Study On The F-P Interferometric Fiber Sensor Based On Special Optical Reflective Diaphragm

Fiber-optic Fabry-Perot interferometric(F-P)sensor has good performance in the measurement of physical parameters such as pressure,temperature,humidity,ultrasonic,electromagnetic field and so on.Significantly,F-P sensor is widely used in medical detection,submarine detection,structural health monitoring and other industrial and military fields.It's noted that,the optical reflection diaphragm structure is the main factor to influence the sensitivity of the sensor.At the same time,many problems exist in the sensor of closed-cavity structure such as the drift of linear working point and the limited bear pressure.In this paper,three kinds of fiber-optic F-P sensors with reflecting diaphragms in different structures are proposed and the influences of pressure and ultrasonic on these F-P sensors are investigated.Due to the limitations of the fabrication process,many sensors have a relatively thick reflective diaphragm,which decreases the sensitivity of sensor so that it's difficult to detect high dynamic ultrasonic signals.In order to improve the ultrasonic sensitivity of the sensor,this paper presents a closed-cavity fiber F-P sensor.The thickness of the reflective silver film is 200 nm and the diameter of the sensor is 125 ?m.As the reflective diaphragm is very thin,the sensor has high sensitivity and high response bandwidth to the ultrasonic signals,where the sensitivity of the sensor to the static pressure can reach 4.944 nm/kPa.The proposed sensor can be used as fiber optic microphone,ultrasonic detector or pressure measuring instrument.For the of the working point drift and pressure limited in the closed-cavity fiber F-P sensor,an open-cavity fiber F-P sensor is proposed in this paper.The reflective diaphragm of the sensor is fabricated by silver film with a fan-leaf structure including three micropores to keep the pressure balance between inside and outside of the cavity which ensure the sensor can work well under the condition of high pressure.Experimentally,the atmosphere pressure sensitivity of the sensor is 0.00411 nm/kPa and the working point is stable and has a high sensitivity to ultrasound in the underwater ultrasonic detection.The sensor is successfully solved the defects of the closed-cavity sensor.The sensor can be used in high pressure environment for gas pressure,gas concentration measurement or in many important applications of military field,for example the submarine in the deep sea for ultrasonic detection.For the problem that the sensitivity of the sensor is restricted by the reflection diaphragm,an optical fiber F-P sensor with gas-liquid interface film is proposed in this paper.The sensor has a simple structure and consists of a single-mode fiber and a fiber-optic tube.When the sensor is immersed in the liquid,the end surface of the optical fiber capillary forms a gas-liquid interface,which has a very high sensitivity to the pressure.In order to increase the stability of the sensor,we let the inwall of the fiber capillary was hydrophobic and oleophobic.The static pressure sensitivity of the sensor is 29.98 nm/kPa,and it has a good response to ultrasound.The sensor can be used as a high-precision pressure or level sensor.Due to the all-fiber structure of the sensor,it can be used in in a strong electromagnetic environment for ultrasonic detection such as embedded in the oil-immersed transformer box for on-line detection.The research work has provided important ideas for the development of fiber optic F-P sensors,and has important guiding significance for the application of optical fiber sensing technology in the field of ultrasonic detection and pressure measurement.