Air Refractive Index Measurement Method Research Based On Fiber Fabry-Perot Cavity Slow Light

In many fields of high precision measurement, it is necessary to compensate the impact of air refractive index at any time. So we need to measure the air refractive index precisely. The resolution of air refractive index measurement can usually reach to 10"7 at present. In practical application, the system needs to achieve higher measurement resolution and also needs to have good instantaneity performance and stability. The current air refractive index measurement methods have the disadvantages of complex large volume, harsh experimental conditions demanding low precision and so on. Otherwise, Edlen formula method is easily influenced by air composition change. So it is hard to modify instantaneity measurement results.According to the special performance in sensing field of fiber Fabry-Perot cavity and emerging slow light technology application in the field of high-precision sensing, this thesis proposes a high accuracy measurement method of air refractive index using fiber Fabry-Perot cavity slow light based on vast literatures at home and abroad. Compared with other means of generating slow light, fiber Fabry-Perot cavity has characteristic of simple structure, ease of integration and controllability. Compared to other air refractive index measurement methods, the method has higher sensitivity and better stability. The research contents of this thesis mainly include:(1) Concluding and reviewing the development of air refractive index measurement methods and the slow light technology. Meanwhile, this thesis analyzes the advantages and disadvantages of various air refractive index measurement methods and different kinds of slow light generating methods. Finally, fiber Fabry-Perot cavity slow light technology is put forward to apply to the measurement of air refractive index.(2) In order to understand the essence of fiber Fabry-Perot cavity slow light principle, the basic Fabry-Perot cavity interference theory is analyzed. The Kramers-Kronig relation which reflects the dispersion properties is introduced. This thesis puts forward two kinds of theory to understand the fiber Fabry-Perot cavity slow light principle. The slow light group index and generating conditions are also thorough analyzed and the structure conditions of slow light of fiber Fabry-Perot cavity are obtained.(3) The air refractive index measurement system based on fiber Fabry-Perot cavity is designed. The influence of the introduction of slow light on the system’s sensitivity and measuring range is analyzed. The end surface reflectivity, cavity length and the incident wavelength is related to the slow light delay time and the bandwidth. The fiber Fabry-Perot cavity structure parameters are optimized and the suitable structural parameters for measuring the air refractive index are determined. Through the simulation analysis, the air refractive index measurement resolution of the designed system can be achieved theoretically 3×10-8 with the 90%-cavity end reflectivity, 100μm-cavity length and 1550.73 nm-incident wavelength. The method has greatly improved than before.(4) Using time domain measurement method, the 2.36ns delay time of fiber Fabry-Perot cavity slow light is observed. Then compared with theoretical analysis, theory correctness of the fiber Fabry-Perot cavity method of producing slow light is verified.The thesis provides a certain reference for the high precision measurement of air refractive index. It also offers a new way for the application of slow light which has a certain practical value. Compared with traditional methods, the design of the sensor overcomes some drawbacks. But there are still some disadvantages, such as surface roughness, the stability of the light source power and micro machining which needs a further consideration.