Optical frequency comb generation using annealing-free Si₃N₄ films for front-end monolithic integration with Si photonics

In this communication, we report on the design, fabrication and testing of silicon-nitride-in-insulator (SiNOI) nonlinear photonic circuits for comb generation in silicon photonics and optoelectronics. The low two-photon absorption when compared with crystalline silicon makes the SiNOI an attractive platform for frequency comb generation.

Kerr combs have been recently used in terabit per second coherent communications demos. Such devices can overcome the intrinsic limitations of nowadays silicon photonics notably concerning the heterogenous integration of III-V on SOI lasers for both datacom and telecom applications. By using monolithically-integrated SiN-based Kerr frequency combs, the generation of tens or even hundreds of new optical frequencies can be obtained in dispersion tailored waveguides and resonators, thus providing an all-optical alternative to the heterointegration of hundreds of standalone III-V on Si lasers.

However, in all the previous SiNOI-based frequency combs, the silicon nitride film is annealed under long and high temperature which made the cointegration with silicon based optoelectronics elusive. The annealing steps used in common SiN fabrication processes are not only incompatible with the front-end of line complementary metal-oxide-semiconductor processes, but also costly and long and thus an important cost factor in non-CMOS compatible processes.

In our work, we present the fabrication and testing of an annealing-free and crack-free SiNOI. Notably, a 800-nm-spanning (1300-2100 nm) frequency comb is generated using 740-nm-thick silicon nitride featuring full compatibility with silicon photonics integrated circuits. This work constitutes a new, decisive step toward time-stable power-efficient Kerr-based broadband sources featuring full process compatibility with Si photonic integrated circuits (Si-PICs) on CMOS-lines.