![]() ![]() The variation in the measured absolute resonance wavelength for nominally identical filters, was found to be <0.4 nm for structures fabricated on the same chip, and <4 nm for structures with the same design across different chips. The performance of the racetrack-based device slightly deteriorated, because of the higher round-trip losses, introduced by the short-radius curves, and because of the change in the modal index between the curved and the straight sections. An example of the OSA-acquired spectra of the drop and through outputs, for the best performing ring filter, is shown in Figure 5. In particular, a significant reduction of the insertion loss was observed for the ring-based filter, thanks to the increased radius. The experimental results obtained using the newly-designed filter, showed some significant differences and improvements with respect to those of the first iteration in Table 3. The results collected from the second iteration devices are shown in Table 5 measurements were carried out on replicas of the same optimized filter structures, fabricated on different chips, and the minimum and maximum values obtained at the central resonance (the one closest to 1550 nm) are reported. It is worth mentioning that the biggest contribution to curvature loss is not represented by radiation loss, which is in our case negligible (<0.01 dB/cm even for a ring radius of 6.0 µm), but by the scattering induced because of the interaction of the optical mode, propagating in the bending section with the waveguide lateral-surface roughness. This allows us to increase the ring radius from 6.0 to 8.4 µm, with the expectation of reducing the possible IL-contribution from the waveguide curvature. As a consequence, once the optical path length is fixed, the corresponding physical length of resonators based on 100-nm waveguides is longer than that of structures based on 220-nm waveguides. This is due to the fact that the effective index in the reduced-height 100-nm waveguides is significantly lower than that in standard 220-nm waveguides, because of the reduced modal confinement in the silicon core. In our initial design the radius-of-curvature could have been increased, while still satisfying the FSR requirement, but only for the rings-based configuration. Two principal constraints limited the range of solutions that could be considered-(i) the bus waveguide width was kept equal to 500 nm, to allow easy matching with the other on-chip components (ii) the waveguide height was set to 100 nm, as this value was achievable without requiring additional processing steps in the fabrication of the overall chip. The ring-based filters were initially defined so that the ring radius matched the physical length of the racetrack filters, and were then analyzed with regard to the dependence of the coupling coefficients on both the bus-to-ring gap, and the ring-waveguide width. Finally, the inter-resonator gap was determined, as once the racetrack shape is fixed, there is only one possible inter-resonator gap which can be used to obtain the desired coupling coefficient k R. ![]() The curvature radius was then calculated, based on the free-spectral-range constraints reported in Table 1. 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.įor racetrack-based filters, the length of the straight section was initially determined through numerical simulations, so as to obtain the required coupling (while keeping the gap between the bus waveguide and the resonator equal to 200 nm). 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. The major challenge in realizing these filters is their relatively high optical losses. Thanks to the HIC in the SOI platform, even micro-resonators with a turn-radius of less than 10 µm can be exploited, which brings the advantage of optical filters with a high free-spectral-range (FSR )-a fundamental requirement for application in real systems. 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. 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. ![]()
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