Tesi sul tema "Cross phase modulation"

The following thesis presents results obtained from both numerical simulation and laboratory experimentation (both of which were carried out by the author). When data is propagated along an optical transmission line some timing irregularities can occur such as timing jitter and phase wander. Traditionally these timing problems would have been corrected by converting the optical signal into the electrical domain and then compensating for the timing irregularity before converting the signal back into the optical domain. However, this thesis posses a potential solution to the problem by remaining completely in the optical domain, eliminating the need for electronics. This is desirable as not only does optical processing reduce the latency effect that their electronic counterpart have, it also holds the possibility of an increase in overall speed. A scheme was proposed which utilises the principle of wavelength conversion to dynamically convert timing irregularities (timing jitter and phase wander) into a change in wavelength (this occurs on a bit-by-bit level and so timing jitter and phase wander can be compensated for simultaneously). This was achieved by optically sampling a linearly chirped, locally generated clock source (the sampling function was achieved using a nonlinear optical loop mirror). The data, now with each bit or code word having a unique wavelength, is then propagated through a dispersion compensation module. The dispersion compensation effectively re-aligns the data in time and so thus, the timing irregularities are removed. The principle of operation was tested using computer simulation before being re-tested in a laboratory environment. A second stage was added to the device to create 3R regeneration. The second stage is used to simply convert the timing suppressed data back into a single wavelength. By controlling the relative timing displacement between stage one and stage two, the wavelength that is finally produced can be controlled.

Long distance repeaterless optical fiber communications systems are currently used to transmit most internet and telephone information worldwide. The growth of photonic telecommunications technology has produced systems with very high bit-rate per fiber, but this still falls short of its potential capacity. Currently systems that are able to transmit even higher bit-rates are being developed utilizing dense wavelength-division-multiplexing (WDM) to maximally utilize the bandwidth potential of optical fibers. One of the most important factors that limits the bit-rate achievable in a such a WDM optical communications system is the cross-talk between channels caused by pulse collisions. In this thesis a consistent mathematical theory is used to analyze the frequency and timing shifts caused collisions between two WDM channels. This theory is applied to the systems currently most promising for next-generation photonic telecommunications; the dispersion managed (DM) soliton and 'quasi-linear' systems. Self-contained formulae are obtained which accurately predict the timing shifts suffered in these systems with a wide range of parameters. These formulae require an order of magnitude less computational time that direct numerical simulations. Several mathematical techniques are introduced to obtain computationally efficient formulae for complete and incomplete collisions in both systems. For complete collisions we use the Poisson sum transform to change the calculation to a sum in the Fourier domain. For incomplete collisions we use asymptotic integration to obtain approximate formulae. For quasi-linear systems we simplify the Laplace method even further to obtain elementary formulae. We show that using a combination of these methods the timing shift for incomplete and complete collisions in a wide range of system configurations can be obtained in comparatively small computational times. We find that for systems with small DM map strength the timing shift from widely separated channels is significant. For quasi-linear systems with large DM map strength this is negligable and the timing shift decreases with the square of the channel frequency separation. We also find the timing shift from closely spaced channels is higher for quasi-linear systems than for DM soliton systems operating at the same average dispersion.

The demand for faster data transmission is ever increasing. Wavelength division multiplexing (WDM) presents as a viable solution to increase the data transmission rate significantly. WDM systems are based on the ability to transmit multiple wavelengths simultaneously down the fibre. Unlike time division multiplexing (TDM) systems, WDM systems do not increase the data transfer by increasing the transmission rate of a single channel. In WDM systems the data rate per channel remains the same, only multiple channels carry data across the link. Dense wavelength division multiplexing (DWDM) promises even more wavelengths packed together in the same fibre. This multiplication of channels increases the bandwidth capacity rapidly. Networks are looking into making use of technology that will ensure no electronic signal regeneration at any point within the DWDM network. Examples are; reconfigurable optical add/drop multiplexers (ROADM) and optical cross connect (OXC) units. These components essentially enable network operators to split, combine and multiplex optical signals carried by optical fibre. WDM allows network operators to increase the capacity of existing networks without expensive re-cabling. This provides networks with the flexibility to be upgraded to larger bandwidths and for reconfiguration of network services. Further, WDM technology opens up an opportunity of marketing flexibility to network operators, where operators not only have the option to rent out cables and fibres but wavelengths as well. Cross phase modulation (XPM) poses a problem to WDM networks. The refractive index experienced by a neighbouring optical signal, not only depends on the signal’s intensity but on the intensity of the co-propagating signal as well. This effect leads to a phase change and is known as XPM. This work investigates the characteristics of XPM. It is shown that, in a two channel WDM network, a probe signal’s SOP can be steered by controlling a high intensity pump signal’s SOP. This effect could be applied to make a wavelength converter. Experimental results show that the degree of polarization (DOP) of a probe signal degrades according to a mathematical model found in literature. The pump and probe signals are shown to experience maximum interaction, for orthogonal probe-pump SOP vector orientations. This may be problematic to polarization mode dispersion compensators. Additionally, experimental results point out that the SOP of a probe signal is much more active in the presence of a high intensity pump, as compared to the single signal transmission scenario.

FundaÃÃo de Amparo à Pesquisa do Estado do CearÃ
Nesta dissertaÃÃo foi estudada a aplicaÃÃo do filtro AcÃstico-Ãptico SintonizÃvel (AOTF), com a ModulaÃÃo de Pulsos por PosiÃÃo (PPM), objetivando implementar, utilizando o mÃtodo de Runge-Kutta de 4a ordem, portas lÃgicas (OR-OU e AND-E) Ãpticas operando com pulsos de luz ultracurtos (2ps). Neste trabalho à investigado o desempenho das portas considerando vÃrios comprimentos do filtro ( L) que integra a sua estrutura interna, com o intuito de obter o comprimento de filtro mais adequado para uma operaÃÃo satisfatÃria, em regime dispersivo, nÃo linear, sem perdas e com modulaÃÃo de fase cruzada (XPM). Esta investigaÃÃo à realizada em duas situaÃÃes: primeiramente, sÃo considerados filtros com automodulaÃÃo de fase (SPM) e GVD (dispersÃo da velocidade de grupo). Em um segundo momento, as mesmas portas sÃo obtidas com efeitos SPM, XPM e GVD agindo juntos no AOTF. Foi observado que para pulsos do tipo sÃliton, os efeitos da dispersÃo, da nÃo linearidade e da modulaÃÃo de fase cruzada exercem juntos uma forte influÃncia na propagaÃÃo do mesmo, provocando a quebra do pulso na saÃda do dispositivo quando utilizamos um comprimento maior para os filtros. Para dispositivos mais curtos, o pulso chaveado apresentou compressÃes e alargamentos temporais e espectrais, bem comodeslocamentos temporais nos dois modo de propagaÃÃo (TE e TM). ApÃs a escolha de um comprimento de filtro adequado, foi selecionado um deslocamento temporal Ãtimo a ser aplicado nos pulsos de entrada para conseguirmos, na saÃda da porta lÃgica, deslocamentos temporais satisfatÃrios (acertos) na aplicaÃÃo da modulaÃÃo PPM. Em seguida, introduzimos fases em um dos pulsos de entrada (TM), provocando um defasamento entre os pulsos TE e TM, reduzindo ainda mais a margem de erro PPM de operaÃÃo das portas. Finalmente, ao analisarmos as fases aplicadas no pulso TM (0 a 2), definirmos o melhor Ãngulo de fase para que as portas operem na regiÃo de acerto da modulaÃÃo PPM.
In this dissertation it was studied the application of the Acoustic Optical Tunable Filter (AOTF), with Pulse Position Modulation (PPM), aiming at to implement, using the method of Runge-Kutta of 4a order, logical gates (OR and AND) optical operating with pulses of light ultra shorts (2ps). In this work the acting of the gates is investigated, considering several lengths of the filter (  L) that integrates your internal structure, with the intention of obtaining the length of more appropriate filter for a satisfactory operation, in dispersion regime, nonlinear, without losses and with Cross Phase Modulation (XPM). This investigation is accomplished in two situations: firstly, filters are considered with Self Phase Modulation (SPM) and GVD (group-velocity dispersion). In a second moment, the same gates are obtained with effects SPM, XPM and GVD, acting together in AOTF. It was observed that for pulses of the type soliton, the effects of the dispersion, of the nonlinearity and of the cross phase modulation exercise together a strong influences in the propagation of the same, provoking the break of the pulse in the exit of the device when we used a larger length for the filters. For shorter devices, the switched pulse presented temporary and spectral compression and spread, as well as, displacement in the time in the two propagation modes (TE and TM). After the choice of a length of appropriate filter, a great temporary displacement was selected to be applied in the input pulses for us to get, in the exit of the logical gate, satisfactory temporary displacements (successes) in the application of the PPM modulation. Soon after, we introduced phases in one of the entrance pulses (TM), provoking a phase displacement among TE and TM pulses, still reducing more the margin of error PPM of operation of the gates. Finally, to the we analyze the applied phases in the pulse TM (0 to 2), we defined the best phase angle for the gates to operate in the success area of the PPM modulation.

Doutoramento em Engenharia Eletrotécnica
All-optical solutions for switching and routing packet-based traffic are crucial for realizing a truly transparent network. To meet the increasing requirements for higher bandwidth, such optical packet switched networks may require the implementation of digital functions in the physical layer. This scenario stimulated us to research and develop innovative high-speed all-optical storage memories, focusing mainly on bistables whose state switching is triggered by a pulsed clock signal. In clocked devices, a synchronization signal is responsible for controlling the enabling of the bistable. This thesis also presents novel solutions to implement optical logic gates, which are basic building blocks of any processing system and a fundamental element for the development of complex processing functionalities. Most of the proposed schemes developed in this work are based on SOA-MZI structures due to their inherent characteristics such as, high extinction ratio, high operation speed, high integration capability and compactness. We addressed the experimental implementation of an all-optical packet routing scheme, with contention resolution capability, using interconnected SOAMZIs. The impact on the system performance of the reminiscent power of the blocked packets, from the non ideal switching performed by the SOA-MZIs, was also assessed.
As soluções totalmente óticas para a comutação e encaminhamento de pacotes de tráfego são cruciais para a realização de uma rede verdadeiramente transparente. Para atender às exigências crescentes de maior largura de banda, tais redes de comutação de pacotes óticos exigem a implementação de funções digitais na camada física. Este cenário estimulou-nos a investigar e a desenvolver memórias totalmente óticas, focando-nos principalmente na implementação de flip-flops óticos síncronos, cujo estado de comutação é accionado por um sinal de relógio. Esta tese também apresenta novas soluções para implementar portas lógicas óticas, visto estas serem um elemento fundamental para o desenvolvimento de funcionalidades complexas de processamento. A maioria dos esquemas propostos neste trabalho são baseados em estruturas interferométricas activas Mach-Zehnder (SOA-MZI) devido às suas características intrínsecas, nomeadamente, razão de extinção elevada bem como elevada capacidade de integração. A implementação experimental de um sistema de encaminhamento de pacotes totalmente ótico foi realizada usando cascatas de SOA-MZIs. O impacto da potência residual, devido à comutação não ideal dos SOA-MZIs, foi também analisado.

Modulation instability (MI) is a general characteristic of wave propagation in nonlinear dispersive media and it has been intensively investigated in several branches of physics due to its fundamental nature as well as technological applications. This phenomenon corresponds to the exponential growth of weak harmonic perturbations in virtue of the interplay between dispersive and nonlinear effects. Hence, despite its important features, MI is also a main source of channel depletion and degradation in optical fiber communications. In this thesis, we investigate the modulational instability (MI) induced by cross-phase-modulation (XPM) of two incoherently coupled optical pulses co-propagating in a lossless fiber with a finite nonlinear response time. The non-instantaneous character of the nonlinear response is introduced through a Debye relaxation process. We analytically obtain the exact dispersion relation for weak harmonic perturbations over the stationary solution. We show that the instability spectrum, present in both normal and anomalous dispersive regimes in instantaneously responding Kerr media, develops a double peak structure whose relative strength and typical frequency range depend on the response time. Further, we reveal that there are two unstable modes in the entire frequency spectrum. We report the dependence of the maximum gain and central frequency within each unstable mode as a function of the group velocity mismatch and response time, showing the crossover between the regimes of fast and slow non-linear responses.
Fundação de Amparo a Pesquisa do Estado de Alagoas
Conselho Nacional de Desenvolvimento Científico e Tecnológico
Instabilidade Modulacional (IM) é um fenômeno característico da propagação de ondas em meios dispersivos não-lineares e tem sido estudado em diversas áreas da Física devido a sua natureza fundamental bem como suas importantes plicações tecnológicas. Esse fenômeno corresponde ao enriquecimento exponencial de pequenas perturbações harmônicas devido a cooperação dos efeitos não-lineares e dispersivos. Portanto, não obstante sua aplicabilidade, IM é, de igual modo, uma fonte importante de degradação em sistemas de comunicação por fibras ópticas. Nesta tese investigamos a instabilidade modulacional (IM) induzida por Modulação de Fase Cruzada (MFC) de dois pulsos ópticos acoplados incoerentemente que se propagam em uma fibra sem perda com tempo de resposta não-linear finito. O caráter não-instantâneo da resposta não-linear é introduzido através de um processo de relaxação de Debye. Obtemos analiticamente, de modo exato, a relação de dispersão para fracas perturbações harmônicas da solução estacionária. Mostramos que o espectro de instabilidade, presente tanto no regime de dispersão normal quanto no anômalo em meios Kerr com resposta instantânea, desenvolve uma estrutura de pico duplo cuja a intensidade relativa e a frequência típica dependem do tempo de resposta considerado. Além do mais, revelamos que existem dois modos instáveis ao longo de todo o espectro de frequência. Apresentamos a dependência do ganho máximo e da frequência correspondente dentro de cada modo instável como função da diferença da velocidade de grupo e do tempo de resposta, mostrando o cruzamento entre os regimes de resposta não-linear rápida e lenta.

Depuis les années 50, la récupération de la phase d’un faisceau optique diffracté par un objet quelconque, est un sujet important dans plusieurs domaines scientifiques, comme la microscopie, l’astronomie et bien d’autres. Généralement, les méthodes qui le permettent se divisent en deux grandes catégories : les méthodes interférométriques et les méthodes itératives basées sur la propagation du faisceau. L’intérêt de ces dernières, réside dans le fait qu’elles sont moins sensibles au bruit, et leur implémentation expérimentale est plus simple. Aussi, le développement des techniques informatiques a rendu cette approche plus rapide et plus intéressante. Cependant, même si l’efficacité de ces méthodes a été démontrée dans plusieurs domaines, leur utilisation est restée limitée à cause de certaines exigences sur les conditions expérimentales, et à la non-convergence de leur algorithme vers une solution unique dans un grand nombre de cas. Ceci est encore plus vrai pour les objets dits "objets complexes", possédant une amplitude et une phase, ce qui réduit fortement leur champ d’application. Afin de surmonter ces problèmes de convergence, diverses stratégies expérimentales ont été développées. Elles ont toutes comme principe d’introduire de nouvelles contraintes bien connues dans le plan de l’objet. Cela permet d’augmenter le nombre de spectres acquis, et donc accroitre et diversifier les sources d’informations sur l’objet de base, ce qui va aider l’algorithme itératif à converger plus rapidement vers une solution finale et unique. Comme exemple de ces stratégies expérimentales, on peut acquérir plusieurs spectres provenant de différentes zones de l’objet, ou moduler la longueur d’onde du faisceau incident, ou même enregistrer les spectres dans des plans parallèles, connectés entre eux par la transformée de Fresnel. Dans ce contexte, le présent travail vise à démontrer expérimentalement une technique connue sous SSPR (Spread Spectrum Phase Retrieval), proposé en 2007 par Zhang, tout en lui introduisant un certain nombre de modifications, afin de la rendre plus pratique. L’idée consiste à moduler le front d’onde de l’objet par M phases aléatoires, générées avec un modulateur spatial de lumière à base de cristaux liquides (LC-SLM), puis enregistrer dans le plan de Fourier les M spectres correspondants. Ces M spectres seront ensuite utilisés dans un algorithme itératif permettant de remonter au front d’onde de l’objet initial, en simulant la propagation du front d’onde entre les deux espaces, spatial et fréquentiel.La première partie de cette thèse comporte une étude détaillée sur les modulateurs spatiaux de lumière, afin de pouvoir choisir le mieux adapté à notre application. Une fois que le modulateur à base de cristaux liquides (LC-SLM) est sélectionné, on présentera ses caractéristiques techniques, ainsi que les tests et les étapes de calibrations nécessaires pour assurer son fonctionnement linéaire et optimal. Ensuite, on montrera plusieurs types d’applications possibles avec ce composant, et dans divers domaines scientifiques, comme l’holographie, la microscopie, l’optique adaptative ainsi que les méthodes interférométriques permettant de reconstruire la phase d’un faisceau lumineux. Dans la deuxième grande partie, on concentre notre travail autour de la méthode itérative SSPR. On montrera comment on peut rendre l’application de cette méthode plus simple en utilisant un modulateur spatial de lumière à base de cristaux liquides, et en travaillant dans le plan de Fourier à la place du plan de Fresnel. Cependant, après avoir appliqué expérimentalement cette méthode, on remarque que les résultats obtenus sont très mauvais par rapport aux résultats des simulations. On effectue donc, une étude détaillée concernant les sources de bruits pouvant être responsable de la dégradation de la qualité des reconstructions obtenues. [...]
Since the 50s, recovering the phase information of a diffracted beam has a major interest in several fields such as microscopy, astronomy and many others. Generally, the solutions fall into two broad categories: interferometric methods and iterative methods based on beam propagation. The advantage of the latter is that they are less sensitive to noise, and their experimental implementation is simpler. Also, the progress in computer technologies as well in digital imaging devices makes the application of this approach easier and more interesting. However, even if the effectiveness of these methods has been demonstrated in several fields, their use remained limited because of certain requirements on the experimental conditions and the non-convergence of their algorithm to a single solution in many cases. This is even more true for the so-called "complex objects", having an amplitude and a phase, which can greatly reduce their field of application. To overcome the convergence problems and improve the robustness of these methods, many experimental strategies have been employed. They are all based on the same principle, which consists of introducing new well-known constraints in the object plane. This increases the number of acquired spectrum, and therefore diversifies the sources of information about the starting object, which will help the iterative algorithm to converge more quickly towards the final solution. As examples of such experimental strategies, one can record several spectra from different areas of the object, or modulate the wavelength of the incident beam, or also acquire the spectrums across two or more parallel planes connected through Fresnel or Fourier transform.In this context, the present work aims to experimentally demonstrate a technique known as SSPR (Spread Spectrum Phase Retrieval), proposed in 2007 by Zhang, while modifying it in order to make it more flexible. The idea is to introduce, using a liquid crystal spatial light modulator M strong phase modulation into the object field, then record in the Fourier plane the M corresponding spectrums. These M acquisitions will then be used in an iterative algorithm what will allow us to recover the object wavefront by simulating the propagation of the light between spatial and frequency spaces. The first part of this thesis includes a complete study on spatial light modulators; in order to select which one will be best suited for our application. Once liquid crystal spatial light modulators are selected, we present their technical characteristics, as well as the calibration tests needed to ensure their linear and optimal functioning. Then we show several possible applications with this type of component, in various scientific fields, like holography, microscopy, adaptive optics and interferometric methods to reconstruct the phase of a beam.In the second part, we focus our work around the SSPR iterative method. We will show how to make the application of this method simpler by using a liquid crystal spatial light modulator, and by working in Fourier plane instead of Fresnel plane. However, after applying SSPR we have noticed that the quality of experimental results is very inferior to the quality of simulation results. Therefore, a detailed study of the noise sources is conducted. Each of these noise sources adds its own contribution, yet modulator cross-talk remains the factor that deteriorates the most the quality of reconstruction. In fact liquid crystal spatial light modulators are known to have a strong cross-talk between their pixels commonly recognized as fringing field effect. As the pixels are micrometric, each addressed one affects its neighbors, and thus, the phase retardation obtained from a pixel will not be uniform over its entire surface. This will result in a blurring effect of the desired sharp edge between the pixels; therefore, the real displayed phase map will be very different from the addressed one. [...]

Die vorliegende Arbeit beschäftigt sich mit dem Konzept der radial nicht-oszillierenden, zeitlich stabilen ultrakurzen Bessel ähnlichen Strahlen oder "Nadelstrahlen" ("needle beams"), die zu einer Klasse von optischen hochlokalisierten Wellenpaketen generalisiert werden. Hierbei wird die Theorie über das räumlich-zeitlichen Ausbreitungsverhaltens von nicht auseinanderdriftenden Nadelstrahlen mit Pulsdauern von kleiner als 10 fs näher diskutiert. Dies wird durch eine systematische Darstellung der Methoden zur Generierung und Detektierung von lokalisierten Wellen komplettiert, die ein optischen Drehmoment tragen. Für die Erzeugung von HLWs kommen räumliche Lichtmodulatoren zum Einsatz, die ein flexibles Zuschneiden von Wellenpaketen mit der Dauer weniger Zyklen des EM-Feldes erlauben. Es wird gezeigt, dass solche optischen Pulse sich über beträchtliche Entfernungen ausbreiten, ohne dass sich dabei signifikant der Strahldurchmesser vergrößert oder der Puls zeitlich verbreitert. In variabler Weise werden verschiedene geometrische (z.B. ringförmige) Lichtverteilungen erzeugt. Anwendungspotential findet sich insbesondere in den Techniken der räumlichen Pulsformung und Diagnostik. Als besonders wichtiger Ansatz ist der Zeit-Wellenfront-Sensor zu erwähnen, welcher die nichtlineare, mehrkanalige Autokorrelation, die Wellenfrontdetektion mittels nichtdiffraktiver Teilstrahlen nach dem Shack-Hartmann-Prinzip und eine adaptive Funktionalität miteinander vorteilhaft verbindet. Das enorme Potential solcher Ansätze wird durch die hohe Genauigkeit orts-, winkel- und zeitabhängiger Rekonstruktionen der Wellenpakete nachgewiesen. Darüber hinaus ermöglicht das räumliche Kodieren und anschließende Verfolgen der Teilstrahlen eine wesentliche Verbesserung der Identifikation relevanter Parameter von Verteilungsfunktionen. Schließlich werden erste Schritte zur experimentellen Generation von optischen "light bullets" mit ganzzahligen und fraktalen orbitalen Drehmomenten präsentiert.
This thesis deals with the concept of radially non-oscillating, temporally stable ultrashort-pulsed Bessel-like beams or "needle pulses", which are an example of a highly localized wave packet (HLW). HLWs are the closest approximation of linear-optical light bullets and provide specific benefits compared to conventional Gaussian-like light bullets. The spatio-temporally nonspreading propagation behavior of few-cycle needle beams of less than 10 fs duration will be theoretically discussed in detail. An overview of the generation and detection of localized waves carrying an orbital angular momentum is also given. High fidelity spatial light modulators are used for the generation of HLWs. The flexible tailoring of few-cycle wave packets at near-infrared wavelengths is reported. It is shown that such pulses propagate over a huge depth of focus, neither significantly changing their spot size or nor the pulse duration. Variable geometrical distributions like circular disks, rings, or bars of light are shaped and exploited as building blocks for structures of higher complexity. Another section of the thesis emphasizes the numerous potential applications of related techniques for an optimized two-dimensional spatial pulse shaping and diagnostics (reduce ambiguities) based on localized waves. As a particularly important example, time-wavefront sensing is used to combine nonlinear multichannel autocorrelation with Shack-Hartmann wavefront sensing by means of localized sub-beams and adaptive functionality. The capabilities of such devices are illustrated by the results of angular and temporal mapping of few-cycle wave packets. Moreover, spatial encoding and subsequent tracking of individual sub-beams, even at incident angles of up to 50°, enables to significantly improve the spot recognition. Finally, first steps towards the generation of optical light bullets carrying integer or non-integer orbital angular momenta are presented.

The spectral, temporal, and spatial characteristics of plasma-induced self-phase and cross-phase modulation in rare gases have been investigated using a femtosecond KrF excimer laser focused to peak intensities of 10$\sp{14}$-10$\sp{15}$ W cm$\sp{-2}.$ The quiver energy of a free electron under these conditions is less than the ionization potential of all rare gases, ensuring that ionization occurs only by optical field-induced processes. Spectral blueshifts of up to 2 nm have been observed, and the blueshifted spectra show an oscillatory structure. The blueshifted spectra are shown to be the result of plasma-induced self-phase modulation and can be modeled by assuming tunneling ionization and one dimensional pulse propagation. The newly discovered oscillatory structure in the spectra is related to that observed in earlier experiments on self-phase modulation in optical fibers. To investigate the temporal behavior of the field ionization process, pump-probe experiments have been performed with a 100 fs probe pulse at 497 nm and a 400 fs pump pulse at 248 nm. Under conditions of weak ionization (Z $\ll$ 1), pump-probe experiments and theoretical calculations show that the ionization rate of the field ionized gas is maximum at the peak of the laser pulse and that the degree of ionization changes over a time equal to about half of the pump pulse width. By observing changes in the transmission of the probe pulse caused by plasma absorption, the electron temperature of a field ionized rare gas is determined to be on the order of 1 eV. The time varying electron density in the pump-probe experiments also causes plasma-induced cross-phase modulation, or spectral blueshifting of the probe pulse spectrum of up to 15 nm. The pump-probe experiments show that plasma defocusing causes the spectral blueshifting to be spatially dependent. Experimental results and a two dimensional pulse propagation model indicate that the most defocused beam components also show the maximum spectral blueshift. Plasma-induced cross-phase modulation has also been used to characterize the amplitude and phase of a 1 ps chirped pulse at 497 nm and the pulse width of a 400 fs pulse at 147 nm generated by four wave frequency mixing in xenon.

Στην εργασία αυτή γίνεται προσομοίωση της Διασταυρούμενης Διαμόρφωσης Φάσης που λαμβάνει χώρα σε ένα SOI κυματοδηγό υπό την προϋπόθεση της μηδενικής διασποράς. Για να καταλήξουμε στο μοντέλο προσομοίωσης παρουσιάζονται τα μη-γραμμικά οπτικά φαινόμενα τα οποία συμμετέχουν στο υπό μελέτη φαινόμενο καθώς επίσης και τα χαρακτηριστικά του κυματοδηγού που επιδρούν στην διάδοση του παλμού. Το τελικό μοντέλο λαμβάνει επίσης υπόψη του τις βασικές ιδιότητες του πυριτίου και τις διεργασίες που γίνονται σ’ αυτό κατά την διάρκεια της αλληλεπίδρασης του υλικού με την οπτική ακτινοβολία. Τα αποτελέσματα που δίνει το μοντέλο είναι η επίδραση της έντασης του παλμού άντλησης στην μεταβολή της φάσης του διαδιδόμενου κύματος και κατά συνέπεια την μεταβολή του μήκους κύματος.
In this work becomes simulation of Cross-Phase Modulation (XPM) that takes place in a silicon-on-insulator (SOI) waveguide under the condition less-dispersion. In order to we lead to the model of simulation are presented the nonlinear optical phenomena which participate in under study phenomenon as well as the characteristics of waveguide that affect in the propagation of pulses. The final model takes into consideration the basic properties of silicon and the activities that become in this at the duration of interaction of material with the optical radiation. The results that it gives the model are the effect of intensity of pump pulse in the change of phase of propagated wave and accordingly the change of wavelength.

Optical regeneration has the potential to significantly increase the reach of long-haul transmission systems. In this thesis, wavelength-preserving polarization-insensitive all-optical 3R regeneration is investigated and demonstrated for 10 and 40 Gb/s signals. The all-optical regenerator utilizes a self-pulsating laser for clock recovery, cross-phase modulation (XPM) based spectral broadening in a highly nonlinear fiber (HNLF) and offset filtering for retiming, and self-phase modulation based spectral broadening in a HNLF and offset filtering for reshaping. Raman amplification is used to increase the XPM-based spectral broadening and thus allow a design that meets the tradeoffs involved in simultaneously achieving good retiming and reshaping performance. The regenerator is shown to reduce amplitude noise and timing jitter while not causing a BER penalty. To fully validate the regeneration scheme, the cascadability is demonstrated using a recirculating loop. For a 10 Gb/s signal, with a regenerator spacing of 240 km, a return-to-zero, on-off-keyed (RZ-OOK) signal was transmitted over 18,000 km (75 loops) with a power penalty of 1.6 dB at a BER of 1E-9 compared to the back-to-back case. For a 40 Gb/s signal, with a regenerator spacing of 80 km, a RZ-OOK signal was transmitted over 8,000 km (100 loops) with a power penalty of 1.2 dB. In addition, all-optical 3R regeneration is demonstrated using a multimode quantum-dot Fabry Petot laser with ultra-low timing jitter.
Thesis (Ph.D, Electrical & Computer Engineering) -- Queen's University, 2009-10-19 14:11:53.826

The dynamical underpinnings of complex computation and information transmission within the brain, while of great interest to the neuroscience community at large, remain poorly understood. One of the striking manifestations of neuronal population activity is that of rhythmic oscillations in the local field potential. It is thought that distinct patterns of these oscillations such as cross-frequency coupling within a given spatial location and coherence between disparate brain regions may represent neuronal computation and communication, respectively. Here we show such dynamics within a human temporal neocortical in vitro model. In specific, we show theta-gamma cross frequency coupling in deep and superficial layers, phase coherence between layers at theta frequencies, and a measure of communication (phase dependent power correlations) between layers at theta frequency. Additionally, we show a novel correlation between communication across layers and cross frequency coupling within layers, demonstrating a dynamic link between local computation and intralaminar communication.

The emergence of quantum physics from the page to the lab and the world at large is an exciting development of recent years. The prospects of absolutely secure communication and efficient simulation of physical systems have spurred great human effort into understanding these possibilities and turning them into realities. Photons are the most easily manipulated quantum particles and are a promising candidate for implementing these technologies. Limitations of photons include the difficulty of keeping objects that move at the speed of light, and producing strong interactions between particles that do not normally interact. The work presented in this thesis is motivated by the possibility of overcoming these limitations. The ability to faithfully store and reproduce a quantum state is essential for many quantum information technologies. Quantum memories for light have been developed over the last two decades to provide this ability. The group at the Australian National University developed the gradient echo memory (GEM): A quantum state of light can be controllably stored and released from an atomic ensemble by the use of additional optical fields and magnetic field gradients. This scheme was previously shown to preserve the quantum characteristics of the light. We used the GEM scheme with a cold rubidium ensemble to create the first optical memory that simultaneously beat the no-cloning limit, a benchmark for many of the technologies relying on quantum memories, and the loss rate for a delay line composed of optical fibre. We also created an analogue to a pulsed optical resonator using GEM with a warm rubidium vapour. This was done by replacing the circulating optical field of a resonator with light stored in the memory, and replacing the coupling of light into and out of that circulating mode with storage and recall from the memory. The bandwidth and repetition rate of this resonator were rapidly tunable as they were controlled by external optical and magnetic fields. We worked on implementing GEM with strings of thousands of atoms strongly coupled to the evanescent field of an optical nanofibre. This raised new possibilities for creating a true random access memory that would allow a more flexible use of the multi-mode capacity of GEM. We developed the theory for a novel type of stationary light in the gradient echo memory. Our stationary light scheme relies on the destructive interference of counter-propagating optical fields throughout the memory. The optical intensity scales with optical depth, as with other forms of stationary light. However, as the destructive interference could be set up over a much greater distance, more of the optical depth is available for generating stationary light. Finally, we studied how a control-phase gate for single-photon optical states could be implemented using a nonlinear interaction with stationary light. The stationary light generated by one state modulates the phase of another state stored in the memory. The second state modifies the stationary light, also producing a back-action on the first state and generating the required cross-phase shift.

碩士
國立中央大學
光電科學研究所
97
At a process of making the optics, no matter what manufacturing method we’ve chosen, we have to measure the surface profile of semi-finished goods or finished items. The most usual test method and measurement of optical element surface profile of optics roughly can be classified into two ways: the interferometry of two wavefronts; and the measurement of transverse ray aberration on the focal plane. The former belongs to the application of wave optics, and the latter belongs to the geometry optics. Our dissertation belongs to the testing method of geometry optics. This experiment is by means of the grating-slit test invented by Dr. Liang to measure the transverse ray aberration. The experiment used incoherent light source to irradiate the liquid crystal modulator. And we used liquid crystal modulator to control the brightness of object and reached the function of grating. By using the cross-slit, we can get the Ronchigram on exit pupil plane. Then through four steps of phase-shifting method, we can mathematically calculate the amount of transverse ray aberration. Finally, we reconstruct the wavefront and get the information of surface profile of optics. We already obtained the wavefront result of mirror’s surface profile by grating-cross slit method. Comparing it with the result measured by interferometry, we found the two surface profiles were very similar. By doing so, we can verify the feasibility of our experiment setup.