Optical micro-ring resonators on silicon for optical communications and optical sensing

Abstract

Silicon-based optical micro-ring resonators (MRRs) are very popular for many application because of the ultra-compact footprint and easy fabrication. The wavelength-selectivity property of MRRs makes it possible to be used not only as an optical filter with a narrow bandwidth but also as an optical sensor with high sensitivity. This paper gives a review of our recent work on the MRRs on silicon. First, a high-order MRR optical filter with a box-like filtering response is realized by introducing bent directional couplers to have sufficient coupling between the access waveguide and the microrings. A efficient thermally-tunable MRR-based optical filter with graphene transparent nano-heater is realized by introducing transparent graphene nanoheaters, which contact the silicon core directly without any isolator layer. As a result, the heating efficiency, the temporal response and the achievable temperature is improved in comparison with a traditional metal heater. MRRs are also very attractive for the applications of optical sensing and two novel designs of MRRs for optical sensing are presented. One is a digital optical sensor based on two cascaded MRRs with different free spectral ranges (FSRs), which was proposed and realized to achieve extremely high sensitivity. The other is a Mach-Zehnder interferometer (MZI) coupled microring, which has only one resonant wavelength with a high extinction ratio in a very large wavelength span and consequently a very large quasi-FSR (free spectral range) is achieved (FSR > 120 nm). This enables a large measurement range for the change of refractive index. In this paper, we give a review of our recent work on the optical MRRs on silicon and their applications.