Spectral engineering of lithographically scribed channel waveguide gratings

We propose and demonstrate a powerful new approach to spectral bandpass engineering (apodization) of one-dimensional channel-waveguide Bragg reflectors. Bandpass engineering is accomplished via precise photolithographic control over the transverse width and longitudinal placement of individual grating lines which, respectively, provide unique line-by-line diffractive amplitude and phase control. Several channel waveguide gratings exhibiting complex filtering functions based on the present apodization method have been fabricated and modeled. They include an essentially polarization -insensitive 4-nm wide flat-top filter with steep roll-off and a multi-passband spectral decoder, useful, e.g., for optical spectral code-division multiplexing or spectral signature recognition. When a second-order apodization effect, comprising effective waveguide refractive index variation with grating-line transverse width, is included in the simulation, extraordinary agreement between predicted and observed spectral passband profiles is obtained.