Model of Bragg grating written in subwavelength-diameter fiber taper

We come up with a model of subwavelength-diameter fiber taper-based Bragg gratings (SWDFT-BGs) and numerically investigated the chirp characteristics of Bragg gratings written in local single-mode fiber tapers based on coupled-mode theory and transfer matrix method. The effective modal refractive index varies along adiabatic subwavelength-diameter fiber tapers (SWDFTs), which results in an effective chirp in grating period. Additionally, SWDFT-BGs with full width at half maximum (FWHM) up to 4.9 nm and time-delay slope over 25 ps/nm were achieved in simulations. This offers an idea for fabricating single-mode chirped fiber Bragg gratings using uniform phase-mask and paves ways for waveguide dispersion engineering and wide-bandwidth optical filters.