6,079 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
Large second harmonic generation enhancement in SiN waveguides by all-optically induced quasi phase matching
Integrated waveguides exhibiting efficient second-order nonlinearities are
crucial to obtain compact and low power optical signal processing devices.
Silicon nitride (SiN) has shown second harmonic generation (SHG) capabilities
in resonant structures and single-pass devices leveraging intermodal phase
matching, which is defined by waveguide design. Lithium niobate allows
compensating for the phase mismatch using periodically poled waveguides,
however the latter are not reconfigurable and remain difficult to integrate
with SiN or silicon (Si) circuits. Here we show the all-optical enhancement of
SHG in SiN waveguides by more than 30 dB. We demonstrate that a Watt-level
laser causes a periodic modification of the waveguide second-order
susceptibility. The resulting second order nonlinear grating has a periodicity
allowing for quasi phase matching (QPM) between the pump and SH mode. Moreover,
changing the pump wavelength or polarization updates the period, relaxing phase
matching constraints imposed by the waveguide geometry. We show that the
grating is long term inscribed in the waveguides, and we estimate a second
order nonlinearity of the order of 0.3 pm/V, while a maximum conversion
efficiency (CE) of 1.8x10-6 W-1 cm-2 is reached
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
Photonic chip based optical frequency comb using soliton induced Cherenkov radiation
By continuous wave pumping of a dispersion engineered, planar silicon nitride
microresonator, continuously circulating, sub-30fs short temporal dissipative
solitons are generated, that correspond to pulses of 6 optical cycles and
constitute a coherent optical frequency comb in the spectral domain. Emission
of soliton induced Cherenkov radiation caused by higher order dispersion
broadens the spectral bandwidth to 2/3 of an octave, sufficient for self
referencing, in excellent agreement with recent theoretical predictions and the
broadest coherent microresonator frequency comb generated to date. In a further
step, this frequency comb is fully phase stabilized. The ability to preserve
coherence over a broad spectral bandwidth using soliton induced Cherenkov
radiation marks a critical milestone in the development of planar optical
frequency combs, enabling on one hand application in e.g. coherent
communications, broadband dual comb spectroscopy and Raman spectral imaging,
while on the other hand significantly relaxing dispersion requirements for
broadband microresonator frequency combs and providing a path for their
generation in the visible and UV. Our results underscore the utility and
effectiveness of planar microresonator frequency comb technology, that offers
the potential to make frequency metrology accessible beyond specialized
laboratories.Comment: Changes: - Added data (new Fig.4) on the first full phase
stabilization of a dissipative Kerr soliton (or dissipative cavity soliton)
in a microresonator - Extended Fig. 8 in the SI - Introduced nomenclature of
dissipative Kerr solitons - Minor other change
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
Ultrafast optical ranging using microresonator soliton frequency combs
Light detection and ranging (LIDAR) is critical to many fields in science and
industry. Over the last decade, optical frequency combs were shown to offer
unique advantages in optical ranging, in particular when it comes to fast
distance acquisition with high accuracy. However, current comb-based concepts
are not suited for emerging high-volume applications such as drone navigation
or autonomous driving. These applications critically rely on LIDAR systems that
are not only accurate and fast, but also compact, robust, and amenable to
cost-efficient mass-production. Here we show that integrated dissipative
Kerr-soliton (DKS) comb sources provide a route to chip-scale LIDAR systems
that combine sub-wavelength accuracy and unprecedented acquisition speed with
the opportunity to exploit advanced photonic integration concepts for
wafer-scale mass production. In our experiments, we use a pair of free-running
DKS combs, each providing more than 100 carriers for massively parallel
synthetic-wavelength interferometry. We demonstrate dual-comb distance
measurements with record-low Allan deviations down to 12 nm at averaging times
of 14 s as well as ultrafast ranging at unprecedented measurement rates of
up to 100 MHz. We prove the viability of our technique by sampling the
naturally scattering surface of air-gun projectiles flying at 150 m/s (Mach
0.47). Combining integrated dual-comb LIDAR engines with chip-scale
nanophotonic phased arrays, the approach could allow widespread use of compact
ultrafast ranging systems in emerging mass applications.Comment: 9 pages, 3 figures, Supplementary information is attached in
'Ancillary files
Microresonator solitons for massively parallel coherent optical communications
Optical solitons are waveforms that preserve their shape while propagating,
relying on a balance of dispersion and nonlinearity. Soliton-based data
transmission schemes were investigated in the 1980s, promising to overcome the
limitations imposed by dispersion of optical fibers. These approaches, however,
were eventually abandoned in favor of wavelength-division multiplexing (WDM)
schemes that are easier to implement and offer improved scalability to higher
data rates. Here, we show that solitons may experience a comeback in optical
communications, this time not as a competitor, but as a key element of
massively parallel WDM. Instead of encoding data on the soliton itself, we
exploit continuously circulating dissipative Kerr solitons (DKS) in a
microresonator. DKS are generated in an integrated silicon nitride
microresonator by four-photon interactions mediated by Kerr nonlinearity,
leading to low-noise, spectrally smooth and broadband optical frequency combs.
In our experiments, we use two interleaved soliton Kerr combs to transmit a
data stream of more than 50Tbit/s on a total of 179 individual optical carriers
that span the entire telecommunication C and L bands. Equally important, we
demonstrate coherent detection of a WDM data stream by using a pair of
microresonator Kerr soliton combs - one as a multi-wavelength light source at
the transmitter, and another one as a corresponding local oscillator (LO) at
the receiver. This approach exploits the scalability advantages of
microresonator soliton comb sources for massively parallel optical
communications both at the transmitter and receiver side. Taken together, the
results prove the significant potential of these sources to replace arrays of
continuous-wave lasers in high-speed communications.Comment: 10 pages, 3 figure
- …