Compact grating coupler using asymmetric waveguide scatterers
Ashutosh Patri, Xiao Jia, Muhammad Mohsin, Stephane Kena-Cohen, Christophe Caloz
CCompact grating coupler using asymmetricwaveguide scatterers
Ashutosh Patri , Xiao Jia , Muhammad Mohsin ,St´ephane K´ena-Cohen , and Christophe Caloz Department of Electrical Engineering, Polytechnique Montr´eal, QC H3T 1J4, Canada Department of Engineering Physics, Polytechnique Montr´eal, QC H3T 1J4, Canada Currently with the National Research Council Canada, Ottawa, ON K1A 0R6, Canada * [email protected] Abstract:
We demonstrate a novel grating coupler design based on double asymmetricand vertically oriented waveguide scatterers to efficiently couple normally incident lightto a fundamental mode silicon waveguide laying on a buried oxide layer. © 2019 TheAuthor(s)
OCIS codes:
1. Introduction
Grating couplers are ubiquitous in integrated photonics for the conversion of free-space propagating light to guidedlight [1]. To achieve unidirectional propagation, grating couplers are typically designed for obliquely incidentlight. This strategy, however, can lead to alignment difficulty and poor stability. For this reason, there have beensignificant efforts to design efficient grating couplers that work at normal incidence [2]. Typical coupling efficien-cies for such couplers are much lower than their oblique angle counterparts, and limited a priori by their lack ofdirectionality.There are different strategies to design normal-incidence grating couplers. The simplest one is to use amomentum-matched binary grating. However, such a structure wastes power both in the undesired direction ofthe waveguide and in the zeroth-order beams. To achieve unidrectionality, one can use a blazed grating with asym-metric saw-tooth profile. This suppresses the undesired coupled mode, but involves complex fabrication. A closelyrelated strategy involves modulating the width of the binary grating elements to achieve unidrectionality. Each in-dividual element within a diffraction-period of a binary-blazed grating works like a waveguide that provides phasematching by proapagtional phase delay [3]. However, due to small diffraction-period size, the coupling betweenthese waveguiding elements does not allow for an appropriate phase-match. Moreover, such designs still suffersfrom loss due to the presence of zeroth diffraction order [4]. To reduce such loss, one may use Bragg reflectors ormetallic mirrors below the waveguide structure, but this requires additional layers and may be incompatible withthe overall fabrication process [5].Here, we propose an alternative approach where the multiple waveguiding elements of a binary-blazed gratingis replaced by a single asymmetric waveguiding structure to provide appropriate phase-match. Such a dielectricgrating coupler could solve the aforementioned issues: 1) it uses double – horizontal and vertical – symmetrybreaking, and hence suppresses all of the undesired diffraction orders; 2) the diffraction period consists of asingle waveguiding element that avoids the inter-waveguide coupling issue of binary-blazed gratings; 3) it iscomposed of purely dielectric material, and hence has negligible absorption loss. As the overall grating couplerstructure utilizes two different types of waveguides; vertically oriented waveguides as its diffractive elements anda horizontal waveguide to which the free-space power will be coupled into, we call them vertical waveguide and slab waveguide, respectively to avoid any confusion.
2. Design Rationale
We designed a single-mode silicon slab waveguide on a buried oxide layer (BOX) based for operation at 1550 nm.The slab waveguide operates in the fundamental transverse electric mode. Applying the grating equation, one findsa period required for coupling to normally-incident light, Λ = λ / n eff , to be ∼
600 nm, where n eff is the effectiverefractive index obtained from the slab waveguide dispersion relation.By applying the reciprocity principle, the grating coupler problem can be transformed into the equivalent, butsimpler analysis of a grating decoupler. First, the slab waveguide coupled to the grating should be impedance-matched only at one of its ends, and fully reflective at its other end, which requires symmetry breaking in thehorizontal plane. Second, the grating should decouple light only toward the top, which requires symmetry breakingin the vertical direction. a r X i v : . [ phy s i c s . op ti c s ] A ug o break the horizontal-plane symmetry, we designed a π -shaped silicon vertical waveguide scatterer. Due tothe presence of two different dielectric media at both sides of the vertical waveguide, air at the top and silicon-BOX at the bottom, the overall scattering of the waveguiding element is also vertically asymmetric, and henceradiates with different phases toward the top and the bottom. However, this does not resolve the issue of thezeroth-order beam, since all the scatterer radiate with same phase toward the bottom. In addition, these scatterers,when coupled to the slab waveguide, radiate more effectively toward the bottom than toward the top due to a higherfield concentration in the substrate as compared to air. To resolve these two issues, we inserted a cylindrical holein every two π -scatterer in such a fashion that the phases radiated toward the bottom by the holey and hole-lessscatterers are out-of-phase; as a result the radiation toward the bottom is significantly suppressed. The combinedstructure of both kind of π -scatterers is shown in Fig. 1a.Fig. 1. Proposed asymmetric dielectric-scatterer grating coupler (a) Structure, (b) Poynting vectorplot for Gaussian beam input.
3. Results
We then performed a full-wave simulation of the grating coupler using periodic boundary conditions in CSTStudio. We chose a 6.5 µ m long grating coupler with 11 dielectric vertical waveguide scatterers to demonstratea compact coupler design. In our reciprocal analysis, the slab waveguide reflects 7% power from the desired end(Port-1), whereas it reflects 75% power when fed from the undesired end (Port-2). In addition, when fed from thedesired end, 10% of the power is transmitted to the undesired end of the slab waveguide. This can be minimized toan almost negligible amount by choosing a longer coupler. The remaining 93% power coming in from the desiredend of slab waveguide splits into 2.5:1 between the radiation toward the top and the bottom. It should be notedthat before introducing cylindrical holes in every two π -scatterers, the splitting ratio was 1:1.8 suggesting higherfield concentration in the substrate.Finally, to calculate the coupling efficiency, we irradiated the grating coupler with a Gaussian input beam, asshown in figure 1b, with a 2.5 µ m radius spot size centered on grating coupler. Our simulation results show40% in-coupling efficiency to the desired waveguide direction. We should highlight that the radiation field profileduring the inverse design is not matched to a that of a Gaussian input beam. Consequently, the overall efficiencycan be further increased by laterally adjusting the grating elements to match the desired free-space profile. References
1. D. Taillaert, F. V. Laere, M. Ayre, W. Bogaerts, D. V. Thourhout, P. Bienstman, and R. Baets,“Gratingcouplers for coupling between optical fibers and nanophotonic waveguides,” Jpn. J. Appl. Phys. (8A),6071–6077 (2006).2. J. Sarathy, R. A. Mayer, K. Jung, S. Unnikrishnan,D. L. Kwong, and J. C. Campbell, “Normal-incidencegrating couplers in Ge-Si,” Opt. Lett. (11), 798–800 (1994).3. P. Lalanne, “Waveguiding in blazed-binary diffractive elements,” J. Opt. Soc. Am. A (10), 2517–2520(1999).4. J. Yang, Z. Zhou, H. Jia, X. Zhang, and S. Qin, “A High-performance and compact binary blazed gratingcoupler based on an asymmetric subgrating structure and vertical coupling,” Opt. Lett. (14), 2614–2617(2011).5. D. Taillaert, W. Bogaerts, P. Bienstman, T. F. Krauss, P. Van Daele, I. Moerman, S. Verstuyft, K. De Mesel,and R. Baets, “An out-of-plane grating coupler for efficient butt-coupling between compact planar waveg-uides and single-mode fibers,” IEEE J. Quantum Electron.38