  Nonreciprocal devices such as isolators and circulators are key enabling
technologies for communication systems, both at microwave and optical
frequencies. While nonreciprocal devices based on magnetic effects are
available for free-space and fibre-optic communication systems, their on-chip
integration has been challenging, primarily due to the concomitant high
insertion loss, weak magneto-optical effects, and material incompatibility. We
show that Kerr nonlinear resonators can be used to achieve all-passive,
low-loss, bias-free, broadband nonreciprocal transmission and routing for
applications in photonic systems such as chip-scale LIDAR. A multi-port
nonlinear Fano resonator is used as an on-chip, all-optical router for
frequency comb based distance measurement. Since time-reversal symmetry imposes
stringent limitations on the operating power range and transmission of a single
nonlinear resonator, we implement a cascaded Fano-Lorentzian resonator system
that overcomes these limitations and significantly improves the insertion loss,
bandwidth and non-reciprocal power range of current state-of-the-art devices.
This work provides a platform-independent design for nonreciprocal transmission
and routing that are ideally suited for photonic integration.
