Gas Sensor Research Based On Microsphere Cavity With High-Q Factor

Ultra-high quality factor optical micro-resonators with Whispering Gallery Mode (WGM) and small mode volume become powerful tools to explore non-linear optics and cavity quantum electrodynamics(CQED). microsphere cavity is appyied in sensor field due to its internal interactions with the or interaction with external factors such as frequency or spectral changes.Generally speaking, many external factors can affect microsphere , such as changing the distance from coupling device to the cavity, or making small objects close to the microsphere. the deformation of the sphere and refractive index changes can affect Cavity's optical path from internal way. it can greatly improved accuracy and sensitivity greatly improved of bio-sensors, temperature sensor and acceleration sensor .The main contents and key points of this thesis are as follows:Firstly,We had a brief review on the history of microsphere resonance in the theoretical and experimental studies .Then we introduced simply the applications of microsphere.Secondly, fusion method is used to fabricate microsphere by laser with wavelength of 10.6 um. A variety of diameter of microsphere were fabricated by controlling the processing time and temperature through modulating the laser pulse. Diameter of microspheres were obtained by measurement software which comes from CCD.so it can meet the requirements of quantitative analysis of the experiment.And we have used RS3100 nano-fiber taper drawing machine to gain nano-fiber taper with 0.5μm -3μm for coupling test.. Low loss biconical fiber was fabricated by melt-pulling method experimently. As the important factors in obtaining the adiabatic taper fiber, pulling length, speed and Hydrogen flow were discussed in detail. Experiment shows that, pulling length with 30mm-32mm, speeds with 0.15mm/s, and Hydrogen flow with 140-150 are tolerant and efficient. With these fabrication parameters, a biconical fiber with 0.5dB-0.7dB loss, 0.6μm-2.5μm taper diameter is obtained. Besides, using the taper fiber to couple the optical model of micro sphere, the transmission spectrum are achieved.Finally, theory of coupling is introduced ,and various factors affecting the Q of optical microsphere cavities were analyzed theoretically and experimentally. and experimental system is designed to test humidity changes on Q values in the external environment。whispering gallery modes (WGM) of microsphere with a diameter of 260μm are achieved through coupling a tapered fiber. Experiments demonstrated that the water and dust around the microsphere cavity have great effects on the Q, which were mainly the optical absorption loss of water and scattering loss of dust, leading to 1-2 orders of magnitude lower of Q. It can provide a new way to detect air quality because microphere cavity is very sensitive to small changes of the external factor.