[Paper Review] A self-starting bi-chromatic LiNbO3 soliton microcomb

Yang He, Qi‐Fan Yang+8 more|arXiv (Cornell University)|Dec 22, 2018

Advanced Fiber Laser Technologies44 citations

TL;DR

The paper demonstrates Kerr soliton microcombs in a monolithic lithium niobate microresonator that self-starts via photorefractive effects, supports bi-directional soliton switching, and enables on-chip second-harmonic generation of the soliton spectrum for f-to-2f self-referencing.

ABSTRACT

For its many useful properties, including second and third-order optical nonlinearity as well as electro-optic control, lithium niobate is considered an important potential microcomb material. Here, a soliton microcomb is demonstrated in a monolithic high-Q lithium niobate resonator. Besides the demonstration of soliton mode locking, the photorefractive effect enables mode locking to self-start and soliton switching to occur bi-directionally. Second-harmonic generation of the soliton spectrum is also observed, an essential step for comb self-referencing. The Raman shock time constant of lithium niobate is also determined by measurement of soliton self-frequency-shift. Besides the considerable technical simplification provided by a self-starting soliton system, these demonstrations, together with the electro-optic and piezoelectric properties of lithium niobate, open the door to a multi-functional microcomb providing f-2f generation and fast electrical control of optical frequency and repetition rate, all of which are critical in applications including time keeping, frequency synthesis/division, spectroscopy and signal generation.

Motivation & Objective

• Demonstrate Kerr soliton mode locking in a high-Q lithium niobate microresonator.
• Exploit photorefractive effect to enable self-starting soliton formation and bi-directional switching.
• Observe and analyze second-harmonic generation of the soliton spectrum within LN.
• Characterize soliton self-frequency shift to determine LN Raman shock time.
• Discuss implications for integrated metrology, frequency synthesis, and on-chip f-to-2f referencing.

Proposed method

• Fabricate a z-cut, high-Q lithium niobate microresonator with loaded Q ≈ 2.2×10^6 and FSR ≈ 199.7 GHz.
• Pump the resonator and scan laser frequency to induce Kerr-driven red detuning while leveraging photorefractive nonlinearity to stabilize and self-start solitons.
• Measure comb spectra, pump transmission, and pulse coherence via FROG and heterodyne beat notes.
• Observe second-harmonic generation of the soliton spectrum and associate spectra with single soliton and soliton crystal states.
• Model dynamics with the Lugiato-Lefever equation to confirm self-starting mechanism.
• Characterize soliton self-frequency shift by varying detuning and pump power to extract the Raman shock time of LN.

Experimental results

Research questions

• RQ1 Can Kerr solitons be stably generated in a lithium niobate microresonator?
• RQ2 Does the photorefractive effect enable self-starting and bidirectional switching of soliton states in LN?
• RQ3 Can LN solitons exhibit on-chip second-harmonic generation enabling f-to-2f self-referencing?
• RQ4 What is the Raman-induced soliton self-frequency shift in LN and its relation to pulse width?

Key findings

• Demonstrated single-soliton and multi-soliton states with a sech^2 spectral envelope in LN microresonators.
• Soliton formation is self-starting due to photorefractive effect, enabling stabilization without external triggering.
• Observed second-harmonic generation of the soliton spectrum, including for a soliton crystal, with comparable sech^2 envelopes.
• Measured a single-soliton 3-dB bandwidth of ~27.9 nm in the telecom band and a second-harmonic 3-dB bandwidth of ~9 nm.
• Found bi-directional soliton switching enabled by the slow photorefractive response coupled with fast Kerr dynamics.
• Determined LN Raman shock time τ_R ≈ 6.3 fs from the linear relation between soliton self-frequency shift and 1/τ_s^4, indicating a larger Raman effect than in fused silica.