Waveguide Integrated Broadband Magneto-Optical Isolators on Silicon Nitride Platforms

Abstract

On-chip optical isolators are key components in photonic integrated circuits (PICs). In recent years, passive magneto-optical (MO) isolators based on nonreciprocal phase shift (NRPS) have been widely studied [1]. Micro-ring and Mach-Zehnder interferometer (MZI) isolators have achieved considerable progress on silicon on insulator (SOI) platforms [2, 3]. As a different material platform, silicon nitride (SiN) based PICs show much lower propagation loss, which are highly promising for telecommunication, data communication and sensing applications [4, 5]. However, high performance MO isolators have not been demonstrated on SiN, which is fundamentally limited by the low Faraday rotation and weak NRPS for MO thin films grown on SiN waveguides. Here, we report high-performance integrated MO isolators on SiN platforms for both TM and TE polarizations using cerium doped yttrium iron garnet (Ce: YIG) thin films deposited by pulsed laser deposition (PLD). By increasing the Ce solubility up to 40% [6], Ce:YIG thin films grown on SiN show high Faraday rotation up to -5500 deg/cm at 1550 nm wavelength, which is 83% higher than previously reported Ce1Y2Fe5O12 thin films [7, 8]. Based on these materials, broadband SiN MZI isolators are designed and fabricated. The TM isolators show 24 dB isolation and 4 dB insertion loss around 1567 nm wavelength, with a footprint of 0.9×0.25 mm2. For TE isolators, a maximum isolation ratio of 28 dB with insertion loss of 7.5 dB is observed at 1565 nm wavelength. These devices have exceeded the performance of their counterparts on SOI [7]. Our work demonstrates that high performance MO thin films and optical isolators can be monolithically integrated on SiN PICs.