Researches On Ultra-Low Threshold Silica Microsphere Laser With Active Doping

Whispering Gallery Mode(WGM)microsphere cavity has triggered great interest in the past decades.The WGM in silica microsphere cavity has very high quality factor(Q)and relatively small Mode Volume(V).These two typical properties enable significant enhancement of the interaction between light and matter,making it an excellent cavity for achieving low threshold and narrow linewidth lasers,and for applications in the fields of lasers,cavity quantum electrodynamics,nonlinear optics,and high-sensitivity biosensing.In this thesis,a new method of doping rare earth ions into silica dielectric was explored,the WGM microsphere cavity with high quality factor Q was taken as the research object.By means of the transition and radiation process of rare earth ions between energy levels,an active microsphere was constructed.The experimental platform for cavity laser pumping,measurement and analysis is set up and a low threshold laser is realized.Finally,the applications of active microlasers in optical sensing are also studied.The main research contents of this thesis are as follows:1.Construction of optical coupling theory between tapered fiber and quartz microsphere cavityFirst,according to the classical electromagnetic field theory,the mode field distribution,characteristic equation and mode characteristics of the quartz WGM microsphere cavity were obtained by analyzing Maxwell equations.The mode characteristics include quality factor Q,mode volume V,free spectral range FSR and fineness F.Then the waveguide theory of the tapered fiber is constructed,the eigenequation of the waveguide mode of the fiber is derived in the cylindrical coordinate system,the cut-off wavelength and cut-off radius of the tapered fiber are analyzed,and the characteristics of the mode field distribution and energy distribution of the tapered fiber are discussed.Then,the optical coupling theory of tapered fiber and quartz microsphere cavity is constructed,the coupling characteristics of the two are analyzed,and the relationship between the normalized transmittance and the quality factor Q of quartz microsphere cavity under the conditions of under coupling,critical coupling and over coupling is derived.2.Drawing of tapered fiber and preparement of doped quartz microsphere cavityHydrogen-molten normal single-mode fiber method is used to fabricate the tapered fiber.Firstly,wire stripping pliers are used to strips the coating layer around the coupling area of optical fiber,and then the stripped fiber is heated over hydrogen flame and additionally stretched by a computer controlled translation stages.In order to fabricate the doped active microsphere cavity,Er3+ ion solution was firstly prepared by mixing Er3+ ion hydrate(Er(NO3)3·5H2O),H2O,and C2H6O,of which the solution oscillating was performed to uniformly dissovle Er3+ ions.The weight ratio of Er3+was 0.013:1.501:2.276.Then a CO2 laser with a working wavelength of 10.6?m was used to melt the ordinary single-mode quartz fiber suspended on a 3D translation stage which has aready been immersed in Er3+ ionic solution for a period of time.Finally,a microsphere cavity was formed with the help of surface tension.A coupling and pumping platform was built and the quality factor Q of erbium-doped microsphere cavity was measured to be?2.28×107.3.Research on active microlaser based on erbium-doped quartz microsphere cavityThe active microspheres doped with Er3+ ion and a diameter of about 150 ?m were pumped with a diode working at wavelength of 980nm.The laser output with a threshold of less than 3?W was measured at 1530nm?1560nm.And a very low threshold doped active micro laser was realized.4.Implementation of 2 ?m Erbium-Doped MicrolaserFirstly,the energy band structure of Tm3+ and the absorption and emission spectra of Tm3+ in the fiber are introduced.Inspired by the successful preparation of erbium-doped active microlasers,experiments on thulium doped lasers were also carried out in this thesis.TmN3O9·5H2O is employed for preparing Tm3+-doped ion solution,and the Quality factor of resonances in doped microsphere is measured to be?1.08×107.Thulium doped quartz microspheres were pumped by a single-frequency tunable external cavity s laser working at wavelength of 1550nm,lasing near 2?m was realized.5.Optical sensing of active microlasersFirstly,we theoretically studied the mode characteristics of two-dimensional WGM cavity with the finite element analysis method,and found that the interference source can effectively reduce the mode Q value and reduce the resonance mode with high Q value,and the laser excitation mode of active microlaser is predicted to be reduced under the same pumping condition,and can achieve single-mode lasing output.Based on this characteristic,the optical sensing of the micro laser is proposed concept.Then we use ordinary single-mode quartz fiber cones as interference sources to study the sensing characteristics of Er3+ ion-doped microsphere cavity laser and external cavity gain-type microsphere wave fiber laser,by adjusting the photon storage capacity of the resonant cavity and resonant mode,to achieve the mechanical control of multi-mode or single-mode laser output.