  We demonstrate that the optical spectrum of a soliton microcomb generated in
a microresonator can be coherently extended by using bichromatic pumping: one
pump with a frequency in the anomalous dispersion regime of the microresonator
is used to generate a bright soliton microcomb while a second pump in the
normal dispersion regime both compensates the thermal effect of the
microresonator and generates a second frequency comb via cross-phase modulation
with the bright soliton, thus extending the spectrum of the microcomb.
Experimental results for different microresonators show that the mode spacings
of the two combs are synchronised, even when the combs are generated in
different mode families. Numerical simulations agree well with experimental
results and reveal that a bright optical pulse from the second pump is
passively formed in the normal dispersion regime and trapped by the bright
soliton. We believe the demonstrated technique provides an alternative way to
generate broadband microcombs and multi-coloured frequency combs that can be
useful for frequency metrology and spectroscopy. In addition, this method
enables the selective enhancement of available optical power in specific parts
of the comb spectrum.
