Study On Pressure Tuning Characteristics Of Calcium Fluoride Resonator In Whispering Gallery Mode

The whispering gallery mode(WGM)resonator has been widely used in optical communication,sensing,laser,and quantum optics due to its high quality factor and small volume.With the development of micro/nano-optical technology,WGM cavity tuning technology has become an important means of controlling the optical field in micro/nano-optical devices.The accurate tuning of the resonant frequency of the WGM cavity is crucial for many applications and is the foundation of many techniques and research.This paper discusses the tuning technology of crystal resonant cavities.Among the existing tuning methods,thermal tuning is applicable to WGM resonant cavities of various materials and shapes,but has a slow response speed.Electro-optic tuning based on the Pockels effect has a fast response speed but is only applicable to crystals with inversion symmetry,and fluoride crystals cannot adopt this method.Pressure tuning is applicable to WGM resonant cavities of various materials,but the effect of refractive index changes on tuning coefficients needs to be considered for fluoride crystal resonant cavities.Therefore,based on pressure tuning,this paper focuses on the tuning technology of fluoride crystal resonant cavities and carries out the following work:The theoretical analysis method of various waveguide structures of calcium fluoride crystal material resonant cavities is studied,and the effects of deformation and elastic-induced refractive index changes are analyzed from two perspectives during the pressure tuning process.The deformation and stress distribution of the calcium fluoride crystal resonator are calculated by finite element simulation,and the pressure tuning coefficients of the calcium fluoride crystal resonator are calculated by the relevant theoretical model of pressure tuning,which are 873.8 MHz/MPa for TM mode and 1038.1 MHz/MPa for TE mode,respectively.The experimental results verify the theoretical calculations and confirm that the refractive index changes of the crystal need to be considered during the pressure tuning process of the crystalline resonator.Meanwhile,the pressure tuning coefficients of the resonator are measured,and the experimental results show good agreement with the theoretical values,with values of 888.2 MHz/MPa for TM mode and 1114.6 MHz/MPa for TE mode.The achieved tuning range is 939 MHz and 1286 MHz,respectively.The experimental measurements also show that the Q factor of the crystal resonator did not decrease during the pressure tuning process,and the maximum pressure capacity of the resonator used in the experiment is not less than 4.9 MPa.Based on the proposed pressure tuning system,a magnetic tuning system is designed,and the frequency response range of the pressure tuning system is successfully measured.The maximum response frequency of the pressure tuning system is demonstrated to be 1.2 MHz.The proposed magnetic tuning system has further application value,and can realize non-contact pressure tuning and magnetic sensing.