1.Project Background

Benefiting from the maturity and scalability of CMOS processes, silicon photonics technology has rapidly advanced in recent years. Compact, low-cost photonic circuits based on silicon chips now integrate mature silicon waveguides, filters, modulators, and detectors, forming a solid foundation for applications in high-speed optical communications, optical interconnects (Optical I/O), photonic computing, and biosensing. However, silicon itself is an indirect bandgap semiconductor incapable of efficient light emission, posing significant challenges in realizing efficient on-chip optical sources. Additionally, photonic chips inherently introduce losses requiring on-chip amplifiers to compensate for signal attenuation. Traditional optical communication systems typically employ erbium-doped fiber amplifiers (EDFAs) or praseodymium-doped fiber amplifiers (PDFAs), which are too bulky for direct chip-level integration and necessitate additional pump sources. Furthermore, integrated lasers must resist optical feedback to maintain performance, yet silicon cannot easily accommodate traditional optical isolators like ferrite materials. Thus, developing high-performance, integrable on-chip silicon-based optical sources and amplifiers remains a critical bottleneck in silicon photonics.

2.Technical Breakthrough: Standardized Quantum Dot Device Integration

To address these challenges, researchers Yang Liu, Jing Zhang, and Gunther Roelkens from IMEC recently published a featured cover article titled "Micro-transfer printing of O-band InAs/GaAs quantum-dot SOAs on silicon photonic integrated circuits" in Photonics Research, reporting their development of standardized InAs/GaAs quantum-dot (QD) active devices integrated onto silicon photonic chips via micro-transfer printing. "Standardization" implies pre-fabricating and optimizing these QD active devices on III-V semiconductor wafers, ensuring consistent dimensions and interfaces suitable for various applications without repetitive custom design. The team chose InAs/GaAs quantum-dot materials as gain media, operating in the telecom O-band (~1.3 μm), which inherently exhibit small linewidth enhancement factors and insensitivity to temperature and feedback, enhancing device robustness. Using micro-transfer printing, these GaAs-based QD active devices are precisely transferred from the III-V source wafer to designated functional regions on CMOS-processed silicon photonic circuit wafers, enabling various types of integrated optical sources and amplifiers. This approach allows silicon photonic components and III-V lasers to be fabricated independently on their respective optimal process lines and integrated subsequently, significantly enhancing process flexibility and yield.

Schematic process flow for the integration of GaAs QD active devices on a SiPh platform

3."Lego-style" On-chip Light Source Solution

The standardized GaAs quantum-dot active devices, when combined with various silicon photonic structures, enable multiple functionalities:

1.Integrated with silicon Bragg-grating waveguides to form distributed feedback (DFB) lasers;

2.Coupled with silicon Sagnac ring reflectors and microring resonators to realize tunable lasers;

3.Directly coupled to silicon straight waveguides to serve as optical amplifiers.

Overall, the team's breakthrough lies in offering a modular on-chip optical source solution. Pre-fabricated quantum-dot "standard units" can be flexibly integrated into silicon photonic circuits, significantly reducing complexity in silicon photonics integration.

4.Performance of DFB Lasers, SOAs, and Tunable Lasers

DFB lasers: At room temperature (20°C), lasing begins at an injection current of approximately 70 mA, achieving a maximum waveguide-coupled output power of around 19.7 mW. Notably, inherent ultrafast carrier dynamics and gain saturation properties of QD media render these devices insensitive to optical feedback. Even with direct feedback around -20 dB without an isolator, laser performance shows minimal variation, supporting error-free data transmission exceeding 30 Gbps. At elevated temperatures (40°C), output power remains robust at 14 mW, demonstrating good thermal stability.

Semiconductor optical amplifiers (SOAs): The device exhibits significant gain in the 1.3 μm band, achieving a small-signal gain of approximately 7.5 dB at 1299 nm. It maintains stable gain over a wide input power range, ideal for on-chip signal amplification. Additionally, the SOA demonstrates excellent current-voltage characteristics and low differential resistance (~3.8 Ω), enabling operation under high current conditions with minimal self-heating, beneficial for achieving higher output powers.

Widely tunable lasers: Utilizing silicon photonic microring Vernier filter arrays and tunable Sagnac ring reflectors as resonant cavities, the team achieved continuous wavelength tuning over 35 nm, ranging from 1280.6 nm to 1315.7 nm, covering the entire O-band.

5.Team Commentary and Outlook

Dr. Yang Liu: This achievement marks a significant step toward practical on-chip optical sources. Firstly, standardized designs combined with micro-transfer printing enable mass production and high-density integration of lasers and amplifiers on silicon photonic chips, supporting parallel optical communication channels and complex photonic integrated circuits. This technology offers scalable optical source solutions for high-speed optical interconnects in data centers, optical transceiver modules, and photonic computing applications. Secondly, the inherent stability of quantum-dot lasers eliminates the need for optical isolators, simplifying optical module design and enhancing system stability. Consequently, on-chip optical sources will no longer be a bottleneck but rather modular components easily integrated for diverse applications. Future work includes:

I. Device Performance Optimization: Improving SOA gain, laser output power, and reducing threshold currents and power consumption through design enhancements.

II. System-level Integration of Diverse Materials: Integrating other materials like piezoelectric and ferromagnetic materials to enable complex on-chip functionalities for comprehensive photonic communication, computing, and memory systems.

III. Industrialization Exploration: Collaborating with industrial partners to incorporate this technology into manufacturing processes, validating stability and cost-effectiveness for mass production, thus advancing toward practical applications.

Through these efforts, we anticipate that micro-transfer integration technology will accelerate photonic chip development, achieving a comprehensive integration of active and passive silicon photonics, paving new avenues for future communication and computing.