Ultra-Broadband Kerr Microcomb Through Soliton Spectral Translation

TL;DR

This paper introduces synthetic dispersion via a second pump laser to extend Kerr microcomb bandwidths beyond traditional limits, demonstrated through simulations and experiments achieving nearly two-octave spans in silicon nitride resonators.

Contribution

The novel concept of synthetic dispersion using a second pump laser enables ultra-broadband Kerr microcombs beyond geometric dispersion constraints.

Findings

Experimental microcombs with nearly two-octave bandwidths

Synthetic dispersion effectively extends comb bandwidths

Phase coherence verified through beat note measurements

Abstract

Broad bandwidth and stable microresonator frequency combs are critical for accurate and precise optical frequency measurements in a compact and deployable format. Typically, broad bandwidths (e.g., octave spans) are achieved by tailoring the microresonator's geometric dispersion. However, geometric dispersion engineering alone may be insufficient for sustaining bandwidths well beyond an octave. Here, we introduce the novel concept of synthetic dispersion, in which a second pump laser effectively alters the dispersion landscape to create Kerr soliton microcombs that extend far beyond the anomalous geometric dispersion region. Through detailed numerical simulations, we show that the synthetic dispersion model captures the system's key physical behavior, in which the second pump enables non-degenerate four-wave mixing that produces new dispersive waves on both sides of the spectrum. We experimentally demonstrate these concepts by pumping a silicon nitride microring resonator at 1060 nm and 1550 nm to generate a single soliton microcomb whose bandwidth approaches two octaves (137 THz to 407 THz) and whose phase coherence is verified through beat note measurements. Such ultra-broadband microcombs provide new opportunities for full microcomb stabilization in optical frequency synthesis and optical atomic clocks, while the synthetic dispersion concept can extend microcomb operation to wavelengths that are hard to reach solely through geometric dispersion engineering.