Study Of Mid-infrared Band Microsphere Cavity Laser

The Whispering gallery mode(WGM)in the microsphere resonator has the characteristics of very small model volume and particularly high quality factor(Q=1010).When WGM is used as the pump sourse,the power density(GW/cm2)is extremely high,the loss is very small,and the pump power threshold for generating laser can be very low.Due to the small size and facilitates integration of microcavity resonator,the research of microcavity resonator become a hotspot in recent years.The mid-infrared laser has application prospects in the fields of medical treatment,biosensing fields and so on.This paper researches 2 ?m-band microsphere lasers.Firstly,the whisper gallery model of microsphere cavity was described,and several physical parameters of the microsphere cavity were described:quality factor(Q value),free spectral range(FSR)and fineness(F).The optical field distribution of biconical optical fiber coupler and the coupling between the optical microsphere cavity and biconical optical fiber were analyzed.The method of preparing biconical optical fiber by the heating and stretching fiber and the Q value measure method of microsphere cavity were introduced.Secondly,several microsphere materials that suitable for mid-infrared lasers are introduced.Three common methods for preparation of doped microspheres in our research group are described in detail:the soaked-sintering method,the sol-gel method and the high-temperature melting method.In this paper,rare earth doped silica microspheres were prepared by sol-gel method.The process is simple and the preparation cycle is short.Secondly,the article describes the experimental scheme and experimental results of a Ho:Tm-codoped silica microsphere laser.A Ho:Tm-codoped active silicon dioxide layer was prepared by sol-gel method on the surface of silica microspheres.The average concentrations of holmium ions and thulium ions were 4 wt.%and 0.4 wt.%,respectively.The Q value of the silica microsphere is 8.06×105.Biconical optical fiber was used as an efficient coupler with rare-earth-doped silica microspheres.Under the CW 808 nm pump source,single-longitudinal mode and multi-longitudinal mode laser oscillation output were realized.The threshold of Ho:Tm-codoped silica microsphere laser(2010.67nm)is 3.1 mW and the slope efficiency is 0.022%.The output power of the single-mode laser is 7.50?W,the side-mode suppression ratio is 14.83 dB,and the full width at half maximum is 0.29 nm.The results show that the single-longitudinal mode and multi-longitudinal mode output of the laser oscillation is closely related to the coupling position between the tapered fiber and microspheres.Different coupling positions will generate single-longitudinal mode or multi-longitudinal mode lasers,when the pump power enhance,the stimulate more longitudinal mode will appear.The multi-longitudinal mode spectrum is modulated by the appearance of microsphere.The mechanism of the energy level transition of holmium ion and thulium ion emitting the 2 ?m band laser was analyzed.Lastly,we study of Tm-doped microspheres laser oscillation emitting near the 2 ?m wavelength.We theoretical analysis of the thulium-doped microsphere laser oscillation,then results in threshold formula and slope efficiency formula.Experimentally,thulium-doped microspheres were also prepared by the sol-gel method.Both 808 nm and 1550 nm laser pumped thulium-doped silica microspheres can achieve laser near 2 ?m,and their energy level transition mechanisms are analyzed.The 1550 nm wide-band light source was used to measure the resonance spectrum of the microspheres around 1550 nm.when tuning the wavelength of the tunable laser to each resonance peak,they all can generate 2 ?m laser.It was found that when turn the pump wavelength from the short-wavelength close to the resonance wavelength,as the pump wavelength gradually approached the resonance wavelength,the number of longitudinal modes gradually decreased and the intensity of longitudinal mode gradually increased,eventually only the single-longitudinal mode was left.The effect of different thulium doping concentrations of laser oscillation to generate laser was investigated.When the doping concentration of the thulium-doped microspheres reached or exceeded 40 wt.%,there is no laser appear due to the concentration quenching effect.Using a 1550 nm broadband light source to pump Tm-doped silica microspheres,2?m multimode spectrum was also obtained.