Low-crosstalk fabrication-insensitive echelle grating multiplexers and passives for the silicon photonics toolbox

In this communication, we report about the design, fabrication, and testing of echelle grating (de-)multiplexers for the 100GBASE-LR4 norm and other passive architectures such as vertical fiber-couplers and slow-wave waveguides in the O-band (1.31-μm) for Silicon-based photonic integrated circuits (Si-PICs). In detail, two-point stigmatic 20^th-order echelle gratings (TPSGs) on the 300-nm-thick SOI platform designed for 4x800-GHz-spaced wavelength division multiplexing featuring extremely low crosstalk (< -30 dB), precise channel spacing and optimized average insertion losses (~ 3 dB) are presented. Distributed Bragg reflectors (DBRs) are used to improve the grating facets reflectivity, while multi-mode interferometers (MMIs) are used in optimized perfectly-chirped echelle gratings (PCGs) for pass-band flattening. Moreover, 200-mm CMOS pilot lines processing tools including VISTEC variable-shape e-beam lithography are employed for the fabrication. In addition, wafer-level statistics of the multiplexers clearly shows the echelle grating to be inherently fabrication-insensitive to processing drifts, resulting in a minimized dispersion of the multiplexer performances over the wafer. In particular, the echelle grating spectral response remains stable over the wafer in terms of crosstalk, channel spacing and bandwidth, with the wavelength dispersion of the filter comb being limited to just 0.8 nm, thus highlighting the intrinsic robustness of design, fab pathways as well as the reliability of modeling tools. As well as that, apodized one-dimensional vertical fiber couplers, optimized multi-mode interferometers (MMIs) and extremely low-losses slow-light waveguides are demonstrated and discussed. The adiabatic apodization of such 1-D gratings is capable to provide band-edge group indices ng as high as 30 with propagation losses equivalent to the indexlike propagation regime.