Novel Nonreciprocal Devices with Integrated Electromagnet for Silicon Photonics

We present a novel approach to construct integrated isolators and circulators based on magneto-optical materials. An integrated electromagnet is designed and fabricated, eliminating the need for a permanent magnet. The fabricated devices exhibit high performance and can be easily integrated/packaged. Introduction Nonreciprocal components, such as optical isolators and circulators, are fundamental building blocks in optics to avoid undesirable back-reflections and to separate counterpropagating optical signals. In integrated optics, their fabrication is still very challenging and several approaches have been proposed. Nonreciprocal devices are characterized by a symmetry breaking for the light that propagates from different directions, which results in a nonreciprocal scattering matrix [1]. Three different approaches can perform this work: i) nonlinear materials, ii) time modulation of the refractive index, iii) magneto-optic (MO) materials. However, only a few nonlinear optical phenomena such as Brillouin-scattering can be effectively used to break the symmetry, and silicon lacks a Pockels effect for efficient modulation [2,3]. On the other hand, MO material can be effectively bonded on silicon-oninsulator (SOI) wafer and the experimental results look very promising [4,5]. The MO material becomes nonreciprocal when it is in a quasi-static magnetic field. When light is transmitted through a magnetized medium, it exhibits a different phase velocity. Properly designed interferometric devices generate constructive interference for forward light and destructive interference for backward light achieving the isolating function [4,5]. However, the large absorption loss (e.g., 60dB/cm) in MO garnet and the use of a permanent magnet for applying an external magnetic field are two important aspects that limit the performance and the integration of those devices. While the propagation loss can be greatly reduced in a device with small footprint, the external biasing magnet is still a significant limiting factor due to possible magnetic field interference with electronics and its large footprint (size>1mm) [4]. In this work, we investigate integrated optical isolators and circulators using a planar spiral electromagnet that provides local control of the magnetic field, and can be easily integrated and packaged. We then experimentally verify our model and explore the implications of these novel devices. Proposed device The proposed devices are shown in Fig. 1. A silicon ring resonator is fabricated on a SOI wafer, having refractive index nSi=3.48 and nSiO2=1.46 at =1550nm, respectively. The ring is bonded with a Ce:YIG garnet (nCe:YIG=2.22) grown on a (Ca,Mg,Zr)-substituted gadolinium gallium garnet (SGGG), (nCe:YIG=1.97), whereas the remaining space is filled by air. The silicon waveguide cross-section (230nm × 600nm) has been designed to maximize the nonreciprocal resonance split between the clockwise (CW) and the counter-clockwise (CCW) transverse magnetic (TM) mode [6,7]. Due to the high SOI index contrast, a high field confinement factor can be achieved even with rather small ring radius (i.e., R=35μm). This small footprint of the device has allowed us to reduce the total excess loss down to 2.3dB [9]. Using a mechanical lapping technique, the SGGG substrate on the bonded die can be thinned down to 5μm and a Ti/Au metal layer is patterned onto the back-side of the SGGG Fig. 1: (a) Perspective view and (b) cross-sectional view of a proposed optical circulator with integrated electromagnet. 704 ECOC 2016 42nd European Conference and Exhibition on Optical Communications September 18 – 22, 2016 Düsseldorf ISBN 978-3-8007-4274-5 © VDE VERLAG GMBH Berlin Offenbach substrate using i-line lithography and metal liftoff. The electromagnet is fabricated using a 1.5μm thick and 3.0μm wide gold microstrip. By injecting a DC current into the metal coil, a magnetic field is locally applied. In order to increase the magnetic field intensity without increasing the electrical current, a planar spiral solution is investigated. For this purpose, 2 layers of metal are needed and the minimum microstrip separation in the spiral is set to 1.0μm. Mathematical model When a static radial magnetic field is applied, the CW and the CCW modes in the ring have a different effective index and a split between their resonance wavelengths occurs, and is given as