[Paper Review] Integrated turnkey soliton microcombs operated at CMOS frequencies

Boqiang Shen, Lin Chang+15 more|CaltechAUTHORS (California Institute of Technology)|Nov 6, 2019

Advanced Fiber Laser Technologies44 references70 citations

TL;DR

The paper demonstrates turnkey generation of soliton microcombs integrated with a pump laser, achieving CMOS-compatible repetition rates (15–40 GHz) without external control circuitry and in a butterfly package.

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.

Motivation & Objective

• Motivate the integration challenges of soliton microcombs for scalable photonic systems.
• Demonstrate a turnkey soliton generation regime that eliminates complex tuning and external control circuitry.
• Achieve CMOS-compatible repetition rates (15–40 GHz) with high-Q Si3N4 resonators.
• Show robustness to temperature and environmental disturbances in unisolated operation.

Proposed method

• Integrates high-Q Si3N4 microresonators with a DFB pump laser in a butterfly package.
• Utilizes backscattered feedback from the resonator to inject into the pump laser cavity, removing the need for optical isolation.
• Develops a nonlinear dynamical model that couples the soliton field, backscattered field, and laser field (S1–S7).
• Derives a turning-point operating regime where solitons form via binary turn-on of the pump, governed by an intersection of detuning and intracavity power (Eq. S1–S6).
• Provides experimental demonstrations of turnkey soliton generation across 40 GHz, 20 GHz, and 15 GHz repetition rates (Fig. 3).
• Evaluates turn-on success probability versus feedback phase to show repeatable turnkey operation (Fig. 4).

Experimental results

Research questions

• RQ1 Can soliton microcombs be generated turnkey without active optical/electronic control when driven by feedback into the pump laser?
• RQ2 What is the new operating point created by nonlinear feedback that enables turn-on of soliton mode locking?
• RQ3 Is CMOS-frequency operation (15–40 GHz) feasible in an integrated, pump-laser–microresonator package with high-Q Si3N4 resonators?
• RQ4 How robust is turnkey soliton generation to temperature and environmental fluctuations?
• RQ5 What are the practical implications for scalable, low-power, chip-scale frequency comb sources?

Key findings

• A new turnkey operating point appears due to nonlinear backscattering, allowing soliton generation by simply turning on the pump laser.
• Repetition rates of 40 GHz, 20 GHz, and 15 GHz are demonstrated with CMOS-compatible integration.
• Observed optical spectra show single- and multi-soliton states with corresponding electrical beatnotes at the repetition rates.
• Backscattered feedback reduces laser frequency noise by ~30 dB compared to a free-running DFB laser, achieving performance surpassing state-of-the-art integrated lasers.
• Turnkey operation is robust over hours in laboratory conditions and works without external feedback control or optical isolation.
• A DFB laser integrated with high-Q Si3N4 resonators in a butterfly package enables chip-to-chip pumping at challenging repetition rates (≤40 GHz).