Electrically pumped photonic integrated soliton microcomb (1810.03909v3)

Abstract: Optical frequency combs have revolutionized frequency metrology and timekeeping, and can be used in a wide range of optical technologies. Advances are under way that allow dramatic miniaturization of optical frequency combs using Kerr nonlinear optical microresonators, where broadband and coherent optical frequency combs can be generated from a continuous wave laser. Such `microcombs', provide a broad bandwidth with low power consumption, unprecedented form factor, wafer scale fabrication compatibility. For future high volume applications, integration and electrical pumping of soliton microcombs is essential. To date, however, microcombs still rely on optical pumping by bulk external laser modules that provide the required coherence, frequency agility and power levels for soliton formation. Electrically-driven, chip-integrated microcombs are inhibited by the high threshold power for soliton formation, typically exceeding the power of integrated silicon based lasers, and the required frequency agility for soliton initiation. Recent advances in high-Q Si3N4 microresonators suggest that electrically driven soliton microcombs are possible. Here we demonstrate an electrically-driven, chip-integrated soliton microcomb by coupling an indium phosphide (III-V) multiple-longitudinal-mode laser diode chip to a high-Q Si3N4 photonic integrated microresonator. We observe that self-injection locking of the laser diode to the microresonator, which is accompanied by a x1000 fold narrowing of the laser linewidth, can simultaneously initiate the formation of dissipative Kerr solitons. By tuning the current, we observe a transition from modulation instability, breather solitons to single soliton states. The system requires less than 1 Watt of electrical power, operates at electronically-detectable sub-100 GHz mode spacing and can fit in a volume of ca. 1cm3.

• The paper presents an integrated InP laser diode and Si₃N₄ microresonator system that achieves self-injection locking and narrows the laser linewidth by a factor of 1000.
• It demonstrates the controlled transition from modulation instability to stable dissipative Kerr solitons with approximately 100 kHz comb linewidth at sub-100 GHz spacing.
• The approach enables scalable, low-power (<1 W) production, paving the way for applications in LIDAR, optical interconnects, and broader photonic integration.

Electrically Pumped Photonic Integrated Soliton Microcomb

The paper presents a significant advancement in the field of optical frequency comb technology, specifically focusing on the integration and electrical pumping of soliton microcombs. Optical frequency combs have been transformative for frequency metrology and timekeeping, with applications expanding across various optical technologies. The work explores the potential of using Kerr nonlinear optical microresonators to miniaturize these combs, aiming to facilitate their deployment in broader industrial and commercial contexts.

Key Findings

1. Integration with Si$_3$N$_4$ Microresonators: The paper demonstrates the coupling of an indium phosphide (InP) multi-longitudinal-mode laser diode with a high-Q Si$_3$N$_4$ photonic integrated microresonator. This coupling achieves self-injection-locking of the diode to the microresonator, resulting in a significant narrowing of the laser linewidth by a factor of 1000.
2. Formation of Dissipative Kerr Solitons: By adjusting the laser diode current, it is possible to transition through states of modulation instability and breather solitons to stable single soliton states. The microcomb generated exhibits narrow linewidth comb teeth (~100 kHz), requiring less than 1 Watt of electrical power and operating at sub-100 GHz mode spacing within a volume of approximately 1 cm$^3$.
3. Potential for Mass Manufacturing: The approach highlights compatibility with existing semiconductor laser diode technology, outlining a feasible pathway towards scalable manufacturing. This could meet the demands of high-volume applications such as laser ranging (LIDAR) and optical interconnects.

Implications

The paper's findings suggest a pivotal step towards the full integration of soliton microcombs. The potential to leverage high-Q Si$_3$N$_4$ microresonators with InP laser diodes simplifies the complex nature of microcomb generation. It also shifts the paradigm from reliance on bulk, external laser modules to entirely chip-integrated solutions. This evolution could substantially reduce cost, size, and power requirements, making ultra-compact soliton frequency combs a viable technology for widespread deployment.

Future Directions

The research underscores a promising future where photonic integrated circuits (PICs) can encompass sophisticated soliton microcombs. This prospect raises several pathways for additional investigation:

• Enhancements to Tunability and Stability: Further examination of tunability and spectral stability could bolster the robustness of microcomb applications in dynamic environments.
• Integration with Other Photonic Components: Exploring the fusion of soliton microcombs with modulator and detector elements within the same PIC might lead to innovative solutions for data communications and signal processing.
• Broadening of Application Domains: As technical barriers diminish, applications could extend into aerospace, defense, healthcare monitoring systems, and beyond.

Conclusion

The paper successfully illustrates a pathway towards high-integration, electrically-driven soliton microcombs using advanced materials and technologies. Its implications are valuable for both theoretical research and practical applications, aiming to embed frequency comb technologies into ubiquitous use cases. The findings pave the way for future explorations into the capabilities of photonic integrated circuits and showcase the continual blurring of boundaries between electronics and photonics.