Heterogeneously Integrated Passively Mode-Locked Quantum Dot Laser on Silicon

Introduction

The ever-increasing demand for high-performance computing and high-speed communication systems has driven significant research efforts towards developing photonic integrated circuits (PICs) that incorporate wavelength-division multiplexing (WDM). Silicon photonics has emerged as a leading platform, offering energy-efficient, cost-effective, compact, and highly scalable solutions due to its compatibility with well-established complementary metal-oxide-semiconductor (CMOS) fabrication technologies.

However, the pursuit of even greater bandwidth and lower energy consumption necessitates the development of fully integrated strategies that incorporate on-chip light sources characterized by low power consumption and high channel count, which are conducive to WDM applications. The heterogeneous integration of III-V materials with silicon substrates is widely acknowledged as the most viable route for fabricating on-chip light sources.

Quantum Dot Mode-Locked Lasers

Traditional high-bandwidth WDM systems typically employ an array of single-wavelength lasers, such as distributed feedback (DFB) lasers, which are collectively multiplexed. However, recent advancements in quantum dot (QD) based multiwavelength lasers have demonstrated markedly low mode partition noise, an intrinsically inhomogeneously broadened gain spectrum, rapid carrier dynamics, a substantial gain-to-saturable absorber (SA) saturation energy ratio, and minimal amplified spontaneous emission (ASE) noise. These attributes render QDs as an exemplary active medium for mode-locked lasers (MLLs).

Figure 1: (a) Outlines a heterogeneously integrated evanescent QD laser, including a simulation of the fundamental TE mode and an SEM image of III-V/Si taper transitions. (b) Depicts a schematic of a second harmonic heterogeneous QD MLL paired with a micro-ring modulator. (c) Shows the fabrication steps for integrating the QD gain elements. (d) Provides a cross-sectional diagram of the laser gain section.

Heterogeneous QD Mode-Locked Laser

The heterogeneous QD MLL features a cavity delineated by a pair of Sagnac loop mirrors, as shown in Figure 1b, with the mirror reflectivity dictated by the directional couplers' length. Within the composite III-V/Si gain segment, SAs are fashioned through targeted etching processes in the laser mesa above the cladding layer that achieve electrical isolation. These SAs are strategically positioned to segment the laser cavity into portions that correspond to the optical lengths requisite for the desired repetition rate.

To facilitate the transition of hybrid optical modes into modes confined exclusively within the Si waveguide, parametrically optimized linear tapers are implemented in both the III-V and Si components, minimizing undesired reflections and optical losses. The light is then efficiently extracted from the PIC via tapered, angled facets designed to suppress parasitic reflections and the excitation of higher-order modes.

Fabrication Process

Figure 1c illustrates the fabrication flow for the heterogeneous integration platform. A principal challenge in the fabrication process is the implementation of protective strategies for the underlying Si waveguides and peripheral Si devices during the III-V integration.

The process commences with an SOI wafer, pre-fabricated with the Si components of the hybrid laser cavities and any Si devices intended for direct laser connection. This wafer is shielded by an oxide layer, which is subsequently selectively etched away to expose the Si surface for bonding. After the GaAs substrate is removed, the QD MLL structure is fabricated, encompassing various steps such as deposition of metal layers, mesa etching, passivation, SA isolation etching, via creation, and probe metallization.

Another critical fabrication concern is the precision dry etching of the III-V mesa to reach the 150-nm n-contact GaAs stratum, ensuring an even etch depth across the wafer. Addressing this etching difficulty is imperative for maintaining the structural fidelity and functional efficacy of the resultant integrated device.

Conclusion and Future Work

The presented heterogeneous integration platform allows the direct integration of QD MLLs with high-performance Si devices. Future ongoing work includes the integration of QD semiconductor optical amplifiers to further boost the power performance and provide more flexibility in system-level design, as well as the integration of high-speed QD photodetectors for a fully integrated link.

This article provides an overview of the heterogeneously integrated passively mode-locked quantum dot laser on silicon, including its design, fabrication process, and future prospects. The integration of QD MLLs with Si photonics promises great advancements for high-performance computing, high-bandwidth communications, and precision spectroscopy in next-generation PICs.

Reference

[1] W. He, X. Ou, Y. Shi, A. Prokoshin, X. Yao, and Y. Wan, "Heterogeneously Integrated Passively Mode-Locked Quantum Dot Laser on Silicon," Integrated Photonics Lab, King Abdullah University of Science and Technology (KAUST), Thuwal, Kingdom of Saudi Arabia, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.