###### Abstract  
Optical resonators are fundamental building-blocks for the development of Si-photonics-integrated circuits, as tunable on-chip optical filters. In addition to the specific spectral shape, which may vary according to a particular application, extremely low losses from these devices are a crucial requirement. In the current state-of-the-art devices, most low-loss filters have only been demonstrated by exploiting ad hoc lithographic and etching techniques, which are not compatible with the standard CMOS (complementary metal-oxide semiconductor) process-flow available at Si-photonic foundries. In this paper, we describe the design and optimization of optical micro-resonators, based on Si-waveguides with a height lower than the standard ones (i.e., less than 220 nm), prepared on SOI (silicon on insulator) platform, which allow the realization of high-performance optical filters with an insertion loss lower than 1 dB, using only previously validated lithographic etch-depths. Silicon photonics; Micro-rings; integrated filters; optical losses; optical components; integrated waveguides +
Footnote †: journal: Applied Physics Letters  
to realize structures with a very high optical refractive-index contrast; and (ii) the exploitation of Si-processing technologies that have already been developed and validated for the microelectronic device industry, and which have the potential to deliver low cost Si-photonic components, at very high volumes. Photonic structures with a high index contrast (HIC) allow modal confinement within sub-wavelength dimensions, which allows for reduced waveguide-turning radii, and small component sizes, both of which lead to very small photonic-chip footprints. The realization of high performance HIC structures requires precise designs and accurate fabrication, to ensure that waveguide propagation losses and reflections are sufficiently low. Additionally, as standard telecom single mode fibers have a mode field diameter of the order of 10\(\upmu\)m, and HIC waveguides typically have a cross-section of the order of 200 nm \(\times\) 500 nm, fiber-to-chip coupling is a significant technical challenge, and often introduces a relevant insertion loss (>3 dB [10; 11]). In this article, we focus our attention on the performance of HIC structures for an optical micro-resonator, with ring and racetrack topology, coupled to two separate waveguides. This 4-port configuration allows for the filtering of the selected channel, without affecting the through-signals, and is thus a promising candidate for the realization of optical filters for wavelength division multiplexing (WDM) telecom systems. Thanks to the HIC in the SOI platform, even micro-resonators with a turn-radius of less than 10 \(\upmu\)m can be exploited, which brings the advantage of optical filters with a high free-spectral-range (FSR [12; 13; 14])--a fundamental requirement for application in real systems. The major challenge in realizing these filters is their relatively high optical losses. These losses can be reduced using ad hoc fabrication protocols, such as the steam oxidation and HF-stripping, to reduce waveguide heights, in addition to the standard processes in the Si-Photonic foundry, but this adds cost and complexity to the process [12; 15; 16]. On the other hand, our target was to implement a carefully designed strategy, so as to obtain the best possible filter performance, whilst only exploiting CMOS-compatible standard processes, as described in Section 2.3. In this article, we present the results obtained in the frame of the European FP7-project FABULOUS, dedicated to the development of complex integrated optical circuits for innovative high bit-rate passive optical networks (PONs), fabricated by standard-CMOS technologies [17; 18]. The final goal, which is the insertion of the chip in a real passive network, imposes very strict requirements on the loss of the component, which has to be kept as low as possible, as the reflective-PON approach yields a very challenging power budget.