  Fiber and bulk-optical isolators are widely used to stabilize laser cavities
by preventing unwanted feedback. However, their integrated counterparts have
been slow to be adopted. While several strategies for on-chip optical isolation
have been realized, these rely on either integration of magneto-optic materials
or high frequency modulation with acousto-optic or electro-optic modulators.
Here, we demonstrate an integrated approach for passively isolating a
continuous wave laser using the intrinsically non-reciprocal Kerr nonlinearity
in ring resonators. Using silicon nitride as a model platform, we achieve
single ring isolation of 17-23dB with 1.8-5.5dB insertion loss, and a cascaded
ring isolation of 35dB with 5dB insertion loss. Employing these devices, we
demonstrate hybrid integration and isolation with a semi-conductor laser chip.
