Quantum dot comb laser for optical interconnects

Abstract

Explosive growth in artificial intelligence (particularly large language models) and consistent growth in optical communication data volume demand higher bandwidth density from interconnects in data centres. Owing to the low propagation loss of silica optical fibres, interconnect between boards and clusters has been improved in terms of power consumption and transmission distance from copper electrical interconnects to optical pluggable. Optical pluggable has evolved a lot during past decades, but the bulky size of discrete optical devices limits higher bandwidth density. Recently, concepts such as co-packaged optics (CPO) and optical input output (OIO) came out, in which a photonics chip is packaged closely with electronic chips. More compact package results in higher shoreline bandwidth density compared to conventional optical pluggable architecture. This also shortens the transmission distance of both the electrical driving signal and modulation signal, leading to reduced signal attenuation and improved signal integrity. Silicon photonics (SiPh) is ideal for such architecture, owing to compatibility with complementary metal oxide semiconductor (CMOS) processes, SiPh devices can be fabricated with advanced nano-scale process nodes. However, laser source on silicon is always challenging since silicon itself is indirect bandgap structured. This means that when an electron jumps from the excited level to ground level, a phonon rather than a photon is generated. Various approaches have been proposed to integrate laser sources to the SiPh chips. For instance, III-V laser on Si substrate and band structure engineering of silicon. Considering feasibility and maturity, importing III-V material to SiPh seems to be a promising method. Heterogeneously grown III-V quantum dot lasers on silicon substrate with high-temperature reliability and dislocation tolerance are studied in this thesis. Key metrics such as feedback insensitivity, comb generation technique and finally transmitter performance are studied concerning the application of QD comb laser in optical interconnect. In this work, we proved the performance of QD laser on silicon under feedback noise without isolator, multiple wavelength injection locking, and finally, together with a microring modulator array, we demonstrate a transmitter with high energy efficiency, dense shoreline bandwidth, compact footprint and high-temperature reliability.