Analysis and optimization of optical frequency comb spectraof magnesium fluoride microbottle resonator

Optical frequency comb has shown great potential applications in many areas including molecularspectroscopy, RF photonics, millimeter wave generation, frequency metrology, atomic clock, and dense/ultra-dense wavelength division multiplexed high speed optical communications. Optical frequency comb in themicroresonator supporting whispering-gallery mode has attracted widespread interest because of its advantagessuch as flexible repetition rate, wide bandwidth, and compact size. The exceptionally long photon lifetime andsmall modal volume enhance light-matter interaction, which enables us to realize intracavity nonlinearfrequency conversions with low pump threshold. With the advantages of small size, low power consumption,wide spectral coverage and adjustable dispersion, the magnesium fluoride microresonator optical frequency combhas potential applications in optical communication and mid-infrared spectroscopy.In this work, the spectral characteristics of the optical frequency comb generated by a magnesium fluoridewhispering-gallery mode microbottle resonator platform are investigated. In order to optimize the spectraldistribution of the optical frequency comb of the magnesium fluoride microbottle resonator, the second-orderdispersion and higher-order dispersion of the bottle resonator structure under different curvatures and axialmodes are solved iteratively by the finite element method, and the spectral evolutions of the optical frequencycomb under different axial mode excitations are simulated by solving the nonlinear Schrodinger equationthrough the split-step Fourier method. The results show that near-zero anomalous dispersion tuning can beachieved in a wide bandwidth range by exciting low-order axial mode at an optimal radius of curvature, whilethe high-order axial mode will lead the microbottle resonator to present the weak normal dispersion. Theweaker anomalous dispersion in the lower-order axial mode broadens the bandwidth of the optical comb,demonstrating that the third-order dispersion and the negative fourth-order dispersion can broaden the Kerrsoliton optical comb; the weak normal dispersion in the higher-order axial mode suppresses the generation of theKerr optical comb, and the Raman optical comb dominates. The selective excitation of Kerr soliton combs andRaman combs can be achieved by modulating the axial mode of the microbottle resonator under suitablepumping conditions. The present work provides guidance for designing the dispersion in magnesium fluoridemicroresonator and the experimental tuning of broadband Kerr soliton optical combs and Raman optical combs