Performance of free-space QKD systems using SIM/BPSK and dual-threshold/direct-detection

This paper proposes a novel free-space quantum key distribution (QKD) system using subcarrier intensity-modulation (SIM) binary phase-shift-keying (BPSK) and dual-threshold/direct-detection (DT/DD) receiver with an avalanche photodiode (APD). The proposed system enables the adoption of well-developed analytical models in conventional free-space optical (FSO) communications and achieves the QKD function with a simple configuration. We analytically investigate the design criteria for transmitter and receiver, in particular, the modulation depth and the setting for dual-threshold in the context of security requirement of QKD systems. The quantum bit error rate (QBER) and the ergodic secret-key rate of the proposed system are analytically derived in closed-form expressions, considering the channel loss, atmospheric turbulence modeled by the log-normal distribution, and receiver noises. Monte-Carlo (M-C) simulations are also implemented to validate the analytical results, and numerical results confirm the feasibility of the proposed system

Development of a 1310 nm Quantum Key Distribution system

Quantum Key Distribution (QKD) represents an innovative technique of generating an encryption key shared between two interlocutors which allows overcoming the problems of classical cryptography. The purpose of the thesis is to build and test a QKD system operating at a wavelength of 1310 nm, which allows a better coexistence of the quantum signal with a classic communication signal at 1550 nm on the same optical fiber. Another objective of the thesis is to demonstrate the operating principle of an advanced version that works for both discrete and continuous variable QKD.Quantum Key Distribution (QKD) represents an innovative technique of generating an encryption key shared between two interlocutors which allows overcoming the problems of classical cryptography. The purpose of the thesis is to build and test a QKD system operating at a wavelength of 1310 nm, which allows a better coexistence of the quantum signal with a classic communication signal at 1550 nm on the same optical fiber. Another objective of the thesis is to demonstrate the operating principle of an advanced version that works for both discrete and continuous variable QKD

New trends in fibre optic sommunications

Přenos optickych vláken je jednou z hlavních metod přenosu informací pro přenos na velké vzdálenosti. Byly vyvinuty různé modulační formáty pro maximalizaci informací přenášenych prostřednictvím komunikačního kanálu. Takové formáty sahají od formátů, které přenášejí jeden bit na symbol (jako je například binární fázové posunování klíčů, BPSK), a nebo vice, jako je například Kvadraturní fázové posunovací klíčování, duální polarizační kvadraturní fázové řazení a mnoho dalších ostatní modulace. Cílem diplomové práce je vytvořit a implementovat přenosovy systém s rychlostí 100 Gbps pomocí modulace formátu DP QPSK. Vysledky jsou shrnuty s ohledem na to, co bylo úspěšně implementováno, a zlepšení, která mohou byt ještě učiněna pro zvyšení přenosové vzdálenosti a snížení BER mimo jiné. Použitysimulační software je PHOTOSS.Optical fiber transmission is one of the leading methods of information transfer for long distance transmission. Various modulation formats have been developed to maximize information transferred through a communication channel. Such formats range from those that transfer one bit per symbol (such as Binary Phase Shift Keying, BPSK), or Quadrature Phase Shift Keying, 3 bits such as in 4 bits per symbol such as Dual Polarization Quadrature Phase Shift Keying and many other modulations. The goal of this thesis is to create and implement a 100 Gbps transmission system using DP QPSK modulation format. The results are discussed with respect to what was successfully implemented and improvements that can still be made to enhance the transmission distance and reduce BER among others. The simulation software used is PHOTOSS.

Optical Communication

Optical communication is very much useful in telecommunication systems, data processing and networking. It consists of a transmitter that encodes a message into an optical signal, a channel that carries the signal to its desired destination, and a receiver that reproduces the message from the received optical signal. It presents up to date results on communication systems, along with the explanations of their relevance, from leading researchers in this field. The chapters cover general concepts of optical communication, components, systems, networks, signal processing and MIMO systems. In recent years, optical components and other enhanced signal processing functions are also considered in depth for optical communications systems. The researcher has also concentrated on optical devices, networking, signal processing, and MIMO systems and other enhanced functions for optical communication. This book is targeted at research, development and design engineers from the teams in manufacturing industry, academia and telecommunication industries

Advanced adaptive compensation system for free-space optical communications

Massive amounts of information are created daily in commercial fields like earth observation, that must be downloaded to earth ground stations in the short time of a satellite pass. Today, much of the collected information must be dropped due to lack of bandwidth, and laser downlinks can offer tenths of gigabits throughput solving this bottleneck limitation. In a down-link scenario, the performance of laser satellite communications is limited due to atmospheric turbulence, which causes fluctuations in the intensity and the phase of the received signal leading to an increase in bit error probability. In principle, a single-aperture phase-compensated receiver, based on adaptive optics, can overcome atmospheric limitations by adaptive tracking and correction of atmospherically induced aberrations. However, under strong-turbulence situations, the effectiveness of traditional adaptive optics systems is severely compromised. In such scenarios, sensor-less techniques offer robustness, hardware simplicity, and easiness of implementation and integration at a reduced cost, but the state-of-the-art approaches still require too many iterations to perform the correction, exceeding the temporal coherence of the field and thus falling behind the field evolution. This thesis proposes a new iterative AO technique for strong turbulence compensation that reduces the correction time, bridging the limitation of similar systems in lasercom applications. It is based on the standard sensor-less system design, but it additionally uses a short-exposure focal intensity image to accelerate the correction. The technique combines basic principles of Fourier optics, image processing, and quadratic signal optimization to correct the wave-front. This novel approach directly updates the phases of the most intense focal-plane speckles, maximizing the power coupled into a single-mode fiber convexly. Numerical analyses show that this method has a robust and excellent performance under very strong turbulence. Laboratory results confirm that a focal speckle pattern can be used to accelerate the iterative compensation. This technique delivers nearly twofold bandwidth reduction compared with standard methods, and sufficient signal gain and stability to allow high throughput data transmission with nearly error-free performance in emulated satellite downlink scenarios. A property highlight is the in-advance knowledge of the required number of iterations, allowing on-demand management of the loop bandwidth in different turbulent regimes. Besides remaining conceptually and technically simple, it opens a new insight to iterative solutions that may lead to the development of new methods. With further refinement, this technique can surely contribute to making possible the use of iterative solutions in laser communicationsSatélites de observación de la tierra diariamente generan gigantescas cantidades de datos que deben ser enviados a estaciones terrenas. La mayoría de la información recolectada debe desecharse debido al reducido tiempo visible de un satélite en movimiento y el limitado ancho de banda de transmisión. Enlaces ópticos pueden solucionar esta limitación ofreciendo multi-gigabit de ancho de banda. Sin embargo, el desempeño de un downlink laser está limitado por la turbulencia atmosférica, la cual induce variaciones en la intensidad y la fase de la señal recibida incrementando la probabilidad de error en los datos recibidos. En principio, un receptor basado en una apertura simple utilizando óptica adaptativa puede corregir las aberraciones de fase inducidas por la atmósfera, mejorando el canal de transmisión. Sin embargo, la eficiencia de los sistemas de óptica adaptativa tradicionales se ve seriamente reducida en situaciones de turbulencia fuerte. En tales escenarios, técnicas iterativas ofrecen mayor robustez, simplicidad de diseño e implementación, así como también facilidad de integración a un costo reducido. Desafortunadamente, dicha tecnología aún requiere demasiadas iteraciones para corregir la fase distorsionada, excediendo el tiempo de coherencia del frente de onda. Esta tesis propone una nueva técnica iterativa de óptica adaptativa capaz de reducir el tiempo de convergencia en escenarios de turbulencia fuerte. La técnica utiliza el diseño tradicional de los sistemas de corrección iterativos, agregando el uso de una imagen focal de intensidad para acelerar el proceso de corrección del campo distorsionado. En dicha técnica se combinan principios básicos de óptica de Fourier, procesamiento de imagen, y optimización cuadrática de la señal para corregir el frente de onda. De esta forma, la fase de los puntos focales de mayor intensidad (speckles) puede modificarse directamente y con ello maximizar de forma convexa la potencia acoplada en fibra. Los análisis numéricos demuestran robustez y un excelente desempeño en escenarios de turbulencia fuerte. Los resultados de laboratorio confirman que el moteado de intensidad puede utilizarse para acelerar la corrección iterativa. Esta técnica utiliza la mitad del ancho de banda requerido con la técnica tradicional, al mismo tiempo que ofrece suficiente ganancia y estabilidad de la señal para lograr enlaces ópticos con muy baja probabilidad de error. Al mismo tiempo, la técnica propuesta permite conocer con anticipación el número total de iteraciones y posibilita la administración bajo demanda del ancho de banda requerido en diferentes escenarios de turbulencia. Esta tesis ofrece una mirada diferente a los métodos iterativos, posibilitando el desarrollo de nuevos conceptos y contribuyendo al uso de soluciones iterativas en comunicaciones laser por espacio libre.Postprint (published version

Design and security analysis of quantum key distribution protocol over free-space optics using dual-threshold direct-detection receiver

This paper proposes a novel design and analyzes security performance of quantum key distribution (QKD) protocol over free-space optics (FSO). Unlike conventional QKD protocols based on physical characteristics of quantum mechanics, the proposed QKD protocol can be implemented on standard FSO systems using subcarrier intensity modulation (SIM) binary phase shift keying (BPSK) and direct detection with a dual-threshold receiver. Under security constraints, the design criteria for FSO transmitter and receiver, in particular, the modulation depth and the selection of dual-threshold detection, respectively, is analytically investigated. For the security analysis, quantum bit error rate (QBER), ergodic secret-key rate, and final key-creation rate are concisely derived in novel closed-form expressions in terms of finite power series, taking into account the channel loss, atmospheric turbulence-induced fading, and receiver noises. Furthermore, Monte-Carlo (M-C) simulations are performed to verify analytical results and the feasibility of the proposed QKD protocol.</p