Selective Out‐of‐Plane Optical Coupling between Vertical and Planar Microrings in a 3D Configuration

Abstract

optoelectronic systems, [4] serving as optical filters, [5] modulators, [6] and frequency comb generators, [7] to name a few. Furthermore, micro/nanoresonator coupling in one plane has been widely investigated [8-13] for applications such as wavelength division multiplexing, [14] sensing, [15] and lasing modulation. [16] In recent years, various schemes have been proposed to conquer the third dimension, such as laser inscription of 3D waveguide circuits, [17] interlayer grating couplers for vertical integration of multiple photonic components, [18-20] and photonic routing architectures based on multiple waveguides for out-of-plane light coupling. [21] However, these schemes mainly rely on simple passive light transfer components, which do not use the 3D space efficiently. For highdensity 3D photonic integration, a new approach is required to not only transfer light from layer to layer but also to fill the space in between the layers with photonic functionalities such as flexible reorientation of light propagation and wavelength-selective transfer between different layers. In contrast to planar ring resonators, a microtube optical ring resonator is able to support out-of-plane whispering gallery modes (WGM) in the vertical direction, [22,23] therefore providing a concept to also exploit the vertical direction with additional 3D photonic integrated circuits are expected to play a key role in future optoelectronics with efficient signal transfer between photonic layers. Here, the optical coupling of tubular microcavities, supporting resonances in a vertical plane, with planar microrings, accommodating in-plane resonances, is explored. In such a 3D coupled composite system with largely mismatched cavity sizes, periodic mode splitting and resonant mode shifts are observed due to mode-selective interactions. The axial direction of the microtube cavity provides additional design freedom for selective mode coupling, which is achieved by carefully adjusting the axial displacement between the microtube and the microring. The spectral anticrossing behavior is caused by strong coupling in this composite optical system and is excellently reproduced by numerical modeling. Interfacing tubular microcavities with planar microrings is a promising approach toward interlayer light transfer with added optical functionality in 3D photonic systems.

Simulation Study on 3D Heterogeneous Photonic Integration with Vertical Microring Coupler

“…Consequently, these limitations impede the achievable performance metrics of the overall photonic integrated chip. Three-dimensional (3D) photonic integration has emerged as a promising approach that enables the integration of multiple photonic components in a vertical stack, offering enhanced functionality and a reduced footprint [10][11][12][13]. Self-rolled-up micro-resonators monolithically integrated on a silicon ridge waveguide have been demonstrated as a viable route for realizing 3D photonic integration [14][15][16][17][18].…”

High-specificity molecular sensing on an individual whispering-gallery-mode cavity: coupling-enhanced Raman scattering by photoinduced charge transfer and cavity effects

“…14,15 (2) It is challenging to couple the light from free space into a WGM cavity due to momentum mismatch. Near-field coupling approaches are generally required to induce light into the cavity, such as prism coupling, 16 tapered fiber coupling, 17 and waveguide coupling, 18 which are complex for optical circuit manipulations and coupling operations. (3) To improve the sensitivity of WGM cavities, a higher Q -factor or a smaller mode volume must be satisfied, requiring high-precision micro- and nanofabrication techniques.…”