Optical Gains In Erbium-doped Whispering-gallery Microcavities

Whispering-gallery-mode(WGM)microcavities with ultra-high quality factors and small mode volumes plays an important role in both fundamental researches and practical applications.By doping active mediums into WGM microcavites,the performances of WGM microcavites can be further improved,which can be used in low-threshold on-chip microlasers,ultra-high sensitive detection and high performance optical devices.This paper focuses on erbium-doped WGM microcavities,which is fabricated by the sol-gel process.Since WGM microcavites can greatly enhance the interaction between erbium ions and optical field,pump power as low as several microwatts can make nearly all erbium ions located in metastable states,and greatly decrease laser threshold.Meanwhile,dy-namic gain and gain competition are both observed in erbium-doped WGM microcavites.This paper will do deep researches on the basic properties of optical gains in erbium-doped WGM microcavites,and the followings are the main works of this paper:1.The participation of building WGM microcavities laboratory,including the fab-rication platform and the optical measurement platform.Furthermore,we fabricated erbium-doped WGM microcavities using sol-gol process based on this platform.Quality factors of these active microcavites are nearly million orders,and laser threshold can be as low as several microwatts.Meanwhile,we investigated basic properties of optical gains under steady state situations,including populations of erbium ions located in different energy levels,lasing threshold,and lasing efficiency.2.The researches on the phenomenon of gain competition between different optical modes in erbium-doped WGM microcavites,and it can be used in the precise control of optical gains.Different from the method by changing pump laser used before,here we introduce another control signal into this system.Through tuning this control signal,transmission spectra of probe signal can exhibit the transition of Lorentz dip,Fano-like resonance and Lorentz peak,while the pump is kept unchanged.With the help of coupled mode theory and solving laser rate equations,we get time-dependent gains for these two signals.Further analyses point out that gain competition between these two signals plays an important role during this process.Gain competition can be used in precisely controlling optical gains with high stability because the pump laser is kept unchanged.3.The measurement of life time for erbium ions located in metastable states through the changes of optical gains for probe signal.Different from photoluminescence method to measure energy life time,here we introduce an probe signal into this system.When the pump laser are closed,optical gains for probe signal obey exceptional decay after the translation transformation,and the life time we measured here is 5.4ms.This probe signal is coupled into WGM microcavites through taper fiber coupling,which has higher pump and collection efficiency than previse method.Meanwhile,we also use optothermal scanning method to measure thermal relaxation time so as to avoid unwanted thermal squeezing or broadening.Thermal relaxation time of the sample we used is 0.33ms.Furthermore,we used optothermal scanning method to tune optical gains in erbium-doped WGM microcavites,and this method is robust to the fluctuations of pump laser.4.The fabrication of a microtoroidal resonator with picometer precesion in resonant wavelength.Here,we added a tuning reflow process after the fabrication of microtoroidal resonators.In this tuning reflow method,when microtoroid is exposed to carbon dioxide laser pulses,its resonant wavelength can have shift with picometer precision,and the direction and amount of this shift can be controlled by both the tuning relfow power and time.Meanwhile,quality factors of microtoroid are kept unchanged during this process.