TiO2-coated shifted Bragg grating in silicon-on-insulator platform as add-drop filter(Conference Presentation)

We present a sidewall patterned shifted Bragg grating based on an add-drop filter in silicon-on-insulator platform with a coating of amorphous titanium dioxide. This particular waveguide grating is equivalent to two identical gratings written across either sides of a waveguide with a longitudinal offset of half of a period. The add-drop operation occurs on the basis of mode conversion due to shifted sidewall structure followed by mode splitting with asymmetric Y-junction. A signal launched through the wide arm (single mode) of an asymmetric Y-junction generates the fundamental mode at the stem of the Y-branch. First order mode is generated at the stem if the signal is launched through the narrow arm. Thus, an asymmetric Y-branch is used as a mode splitter fulfilling proper limiting condition for an adiabatic operation. A signal at the Bragg wavelength launched through the wide arm of asymmetric Y-junction generates fundamental mode at the stem. The fundamental mode converted to first order upon reflection from the shifted Bragg grating. The reflected mode couples into the narrow arm of the Y-junction. The bandwidth of the reflected signal depends on the grating strength. We used 80 nm grating amplitude for 800 nm wide waveguide. The height of the guiding layer is 220 nm. The TiO2 thickness is set to 180 nm. A reflection bandwidth of 2.2 nm with 14 dB extinction ratio is obtained at 1552.5 nm for 300 µm long grating. We further demonstrate the potential of TiO2 recoating with atomic layer deposition as a method of fine tuning the spectrum.