  Non-reciprocal optical components such as isolators and circulators are
crucial for preventing catastrophic back-reflection and controlling optical
crosstalk in photonic systems. While non-reciprocal devices based on Brillouin
intermodal transitions have been experimentally demonstrated in chip-scale
platforms, harnessing such interactions has required a suspended waveguide
structure, which is challenging to fabricate and is potentially less robust
than a non-suspended structure, thereby limiting the design flexibility. In
this paper, we numerically investigate the performance of a Brillouin-based
isolation scheme in which a dual-pump-driven optoacoustic interaction is used
to excite confined acoustic waves in a traditional ridge waveguide. We find
that acoustic confinement, and therefore the amount of Brillouin-driven mode
conversion, can be enhanced by selecting an appropriate optical mode pair and
waveguide geometry of two arsenic based chalcogenide platforms. Further, we
optimize the isolator design in its entirety, including the input couplers,
mode filters, the Brillouin-active waveguide as well as the device fabrication
tolerances. We predict such a device can achieve 30 dB isolation over a 38 nm
bandwidth when 500 mW pump power is used; in the presence of a +/- 10 nm
fabrication-induced width error, such isolation can be maintained over a 5-10
nm bandwidth.
