Enhancing Energy Transfer in Hybrid Photonic Integration using Grating-Assisted Codirectional Couplers

Abstract

Hybrid integration of active III-V materials on a silicon-on-insulator (SOI) platform has been extensively explored in recent years due to its potential to combine the advantages of both material systems. However, one of the critical challenges in this integration is achieving efficient coupling of light between the distinct waveguide structures of the active III-V and passive SOI materials. This tutorial article discusses the use of grating-assisted codirectional couplers (GACC) to overcome the phase mismatching between highly asynchronous waveguides and enhance the energy transfer for hybrid photonic integration.

Introduction

Traditional approaches to achieve high-efficiency coupling between III-V and SOI waveguides often involve using a relatively thick Si waveguide layer or adiabatic tapered couplers with a very narrow tip in the III-V or SOI structures. These methods require critical fabrication processes and can encounter multimode waveguide issues or the need for dedicated and costly fabrication. The proposed GACC method aims to enable efficient hybrid integration while using common multi-project wafer (MPW) services in a generic semiconductor foundry.

Device Structure

The hybrid structure consists of active III-V materials bonded to a passive SOI platform with a standard 220nm-thick Si rib waveguide, as shown in Fig. 1. The two materials are bonded together with a 100-nm thick divinylsiloxane-benzocyclobutene (DVS-BCB) adhesive layer.

Fig. 1 Schematic of hybrid III-V/SOI waveguides at (a) side view (b) crosssectional view and (c) detailed lateral grating structure.

The GACC grating structure is formed on the rib of the SOI waveguide, as indicated in Fig. 1(a) and (c). The grating structure has a width of 1.5 μm with 0.5-μm corrugation and a 50% duty cycle.

Operation Principles

The effective index of the III-V waveguide is usually greater than that of the 220-nm thick SOI waveguide, leading to poor coupling efficiency without any compensation. The GACC helps to match the group velocities and produce mode interaction at the resonance condition between the two waveguides.

The grating period (Λ) of the GACC structure can be determined using the coupled mode theory (CMT) and the equation:

Where K is the grating wave-vector, β is the propagation constant of each super-mode, and δ is the deviation wavenumber from the nominal resonant condition.

The coupling length (Lc) can be estimated using the equation:

Simulation Results and Analysis

Modal analysis and Eq. (1) were used to estimate the grating period (Λ) and coupling length of the device. The effective refractive indices of the III-V and SOI waveguides were calculated to be 3.18158 and 2.85373, respectively, at 1550 nm wavelength.

The simulation results indicate that a grating period of 5.42 μm can achieve an optimal coupling coefficient of 82.30% at a coupling length of 275 μm. Fig. 2 compares the power evolution along the propagation direction with and without the GACC structure, highlighting the importance of the GACC for efficient energy transfer between the asynchronous waveguides.

Fig. 2. Power evolution along the propagation direction of hybrid III-V/SOI device (a) with- and (b) without GACC structure at 1550nm.

Fig. 3 shows that >80% coupling efficiency can be obtained over a 20 nm wavelength range, demonstrating the broadband operation of the GACC structure.

Fig. 3. Wavelength dependency of hybrid integration III-V/SOI device.

Further analysis was performed by varying the rib waveguide width in the grating region while fixing the width corrugation at 0.5 μm. As shown in Fig. 4, a narrower rib width results in higher coupling efficiency, shorter coupler length, and smaller grating period. The coupling efficiency can exceed 85% with an optimized design.

Fig. 4. Correlation between grating width variation propagation profiles, on the grating period, coupling efficiency, and coupling length.

Conclusion

The proposed GACC structure enables efficient power coupling of 82.30% from a III-V waveguide to a standard 220nm-thick SOI waveguide, even when the two materials are bonded with a 100-nm thick BCB layer. The GACC can be fabricated using common procedures provided by most silicon photonics foundries, making it a promising solution for hybrid photonic integration. By overcoming the phase mismatching between highly asynchronous waveguides, the GACC structure enhances the energy transfer and enables the realization of high-performance hybrid photonic integrated circuits.

Reference

[1] Novitasari and S.-L. Lee, "Grating-assisted codirectional couplers for hybrid integration of active III-V on a normal SOI platform," Department of Graduate Institute of Electro-Optical Engineering, National Taiwan University of Science and Technology, Taipei City, Taiwan, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.