  The singlemode condition is one of the most important design rules for
optical waveguides in guided-wave optics. The reason following the singlemode
condition is that higher-order modes might be excited and thus introduce some
undesired mode-mismatching loss as well as inter-mode crosstalk when light
propagates along an optical waveguide beyond the singlemode regime. As a
result, multimode photonic waveguides are usually not allowed. In this paper,
we propose the concept of silicon photonics beyond the singlemode regime,
developed with low-loss and low-crosstalk light propagation in multimode
photonic waveguides with broadened silicon cores. In particular, silicon
photonic waveguides with a broadened core region have shown an ultra-low-loss
of ~0.1 dB/cm for the fundamental mode even without any special fabrication
process. A micro-racetrack resonator fabricated with standard 220-nm-SOI
MPW-foundry processes shows a record intrinsic Q-factor as high as 1.02*107 for
the first time, corresponding to ultra-low waveguide propagation loss of only
0.065 dB/cm. A high-performance microwave photonic filter on silicon is then
realized with an ultra-narrow 3-dB bandwidth of 20.6 MHz as well as a tuning
range of ~20 GHz for the first time. An on-chip 100-cm-long delayline is also
demonstrated by using the present broadened SOI photonic waveguides with
compact Euler-curve bends, the measured propagation loss is ~0.14 dB/cm. The
proposed concept of silicon photonics beyond the singlemode regime helps solve
the issue of high propagation loss and also significantly reduces the random
phase errors of light due to the random variations of waveguide dimensions. In
particularity it enables silicon photonic devices with enhanced performances,
which paves the way for new-generation silicon photonics realizing the
large-scale photonic integration.

Optics;Applied Physics