Electrically-pumped soliton microcombs on thin-film lithium niobate

TL;DR

This work demonstrates electrically pumped Kerr soliton microcombs on thin-film lithium niobate by coupling a DFB laser directly to a high-$Q$ TFLN microring and using self-injection locking for turnkey soliton initiation. With edge-coupler optimization, a target $FSR$ of $\sim200~\mathrm{GHz}$, and a measured on-chip pump of about $25~\mathrm{mW}$, the system achieves $200~\mathrm{GHz}$ repetition and >$180~\mathrm{nm}$ optical span, along with a narrowed laser linewidth to $4.7~\mathrm{kHz}$. Coherence characterization via delayed self-heterodyne shows $-130~\mathrm{dBc/Hz}$ phase noise at $1~\mathrm{MHz}$ on the soliton repetition rate and a transduction of pump-noise to repetition-noise by ~40 dB at $2\times10^5$ Hz. These results establish integrated TFLN comb sources suitable for communications, computation, and metrology, and point toward fully integrated photonic circuits combining EO modulation, nonlinear frequency conversion, and frequency references on a single chip.

Abstract

Thin-film lithium niobate (TFLN) has enabled efficient on-chip electro-optic modulation and frequency conversion for information processing and precision measurement. Extending these capabilities with optical frequency combs unlocks massively parallel operations and coherent optical-to-microwave transduction, which are achievable in TFLN microresonators via Kerr microcombs. However, fully integrated Kerr microcombs directly driven by semiconductor lasers remain elusive, which has delayed integration of these technologies. Here we demonstrate electrically pumped TFLN Kerr microcombs without optical amplification. With optimized laser-to-chip coupling and optical quality factors, we generate soliton microcombs at a 200 GHz repetition frequency with an optical span of 180 nm using only 25 mW of pump power. Moreover, self-injection locking enables turnkey initiation and substantially narrows the laser linewidth. Our work provides integrated comb sources for TFLN-based communicational, computational, and metrological applications.

Figures (6)

• Figure 1: Electrically pumped TFLN soliton microcombs and representative applications.
• Figure 2: Device layout. (a) The DFB laser and the microresonator. The LN waveguide width is reduced from $W_{1}$ to $W_{2}$ over a length $L$ to enable efficient coupling to the laser. Insets: simulated fundamental transverse-electric (TE) modes of the DFB laser (left), the input facet (middle), and the bus waveguide (right). (b) Simulated laser-chip coupling efficiency as a function of the facet size. (c) Simulated transition efficiency as a function of the taper length.
• Figure 3: Design optimization for DFB-laser–pumped parametric oscillation. Threshold power map $P_\mathrm{th}$ as a function of intrinsic quality factor $Q_o$ and FSR under the assumption of optimal coupling $\eta=1/3$ . The blue region indicates the soliton generation space, bounded by the red dashed line at $P_\mathrm{th}=12.5~\mathrm{mW}$. The star marker denotes the target operating point ($Q_o=3\times10^{6}$, $\mathrm{FSR}=200~\mathrm{GHz}$) with a calculated threshold of $5.4~\mathrm{mW}$.
• Figure 4: Device characterization. (a) Scanning electron micrograph of the microresonator. (b) Normalized transmission near $\lambda=1550.61~\mathrm{nm}$ with a fit yielding $Q_o=2.93\times10^6$ and $Q_e=6.57\times10^6$ (blue: data; red: fit). (c) Integrated dispersion $D_\mathrm{int}/2\pi$ of the fundamental TE mode.
• Figure 5: Experimental soliton microcomb generation. (a) Experimental setup. (b) Measured comb power versus driving current of the DFB laser. (c) Optical spectra for (i) a two-soliton state and (ii) a single-soliton state, corresponding to the plateaus in panel (b). (d) Repeatable turnkey initiation tests.