Based on a full-wave formulation, a vector boundary element method (VBEM) is proposed to model the photonic crystal fibers (PCFs) (microstructured fibers). The accuracy and efficiency of the approach are confirmed by comparing the results calculated with those in previous literatures. With employing the VBEM, the guiding characteristics, including the effective indexes, vector mode patterns, and the polarization properties of the PCFs are investigated. There polarization characteristics of the PCFs with elliptical air holes (EPCFs) and the one ring air-hole EPCF embedded in the step-index core are studied and discussed. In addition, based on the VBEM formulations, a novel and efficient numerical approach to calculate the dispersion parameters of the PCFs is also proposed. The effect of the PCF geometrical structure on the group velocity dispersion property is reviewed, and then the one-ring defect and two-ring defect PCFs are studied and designed for the ultra-flattened dispersion applications. As an example, a four-ring (two-ring defect) PCF with flattened dispersion of ¡Ó0.25 ps/km/nm from 1.295£gm to 1.725£gm wavelength is numerically demonstrated.

The presence of various impairments in fiber channel has forced researchers to uncover solutions to minimize those effects. With the advancement of technology, optical solutions were finally easier to implement in the system. To this day, optical compensation methods are still found to be as the best way to minimize fiber impairments. With the development of digital signal processing (DSP) and FIR techniques, coherent detection with digital signal processing (DSP) is developed, analyzed theoretically and numerically and experimentally demonstrated in long-haul high speed fiber‐optic transmission system. The use of DSP in conjunction with coherent detection unleashes the benefits of coherent detection which rely on the preservation of full information of the transmitted field. These benefits include high receiver sensitivity, the ability to achieve high spectral‐efficiency and the use of advanced modulation formats. The local oscillator (LO) of coherent receiver alleviates the need for hardware phase‐locking and polarization tracking, which can now be achieved in the digital domain. The computational complexity previously associated with coherent detection is hence significantly diminished and coherent detection is once again considered a feasible detection alternative. In this thesis, an optical fiber communication scheme using the coherent detection method is simulated. Firstly, at the beginning of each chapter, we introduce the various compensation methods for certain optical fiber impairments which is developed by the pioneers. However, such technique does introduce enormous complexity to the system, in addition to a large cost. For that reason, the main focus had to shift to an alternative method. DSP techniques has enabled simple techniques to mitigate various impairments in fiber-optical systems. In this thesis, the background knowledge about the structure of fiber-optical transmission system is provided. After the mathematical analysis of the various impairments (laser noise, chromatic dispersion, polarization mode dispersion and nonlinearity) in fiber-optical links, the compensation methods by using DSP techniques are provided. By the methods of fourth-power carrier recovery algorithm and feedforward carrier recovery algorithm, the phase rotation in constellation due to laser noise is compensated in QPSK systems and QAM systems, respectively. The feedforward carrier recovery algorithm has a high tolerance for laser linewidth in high-order QAM system. As for PMD compensation, on the basis of adaptive equalizers in both time domain and frequency domain achiever by the pioneers, a novel LMS algorithm is proposed in this thesis. It has a fair comparative and steady computational complexity with the increase in the number of training blocks. The last part is the nonlinearity compensation. The DBP compensation is a popular method for nonlinearity compensation but its computational complexity is fair high (Shao J, Kumar S and Liang X., 2013). We adopt two kinds of fold-DBP which are distance-folded DBP and dispersion-folded DBP to compensate the joint impairments of chromatic dispersion and nonlinearity in dispersion-managed system. The distance-folded DBP works well in the full compensation dispersion-managed system but in the presence of RDPS, only the dispersion-folded DBP is efficient. / Thesis / Master of Applied Science (MASc)

Coherent detection employing multilevel modulation formats has become one of the most promising technologies for next generation high speed transmission systems due to the high power and spectral efficiencies. Using the powerful digital signal processing (DSP), coherent optical receivers allow the significant equalization of chromatic dispersion (CD), polarization mode dispersion (PMD), phase noise (PN) and nonlinear effects in the electrical domain. Recently, the realizations of these DSP algorithms for mitigating the channel distortions in the coherent transmission systems are the most attractive investigations. The CD equalization can be performed by the digital filters developed in the time and the frequency domain, which can suppress the fiber dispersion effectively. The PMD compensation is usually performed in the time domain with the adaptive least mean square (LMS) and constant modulus algorithms (CMA) equalization. Feed-forward and feed-back carrier phase estimation (CPE) algorithms are employed to mitigate the phase noise (PN) from the transmitter (TX) and the local oscillator (LO) lasers. The fiber nonlinearities are compensated by using the digital backward propagation methods based on solving the nonlinear Schrödinger (NLS) equation and the Manakov equation. In this dissertation, we present a comparative analysis of three digital filters for chromatic dispersion compensation, a comparative evaluation of different carrier phase estimation methods considering digital equalization enhanced phase noise (EEPN) and a brief discussion for PMD adaptive equalization. To implement these investigations, a 112-Gbit/s non-return-to-zero polarization division multiplexed quadrature phase shift keying (NRZ-PDM-QPSK) coherent transmission system with post-compensation of dispersion is realized in the VPI simulation platform. In the coherent transmission system, these CD equalizers have been compared by evaluating their applicability for different fiber lengths, their usability for dispersion perturbations and their computational complexity. The carrier phase estimation using the one-tap normalized LMS (NLMS) filter, the differential detection, the block-average (BA) algorithm and the Viterbi-Viterbi (VV) algorithm is evaluated, and the analytical predictions are compared to the numerical simulations. Meanwhile, the phase noise mitigation using the radio frequency (RF) pilot tone is also investigated in a 56-Gbit/s NRZ single polarization QPSK (NRZ-SP-QPSK) coherent transmission system with post-compensation of chromatic dispersion. Besides, a 56-Gbit/s NRZ-SP-QPSK coherent transmission system with CD pre-distortion is also implemented to analyze the influence of equalization enhanced phase noise in more detail. / QC 20120528

Photonic crystal fibers (PCFs) are divided into two different kinds of fibers. The first one, index-guiding PCF, guides light by total internal reflection between a solid core and a cladding region with multiple air-holes. On the other hand, the second one uses a perfectly periodic structure exhibiting a photonic band-gap (PBG) effect at the operating wavelength to guide light in a low index core-region. A compact 2D-FDTD method based on finite-difference time-domain method is formulated and is effectively applied to analysis PCFs and PBGFs. We study the propagation features of fundamental mode and the fundamental characteristics such as effective index, modal-field diameter and chromatic dispersion in index-guiding PCFs. By optimizing the air-hole diameters and the hole-to-hole spacing of index-guiding PCFs, both the dispersion and the dispersion slope can be controlled in a wide wavelength range. We also investigate the propagation features of fundamental mode and band-gap effect of PBGFs.

[pt] Neste trabalho, uma análise do fenômeno de mistura de quatro ondas (FWM) em sistemas ópticos multicanais é apresentada. Inicialmente, uma série de simulações determina as limitações impostas aos sistemas ópticos, sendo adotado um nível máximo de crosstalk permitido. Em seguida, os diversos parâmetros que influenciaram a eficiência da geração de FWM são estudados detalhadamente, com resultados apresentados em forma de gráficos. Nestes últimos, o nível de FWM obtido é especificado em função da potência de entrada por canal, comprimento do enlace, número de canais, presença de amplificadores, número de amplificadores e dispersão cromática da fibra utilizada. Um breve estudo discute a deterioração da qualidade do serviço, causada pro FWM, em redes ópticas. O trabalho se encerra com algumas aplicações que utilizam, de forma favorável, o fenômeno de FWM. / [en] This work adresses the four-wave mixing (FWM) issue in optical multichannel systems. First, some simulations are carried out for specifying which are the optical system limitations coping with a maximum allowed crosstalk level. Next, the different parameters affecting the FWM are throughly studied, with these last pertinent results being graphicallly presented. In these, the obtained FWM levels are specified as a function of the channel input power, link span, channel rumber, presence of amplifiers, number of amplifiers and fiber chromatic dispersion. A concise discussion, concerning the quality of service deterioration on optical networks, due to FWM, is also presented. The work is finally closed by presenting some applications that favorably use the FWM phenomena.

<p> This thesis deals with the limiting factors in the design of a long-haul fiber-optic communication system, and the techniques used to suppress their resulting impairments. These limiting factors include both linear and nonlinear effects, such as fiber chromatic dispersion and the Kerr nonlinearity, and the modulator-induced nonlinearity. </p> <p> In Chapter 3, the conditional probability density function (PDF) of the received elect rical signal given transmitted bit '1 '/'0' for a coherent fiber-optic transmission system based on binary phase shift keying (BPSK) is mathematically derived. Both amplified spontaneous emission (ASE) noise and fiber nonlinearity are taken into account . The results show that the conditional PDF of given bit '1' or '0' is asymmetric when intrachannel four-wave mixing (IFWM) is dominant, while it becomes nearly symmetric when the variance of ASE is much larger than that due to IFWM. The standard deviation of the received signal is calculated analytically. The system parameters, including optimum dispersion map and pre-compensation ratio, are optimized by analytically calculating variance of IFWM. Significant computation efforts can be saved using this approach as compared to full numerical simulations of the nonlinear Schrodinger equation, without losing much accuracy. </p> <p> In Chapter 4, an improved 4-f time-lens configuration is proposed. Fourier transform (FT) and inverse Fourier transform (IFT) can be realized using time lenses such that there is no need for time reversal at the end. A typical 4-f configuration consists of two 2-f systems and a temporal filter. The first 2-f system consisting of a time lens and two dispersive elements produces the Fourier transform (FT) of the input signal. The temporal filter modifies the spectrum. The next 2-f system produces the inverse Fourier transform (IFT). A wavelength division demultiplexer and a higher-order dispersion compensator based on 4-f configuration are numerical implemented. One of the advantages of the time-lens-based temporal filtering technique is that the transfer function of the temporal filter can be dynamically altered by changing the input voltage to the temporal filter (amplitude/phase modulator) and therefore, this technique could be used for dynamic switching and multiplexing in optical networks. </p> <p> In chapter 5, a direct-detection optical orthogonal frequency division multiplexing (DD-0-0FDM) is realized using time lenses. Typically, in OFDM systems, discrete Fourier transform (DFT) is used at the transmitter and inverse discrete Fourier transform (IDFT) is used at the receiver. In this chapter, it is proposed to use continuous Fourier transform (FT) and inverse Fourier transform (IFT) using time lenses that replace DFT and IDFT in the electrical domain. The third- and higher-order dispersive effects can be considerably reduced using the proposed DD-0-0FDM scheme. </p> <p> In Chapter 6, a coherent optical orthogonal frequency division multiplexing (OFDM) (C0-0-0FDM) scheme using time lenses is analyzed. The comparison of performance between the proposed scheme and the conventional optical OFDM scheme using fast Fourier transform (FFT) and inverse FFT in the electrical domain is made. Both the Mach-Zehnder modulator (MZM) induced and fiber induced nonlinearities are investigated. Results show that the time-lens-based C0-0-0FDM performs almost the same as the FFT-based C0-0-0FDM when the message signal launched to MZM is low so that MZM operates in the linear region. The nonlinearity of MZM degrades the performance of FFT-based C0-0-0FDM drastically when the power of message signal becomes sufficiently large, but only has negligible impact on the time-lens-based C0-0-0FDM. A periodical driving voltage has been proposed to set up the time lens such that the maximally required driving voltage level is kept low within the time frame. The advantages using the time-lens-based C0-0-0FDM are that (i) FT can be done in optical domain almost instantaneously, whereas the FFT in digital domain is slow and requires significant computational efforts, (ii) optical domain Fourier transform has a large bandwidth (~THz) and therefore, FT /IFT can be performed at a large symbol rate. </p> <p> In Chapter 7, the digital backward propagation (DBP) has been studied both in orthogonal frequency-division multiplexing ( OFDM) and single-carrier (SC) fiber-optic transmission systems. 16 quadrature amplitude modulation (QAM) is used for both systems with the bit rate of 100 Gbjs. The results show that OFDM and SC with Nyquist pulses (SC-Nyquist) have a superior performance as compared to SC with raisedcosine pulses (SC-NRZ) when the DBP is used. The impact of electrical filter bandwidth and nonlinear phase/amplitude noise has also been investigated. The performance of perfect-BP-based OFDM/SC initially improves when the electrical filter bandwidth increases at high signal-to-noise ratio (SNR). The comparison of the effects of nonlinear phase/amplitude noise among OFDM, SC-Nyquist and SC-NRZ systems is made and it is shown that SC-NRZ systems significantly suffer from the effects of nonlinear phase/amplitude noise, which explains the performance advantage of OFDM/SC-Nyquist over SC-NRZ when the DBP used. </p> / Thesis / Doctor of Philosophy (PhD)

A digital filter is a system or a device that modifies a signal. This is an essential feature in digital communication. Using optical fibers in the communication has various advantages like higher bandwidth and distance capability over copper wires. However, at high-rate transmission, chromatic dispersion arises as a problem to be relieved in an optical communication system. Therefore, it is necessary to have a filter that compensates chromatic dispersion. In this thesis, we introduce the implementation of a new architecture of the filter and compare it with a previously proposed architecture.

Η παρούσα διατριβή εστιάζεται σε οπτικά τηλεπικοινωνιακά συστήματα με σύμφωνο δέκτη, τετραδική διαμόρφωση φάσης και πολύπλεξη κατά πόλωση. Το κυριότερο πλεονέκτημα αυτών των συστημάτων σε σύγκριση με τα συμβατικά οπτικά τηλεπικοινωνιακά συστήματα με δέκτες άμεσης φώρασης είναι οι αυξημένες δυνατότητες ψηφιακής επεξεργασίας σήματος στο δέκτη, που καθιστά εφικτή την εξίσωση των φαινομένων διάδοσης και την αποπολύπλεξη πόλωσης με ηλεκτρονικά μέσα. Μελετώνται διατάξεις ηλεκτρονικής επεξεργασίας σήματος. Η μελέτη διεξάγεται με χρήση προσομοίωσης με τη βοήθεια ηλεκτρονικού υπολογιστή. Προτείνεται η χρήση δύο πρωτότυπων μεθόδων αποπολύπλεξης σημάτων πολυπλεγμένων κατά πόλωση, με πλεονεκτήματα ως προς την απόδοση, την αξιοπιστία και την ταχύτητα σύγκλισης σε σύγκριση με τις μεθόδους που έχουν προταθεί στη βιβλιογραφία Στη συνέχεια μελετάται η απόδοση των ηλεκτρονικών εξισωτών ελαχίστου μέσου τετραγώνου (least-mean square) και σταθερής περιβάλλουσας (constant modulus algorithm) στην εξίσωση της χρωματικής διασποράς και της διασποράς τρόπων πόλωσης, τόσο μεμονωμένα, όσο και από κοινού. Στη μελέτη μας συμπεριλαμβάνουμε το θόρυβο φάσης και την ενδιάμεση συχνότητα, αίροντας αντίστοιχες απλοποιήσεις που συνήθως γίνονται στη βιβλιογραφία. Ακολούθως αναφερόμαστε στο πρόβλημα του εκφυλισμού του πίνακα συντελεστών των εξισωτών ελαχίστου μέσου τετραγώνου και σταθερής περιβάλλουσας παρουσία έντονης διεξασθένησης. O πίνακας συντελεστών στον οποίο έχει συγκλίνει ο εξισωτής ονομάζεται εκφυλισμένος όταν παράγεται η ίδια απεσταλμένη ακολουθία συμβόλων και στις δύο εξόδους του εξισωτή. Προτείνουμε τη χρήση του πολυχρηστικού εξισωτή σταθερής περιβάλλουσας (multiuser constant modulus algorithm equalizer). Η λύση που προτείνεται έχει πλεονεκτήματα αξιοπιστίας και απλότητας σε σχέση με αντίστοιχες προτάσεις στη βιβλιογραφία, και αποδεικνύεται η άριστη απόδοσή της. Στη συνέχεια, γίνεται αναφορά στο φαινόμενο της ανισοσταθμίας ορθογωνιότητας που παρατηρείται στα οπτικά συστήματα με σύμφωνους δέκτες και αποτελεί ένα φαινόμενο το οποίο, κατά κανόνα, παραβλέπεται στις μελέτες της βιβλιογραφίας και αποδεικνύεται το ότι οι συμβατικοί ηλεκτρονικοί εξισωτές δεν μπορούν να το αντιμετωπίσουν. Τέλος, παραθέτουμε αποτελέσματα από την επεξεργασία πραγματικών πειραματικών δεδομένων, για την επαλήθευση των συμπερασμάτων που προέκυψαν θεωρητικά. / This PhD dissertation focuses on optical communication systems utilizing coherent receiver, quadrature phase modulation and polarization multiplexing. Their main advantage compared to conventional systems is the ability to perform extended digital signal processing, in order to compensate for transmission effects and to successfully demultiplex polarization multiplexed signals. We study such DSP modules with the help of computer simulations. We propose two novel polarization demultiplexing algorithms, with advantages in performance reliability and convergence speed compared to conventional methods. One of the two methods utilizes training procedure, an approach not yet studied enough in the literature. Next, we study the performance of least mean square and constant modulus algorithm equalizers in mitigating chromatic dispersion and polarization mode dispersion, both separately and in combination. We include in our study phase noise and intermediate frequency offset, without the simplifications usually found in the literature. Next, we address the subject of equalizer degeneration, which describes the phenomenon when both equalizer outputs transmit one of the two polarization multiplexed signals. We propose the use of multi-user constant modulus algorithm, which is an enhanced version of the conventional constant modulus algorithm. We prove that our solution has excellent performance as well as reliability and simplicity advantages compared to other methods. Next, we prove that quadrature imbalance is a ubiquitous phenomenon that cannot be addressed with conventional electronic equalizers and should not be omitted as is often the case. Finally, we present results derived from the application of some of the proposed algorithms on real experimental data, and prove the consistency of the theoretical derived results.

Coherent detection employing multilevel modulation format has become one of the most promising technologies for next generation high speed transmission system due to the high power and spectral efficiencies. With the powerful digital signal processing (DSP), coherent optical receivers allow the significant equalization of chromatic dispersion (CD), polarization mode dispersion (PMD), phase noise (PN) and nonlinear effects in the electrical domain. Recently, the realizations of these DSP algorithms for mitigating the channel distortions in the transmission system are the most attractive investigations.  The CD equalization can be performed by the digital filters developed in the time and the frequency domain, which can suppress the fiber dispersion effectively. The PMD compensation is usually performed in the time domain with the adaptive least mean square (LMS) and constant modulus algorithms (CMA) equalization. Feed-forward and feed-back carrier phase estimation algorithms are employed to mitigate the phase noise from the transmitter and local oscillator lasers. The fiber nonlinearities are compensated by using the digital backward propagation methods based on solving the nolinear Schrodinger (NLS) equation and the Manakov equation.  In this dissertation, we present a comparative analysis of three digital filters for chromatic dispersion compensation, an analytical evaluation of carrier phase estimation with digital equalization enhanced phase noise and a brief discussion for PMD adaptive equalization. To implement these investigations, a 112-Gbit/s non-return-to-zero polarization division multiplexed quadrature phase shift keying (NRZ-PDM-QPSK) coherent transmission system is realized in the VPI simulation platform. With the coherent transmission system, these CD equalizers have been compared by evaluating their applicability for different fiber lengths, their usability for dispersion perturbations and their computational complexity. Meanwhile, the bit-error-rate (BER) floor in carrier phase estimation using a one-tap normalized LMS filter is evaluated analytically, and the numerical results are compared to a differential QPSK detection system. / QC 20110629

Cette étude se situe au niveau de la couche physique d'un système de transmission optique terrestre de type réseau métropolitain. Il s'agit d'y intégrer un composant, le laser modulateur intégrés (Electroabsorption Modulated Laser ou EML) du III-V Lab Alcatel-Thales, un laboratoire industriel de composants optoélectroniques. Et ceci pour en diminuer la complexité par pré-distorsion apportée par la modulation du laser. Les distances de transmission étant limités par des phénomènes physiques intrinsèques aux fibres optiques, le but est de développer un schéma de fonctionnement des EML pouvant dépasser les limites fixées par la dispersion chromatique des fibres et ses effets non-linéaires, principe qui a été démontré avec un laser et un modulateur discret mais jamais avec un composant intégré. La technique est basée sur une pré-compensation de la dispersion chromatique en appliquant une modulation sur le laser de l'EML qui génère la porteuse optique préchirpée qui est ensuite modulée par le modulateur à électro-absorption, raison du nom donné à l'EML : Dual Electroabsorption Modulated Laser ou D-EML. C'est l'axe majeur de recherche car il met en évidence le principe de la compensation de dispersion permettant d'incrémenter les distances de transmission de 80 km à 160 km pour la première fois avec un composant monolithique. Une application Radio-sur-Fibre pour les réseaux d'accès a été explorée, elle est basée sur une modulation duale analogique, permettant l'extinction d'une raie de modulation optique générant ainsi un signal à bande latérale unique Single Side Band. Ce format permet de transporter des signaux I-Q très haut-débit sur des distances de plus de 100 km.