Study On The Fabrication And Characteristics Of Tunable Optical Whispering Gallery Mode Microcavity Filter

In recent years,whispering gallery mode(WGM)filters have been widely used in optical fiber communications and lasers because WGM filters have advantages of ultrahigh Q,ultrasmall volume,simple structures and narrow bandwidth.Wavelength tuning is critical to improve the flexibility and additivity of the optical fiber communications system and lasers.In this article,we fabricated integrated tunable WGM microtube filters and lithium niobate microdisk filters by micromaching process such as femtosecond laser pulses machining,optical fiber tapering,mechanically milling methods.The wavelength tuning was realized by changing the size and refractive index of the WGM resonators.The main research contents are as follows:(1)Based on the mode theory and coupling theory of cylinder WGM resonators,finite difference time domain method(FDTD)was used to simulate the whispering gallery modes.Different coupling distance between the taper and resonator,radius and refractive index of the resonator had been simulated to study their influences on the resonant wavelength and mode field distribution.It could be found that the wavelength shifted with the change of radius or refractive index of the resonator.(2)D-shaped fiber was fabricated by femtosecond laser and was used to couple light into WMG resonator.This coupling structure had more than 20 dB extinction ratio,good robustness and high stability.Moreover,D-shaped fiber and WGM resonator can be integrated on the substrate to form high-Q factor,robust,applicable tunable WGM filters by further controlling the coupling distance between the D-shaped fiber and WGM resonator.Taking advantage of D-shaped fiber,integrated mechanically tunable filter and electronically thermal-optical tunable filter were fabricated.The wavelength tuning of mechanically tunable filter was realized by changing the radius of the resonator while the electronically thermal-optical tunable filter was achieved by changing the refractive index of the microtube through applying the voltage to the heating wire.(3)Lithium niobate WMG filters had been fabricated by femtosecond laser and mechanically milling methods.Lithium niobate WGM resonator fabricated by femtosecond laser had low Q factor and unitability,however lithium niobate WGM resonator fabricated by mechanically milling had high Q factor and stability.Electro-optic tuning and thermal-optic tuning of the lithium niobate WGM resonator had been theoretically and experimentally studied,which would lay a foundation for further integrating D-shaped fiber and lithium niobate WGM resonator on the substrate to fabricate practical lithium niobate tunable WGM filters.