218 research outputs found
Coupling ideality of integrated planar high-Q microresonators
Chipscale microresonators with integrated planar optical waveguides are
useful building blocks for linear, nonlinear and quantum optical devices. Loss
reduction through improving fabrication processes has resulted in several
integrated micro resonator platforms attaining quality (Q) factors of several
millions. However only few studies have investigated design-dependent losses,
especially with regard to the resonator coupling section. Here we investigate
design-dependent parasitic losses, described by the coupling ideality, of the
commonly employed microresonator design consisting of a microring resonator
waveguide side-coupled to a straight bus waveguide. By systematic
characterization of multi-mode high-Q silicon nitride microresonator devices,
we show that this design can suffer from low coupling ideality. By performing
full 3D simulations to numerically investigate the resonator to bus waveguide
coupling, we identify the coupling to higher-order bus waveguide modes as the
dominant origin of parasitic losses which lead to the low coupling ideality.
Using suitably designed bus waveguides, parasitic losses are mitigated, and a
nearly unity ideality and strong overcoupling (i.e. a ratio of external
coupling to internal resonator loss rate > 9) are demonstrated. Moreover we
find that different resonator modes can exchange power through the coupler,
which therefore constitutes a mechanism that induces modal coupling, a
phenomenon known to distort resonator dispersion properties. Our results
demonstrate the potential for significant performance improvements of
integrated planar microresonators, achievable by optimized coupler designs.Comment: 8 pages, 3 figures, 1 tabl
Probing the loss origins of ultra-smooth integrated photonic waveguides
On-chip optical waveguides with low propagation losses and precisely
engineered group velocity dispersion (GVD) are important to nonlinear photonic
devices such as soliton microcombs. Yet, despite intensive research efforts,
nonlinear integrated photonic platforms still feature propagation losses orders
of magnitude higher than in standard optical fiber. The tight confinement and
high index contrast of integrated waveguides make them highly susceptible to
fabrication induced surface roughness. Therefore, microresonators with
ultra-high Q factors are, to date, only attainable in polished bulk
crystalline, or chemically etched silica based devices, that pose however
challenges for full photonic integration. Here, we demonstrate the fabrication
of silicon nitride () waveguides with unprecedentedly smooth
sidewalls and tight confinement with record low propagation losses. This is
achieved by combining the photonic Damascene process with a novel reflow
process, which reduces etching roughness, while sufficiently preserving
dimensional accuracy. This leads to previously unattainable \emph{mean}
microresonator Q factors larger than for tightly confining
waveguides with anomalous dispersion. Via systematic process step variation and
two independent characterization techniques we differentiate the scattering and
absorption loss contributions, and reveal metal impurity related absorption to
be an important loss origin. Although such impurities are known to limit
optical fibers, this is the first time they are identified, and play a tangible
role, in absorption of integrated microresonators. Taken together, our work
provides new insights in the origins of propagation losses in
waveguides and provides the technological basis for
integrated nonlinear photonics in the ultra-high Q regime
Dual chirped micro-comb based parallel ranging at megapixel-line rates
Laser based ranging (LiDAR) - already ubiquitously used in industrial
monitoring, atmospheric dynamics, or geodesy - is key sensor technology.
Coherent laser ranging, in contrast to time-of-flight approaches, is immune to
ambient light, operates continuous wave allowing higher average powers, and
yields simultaneous velocity and distance information. State-of-the-art
coherent single laser-detector architectures reach hundreds of kilopixel per
second rates. While emerging applications such as autonomous driving, robotics,
and augmented reality mandate megapixel per second point sampling to support
real-time video-rate imaging. Yet, such rates of coherent LiDAR have not been
demonstrated. Here we report a swept dual-soliton microcomb technique enabling
coherent ranging and velocimetry at megapixel per second line scan measurement
rates with up to 64 spectrally dispersed optical channels. It is based on
recent advances in photonic chip-based microcombs that offer a solution to
reduce complexity both on the transmitter and receiver sides.
Multi-heterodyning two synchronously frequency-modulated microcombs yields
distance and velocity information of all individual ranging channels on a
single receiver alleviating the need for individual separation, detection, and
digitization. The reported LiDAR implementation is hardware-efficient,
compatible with photonic integration, and demonstrates the significant
advantages of acquisition speed afforded by the convergence of optical
telecommunication and metrology technologies. We anticipate our research will
motivate further investigation of frequency swept microresonator dual-comb
approach in the neighboring fields of linear and nonlinear spectroscopy,
optical coherence tomography
Nanophotonic soliton-based microwave synthesizers
Microwave photonic technologies, which upshift the carrier into the optical
domain to facilitate the generation and processing of ultrawide-band electronic
signals at vastly reduced fractional bandwidths, have the potential to achieve
superior performance compared to conventional electronics for targeted
functions. For microwave photonic applications such as filters, coherent
radars, subnoise detection, optical communications and low-noise microwave
generation, frequency combs are key building blocks. By virtue of soliton
microcombs, frequency combs can now be built using CMOS compatible photonic
integrated circuits, operated with low power and noise, and have already been
employed in system-level demonstrations. Yet, currently developed photonic
integrated microcombs all operate with repetition rates significantly beyond
those that conventional electronics can detect and process, compounding their
use in microwave photonics. Here we demonstrate integrated soliton microcombs
operating in two widely employed microwave bands, X- and K-band. These devices
can produce more than 300 comb lines within the 3-dB-bandwidth, and generate
microwave signals featuring phase noise levels below 105 dBc/Hz (140 dBc/Hz) at
10 kHz (1 MHz) offset frequency, comparable to modern electronic microwave
synthesizers. In addition, the soliton pulse stream can be injection-locked to
a microwave signal, enabling actuator-free repetition rate stabilization,
tuning and microwave spectral purification, at power levels compatible with
silicon-based lasers (<150 mW). Our results establish photonic integrated
soliton microcombs as viable integrated low-noise microwave synthesizers.
Further, the low repetition rates are critical for future dense WDM channel
generation schemes, and can significantly reduce the system complexity of
photonic integrated frequency synthesizers and atomic clocks
Soliton microcomb based spectral domain optical coherence tomography
Spectral domain optical coherence tomography (SD-OCT) is a widely used and
minimally invaive technique for bio-medical imaging [1]. SD-OCT typically
relies on the use of superluminescent diodes (SLD), which provide a low-noise
and broadband optical spectrum. Recent advances in photonic chipscale frequency
combs [2, 3] based on soliton formation in photonic integrated microresonators
provide an chipscale alternative illumination scheme for SD-OCT. Yet to date,
the use of such soliton microcombs in OCT has not yet been analyzed. Here we
explore the use of soliton microcombs in spectral domain OCT and show that, by
using photonic chipscale Si3N4 resonators in conjunction with 1300 nm pump
lasers, spectral bandwidths exceeding those of commercial SLDs are possible. We
demonstrate that the soliton states in microresonators exhibit a noise floor
that is ca. 3 dB lower than for the SLD at identical power, but can exhibit
significantly lower noise performance for powers at the milliWatt level. We
perform SD-OCT imaging on an ex vivo fixed mouse brain tissue using the soliton
microcomb, alongside an SLD for comparison, and demonstrate the principle
viability of soliton based SD-OCT. Importantly, we demonstrate that classical
amplitude noise of all soliton comb teeth are correlated, i.e. common mode, in
contrast to SLD or incoherent microcomb states [4], which should, in theory,
improve the image quality. Moreover, we demonstrate the potential for circular
ranging, i.e. optical sub-sampling [5, 6], due to the high coherence and
temporal periodicity of the soliton state. Taken together, our work indicates
the promising properties of soliton microcombs for SD-OCT
Dynamics of soliton crystals in optical microresonators
Dissipative Kerr solitons in optical microresonators provide a unifying
framework for nonlinear optical physics with photonic-integrated technologies
and have recently been employed in a wide range of applications from coherent
communications to astrophysical spectrometer calibration. Dissipative Kerr
solitons can form a rich variety of stable states, ranging from breathers to
multiple-soliton formations, among which, the recently discovered soliton
crystals stand out. They represent temporally-ordered ensembles of soliton
pulses, which can be regularly arranged by a modulation of the continuous-wave
intracavity driving field. To date, however, the dynamics of soliton crystals
remains mainly unexplored. Moreover, the vast majority of the reported crystals
contained defects - missing or shifted pulses, breaking the symmetry of these
states, and no procedure to avoid such defects was suggested. Here we explore
the dynamical properties of soliton crystals and discover that often-neglected
chaotic operating regimes of the driven optical microresonator are the key to
their understanding. In contrast to prior work, we prove the viability of
deterministic generation of soliton crystal states, which
correspond to a stable, defect-free lattice of optical pulses inside the
cavity. We discover the existence of critical pump power, below which the
stochastic process of soliton excitation suddenly becomes deterministic
enabling faultless, device-independent access to perfect soliton crystals.
Furthermore, we demonstrate the switching of soliton crystal states and prove
that it is also tightly linked to the pump power and is only possible in the
regime of transient chaos. Finally, we report a number of other dynamical
phenomena experimentally observed in soliton crystals including the formation
of breathers, transitions between soliton crystals, their melting, and
recrystallization
Massively parallel coherent laser ranging using soliton microcombs
Coherent ranging, also known as frequency-modulated continuous-wave (FMCW)
laser based ranging (LIDAR) is currently developed for long range 3D distance
and velocimetry in autonomous driving. Its principle is based on mapping
distance to frequency, and to simultaneously measure the Doppler shift of
reflected light using frequency chirped signals, similar to Sonar or Radar.
Yet, despite these advantages, coherent ranging exhibits lower acquisition
speed and requires precisely chirped and highly-coherent laser sources,
hindering their widespread use and impeding Parallelization, compared to modern
time-of-flight (TOF) ranging that use arrays of individual lasers. Here we
demonstrate a novel massively parallel coherent LIDAR scheme using a photonic
chip-based microcomb. By fast chirping the pump laser in the soliton existence
range of a microcomb with amplitudes up to several GHz and sweep rate up to 10
MHz, the soliton pulse stream acquires a rapid change in the underlying carrier
waveform, while retaining its pulse-to-pulse repetition rate. As a result, the
chirp from a single narrow-linewidth pump laser is simultaneously transferred
to all spectral comb teeth of the soliton at once, and allows for true
parallelism in FMCW LIDAR. We demonstrate this approach by generating 30
distinct channels, demonstrating both parallel distance and velocity
measurements at an equivalent rate of 3 Mpixel/s, with potential to improve
sampling rates beyond 150 Mpixel/s and increase the image refresh rate of FMCW
LIDAR up to two orders of magnitude without deterioration of eye safety. The
present approach, when combined with photonic phase arrays based on
nanophotonic gratings, provides a technological basis for compact, massively
parallel and ultra-high frame rate coherent LIDAR systems.Comment: 18 pages, 12 Figure
- …