Nanophotonic Bragg grating assisted Mach-Zehnder interferometers for O-band add-drop filters.

Spectral filters are fundamental building blocks in integrated photonics. Bragg grating filters have been demonstrated in silicon waveguides with a wide range of spectral responses and are suitable for wavelength division multiplexing applications. However, retrieving Bragg grating reflections typically requires external components such as fiber optic circulators. In this work, we develop fully integrated add-drop filters based on cladding-modulated Bragg gratings incorporated in a Mach-Zehnder interferometer configuration. We design complex spectral filtering devices with single and dual-band flat-top responses for the specified bandwidth. Additionally, we propose a novel design methodology which aims to minimize phase errors within the filters. We experimentally demonstrate add-drop filters with single-band and two-band rejection spectra at the datacom O-band, fabricated on a 220-nm thick silicon-on-insulator platform. Our results show an insertion loss below 1 dB and a crosstalk of around -20 dB at the channel center for a 4.5-nm wavelength grid and 3-nm wide channels.