Kavli Affiliate: Alireza Marandi
| First 5 Authors: Ryoto Sekine, Robert M. Gray, Luis Ledezma, Selina Zhou, Qiushi Guo
| Summary:
Ultrabroadband frequency combs coherently unite distant portions of the
electromagnetic spectrum. They underpin discoveries in ultrafast science and
serve as the building blocks of modern photonic technologies. Despite
tremendous progress in integrated sources of frequency combs, achieving
multi-octave operation on chip has remained elusive mainly because of the
energy demand of typical spectral broadening processes. Here we break this
barrier and demonstrate multi-octave frequency comb generation using an optical
parametric oscillator (OPO) in nanophotonic lithium niobate with only
femtojoules of pump energy. The energy-efficient and robust coherent spectral
broadening occurs far above the oscillation threshold of the OPO and detuned
from its linear synchrony with the pump. We show that the OPO can undergo a
temporal self-cleaning mechanism by transitioning from an incoherent operation
regime, which is typical for operation far above threshold, to an ultrabroad
coherent regime, corresponding to the nonlinear phase compensating the OPO
cavity detuning. Such a temporal self-cleaning mechanism and the subsequent
multi-octave coherent spectrum has not been explored in previous OPO designs
and features a relaxed requirement for the quality factor and relatively narrow
spectral coverage of the cavity. We achieve orders of magnitude reduction in
the energy requirement compared to the other techniques, confirm the coherence
of the comb, and present a path towards more efficient and wider spectral
broadening. Our results pave the way for ultrashort-pulse and ultrabroadband
on-chip nonlinear photonic systems for numerous applications.
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