Re-shapeable double-hump Bragg-spectrum using a partial-width entrenched-core waveguide

Abstract

Bragg gratings are well known with several unique characteristics that make them very crucial components in various optical systems and applications. Here, a novel partial-width entrenched core (PWEC) structure within a few-mode waveguide is demonstrated. The core of silicon-on-insulator planar waveguide is simply entrenched with nano-wide rectangular air-gaps that partially fill the core width to form a stand-alone short and uniform entrenched-core Bragg grating. The periodic perturbation of trenches to input fundamental mode couples to two guided even-modes, and in turn generates a double-hump spectrum with two closely separated Bragg wavelengths. The spectrum can be tuned over a wide wavelength range from 1 to 2 µm within the shortwave infrared band. It shows a very small induced dispersion. The trenches’ width and period are found to be very crucial in the design of double-hump spectral characteristics (e.g., Bragg wavelengths, humps bandwidths, and separation). The double-hump reflection and transmission spectra with their two modes can be simply reshaped to form new spectra such as double stop-bands within the optical communication standard bands, single wide (200 nm)/ narrow (10 nm) pass-bands, notch-bands, or ultra-wide bandwidth (530 nm) full-mirrors.