High-Gyrotropy Seedlayer-Free Ce:TbIG for Monolithic Laser-Matched SOI Optical Isolators

Monolithic optical isolators that provide modal (transverse electric, TE) and dimensional (500 nm core) matching to on-chip lasers have been realized with one step seedlayer-free garnets. To date, seedlayer-free garnet claddings have required thinner (<340 nm) silicon-on-insulator (SOI) cores because mode-cladding interactions were too weak for laser-matched cores. However, laser matching is important because tapers and mode converters between the laser and the isolator can cause detrimental reflections prior to isolation. This paper reports the use of cerium-doped terbium iron garnet (Ce:TbIG) in a quasi-phase matched nonreciprocal mode conversion (NRMC) isolator that operates on both TE and TM modes without an external field. A key innovation presented here is a repeatable process for foundry-friendly sputter deposition of Ce:TbIG, which enables this high Faraday rotation material (-3200 degrees/cm) to be synthesized in any isolator design that would benefit from one-step lithographical manufacturing. A proof-of-feasibility 500 nm SOI NRMC device is demonstrated with seedlayer-free Ce:TbIG that achieves an isolation ratio of 11 dB. With an optimal length, this NRMC design can provide greater than 30 dB isolation.