Integrated turnkey soliton microcombs operated at CMOS frequencies (1911.02636v1)

Abstract: While soliton microcombs offer the potential for integration of powerful frequency metrology and precision spectroscopy systems, their operation requires complex startup and feedback protocols that necessitate difficult-to-integrate optical and electrical components. Moreover, CMOS-rate microcombs, required in nearly all comb systems, have resisted integration because of their power requirements. Here, a regime for turnkey operation of soliton microcombs co-integrated with a pump laser is demonstrated and theoretically explained. Significantly, a new operating point is shown to appear from which solitons are generated through binary turn-on and turn-off of the pump laser, thereby eliminating all photonic/electronic control circuitry. These features are combined with high-Q $Si_3N_4$ resonators to fully integrate into a butterfly package microcombs with CMOS frequencies as low as 15 GHz, offering compelling advantages for high-volume production.

• The paper introduces a turnkey method that enables stable soliton microcomb generation by integrating the pump laser directly with the microresonator.
• Experimental results validate consistent soliton states and various repetition rates in a CMOS-compatible design, reducing startup complexity.
• The breakthrough simplifies microcomb operation, lowering cost and complexity for applications in frequency metrology and on-chip spectroscopy.

Integrated Turnkey Soliton Microcombs Operated at CMOS Frequencies

The paper "Integrated turnkey soliton microcombs operated at CMOS frequencies" addresses significant challenges related to the integration and operation of soliton microcombs, which are crucial for advanced applications in frequency metrology and spectroscopy. Soliton microcombs have faced roadblocks due to the demanding requirements for power and the complexity of startup and stabilization processes. The breakthrough presented in this paper is the demonstration of a novel regime for soliton microcombs that facilitates simplified operation by eliminating complex controls typically necessary for initiating and maintaining soliton states.

Key Contributions and Findings

This research introduces a new operating point for soliton microcombs that allows for a highly simplified "turnkey" operation. By co-integrating soliton microcombs with a pump laser, a deterministic and stable generation of solitons is achieved. This mode of operation bypasses the need for complex electronic or photonic feedback control systems. The key innovation lies in the utilization of feedback effects between the high-Q microresonator and a pump laser without optical isolation. The impact of non-linear dynamics is fully considered, leading to the realization of this stable operating point defined by the system's parameters.

The paper provides substantial numerical and experimental evidence showing that soliton generation can be initiated through a straightforward binary action of turning the pump laser on. The confirmation comes from measurements analyzing soliton power and beatnote signals, which are consistent with the predicted stable operational state.

Technical Advancements and Experimental Results

Experimental results demonstrate the ability to produce soliton microcombs within a butterfly package, yielding significant advantages for scalable manufacturing. A range of soliton states is achieved, each operating at different repetition rates while maximizing the integration with CMOS-compatible circuitry. Crucially, the system is shown to be robust enough to maintain soliton generation over extended periods without external interference, underscoring its suitability for practical applications.

The turnkey soliton mode-locking is shown to function reliably, as exhibited by successful consecutive switching-on tests. The relationship between feedback phase and pump power is explored to map the phase diagram of accessible comb states, indicating that soliton generation can be precisely controlled.

Practical and Theoretical Implications

The implications of this research are multifaceted. Practically, this development advances the technology toward scalable production of microcombs that could be widely used in applications such as miniaturized frequency synthesizers, precision timekeeping, and on-chip spectroscopy. Theoretically, the findings enhance the understanding of nonlinear dynamic behaviors in unisolated laser-microcomb systems, offering new insights into managing resonance detuning and feedback phenomena.

Eliminating the need for isolators and decreasing dependence on electronic control circuits not only reduces the complexity and cost of soliton microcomb systems but also improves their reliability. This paper could pave the way for future work in integrating this system within monolithic platforms and exploring the adaptability of turnkey operations in different resonator materials and architectures.

Future Directions

The seamless operation enabled by feedback locking introduces opportunities for further integration with other photonic and electronic devices, potentially leading to fully chip-based systems. Future research may explore integrating active phase control to further refine soliton state selection or enhance the robustness of soliton microcombs under varying operational conditions. The implications on the design of feedback control strategies could also be investigated to accommodate deviances from ideal conditions, facilitating commercial applications across broader operational spectra.

In conclusion, the paper provides a critical step toward simple soliton microcomb operation, making the technology more accessible for a broader range of applications and setting the stage for continuous advancements in integrated photonics.