Sputter-deposited magneto-optical garnet for all-mode (transverse electric/transverse magnetic) Faraday rotators
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Elements of a Faraday rotator Nonreciprocal optical isolators are necessary and ubiquitous components in optical fiber systems, primarily using Faraday rotation of linearly polarized light to protect laser sources from back reflections, which can cause unwanted noise and instabilities (Figure 1a). Typical implementations of optical isolators involve the assembly of discrete elements into a single optical-fiber-coupled device without waveguiding structures (Figure 1b).1 Integrated photonics is a new field that offers a route to mass production of multiple isolator devices on a single chip, and also to realize photonic integrated circuits (PICs) where lithography and microfabrication are used to define and align elements, similar to electronic integrated circuit (ICs). Unfortunately, isolators and other devices that break time-reversal symmetry are not currently available in PIC systems,2 such as silicon-on-insulator (SOI) chips.
Unlike fiber isolators, PIC devices consist of waveguide structures, and the light is guided in modes whose polarization is defined by the alignment between the electric field of the light and the plane of the chip. Implementing nonreciprocity therefore involves additional complexities3 (Figure 2a). Most integrated lasers emit transverse electric (TE)-polarized modes (the electric field lies predominantly in the plane of the chip), and a majority of integrated PIC devices also operate in the TE mode. An ideal integrated isolator should allow forward propagation of TE-polarized modes, and it should either convert reflections to transverse magnetic (TM) polarization (the electric field lies predominantly normal to the plane of the chip), which does not normally interact with TE-mode (quantum well) lasers, or block reflections altogether. Many proposed integrated isolators use nonreciprocal phase shift (NRPS)—a phenomenon that induces a phase shift in TM-polarized modes using a transverse magnetic field,
Bethanie J.H. Stadler, University of Minnesota, USA; [email protected] David C. Hutchings, University of Glasgow, Scotland; [email protected] doi:10.1557/mrs.2018.121
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• VOLUME 43 • JUNE 2018 University • www.mrs.org/bulletin ©terms 2018 of Materials Research Downloaded MRS fromBULLETIN https://www.cambridge.org/core. of Massachusetts Amherst, on 13 Jun 2018 at 16:52:41, subject to the Cambridge Core use, available at Society https://www.cambridge.org/core/terms. https://doi.org/10.1557/mrs.2018.121
Sputter-deposited magneto-optical garnet for all-mode Faraday rotators
to these structures via vacuum deposition, such as pulsed layer deposition (PLD), or wafer bonding, both of which are best suited to depositing the cladding on top of the waveguides rather than on sidewalls. Some attempts have been made to tilt samples during PLD for sidewall coating which enables TE functionality.8,9 More often, these devices require reciprocal polarization converters (RPCs) before and after the isolator5 to convert TE modes into TM Figure 1. (a) Operation of a generic Faraday rotation isola
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