  Perturbations to the effective refractive index from nanometer-scale
fabrication variations in waveguide geometry plague high index-contrast
photonic platforms including the ubiquitous sub-micron silicon-on-insulator
(SOI) process. Such variations are particularly troublesome for phase-sensitive
devices such as interferometers and resonators, which exhibit drastic changes
in performance as a result of these fabrication-induced phase errors. In this
Letter, we propose and experimentally demonstrate a design methodology for
dramatically reducing device sensitivity to silicon width variations. We apply
this methodology to a highly phase-sensitive device, the ring-assisted Mach
Zehnder interferometer (RAMZI), and show comparable performance and footprint
to state-of-the-art devices while substantially reducing stochastic phase
errors from etch variations. This decrease in sensitivity is directly realized
as energy savings by significantly lowering the required corrective thermal
tuning power, providing a promising path towards ultra-energy-efficient
large-scale silicon photonic circuits.
