41 research outputs found
Devices and Fibers for Ultrawideband Optical Communications
Wavelength-division multiplexing (WDM) has
historically enabled the increase in the capacity of optical
systems by progressively populating the existing optical bandwidth
of erbium-doped fiber amplifiers (EDFAs) in the C-band.
Nowadays, the number of channels—needed in optical
systems—is approaching the maximum capacity of standard
C-band EDFAs. As a result, the industry worked on novel
approaches, such as the use of multicore fibers, the extension
of the available spectrum of the C-band EDFAs, and the
development of transmission systems covering C- and L-bands
and beyond. In the context of continuous traffic growth,
ultrawideband (UWB) WDM transmission systems appear
as a promising technology to leverage the bandwidth of
already deployed optical fiber infrastructure and sustain the
traffic demand for the years to come. Since the pioneering
demonstrations of UWB transmission a few years ago, long
strides have been taken toward UWB technologies. In this
review article, we discuss how the most recent advances in
the design and fabrication of enabling devices, such as lasers,
amplifiers, optical switches, and modulators, have improved
the performance of UWB systems, paving the way to turn
research demonstrations into future products. In addition,
we also report on the advances in UWB optical fibers, such as
the recently introduced nested antiresonant nodeless fibers
(NANFs), whose future implementations could potentially provide
up to 300-nm-wide bandwidth at less than 0.2 dB/km loss
Acute mesenteric ischaemia in refractory shock on veno-arterial extracorporeal membrane oxygenation
Background: Acute mesenteric ischaemia is a severe complication in critically ill patients, but has never been evaluated in patients on veno-arterial extracorporeal membrane oxygenation (V-A ECMO). This study was designed to determine the prevalence of mesenteric ischaemia in patients supported by V-A ECMO and to evaluate its risk factors, as well as to appreciate therapeutic modalities and outcome. Methods: In a retrospective single centre study (January 2013 to January 2017), all consecutive adult patients who underwent V-A ECMO were included, with exclusion of those dying in the first 24 hours. Diagnosis of mesenteric ischaemia was performed using digestive endoscopy, computed tomography scan or first-line laparotomy. Results: One hundred and fifty V-A ECMOs were implanted (65 for post-cardiotomy shock, 85 for acute cardiogenic shock, including 39 patients after refractory cardiac arrest). Overall, median age was 58 (48-69) years and mortality 56%. Acute mesenteric ischaemia was suspected in 38 patients, with a delay of four (2-7) days after ECMO implantation, and confirmed in 14 patients, that is, a prevalence of 9%. Exploratory laparotomy was performed in six out of 14 patients, the others being too unstable to undergo surgery. All patients with mesenteric ischaemia died. Independent risk factors for developing mesenteric ischaemia were renal replacement therapy (odds ratio (OR) 4.5, 95% confidence interval (CI) 1.3-15.7, p=0.02) and onset of a second shock within the first five days (OR 7.8, 95% CI 1.5-41.3, p=0.02). Conversely, early initiation of enteral nutrition was negatively associated with mesenteric ischaemia (OR 0.15, 95% CI 0.03-0.69, p=0.02). Conclusions: Acute mesenteric ischaemia is a relatively frequent but dramatic complication among patients on V-A ECMO
Deterministic polarization chaos from a laser diode
Fifty years after the invention of the laser diode and fourty years after the
report of the butterfly effect - i.e. the unpredictability of deterministic
chaos, it is said that a laser diode behaves like a damped nonlinear
oscillator. Hence no chaos can be generated unless with additional forcing or
parameter modulation. Here we report the first counter-example of a
free-running laser diode generating chaos. The underlying physics is a
nonlinear coupling between two elliptically polarized modes in a
vertical-cavity surface-emitting laser. We identify chaos in experimental
time-series and show theoretically the bifurcations leading to single- and
double-scroll attractors with characteristics similar to Lorenz chaos. The
reported polarization chaos resembles at first sight a noise-driven mode
hopping but shows opposite statistical properties. Our findings open up new
research areas that combine the high speed performances of microcavity lasers
with controllable and integrated sources of optical chaos.Comment: 13 pages, 5 figure
Mode oscillation and harmonic distortions associated with sinusoidal modulation of semiconductor lasers
This paper investigates mode dynamics, operation characteristics and signal distortions associated with sinusoidal modulation of semiconductor lasers. The study is based on intensive integrations of the multimode rate equation model of semiconductor lasers over wide ranges of the modulation frequency and depth. The rate equations take into account both spectral symmetric and asymmetric suppressions of modal gain. The higher harmonic distortions as well as the half harmonic distortion associated with the period doubling effect are investigated. The study is applied to both cases of single-mode and multimode oscillations of the non-modulated laser. The obtained results showed that the modulated signal has six distinct waveforms depending on the modulation conditions; three types have continuous periodic waveforms and the others have periodic pulsing waveforms. The modulated laser is found to oscillate in a single mode under weak modulation where the modulated signal is continuous, whereas the pulsing signals are associated with multimode oscillation. The higher harmonic distortions of single-mode laser are lower than those of two-mode lasers, and become serious at modulation frequencies around the relaxation oscillation frequency. These distortions are highest when the laser output is pulsating and the pulses are superposed by relaxation oscillations. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2012
Integrated frequency comb laser with narrow intrinsic optical linewidth based on a dielectric waveguide feedback circuit
We present an integrated hybrid semiconductor-dielectric (InP-SiN)
waveguide laser that generates frequency combs at a wavelength around 1.5
m with a record-low intrinsic optical linewidth of 34 kHz. This is
achieved by extending the cavity photon lifetime using a low-loss dielectric
waveguide circuit. In our experimental demonstration, the on-chip, effective
optical path length of the laser cavity is extended to 6 cm. The resulting
linewidth narrowing shows the high potential of on-chip, highly coherent
frequency combs with direct electrical pumping, based on hybrid and
heterogeneous integrated circuits making use of low-loss dielectric waveguides
Demonstration of a self-pulsing photonic crystal Fano laser
Semiconductor lasers in use today rely on mirrors based on the reflection at
a cleaved facet or Bragg reflection from a periodic stack of layers. Here, we
demonstrate an ultra-small laser with a mirror based on the Fano resonance
between a continuum of waveguide modes and the discrete resonance of a
nanocavity. The Fano resonance leads to unique laser characteristics. Since the
Fano mirror is very narrow-band compared to conventional lasers, the laser is
single-mode and in particular, it can be modulated via the mirror. We show,
experimentally and theoretically, that nonlinearities in the mirror may even
promote the generation of a self-sustained train of pulses at gigahertz
frequencies, an effect that was previously only observed in macroscopic lasers.
Such a source is of interest for a number of applications within integrated
photonics
Coherent transponder for optical PS-QPSK signals
The present document relates to optical transmission
systems. In particular, the present document relates
to a transmitter and/or a receiver for optical polarization
switches (PS) QPSK (quadrature phase shift keying)
signals. An apparatus (500) for an optical transmitter
of an optical polarization switched M-PSK signal comprising
a first and a second polarization, with M = 2m and
m≥ 1 is described. The apparatus (500) comprises mapping
means (200) configured to map, at a time instant
k=1,...,K, m information bits to an M-PSK constellation
point, thereby generating a first symbol at time instant k
for the first polarization; assign, at the time instant k, one
further information bit by generating a second symbol at
time instant k for the second polarization, wherein the
second symbol at time instant k corresponds to the first
symbol at time instant k, or corresponds to the constellation
point opposite the constellation point of the first
symbol at time instant k; and repeat the mapping and
assigning steps for all k=1,...,K to generate a sequence
of first and second symbols for the first and second polarization,
respectively. The apparatus (500) further comprises
decorrelation means (501, 502) configured to
decorrelate the first symbol at time instant k and the second
symbol at time instant k, for all k=1,...,K
Intra- versus Inter-channel PMD in LinearlyCompensated Coherent PDM-PSK NonlinearTransmissions
We investigate the interaction between Kerr nonlinearity
and polarization mode dispersion (PMD) in homogeneous
multi-channel polarization division multiplexing coherent phase
shift keying transmissions. Assuming linear distortions can be
fully equalized by the coherent receiver, we concentrate the
investigation on the residual nonlinear penalty. We introduce a
novel simulation procedure to discriminate intra-channel PMD,
i.e., PMD within the channel, from inter-channel PMD, i.e., PMD
among different channels, showing their relative impact on system
performance in the nonlinear regime. A simple yet effective description
of intra-channel PMD is proposed, which helps identify
the PMD realizations giving the best/worst performance in the
nonlinear regime. We show that on average PMD improves the
performance of dispersion-managed links by reducing both nonlinear
interactions among signals and nonlinear phase noise, while
it has no effect in non-dispersion managed links