Silicon photonic add-drop filter based on a grating-assisted co-directionally coupled vertical hybrid structure.

Add-drop filters (ADFs) are an essential component in optical interconnection using dense wavelength-division multiplexing. Silicon photonic ADFs based on contra-directional coupling have been well developed, but those based on grating-assisted co-directional coupling (GACC) have never been studied. This paper reports an ADF based on GACC in a vertical hybrid structure (VHS). which consists of two width-modulated silicon strip waveguides with a large lateral gap and a wide silicon nitride strip waveguide above them. The VHS makes it possible for the ADF to have a narrow 3-dB bandwidth as well as a short grating length. An efficient analysis method for design is explained, and the ADF is designed. Theoretical investigation of the ADF demonstrates that the ADF has a 3-dB bandwidth of 1.16 nm and a grating length of 1.13 mm, which are similar to those of ADFs based on contra-directional coupling. As an application, the ADF is used for a nonvolatile switchable ADF by adding an optical phase change material strip above the silicon nitride waveguide. The nonvolatile switchable ADF is shown to have an extinction ratio larger than 30 dB. The investigated ADF requires neither waveguides in close proximity nor grating teeth with dimensions close to the resolution of deep UV lithography. In this regard, it has the advantage of ease of fabrication as compared to ADFs based on contra-directional coupling. Therefore, the ADF is expected to play a key role in optical interconnection using dense wavelength-division multiplexing, prevailing over ADFs based on contra-directional coupling.