  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.
