IQ Imbalance Compensation Based on Maximum SNR Estimation in Coherent QPSK Systems

International audienceWe present a simple alternative method for the compensation of quadrature imbalance in optical quadrature phase-shift-keying (QPSK) coherent systems. The method is based on the determination and the compensation of the phase mismatch by the introduction of a relevant signal-to-noise ratio metric. The principle is validated numerically and the algorithm is validated experimentally through bit-error-rate (BER) and error vector magnitude (EVM) measurements. A 20 Gb/s optical QPSK experiment reveals a good agreement of theproposed method with the Gram-Schmidt orthogonalization procedure (GSOP). Moreover, the robustness of both methods was verified with up to 30° phase misalignment by comparing the signal after phase imbalance compensation to that without compensation. A 10% reduction of EVM is achieved with our method for a high phase misalignment of 30°

Digital Signal Processing for Front-end Non-idealities in Coherent Optical OFDM system

Ph.DDOCTOR OF PHILOSOPH

Impacto de imperfeições do laser em receptores ópticos coerentes com formatos de modulação de alta ordem

Orientador: Darli Augusto de Arruda MelloDissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de ComputaçãoResumo: Atualmente, os sistemas ópticos coerentes transmitem grandes volumes de informação graças à utilização de formatos de modulação de alta ordem. No entanto, esses formatos de modulação são mais suscetíveis a perturbações de fase geradas por imperfeições nos lasers utilizados no transmissor e receptor. Este trabalho centrou-se em uma análise das imperfeições do laser e seu impacto sobre o desempenho de receptores ópticos coerentes com formatos de modulação de alta ordem. Em especial, avaliaram-se as duas fontes principais de perturbações de fase: o ruído de fase do laser e as flutuações na frequência de operação, efeito conhecido como jitter de frequência da portadora. Primeiramente, investigou-se o impacto das imperfeições do laser por meio de simulações. O ruído de fase foi simulado como um processo discreto de Wiener, e o jitter de frequência foi modelado como uma forma de onda senoidal. Os resultados permitiram avaliar o comportamento do sistema sob diversas condições de frequência e amplitude do sinal de jitter. Posteriormente, o impacto das perturbações de fase foi avaliado por meio de experimentos. Observou-se que parâmetro de largura de linha calculado por métodos existentes não é suficiente para prever o comportamento dos algoritmos de processamento digital de sinais sob condições intensas de jitter. Alternativamente, o trabalho sugeriu uma metodologia mais conveniente para prever o impacto das perturbações do laser no desempenho do sistema, que leva em consideração a composição de ruído de fase e jitter de frequênciaAbstract: Currently, coherent optical systems transmit large volumes of information thanks to the use of high-order modulation formats. However, such modulation formats are more susceptible to phase perturbations generated by imperfections in the lasers used in the transmitter and receiver. This work focused on an analysis of laser imperfections and their impact on the performance of coherent optical receivers with high-order modulation formats. In particular, the two main sources of phase perturbations were evaluated: laser phase noise and fluctuations in the operating frequency, an effect known as carrier frequency jitter. First, the impact of laser imperfections was evaluated by simulations. Phase noise was modeled as a Wiener process, and frequency jitter was assumed to be sinusoidal. The results allowed to evaluate the behavior of the system under different conditions of frequency and amplitude of the jitter signal. Later, the impact of phase perturbations was evaluated through experiments. It was observed that the laser linewidth calculated by existing methods is not sufficient to predict the behavior of the digital signal processing algorithms under intense jitter conditions. Alternatively, the work suggested a more convenient methodology for predicting the impact of laser perturbations on system performance, which takes into account the composition of phase noise and carrier frequency jitterMestradoTelecomunicações e TelemáticaMestra em Engenharia ElétricaCAPE

Transmissores-recetores de baixa complexidade para redes óticas

Traditional coherent (COH) transceivers allow encoding of information in both quadratures and the two orthogonal polarizations of the electric field. Nevertheless, such transceivers used today are based on the intradyne scheme, which requires two 90o optical hybrids and four pairs of balanced photodetectors for dual-polarization transmission systems, making its overall cost unattractive for short-reach applications. Therefore, SSB methods with DD reception, commonly referred to as self-coherent (SCOH) transceivers, can be employed as a cost-effective alternative to the traditional COH transceivers. Nevertheless, the performance of SSB systems is severely degraded. This work provides a novel SCOH transceiver architecture with improved performance for short-reach applications. In particular, the development of phase reconstruction digital signal processing (DSP) techniques, the development of other DSP subsystems that relax the hardware requirement, and their performance optimization are the main highlights of this research. The fundamental principle of the proposed transceiver is based on the reception of the signal that satisfies the minimum phase condition upon DD. To reconstruct the missing phase information imposed by DD, a novel DCValue method exploring the SSB and the DC-Value properties of the minimum phase signal is developed in this Ph.D. study. The DC-Value method facilitates the phase reconstruction process at the Nyquist sampling rate and requires a low intensity pilot signal. Also, the experimental validation of the DC-Value method was successfully carried out for short-reach optical networks. Additionally, an extensive study was performed on the DC-Value method to optimize the system performance. In the optimization process, it was found that the estimation of the CCF is an important parameter to exploit all advantages of the DC-Value method. A novel CCF estimation technique was proposed. Further, the performance of the DC-Value method is optimized employing the rate-adaptive probabilistic constellation shaping.Os sistemas de transcetores coerentes tradicionais permitem a codificação de informação em ambas quadraturas e em duas polarizações ortogonais do campo elétrico. Contudo, estes transcetores utilizados atualmente são baseados num esquema intradino, que requer dois híbridos óticos de 90o e quatro pares de foto detetores para sistemas de transmissão com polarização dupla, fazendo com que o custo destes sistemas seja pouco atrativo para aplicações de curto alcance. Por isso, métodos de banda lateral única com deteção direta, também referidos como transcetores coerentes simplificados, podem ser implementados como uma alternativa de baixo custo aos sistemas coerentes tradicionais. Contudo, o desempenho de sistemas de banda lateral única tradicionais é gravemente degradado pelo batimento sinal-sinal. Nesta tese foi desenvolvida uma nova arquitetura de transcetor coerente simplificada com um melhor desempenho para aplicações de curto alcance. Em particular, o desenvolvimento de técnicas de processamento digital de sinal para a reconstrução de fase, bem como de outros subsistemas de processamento digital de sinal que minimizem os requerimentos de hardware e a sua otimização de desempenho são o foco principal desta tese. O princípio fundamental do transcetor proposto é baseado na receção de um sinal que satisfaz a condição mínima de fase na deteção direta. Para reconstruir a informação de fase em falta causada pela deteção direta, um novo método de valor DC que explora sinais de banda lateral única e as propriedades DC da condição de fase mínima é desenvolvido nesta tese. O método de valor DC facilita a reconstrução da fase à frequência de amostragem de Nyquist e requer um sinal piloto de baixa intensidade. Além disso, a validação experimental do método de valor DC foi executada com sucesso em ligações óticas de curto alcance. Adicionalmente, foi realizado um estudo intensivo do método de valor DC para otimizar o desempenho do sistema. Neste processo de otimização, verificou-se que o fator de contribuição da portadora é um parâmetro importante para explorar todas as vantagens do método de valor DC. Neste contexto, é proposto um novo método para a sua estimativa. Por último, o desempenho do método de valor DC é otimizado recorrendo a mapeamento probabilístico de constelação com taxa adaptativa.Programa Doutoral em Engenharia Eletrotécnic

Transmission optique longue distance avec le format MB-OFDM cohérent à 100 Gbps

Today the 100 Gbps coherent dual polarization quadrature phase shift keying (Co-DP-QPSK) is standardized as the industrial solution for long-haul WDM transmission. Another alternative format to DP-QPSK that permits also to reach a data rate of 100 Gbps and beyond is the coherent orthogonal frequency division multiplexing (OFDM) format. However a doubt exists over the ability of OFDM to be as efficient as QPSK for long-haul WDM transmission due to its supposed higher sensitivity to nonlinear effects . In this thesis, we have investigated the potential of Co-DP-OFDM for 100 Gbps WDM transport. The digital signal processing algorithms are detailed as well as the various experimental set-ups required to carry out and validate the 100 Gbps transceiver. We also present the transmission results obtained with several configurations. In one of these configurations, the 100 Gbps Co-DP-OFDM channel is multiplexed with forty 100 Gbps DP-QPSK channels and all these channels are transmitted over 1000 km of DCF-free G.652 fiber, while in another configuration, the Co-DP-OFDM and Co-DP-QPSK channels are combined with seventy eight 10 Gbps NRZ-OOK channels and transmitted over 1000 km of dispersion managed G.652 fiber line. We have demonstrated that OFDM and QPSK have nearly the same performance after a transmission over 1000 km, and also we have demonstrated that the transmission of these two formats over legacy fiber infrastructure is possible under the condition of decreasing by 5 dB the 10 Gbps NRZ-OOK channel power with respect to the 100 Gbps channels. The results presented in this thesis are very valuable when considering the next generation of 400 Gbps or 1 Tbps for WDM systems.Aujourd'hui, le format « Quadrature Phase Shift Keying » avec multiplexage de polarisation (DP-QPSK) opérant à 100 Gbps est devenue un standard pour la transmission WDM longue distance. Une alternative au format DP-QPSK permettant d’atteindre des débits de 100 Gbps et plus (400 G & 1Tbps) est l’ « Orthogonal Frequency Division Multiplexing » (OFDM). Mais, des interrogations subsistent quant à sa robustesse aux effets non linéaires. Dans cette thèse nous avons étudié le potentiel de la technologie OFDM pour la transmission WDM longue distance à 100 Gbps. Le traitement du signal est détaillé ainsi que la mise en œuvre du transmetteur et récepteur OFDM cohérent. Nous présentons aussi les résultats expérimentaux de la transmission obtenus dans plusieurs configurations. Dans l’une de ces configurations, le canal modulé avec le format DP-OFDM coherent (Co-DP-OFDM) est multiplexé avec 40 canaux modulés en DP-QPSK à 100 Gbps. Les canaux ont ensuite été transmis sur 1000 km de fibre G.652 sans gestion de dispersion chromatique. Dans une autre configuration, les canaux Co-DP-OFDM et Co-DP-QPSK sont combinés avec 78 canaux 10 Gbps NRZ-OOK et transmis sur 1000 km de fibre G.652 avec gestion de dispersion. Nous avons montré que le Co-DP-OFDM et Co-DP-QPSK ont des performances similaires après une transmission de 1000 km sur une ligne sans gestion de dispersion, et nous avons aussi montré que la transmission de ces formats sur une infrastructure de fibre deployée est possible à condition de réduire de 5 dB la puissance des canaux 10 Gbps NRZ-OOK par rapport aux canaux à100 Gbps. Ces résultats sont précieux pour la prochaine génération de systèmes WDM à 400 Gbps ou 1 Tbps

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

Compensation of Laser Phase Noise Using DSP in Multichannel Fiber-Optic Communications

One of the main impairments that limit the throughput of fiber-optic communication systems is laser phase noise, where the phase of the laser output drifts with time. This impairment can be highly correlated across channels that share lasers in multichannel fiber-optic systems based on, e.g., wavelength-division multiplexing using frequency combs or space-division multiplexing. In this thesis, potential improvements in the system tolerance to laser phase noise that are obtained through the use of joint-channel digital signal processing are investigated. To accomplish this, a simple multichannel phase-noise model is proposed, in which the phase noise is arbitrarily correlated across the channels. Using this model, high-performance pilot-aided phase-noise compensation and data-detection algorithms are designed for multichannel fiber-optic systems using Bayesian-inference frameworks. Through Monte Carlo simulations of coded transmission in the presence of moderate laser phase noise, it is shown that joint-channel processing can yield close to a 1 dB improvement in power efficiency. It is further shown that the algorithms are highly dependent on the positions of pilots across time and channels. Hence, the problem of identifying effective pilot distributions is studied.The proposed phase-noise model and algorithms are validated using experimental data based on uncoded space-division multiplexed transmission through a weakly-coupled, homogeneous, single-mode, 3-core fiber. It is found that the performance improvements predicted by simulations based on the model are reasonably close to the experimental results. Moreover, joint-channel processing is found to increase the maximum tolerable transmission distance by up to 10% for practical pilot rates.Various phenomena decorrelate the laser phase noise between channels in multichannel transmission, reducing the potency of schemes that exploit this correlation. One such phenomenon is intercore skew, where the spatial channels experience different propagation velocities. The effect of intercore skew on the performance of joint-core phase-noise compensation is studied. Assuming that the channels are aligned in the receiver, joint-core processing is found to be beneficial in the presence of skew if the linewidth of the local oscillator is lower than the light-source laser linewidth.In the case that the laser phase noise is completely uncorrelated across channels in multichannel transmission, it is shown that the system performance can be improved by applying transmitter-side multidimensional signal rotations. This is found by numerically optimizing rotations of four-dimensional signals that are transmitted through two channels. Structured four-dimensional rotations based on Hadamard matrices are found to be near-optimal. Moreover, in the case of high signal-to-noise ratios and high signal dimensionalities, Hadamard-based rotations are found to increase the achievable information rate by up to 0.25 bits per complex symbol for transmission of higher-order modulations

Technology Requirements for an Alamouti-Coded 100 Gb/s Digital Coherent Receiver Using 3 × 3 Couplers for Passive Optical Networks

Digital coherent receivers employing Alamouti-coding are a promising solution for passive optical networks (PON) with downstream rates of 100Gb/s/λ. To reduce cost by utilizing high-volume photodiodes designed for 25Gb/s data-center applications, we consider 28-Gbaud 16-QAM modulation format detected with a single-polarization coherent receiver based on a 3×3 coupler with single-ended detection. We investigate two alternative configurations: first, a receiver with a symmetric 3×3 coupler and subsequently an IQ coupler (asymmetric 3×3 coupler) with Alamouti-coding based polarization diversity. In this paper, we present a detailed investigation as to the feasibility of both the receivers for a wide range of parameters including quantization noise, relative intensity noise (RIN), I/Q mismatch etc. It is found that such receivers can be realized with commercially available components. We also quantify the penalty associated with non-ideal components to allow for lower cost designs with reduced specifications suitable for access applications. In addition, we present the adaptive DSP algorithms required to recover data for single-carrier Alamouti-coded signal. Considering both linear and nonlinear impairments, a power budget of 36.4dB and 33.1dB is found at BER of 3.8×10−3 for receiver with symmetric 3×3 coupler and IQ coupler, respectively and thereby proving their applicability as receivers for 100Gb/s/λ PON