Silicon nitride-based integrated components for optical communication

Doctoral thesis, 2023

To follow the trend of the data traffic and to limit the size of the hyperscale data centers, communication solutions offering small footprint, low cost and low power consumption are needed. Optical interconnects used in data centers are mostly short reach (approximately 100 m) based

on GaAs-based 850 nm vertical-cavity surface emitting lasers (VCSELs) and OM4 multimode fibers (MMF). However, with 1 km-long optical links, the use of VCSEL-MMF at 850 nm becomes challenging at high data rates (Tb/s) due to large modal dispersion and high propagation loss. Therefore, other cost-effective methods are needed to compensate these limits. Single mode GaAs-based VCSELs have been demonstrated at 1060 nm of wavelength, where the chromatic dispersion is lower, for optical links ranging between 300 m and 10 km. This solution could be a better alternative than InP-based distributed feedback laser sources at 1310 nm in terms of cost and energy dissipation. As the modulation bandwidth of GaAs-based single mode VCSELs is limited to around 30 GHz, reaching the capacity target then requires a wavelength division multiplexing scheme with parallel single-core fibers (SCFs) or even multi-core fibers (MCFs).

 
In this thesis we discuss different types of demultiplexers at 1060 nm of wavelength. The proposed designed demultiplexers are arrayed waveguide gratings (AWGs) and cascaded Mach-Zehnder interferometers (MZIs). These two technologies are compared in terms of transmission, bandwidth, crosstalk and footprint with the number of output channels. Grating couplers at 1060 and 850 nm for on-chip coupling are also studied. The goal is to couple the light coming from a single mode fiber or a VCSEL with the lowest possible loss and back reflection.