63 research outputs found
On the Optimization of Underwater Quantum Key Distribution Systems with Time-Gated SPADs
In this paper, we study the effect of various transmitter and receiver
parameters on the quantum bit error rate (QBER) performance of underwater
quantum key distribution. We utilize a Monte Carlo approach to simulate the
trajectories of emitted photons transmitting in the water from the transmitter
towards receiver. Based on propagation delay results, we first determine a
proper value for bit period to avoid the intersymbol interference as a result
of possible multiple scattering events. Then, based on the angle of arrival of
the received photons, we determine a proper field-of-view to limit the average
number of received background noise. Finally, we determine the optimal value
for the single photon avalanche diode (SPAD) gate time in the sense of
minimizing the QBER for the selected system parameters and given propagation
environment
Single-photon avalanche diode receivers for optical wireless communications
Single-photon avalanche diodes (SPADs) have been widely applied in many applications over
the past few decades thanks to their high sensitivity, high photon detection efficiency and
high timing resolution. Nowadays, they are drawing particular attention in the field of optical
wireless communication (OWC), resulting in wider and deeper studies among the scientific
research community. Compared with positive-intrinsic-negative (PIN) diodes and avalanche
photodiodes (APDs), SPADs provide much higher internal gains and sensitivities, thereby
easily overcoming the thermal noise and enabling the detection of individual photons without
the need for transimpedance amplifiers (TIAs). However, upon detecting a photon, the SPAD is
unable to respond to subsequent incident photons for a certain period of time, called dead time.
This dead time is caused by the quenching circuit, which is of two principal modes: active
quenching (AQ) and passive quenching (PQ). Depending on the structure of this circuit, the
dead time can be constant or variable, in any case, it degrades the photon counting performance
of the SPAD.
In this thesis, a comprehensive analytical approach is presented for modelling the counting
statistics of SPAD detectors in the presence of dead time. To the best of author’s knowledge, this
is the first in-depth study of the impact of dead time in the context of OWC. Using the concepts
of arrival processes and renewal theory, the exact photocount distributions and the count rate
models are derived for AQ and PQ single SPADs. It is shown that, unlike ideal photon counting
detectors, in AQ and PQ single SPADs, the photocounts do not follow a Poisson distribution.
The results confirm that AQ single SPADs generally exhibit less counting losses and therefore,
higher count rates compared to PQ single SPADs and the count rate gap in high photon rate
regimes is substantial. It is also shown that the photocount distribution of a SPAD array can
be well approximated by a Gaussian distribution, for which the mean and variance are dead
time dependent. The numerical results suggest that as the size of the array increases, the gap
between the photon counting performance of AQ and PQ SPAD arrays tends to vanish.
Furthermore, in this thesis, the bit error performance of SPAD-based OWC systems with AQ
single SPADs, PQ single SPADs and AQ SPAD arrays are evaluated. The results show that the
SPAD dead time significantly degrades the bit error ratio (BER) of the systems. The system
with an AQ single SPAD exhibits better BERs compared to the system with a PQ single SPAD.
The effect of dead time is mitigated to some extent when an array is employed. The analytical
and Monte Carlo simulation results are provided for various dead time values, background
count levels and SPAD array sizes.
From a communication theory point of view, the dead time also limits the achievable data
rate of SPAD-based systems. In this thesis, the information transfer rate of SPAD detectors
is also investigated. To this end, the SPAD is modelled as a communication channel. Using
an information theoretic approach, the channel capacity and the capacity-achieving input
distributions for AQ single SPADs and AQ SPAD arrays are obtained for various dead time
values, background count levels, and array sizes
Imaging through obscurants using time-correlated single-photon counting in the short-wave infrared
Single-photon time-of-flight (ToF) light detection and ranging (LiDAR) systems have
emerged in recent years as a candidate technology for high-resolution depth imaging in
challenging environments, such as long-range imaging and imaging in scattering media.
This Thesis investigates the potential of two ToF single-photon depth imaging systems
based on the time-correlated single-photon (TCSPC) technique for imaging targets in
highly scattering environments. The high sensitivity and picosecond timing resolution
afforded by the TCSPC technique offers high-resolution depth profiling of remote targets
while maintaining low optical power levels. Both systems comprised a pulsed picosecond
laser source with an operating wavelength of 1550 nm, and employed InGaAs/InP SPAD
detectors. The main benefits of operating in the shortwave infrared (SWIR) band include
improved atmospheric transmission, reduced solar background, as well as increased laser
eye-safety thresholds over visible band sensors.
Firstly, a monostatic scanning transceiver unit was used in conjunction with a
single-element Peltier-cooled InGaAs/InP SPAD detector to attain sub-centimetre
resolution three-dimensional images of long-range targets obscured by camouflage
netting or in high levels of scattering media. Secondly, a bistatic system, which employed
a 32 × 32 pixel format InGaAs/InP SPAD array was used to obtain rapid depth profiles
of targets which were flood-illuminated by a higher power pulsed laser source. The
performance of this system was assessed in indoor and outdoor scenarios in the presence
of obscurants and high ambient background levels.
Bespoke image processing algorithms were developed to reconstruct both the depth and
intensity images for data with very low signal returns and short data acquisition times,
illustrating the practicality of TCSPC-based LiDAR systems for real-time image
acquisition in the SWIR wavelength region - even in the photon-starved regime.The Defence Science and Technology Laboratory ( Dstl) National PhD Schem
Microparticle photometry in a CMOS microsystem combining magnetic actuation and in-situ optical detection
We present a hybrid CMOS-based microfluidic system that combines magnetic actuation of microparticles with in situ optical detection using single photon avalanche diodes (SPADs). The decoupling of the principles used for actuation and sensing permits a high sensitivity with respect to detection and particle handling. Single magnetic microparticles are transported within a glass micro-capillary positioned over an array of actuation coils and are detected upon passage over a SPAD, where they block incident light and thus lower the photon count. Use of the photometry method allows the determination of the particle size, which, in combination with a simultaneous measurement of the particle velocity, enables us to estimate further particle properties, such as their magnetization.We present the successful manipulation, detection and evaluation of magnetic particles with diameters ranging from 1 to 30 um
Photovoltaic Energy Harvesting for Millimeter-Scale Systems
The Internet of Things (IoT) based on mm-scale sensors is a transformational technology that opens up new capabilities for biomedical devices, surveillance, micro-robots and industrial monitoring. Energy harvesting approaches to power IoT have traditionally included thermal, vibration and radio frequency. However, the achievement of efficient energy scavenging for IoT at the mm-scale or sub mm-scale has been elusive. In this work, I show that photovoltaic (PV) cells at the mm-scale can be an alternative means of wireless power transfer to mm-scale sensors for IoT, utilizing ambient indoor lighting or intentional irradiation of near-infrared (NIR) LED sources through biological tissue. Single silicon and GaAs photovoltaic cells at the mm-scale can achieve a power conversion efficiency of more than 17 % for silicon and 30 % for GaAs under low-flux NIR irradiation at 850 nm through the optimized device structure and sidewall/surface passivation studies, which guarantees perpetual operation of mm-scale sensors. Furthermore, monolithic single-junction GaAs photovoltaic modules offer a means for series-interconnected cells to provide sufficient voltage (> 5 V) for direct battery charging, and bypassing needs for voltage up-conversion circuitry. However, there is a continuing challenge to miniaturize such PV systems down to the sub mm-scale with minimal optical losses from device isolation and metal interconnects and efficient voltage up-conversion. Vertically stacked dual-junction PV cells and modules are demonstrated to increase the output voltage per cell and minimize area losses for direct powering of miniature devices for IoT and bio-implantable applications with low-irradiance narrowband spectral illumination. Dual-junction PV cells at small dimensions (150 µm x 150 µm) demonstrate power conversion efficiency greater than 22 % with more than 1.2 V output voltage under low-flux 850 nm NIR LED illumination, which is sufficient for batteryless operation of miniaturized CMOS IC chips. The output voltage of dual-junction PV modules with eight series-connected single cells is greater than 10 V while maintaining an efficiency of more than 18 %. Finally, I demonstrate monolithic PV/LED modules at the µm-scale for brain-machine interfaces, enabling two-way optical power and data transfer in a through-tissue configuration. The wafer-level assembly plan for the 3D vertical integration of three different systems including GaAs LED/PV modules, CMOS silicon chips, and neural probes is proposed.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/163261/1/esmoon_1.pd
Multi-Uncrewed Underwater Vehicle (UUV) Optical Communication System Design
Over the past few decades the state of art of Uncrewed Underwater Vehicles (UUVs) has grown significantly, and one of the major challenges remains establishing reliable underwater communication among UUVs. This case is especially true in a multi-UUV setting where tethered communication is not an option. This research focuses on designing a cost-efficient, short distance optical communication system capable of supporting formation control of multiple UUVs. Although light attenuation underwater significantly degrades communication ranges, experimental results show that optical communication can achieve distances of almost 20 meters in clear water by utilizing a simple 10-Watt LED transmitter (with larger distances being tenable given more powerful light sources). Furthermore, a signal processing scheme and protocol is designed and tested. This scheme includes a timing sequence capable of supporting multiple UUVs, all utilizing the same transmitter wavelength and carrier frequency. This optical communication scheme is tested in air in a static three-node network. All nodes are able to send, receive and interpret digital packets at a speed of 5kbps. Although further fine-tuning of the system is required due to divergence angle limitations and timing inefficiencies, the experiments presented in this work show a successful proof-of-concept of a short distance multi-UUV optical communication syste
Feasibility of quantum key distribution from high altitude platforms
This paper presents the feasibility study of deploying Quantum Key
Distribution (QKD) from High Altitude Platforms (HAPs), as a way of securing
future communications applications and services. The paper provides a thorough
review of the state of the art HAP technologies and summarises the benefits
that HAPs can bring to the QKD services. A detailed link budget analysis is
presented in the paper to evaluate the feasibility of delivering QKD from
stratospheric HAPs flying at 20 km altitude. The results show a generous link
budget under most operating conditions which brings the possibility of using
diverged beams, thereby simplifying the Pointing, Acquisition and Tracking
(PAT) of the optical system on the HAPs and ground, potentially widening the
range of future use cases where QKD could be a viable solution.Comment: 12 pages, 11 figures. Comments are welcom
Design and implementation of a high-speed free-space quantum key distribution system for urban scenarios
Tesis doctoral inédita leída en la Universidad Autónoma de Madrid. Facultad de Ciencias, Departamento de Física de Materiales. Fecha de lectura: 21-06-201
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