Research And Implementation Of High-Q Whispering Gallery Mode Optical Microcavities

Whispering Gallery Mode Optical Micro Cavity(WGMOMC)is of ultra-small mode volume and high quality factor(QF),enabling tremendous potential application in many aspects,e.g.optical sensing,optical telecommunication and low threshold laser source.And all these superiorities attract researchers intensive attentions from different scientific communities.In the aforementioned characteristics,achieving high QF is a key parameter to increase sensitivity of sensors,lower laser threshold value,enhance various optical effects.This work focuses on how to realize WGMOMC with high QF,detailed in-depth theoretical and experimental study is conducted on two typical WGMOMC,SOI waveguide circular resonator(WGCR)and silicon oxide based Hollowed micro bubble cavity(HMBC).SOI WGCR is of high integration and,compatible with CMOS processing.However,limited by the bottleneck of fabricating technique,some roughness does exist on the surface of the waveguide.Surface roughness(SF)drastically increases the scattering loss of waveguide(WG),resulting in the decrease of quality factor(QF)of resonator cavity.Meanwhile,silicon oxide based HMBC can achieve high QF with simple fabrication technique.However,it is still remained to be found an accepted method to fabricate micro bubble resonant cavity(mBRC)with symmetrical and uniform thickness.In this work,we carried out both theoretical and experimental study to resolve the mentioned problems that two optical cavities are facing.The major contects of the thesis are listed as follows:1.Theoretical analysis on propagation loss of optical waveguide(WG).A high-precision modeling and analyzing method based on Fourier transformation and Finite Differentiation Time Domain(FDTD)is proposed.The surface roughness of WG is modeled and analyzed based our algorithm.Impacts of correlation length and root mean square(RMS)roughness on the loss of propagation in straight WG and resonating characteristics of WGRC,are fully investigated.Experimental testing data are compared to simulation results.Consistent results demostrate the correctness of proposed models and analytic methods.2.Investigations on characterizing method of WG surface morphology are carried out.With comprehensive comparison,some existing difficulties in terms of measurements in characterizing sidewalls morphology are indicated.Specific analysis is conducted on mechanism of surface smoothing technique utilizing hydrogen annealing.Particularly,the testing platform of hydrogen annealing is established.With the equipment,high temperature annealing is conducted on nano-WG.It is observed from AFM testing results that the surface roughness(SF)of WG decreased from 1.75 nm to 0.59 nm,which verifies the validity of hydrogen annealing in smoothing technique.3.Aiming at resolving drawbacks and current problems during fabrication,a low cost and high pressure gas assisted electrical arc discharging method is proposed,specializing in fabricating ultra-thin and hollow optical micro bubble.Several processing parameters,e.g.,weak discharging,release pressure,are adopted in our experiments.An symmetrical micro bubble resonant cavity mBRC(approximately 2?m)with uniform thickness is eventually fabricated,the Q value of which is as high as 107.4.Focusing on mechanical characteristics of micro bubble,we developed an analytic way in commercialized software,ANSYS,based on the basic principle of pressure sensing of optical micro bubble.The relationship between filled gas pressure in micro bubble cavity(MBC)and the deformation of micro bubble is experimentally performed and discussed in detail by two means: FP interferometry and cone optical coupling,respectively.The results of which demonstrated the possibility of MBC served as pressure sensors with high sensitivity.In this study,the key problem of how to effectively enhance QF of WGMOMC is successfully tackled.Major contributes is devoted in the following two aspects: firstly,the impact of surface roughness on waveguide transportation characteristics is theoretically revealed by simulation.Experimentally,surface roughness of waveguide is significantly declined by hydrogen annealing.Scattering loss on waveguide surface is decreased while the QF is substantially enhanced.Secondly,in-symmetric structures and non-uniform in thickness during fabrication is also resolved.As a whole,the results could provide theoretical basis and technical supports in fabricating WGMOMC with high QF.This investigation could further promoting the application and development on micro optical cavity.