31 research outputs found
Tailoring microcombs with inverse-designed, meta-dispersion microresonators
Nonlinear-wave mixing in optical microresonators offers new perspectives to
generate compact optical-frequency microcombs, which enable an ever-growing
number of applications. Microcombs exhibit a spectral profile that is primarily
determined by their microresonator's dispersion; an example is the spectrum of dissipative Kerr solitons under anomalous
group-velocity dispersion. Here, we introduce an inverse-design approach to
spectrally shape microcombs, by optimizing an arbitrary meta-dispersion in a
resonator. By incorporating the system's governing equation into a genetic
algorithm, we are able to efficiently identify a dispersion profile that
produces a microcomb closely matching a user-defined target spectrum, such as
spectrally-flat combs or near-Gaussian pulses. We show a concrete
implementation of these intricate optimized dispersion profiles, using
selective bidirectional-mode hybridization in photonic-crystal resonators.
Moreover, we fabricate and explore several microcomb generators with such
flexible `meta' dispersion control. Their dispersion is not only controlled by
the waveguide composing the resonator, but also by a corrugation inside the
resonator, which geometrically controls the spectral distribution of the
bidirectional coupling in the resonator. This approach provides programmable
mode-by-mode frequency splitting and thus greatly increases the design space
for controlling the nonlinear dynamics of optical states such as Kerr solitons.Comment: 16 pages, includes S
Stably accessing octave-spanning microresonator frequency combs in the soliton regime
Microresonator frequency combs can be an enabling technology for optical
frequency synthesis and timekeeping in low size, weight, and power
architectures. Such systems require comb operation in low-noise, phase-coherent
states such as solitons, with broad spectral bandwidths (e.g., octave-spanning)
for self-referencing to detect the carrier-envelope offset frequency. However,
stably accessing such states is complicated by thermo-optic dispersion. For
example, in the Si3N4 platform, precisely dispersion-engineered structures can
support broadband operation, but microsecond thermal time constants have
necessitated fast pump power or frequency control to stabilize the solitons. In
contrast, here we consider how broadband soliton states can be accessed with
simple pump laser frequency tuning, at a rate much slower than the thermal
dynamics. We demonstrate octave-spanning soliton frequency combs in Si3N4
microresonators, including the generation of a multi-soliton state with a pump
power near 40 mW and a single-soliton state with a pump power near 120 mW. We
also develop a simplified two-step analysis to explain how these states are
accessed in a thermally stable way without fast control of the pump laser, and
outline the required thermal properties for such operation. Our model agrees
with experimental results as well as numerical simulations based on a
Lugiato-Lefever equation that incorporates thermo-optic dispersion. Moreover,
it also explains an experimental observation that a member of an adjacent mode
family on the red-detuned side of the pump mode can mitigate the thermal
requirements for accessing soliton states
A Kerr-microresonator optical clockwork
Kerr microresonators generate interesting and useful fundamental states of
electromagnetic radiation through nonlinear interactions of continuous-wave
(CW) laser light. Using photonic-integration techniques, functional devices
with low noise, small size, low-power consumption, scalable fabrication, and
heterogeneous combinations of photonics and electronics can be realized. Kerr
solitons, which stably circulate in a Kerr microresonator, have emerged as a
source of coherent, ultrafast pulse trains and ultra-broadband
optical-frequency combs. Using the f-2f technique, Kerr combs support
carrier-envelope-offset phase stabilization for optical synthesis and
metrology. In this paper, we introduce a Kerr-microresonator optical clockwork
based on optical-frequency division (OFD), which is a powerful technique to
transfer the fractional-frequency stability of an optical clock to a lower
frequency electronic clock signal. The clockwork presented here is based on a
silicon-nitride (SiN) microresonator that supports an optical-frequency
comb composed of soliton pulses at 1 THz repetition rate. By electro-optic
phase modulation of the entire SiN comb, we arbitrarily generate
additional CW modes between the SiN comb modes; operationally, this
reduces the pulse train repetition frequency and can be used to implement OFD
to the microwave domain. Our experiments characterize the residual frequency
noise of this Kerr-microresonator clockwork to one part in , which
opens the possibility of using Kerr combs with high performance optical clocks.
In addition, the photonic integration and 1 THz resolution of the SiN
frequency comb makes it appealing for broadband, low-resolution liquid-phase
absorption spectroscopy, which we demonstrate with near infrared measurements
of water, lipids, and organic solvents