Silicon photonics technology has demonstrated, over the years, Photonic Integrated Circuits (PICs) relying on Si or Si3N4 materials that feature advanced functionalities for a wide area of applications. However, the fabrication of such PICs is usually compatible only with Front-End-of-Line (FEOL) processes that render very difficult post processing of the involved chips towards providing efficient interfaces with optical sources. This is a major problem for the next generation photonic circuits that are expected to co-integrate III-V laser sources on the Si substrate in a monolithic way, as the coupling interface between the active and the passive part of the PIC should be developed after the epitaxy and the fabrication of the lasers. In this work, we report on the development of a novel Silicon Rich Nitride (SRN) material with low stress and high refractive index (n<3.16), close to that of InP and InGaAsP which are commonly utilized for the laser sources. The SRN has been characterized with spectroscopic ellipsometry and Fourier-Transform Infrared Spectroscopy for estimation of complex refractive index and hydrogen content in the film. Based on this material, a trilayer stack has been developed for the formation of waveguides compatible with the Back-End-of-Line (BEOL) processes, while propagation losses have been extracted through cut-back measurements. These experimental results were then inserted as input parameters in 2D- and 3D-FDTD simulations for the design of efficient interfaces between III-V lasers and Si3N4 waveguides providing coupling efficiencies that can reach 83.81% and back-reflections of 0.032%.
Free space optical (FSO) communications are a potential application envisioned for Quantum Cascade Lasers technology. In this paper, we sketch out the main market scenarii for FSO communications, where data channels should reach up to Tbit/s over distances that range from 0.1 to 36000 km. We analyze where QCLs can be relevant, and what standards they must meet to be relevant at the industrial level. The following topics are discussed: competing technologies, atmospheric transmission physics, signal processing and multiplexing. We try and translate the constraints of the system-level into challenges for device-level research and development.