21,580 research outputs found
Soliton propagation and polarisation mode-locking in birefringent optical fibres
Soliton propagation in polarization-preserving fibres is analysed. Based on the coupled nonlinear Schrodinger equations we derive an analytical approximation for such type of soliton propagation. Exploitation of soliton polarization properties for passive mode-locking in fibre lasers is also considered
A Machine Learning Approach for Automated Fine-Tuning of Semiconductor Spin Qubits
While spin qubits based on gate-defined quantum dots have demonstrated very
favorable properties for quantum computing, one remaining hurdle is the need to
tune each of them into a good operating regime by adjusting the voltages
applied to electrostatic gates. The automation of these tuning procedures is a
necessary requirement for the operation of a quantum processor based on
gate-defined quantum dots, which is yet to be fully addressed. We present an
algorithm for the automated fine-tuning of quantum dots, and demonstrate its
performance on a semiconductor singlet-triplet qubit in GaAs. The algorithm
employs a Kalman filter based on Bayesian statistics to estimate the gradients
of the target parameters as function of gate voltages, thus learning the system
response. The algorithm's design is focused on the reduction of the number of
required measurements. We experimentally demonstrate the ability to change the
operation regime of the qubit within 3 to 5 iterations, corresponding to 10 to
15 minutes of lab-time
Caracterização, modelação e compensação de efeitos de memória lenta em amplificadores de potência baseados em GAN HEMTS
Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) have
emerged as the most compelling technology for the transmission of highpower
radio-frequency (RF) signals for cellular mobile communications and
radar applications. However, despite their remarkable power capabilities, the
deployment of GaN HEMT-based RF power amplifiers (PAs) in the mobile
communications infrastructure is often ruled out in favor of alternative siliconbased
technologies. One of the main reasons for this is the pervasiveness of
nonlinear long-term memory effects in GaN HEMT technology caused by thermal
and charge-trapping phenomena. While these effects can be compensated
for using sophisticated digital predistortion algorithms, their implementation
and model-extraction complexity—as well as the power necessary for
their real-time execution—make them unsuitable for modern small cells and
large-scale multiple-input multiple-output transceivers, where the power necessary
for the linearization of each amplification element is of great concern.
In order to address these issues and further the deployment of high-powerdensity
high-efficiency GaN HEMT-based RF PAs in next-generation communications
and radar applications, in this thesis we propose novel methods for
the characterization, modeling, and compensation of long-term memory effects
in GaN HEMT-based RF PAs. More specifically, we propose a method
for the characterization of the dynamic self-biasing behavior of GaN HEMTbased
RF PAs; multiple behavioral models of charge trapping and their implementation
as analog electronic circuits for the accurate real-time prediction
of the dynamic variation of the threshold voltage of GaN HEMTs; a method
for the compensation of the pulse-to-pulse instability of GaN HEMT-based
RF PAs for radar applications; and a hybrid analog/digital scheme for the
linearization of GaN HEMT-based RF PAs for next-generation communications
applications.Os transístores de alta mobilidade eletrónica de nitreto de gálio (GaN HEMTs)
são considerados a tecnologia mais atrativa para a transmissão de sinais
de radiofrequência de alta potência para comunicações móveis celulares e
aplicações de radar. No entanto, apesar das suas notáveis capacidades de
transmissão de potência, a utilização de amplificadores de potência (PAs)
baseados em GaN HEMTs é frequentemente desconsiderada em favor de
tecnologias alternativas baseadas em transístores de silício. Uma das principais
razões disto acontecer é a existência pervasiva na tecnologia GaN
HEMT de efeitos de memória lenta causados por fenómenos térmicos e de
captura eletrónica. Apesar destes efeitos poderem ser compensados através
de algoritmos sofisticados de predistorção digital, estes algoritmos não são
adequados para transmissores modernos de células pequenas e interfaces
massivas de múltipla entrada e múltipla saída devido à sua complexidade
de implementação e extração de modelo, assim como a elevada potência
necessária para a sua execução em tempo real. De forma a promover a
utilização de PAs de alta densidade de potência e elevada eficiência baseados
em GaN HEMTs em aplicações de comunicação e radar de nova geração,
nesta tese propomos novos métodos de caracterização, modelação,
e compensação de efeitos de memória lenta em PAs baseados em GaN
HEMTs. Mais especificamente, nesta tese propomos um método de caracterização
do comportamento dinâmico de autopolarização de PAs baseados
em GaN HEMTs; vários modelos comportamentais de fenómenos de captura
eletrónica e a sua implementação como circuitos eletrónicos analógicos para
a previsão em tempo real da variação dinâmica da tensão de limiar de condução
de GaN HEMTs; um método de compensação da instabilidade entre
pulsos de PAs baseados em GaN HEMTs para aplicações de radar; e um
esquema híbrido analógico/digital de linearização de PAs baseados em GaN
HEMTs para comunicações de nova geração.Programa Doutoral em Telecomunicaçõe
Rate-Accuracy Trade-Off In Video Classification With Deep Convolutional Neural Networks
Advanced video classification systems decode video frames to derive the
necessary texture and motion representations for ingestion and analysis by
spatio-temporal deep convolutional neural networks (CNNs). However, when
considering visual Internet-of-Things applications, surveillance systems and
semantic crawlers of large video repositories, the video capture and the
CNN-based semantic analysis parts do not tend to be co-located. This
necessitates the transport of compressed video over networks and incurs
significant overhead in bandwidth and energy consumption, thereby significantly
undermining the deployment potential of such systems. In this paper, we
investigate the trade-off between the encoding bitrate and the achievable
accuracy of CNN-based video classification models that directly ingest
AVC/H.264 and HEVC encoded videos. Instead of retaining entire compressed video
bitstreams and applying complex optical flow calculations prior to CNN
processing, we only retain motion vector and select texture information at
significantly-reduced bitrates and apply no additional processing prior to CNN
ingestion. Based on three CNN architectures and two action recognition
datasets, we achieve 11%-94% saving in bitrate with marginal effect on
classification accuracy. A model-based selection between multiple CNNs
increases these savings further, to the point where, if up to 7% loss of
accuracy can be tolerated, video classification can take place with as little
as 3 kbps for the transport of the required compressed video information to the
system implementing the CNN models
Few-cycle Pulses Amplification For Attosecond Science Applications Modeling And Experiments
The emergence of mode-locked oscillators providing pulses with durations as short as a few electric-field cycles in the near infra-red has paved the way toward electric-field sensitive physics experiments. In addition, the control of the relative phase between the carrier and the pulse envelope, developed in the early 2000’s and rewarded by a Nobel price in 2005, now provides unprecedented control over the pulse behaviour. The amplification of such pulses to the millijoule level has been an on-going task in a few world-class laboratories and has triggered the dawn of attoscience, the science of events happening on an attosecond timescale. This work describes the theoretical aspects, modeling and experimental implementation of HERACLES, the Laser Plasma Laboratory optical parametric chirped pulse amplifier (OPCPA) designed to deliver amplified carrier-envelope phase stabilized 8-fs pulses with energy beyond 1 mJ at repetition rates up to 10 kHz at 800 nm central wavelength. The design of the hybrid fiber/solid-state amplifier line delivering 85-ps pulses with energy up to 10 mJ at repetition rates in the multi-kHz regime tailored for pumping the optical parametric amplifier stages is presented. The novel stretcher/compressor design of HERACLES, suitable for handling optical pulses with spectra exceeding 300 nm of bandwidth with unprecedented flexibility, is fully modeled and also presented in the frame of this thesis. Finally, a 3D model of the multistage non-collinear optical parametric amplifier is also reported. The current and foreseen overall performances of HERACLES are presented. This facility is designed to enable attosecond physics experiments, high-harmonic generation and physics of plasma studies
Respiratory organ motion in interventional MRI : tracking, guiding and modeling
Respiratory organ motion is one of the major challenges in interventional MRI, particularly in interventions with therapeutic ultrasound in the abdominal region. High-intensity focused ultrasound found an application in interventional MRI for noninvasive treatments of different abnormalities. In order to guide surgical and treatment interventions, organ motion imaging and modeling is commonly required before a treatment start. Accurate tracking of organ motion during various interventional MRI procedures is prerequisite for a successful outcome and safe therapy.
In this thesis, an attempt has been made to develop approaches using focused ultrasound which could be used in future clinically for the treatment of abdominal organs, such as the liver and the kidney. Two distinct methods have been presented with its ex vivo and in vivo treatment results. In the first method, an MR-based pencil-beam navigator has been used to track organ motion and provide the motion information for acoustic focal point steering, while in the second approach a hybrid imaging using both ultrasound and magnetic resonance imaging was combined for advanced guiding capabilities.
Organ motion modeling and four-dimensional imaging of organ motion is increasingly required before the surgical interventions. However, due to the current safety limitations and hardware restrictions, the MR acquisition of a time-resolved sequence of volumetric images is not possible with high temporal and spatial resolution. A novel multislice acquisition scheme that is based on a two-dimensional navigator, instead of a commonly used pencil-beam navigator, was devised to acquire the data slices and the corresponding navigator simultaneously using a CAIPIRINHA parallel imaging method. The acquisition duration for four-dimensional dataset sampling is reduced compared to the existing approaches, while the image contrast and quality are improved as well.
Tracking respiratory organ motion is required in interventional procedures and during MR imaging of moving organs. An MR-based navigator is commonly used, however, it is usually associated with image artifacts, such as signal voids. Spectrally selective navigators can come in handy in cases where the imaging organ is surrounding with an adipose tissue, because it can provide an indirect measure of organ motion. A novel spectrally selective navigator based on a crossed-pair navigator has been developed. Experiments show the advantages of the application of this novel navigator for the volumetric imaging of the liver in vivo, where this navigator was used to gate the gradient-recalled echo sequence
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