  Optical isolators and circulators are indispensable for photonic integrated
circuits (PICs). Despite of significant progress in silicon-on-insulator (SOI)
platforms, integrated optical isolators and circulators have been rarely
reported on silicon nitride (SiN) platforms. In this paper, we report
monolithic integration of magneto-optical (MO) isolators on SiN platforms with
record high performances based on standard silicon photonics foundry process
and magneto-optical thin film deposition. We successfully grow high quality MO
garnet thin films on SiN with large Faraday rotation up to -5900 deg/cm. We
show a superior magneto-optical figure of merit (FoM) of MO/SiN waveguides
compared to that of MO/SOI in an optimized device design. We demonstrate TM/TE
mode broadband and narrow band optical isolators and circulators on SiN with
high isolation ratio, low cross talk and low insertion loss. In particular, we
observe 1 dB insertion loss and 28 dB isolation ratio in a SiN racetrack
resonator-based isolator at 1570.2 nm wavelength. The low thermo-optic
coefficient of SiN also ensures excellent temperature stability of the device.
Our work paves the way for integration of high performance nonreciprocal
photonic devices on SiN platforms.
