Communication-ready high-power soliton microcombs in highly-dispersive Fabry-Perot-microresonators

TL;DR

The paper tackles the challenge of achieving high power per comb line in Kerr soliton microcombs for practical communications. It introduces an integrated Fabry-Pérot microresonator with chirped Bragg gratings to engineer large anomalous dispersion, achieving $D_2/2\pi \approx 90~\mathrm{MHz}$ and enabling $>10$ lines at the $1~\mathrm{mW}$ level, supporting an aggregate $2~\mathrm{Tb/s}$ transmission without amplification. Key results include a total on-chip soliton power of $29.2~\mathrm{mW}$, ten lines above $1~\mathrm{mW}$, low RIN, and ~250 Hz linewidth per line, with a direct 2 Tb/s PON demonstration showing practical viability. The work establishes FP-based high-power soliton microcombs as viable, integration-friendly light sources for next-generation coherent photonic systems and points to pathways for scaling to multi- to petabit-per-second networks.

Abstract

Microcombs generated in optical microresonators are widely regarded as promising light sources for next-generation communication systems, but the optical power available per comb line has so far fallen short of practical requirements. Here we introduce an integrated Fabry-Pérot microresonator platform that overcomes fundamental dispersion-engineering constraints and enables bright soliton microcombs with unprecedented power per line. The resonator is defined by chirped Bragg gratings that provide exceptionally large anomalous group-velocity dispersion, allowing more than ten comb lines to reach the milliwatt level. These combs can be used directly in coherent communication systems without additional amplification, achieving an aggregate data rate of 2 Tb/s. Once integrated, our high-power soliton microcombs could be instantly ready for communications as well as a broad range of practical comb-based applications.

Figures (5)

• Figure 1: Applications and implementation of high-power soliton microcombs. (a) Representative microcomb-enabled applications and their required power per comb line. (b) Conceptual spectra of microcombs under different group-velocity-dispersion (GVD) conditions. (c) Schematic comparison between a ring microresonator (left) and a chirped Bragg-grating Fabry-Pérot (FP) microresonator (right). (d) Illustration of the dispersion-engineering degrees of freedom and accessible range afforded by FP versus ring microresonator geometries.
• Figure 2: Design of an integrated chirped Bragg-grating Fabry--Pérot microresonator. (a) Schematic of the integrated FP microresonator formed by two chirped Bragg gratings (CBGs) and a uniform waveguide section. (b) Enlarged view of a CBG, whose modulation depth is apodized using a Blackman function. (c) Simulated dependence of the GVD on the chirp rate. (d) Reflectivity of the Bragg grating as a function of grating length. (e) Simulated coupling $Q$ factor versus grating length. (f) Integrated dispersion and its deviation from a purely quadratic profile for different apodization schemes.
• Figure 3: Characterization of the integrated FP microresonator. (a) Optical micrograph of the chip, zoomed-in view of the integrated FP microresonator, and scanning electron micrograph of the chirped Bragg grating. (b) Normalized reflection spectrum of the microresonator. (c) Intrinsic dissipation and external coupling rates of the microresonator. (d) Measured integrated dispersion of the FP microresonator compared to a ring resonator with the same waveguide cross-section.
• Figure 4: Generation and characterization of the soliton microcombs. (a) Experimental setup for soliton generation and characterization. EDFA, erbium-doped fiber amplifier; BPF, bandpass filter; FPC, fiber polarization controller; PD, photodetector; OSC, oscilloscope; OSA, optical spectral analyzer. (b) Measured transmitted power versus scan time during laser-frequency tuning. Different intracavity optical states are indicated. (c) Optical spectra of solitons generated in the FP microresonator and in a ring microresonator with a similar $Q$ factor and cross-section. (d) Measured single-sideband frequency-noise spectra of selected comb lines. (e) Measured relative intensity noise (RIN) spectra of the same comb lines.
• Figure 5: Comb-driven transmission experiments. (a) Experimental setup for passive optical network transmission. AWG, arbitrary waveform generator; LO, local oscillator provided by a tunable laser. (b) Optical spectrum of the ten comb lines used in the transmission experiments, sampled before the bandpass filter. (c) Measured bit-error ratios (BERs) of the data channels in the X polarization (blue) and Y polarization (red). The dashed line denotes a forward-error-correction (FEC) threshold at $4\times10^{-2}$, corresponding to a 20% soft-decision FEC based on spatially coupled low-density parity-check codes schuh2017single. (d) Representative constellation diagrams for both polarizations at a carrier wavelength of 1551.77 nm, with color indicating the relative symbol density.