensors, Fabry-Perot interferometric sensors are better than conventional ultrasound sensors in the aspects of resolution, accuracy and response frenquency, etc. Therefore, Fabry-Perot ultrasound sensors developed rapidly and have great prospect in many fields.The work is supported by National Natural Science Foundation (60537040). Based on the need of ultrasound sensor with superior performance, we proposed to detect the ultrasound field by in-line fiber optic Fabry-Perot sensor formed by hollow core photonic crystal fiber (HCPCF-ILFP) for the first time, which is a totally new sensor in the ultrasound detection field.The main contents are given as follows:I.The distribution of the ultrasound field on the central axis of the transducer, arbitrary plane and the cross section of sound axis are gained in theory by the use of the longitudinal wave sound field formula of the circular piston transducer. The simulation results show that the sound field is Gaussian distribution on the cross section perpendicular to the axis of the transducer. The ultrasound field on the central axis is oscillating largely inner the near field and monotone attenuated out of the near field. Therefore the sensor should be mounted out of the near field.II.Compared the temperature and strain characteristics of HCPCF-ILFP with that of fiber Bragg gratings (FBG), it is found that the former one has lower temperature sensitivity (HCPCF-ILFP: 0.9pm/℃,FBG:10.7pm/℃) and higher strain sensitivity (HCPCF-ILFP: 2.8pm/με,FBG: 1.3 pm/με), which illustrates HCPCF-ILFP is more suitable for ultrasound detection. The experiment system for ultrasound detection is setted up which included the system generating ultrasound, the lifting device of the fiber sensor, the holding device of the ultrasound transducer and the optical detection system. The experiment results show that HCPCF-ILFP could be used todetect the ultrasound and it has higher sensitivity than FBG. Othewise, the action mechanism of HCPCF-ILFP and FBG is investigated by the use of the reflective and refractive theory of the sound wave on the interface in the aspect of acoustic impedance of different medium.III.HCPCF-ILFP sensor combined with fiber Bragg grating pairs is proposed to optimize the HCPCF-ILFP sensor. By the use of transmission matrix method and the coupling theory of FBG, it is found that a resonant peak inner the reflection band of the FBG could be attained when HCPCF-ILFP cavity length, the central wavelength of the FBG and the length of the single mode between FBGs are mathed with each other. A resonant peak with 3 dB bandwith of only 28 pm is gained through the numerical simulation.