Study of waveguide crosstalk in silicon photonics integrated circuits

Silicon photonics is going trough a terrific expansion driven by several applications, from chip wiring to integrated sensors and telecommunications. Some applications, e. g. intra and inter chip connections and sensing, require long parallel waveguides for wiring or for connecting grating couplers (GCs) to devices situated in sensing micro-channels. In well packed photonics chips there are often long wiring waveguides parallel for several mm, so loss can be caused by light coupled back and forth between them (cross-talk), by scattering, wall roughness, mode mismatch, etc. This work aims to investigate cross-talk for long parallel waveguides, and to propose methods to reduce cross-talk loss when high integration density is required. We have designed and fabricated about 200 testing structures exploiting e-beam on silicon on insulator (SOI) chip, in order to test several parameters and to find out dominant loss mechanisms. All devices have been tested and measured using an automatic optical bench, in the wavelength range between 1500-1600 nm. Achieved results are promising, since they allow for comparing cross-talk for short as well as long interaction lengths (up to 5 mm), different waveguide width pairs, several separation distances, and for TE and TM polarization. For smaller gaps, having not symmetric pair of waveguides is very beneficial, since it results in a lower power coupling, e. g. about 20/14 dB of crosstalk reduction for TE/TM waveguides after 5 mm of propagation and gap of 0.5 mu m. This can be very useful for the design of integrated photonics chips requiring high-density packaging of devices and waveguides.