Fabrication, characterization and sensor applications of optical whispering gallery mode coupling system

Micro/nano optical whispering gallery mode (WGM) resonators have attracted tremendous attention in the past two decades 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 fabrications of high quality silica microsphere (50mum ∼500mum in diameter, quality factor ∼107-108) and submicron fiber taper, which are examined utilizing both optical microscope and scanning electron microscope. Various WGM spectra are recorded 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 are achieved experimentally.;Temperature measurement based on WGM resonance wavelength shift is studied in a range of cryogenic temperatures (∼110K) to near room temperatures (310K). The experimental results match with theory well. The ultra high temperature measurement resolution (potentially ∼10-6K) is discovered and discussed. Other unprecedented advantages of the sensor are also addressed. A vacuum chamber is fabricated to enclose the WGM system. WGM instability is characterized in vacuum, using reconstructed WGM spectra. A resonance wavelength shift noise level within 0.4pm is measured and analyzed. Gas phase molecule detection utilizing spectral shift of WGM is explored. A Mie theory analysis is carried out to interpret the experiment. Water vapor sensing is realized by SiO2 nanoparticle coating on the microsphere. High measurement resolution (1ppm H2O) in low humidity level (0%∼10%) is found possible. Finally, gas phase molecule detection utilizing cavity enhanced absorption spectroscopy of WGM is also addressed by illustrating CRD, Q-spoiling and dip depth variation. An experiment system is designed and initial test result provides insights of future endeavors.