Fully-integrated Isolators for Silicon Photonics using WAMO (Wrap Around Magneto-Optics)

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

Title: Fully-integrated Isolators for Silicon Photonics using WAMO (Wrap Around Magneto-Optics) Abstract: Nontechnical: Imagine an integrated circuit where light carries the signal rather than electrons. This promises to allow many signals to transmit faster, simultaneously, and without heat. The major missing link in achieving impactful applications of such "photonic integrated circuits" with high densities is an integrated isolator. Isolators use magneto-optical garnets to control the direction of light, similar to diodes in electronics. Although several integrated isolators have been proposed, they do not work for all light polarizations, and in fact the large majority only work for the polarization that is the opposite of all silicon-integrated lasers. Here, novel isolators are proposed starting with a uniquely simple design that has a high probability of success with today's lasers, and escalating to a final design that includes an integrated magnet and offers polarization diversity (meaning all polarizations can be isolated with one device). The technical novelty of this project are three-fold. First, materials challenges will be overcome to integrate magneto-optical garnets onto silicon. Second, new silicon photonic designs will initially enable the isolation of the laser-matched polarization and later the isolation of all polarizations. Third, magnetic design will be used to incorporate closed flux structures to magnetize the garnets while minimizing the magnetic cross-talk between devices. Technical: Photonic systems keep society connected with optical fibers and inside computers with optical interconnects. Photonics also impacts medicine, chemistry, and many other fields. An integrated isolator is the missing link that needs to be solved before photonic systems with fully-integrated laser sources can be a reality, and this means the results here will have very broad impact. The only passive (zero-power) way to produce a nonreciprocal device, such as an isolator, is to include a nonreciprocal (magneto-optic) material. Magneto-optic garnets (e.g., Cerium-doped Yttrium Iron Garnet) yield orders of magnitude better performance than any other material. However, these garnets are difficult to integrate with silicon as single phase films, and multiple phases cause loss. Also, isolator designs proposed to date are too large for practical implementation, and they mostly use non-reciprocal phase shift in a geometry that only applies to transverse magnetic polarizations while integrated lasers output only transverse electric light. Here, the combination of proven high-gyrotropy, low-loss, and silicon-integrated garnet films (Stadler) with novel silicon photonic designs (Li) will ensure all-passive silicon isolators with polarization diverse functionality. Three layers of innovation will be explored. First, a transverse electric isolator is proposed where garnet is deposited between the branches of an interferometer to provide a simple asymmetry such that, with one applied magnetic field, a "push-pull" non-reciprocal transverse electric phase shift will occur. The next layer of innovation will apply the fundamental photonic results from above and will extend the design to include an integrated magnetostatically-engineered magnetic bias. Specifically, a closed flux loop structure (similar to a closed horseshoe magnet) will be used to magnetize the garnet claddings without requiring an external field and without producing fringing fields in the circuit. The final step towards polarization diversity is a nearly closed flux design that will yield the first device to provide polarization diversity in an all-passive, fully-integrated isolator with wrap around magneto-optics.