Fabrication, characterization and sensor applications of optical whispering gallery mode coupling system
Creators: name: Ma, Qiulin
Publisher: name: No Publisher Supplied
Year: 2010

description: Micro/nano optical whispering gallery mode (WGM) resonators based on total internal reflection have attracted tremendous attention in the past two decades in the area of cavity quantum electrodynamics, micro-lasers and micro-sensors, due to their distinct feature of high quality factor in small mode volumes. However, few studies have been done on temperature sensitivity and measurement of WGM, instability characterization of WGM resonance and gas phase molecules detection using WGM, all of which are explored in the present study. A complete analytical description of optical WGM resonance in micro spherical resonators as well as an analysis of optical coupling between fiber taper and micro spherical resonator is reviewed and discussed. Experimental systems and methods are developed for well controlled and efficient fabrications of high quality silica microsphere (50μm ~500μm in diameter) and submicron fiber taper, which are examined utilizing both optical microscope and scanning electron microscope. High quality WGM resonance (loaded quality factor~107-108) is obtained with microscale optical coupling. A cavity ring down (CRD) measurement system is designed and built for WGM photon life time measurement. Various WGM spectra are recorded to characterize the microscale system for size matching between microsphere and fiber taper. Free spectrum range of the resonance is experimentally verified. Switching between TE mode and TM mode coupling is demonstrated. Three different coupling regimes by leaving a micro/nano air gap between microsphere and fiber taper are achieved experimentally. Temperature measurement and sensitivity of WGM based on resonance wavelength shift are analytically and experimentally studied in a range of cryogenic temperatures (~110K) to near room temperatures (310K), utilizing the fabricated WGM coupling system. The experimental results match with theory well. The unique feature of ultra high temperature measurement resolution (potentially ~10-6K) is discovered and discussed. Other unprecedented advantages such as fast response, great integration capability and cryogenic temperature measurement are addressed. A vacuum chamber (~0.03torr vacuum level) is designed and fabricated to enclose the delicate WGM system. WGM resonance instability of the current coupling system (microsphere-fiber taper) is characterized in vacuum, using reconstructed WGM spectra obtained with the help of a fiber ring resonator (a Fabry-Perot type interferometer). A resonance wavelength shift noise level within 0.4pm is measured which is analyzed to result from the coupling contact instability. Gas phase molecule detection utilizing spectral shift of WGM based on coated microsphere is explored for the first time. A Mie theory analysis on the phenomenon is successfully carried out to interpret and direct the experiment. An example of water vapor sensing is realized by SiO2 nanoparticle coating on the microsphere. High measurement resolution (1ppm H2O change) in very low humidity level (0%~10%) is found possible which far exceeds previous studies based on the same coating. Finally, gas phase molecule detection utilizing cavity enhanced absorption spectroscopy of WGM is also addressed by illustrating three typical techniques, i.e., CRD, Q-spoiling and dip depth variation based on absorption spectral profile simulation. An experiment system is designed and initial test result provides insights of future endeavors.