Дисертації з теми "Coherent optical pulses"

2006/2007
The need of coherent and intense pulsed radiation is spread among many research disciplines, such as biology, nanotechnology, physics, chemistry and medicine. The synchrotron light from traditional sources only partially meets these characteristics. A new kind of light source has been conceived and developed in the last decades: the Free-Electron Laser (FEL). The FEL process relies on the interaction between a relativistic electron beam and an electromagnetic wave in presence of a static and periodic magnetic field, produced by a device called undulator. This interaction generates coherent radiation at a fundamental frequency and its higher harmonics. In the standard configuration, the electron beam is generated by a linear accelerator and the interaction occurs in a single passage through one or several undulators. An alternative configuration can be obtained if the electrons are supplied by a storage ring. This work has been carried out at the Elettra laboratory within the ``new light sources'' group. My thesis focuses on both numerical and experimental issues about the generation of coherent harmonics on storage-ring FELs. The Elettra SRFEL has been originally designed to operate in ``oscillator configuration'' where the radiation is stored in an optical cavity (made of two mirrors). This process also drives the emission of radiation in the harmonics. In this work, different experimental methods have been implemented at Elettra to concentrate the power in giant pulses, both for the fundamental wavelength and its harmonics. Using this technique, it has been possible to generate fundamental radiation at 660 nm and 450 nm with (intra-cavity) power of few mJ and third harmonic radiation at 220 nm and 150 nm with few nJ of power. This process has been studied numerically by using a tri-dimensional simulation which also accounts for the re-circulation of the beam. The results of simulations are in good agreement with experimental measurements and allow to investigate the inner structure of the light below the picoseconds scale, where the instrumentation resolution reaches its limit. Structures of hundreds of femtoseconds inside the laser pulse have been found and this implies a higher peak power. Moreover, the numerical results have been confirmed by spectral measurements. By removing the optical cavity and focusing an external laser in the first undulator, a ``seeded single-pass'' configuration has been implemented. In the first undulator, the interaction with the external laser (``seed'') modulates the electron energy which is converted to spatial modulation (``bunching''). A Fourier analysis of the bunched electron-beam shows the presence of components at all harmonics (even and odd) and this explains why electrons in the second undulator can emit at any harmonic. To implement this configuration a design and layout plus tri-dimensional simulations were performed. Followed by the installation of the seed laser (Ti:Sapphire, lambda = 796 nm), the timing and the diagnostics. The commissioning focused on optimizing the spatial overlap and the synchronization between the electrons and the seed laser. Coherent harmonic radiation has been obtained at 265 nm, the third harmonic of the seed laser. After the characterization of this light, the seed frequency has been doubled by means of a nonlinear crystal. With this setup, radiation down to 99.5 nm (the fourth harmonic of the seed) has been generated. The shot-to-shot stability is comparable to the stability of the synchrotron radiation (fluctuations of few %) but the number of photons per pulse (~10^9) is about two-three orders of magnitude bigger than the synchrotron one. Thus this coherent radiation can be used for experiments similar to those suggested for the next generation FELs. Summarizing, the light source developed during my thesis is a unique facility able to generate coherent radiation with variable polarization, variable duration (between 100 fs and 1 ps), with peak power of the order of mega-Watts in a wide spectral VUV range. In the latest implementation, this radiation source has been used for two different kind of experiments, one in gas-phase, the other of solid state. The obtained results demonstrate the appealing of this source for user experiments. In perspective, there is a plan to extend the wavelength range below 100 nm and to improve the tunability of the source.
Vari ambiti della ricerca scientifica, dalla biologia alle nanotecnologie, passando per la fisica, la chimica e la medicina, richiedono per le loro indagini una radiazione spazialmente coerente con un elevato numero di fotoni per impulso. Poiché la radiazione di sincrotrone non possiede queste caratteristiche, negli ultimi anni gli sforzi si sono concentrati nello sviluppo delle cosiddette sorgenti di quarta generazione: i laser a elettroni liberi (LEL). Il processo LEL avviene per l'interazione di un'onda elettromagnetica con un fascio di elettroni relativistici in presenza di un campo magnetico. Tale campo, statico e periodico, viene generato da un dispositivo detto ondulatore. L'interazione produce emissione di luce coerente ad una frequenza fondamentale e alle sue armoniche superiori. La configurazione standard prevede che gli elettroni siano prodotti da un acceleratore lineare e l'interazione si risolve tipicamente in un singolo passaggio attraverso uno o più ondulatori. Una configurazione alternativa si ottiene quando gli elettroni sono forniti da un anello di accumulazione. La tesi si è svolta presso il laboratorio Elettra, nel gruppo che si occupa dello sviluppo di nuove sorgenti di luce. La mia attività di ricerca comprende sia aspetti teorico-numerici che sperimentali relativi alla generazione di armoniche coerenti su LEL installati su anelli di accumulazione. Storicamente il laser a elettroni liberi ad Elettra è nato in ``configurazione oscillatore'' (la radiazione è immagazzinata in una cavità ottica formata da due specchi). Ad ogni passaggio successivo gli elettroni interagiscono con l'onda electtromagnetica amplificandola fino all'instaurarsi dell'effetto laser. Questo processo guida anche l'emissione alle armoniche superiori. Diversi metodi sperimentali possono essere usati per concentrare la potenza in impulsi giganti, sia per la fondamentale che per le armoniche. Questa tecnica, che ho affinato durante il mio lavoro di tesi, ci ha permesso di generare potenze dell'ordine di alcuni mJ per la fondamentale (nella cavità) e di alcuni nJ alla terza armonica di 660 nm e di 450 nm, cioè 220 nm e 150 nm rispettivamente. Dal punto di vista numerico, per studiare questo processo abbiamo modificato un codice per simulare tridimensionalmente la nostra configurazione ed abbiamo aggiunto una parte che propaga gli elettroni lungo l'anello. Le simulazioni sono in ottimo accordo con i dati sperimentali e ci permettono di investigare più nel dettaglio l'impulso, nella scala temporale dei femtosecondi dove si arresta la risoluzione strumentale. Dalle simulazioni risulta che all'interno degli impulsi laser sono presenti delle substrutture della durata di alcune centinaia di femtosecondi. La presenza di tali strutture implica una potenza di picco maggiore. Abbiamo inoltre una conferma indiretta dei risultati numerici tramite le misure spettrali. Rimuovendo la cavità ottica e focalizzando un laser esterno nel primo ondulatore si può passare alla cosiddetta configurazione in ``singolo passaggio''. Nel primo ondulatore, l'interazione con il laser esterno (``seed'') produce una modulazione nell'energia degli elettroni, la quale viene trasformata in separazione spaziale (``bunching''). Un'analisi di Fourier del fascio di elettroni mostra componenti a tutte le armoniche (pari e dispari), per cui gli elettroni sono in grado di emettere a qualsiasi armonica nel secondo ondulatore. In questa configurazione la prima parte del lavoro di tesi è stata il design della linea e lo studio numerico dei risultati attesi. A questo studio preliminare è seguita l'installazione dell'esperimento, a partire dall'alloggiamento e la messa in funzione del laser esterno (Ti:Sapphire, lambda = 796 nm) fino alla realizzazione del sistema di sincronizzazione del seed con gli elettroni. Prima di ottenere la radiazione armonica coerente e poter confrontare le aspettative con i risultati sperimentali abbiamo dovuto dedicare molti turni di fisica di macchina al perfezionamento della sovrapposizione spaziale e temporale tra elettroni e laser esterno. La prima radiazione armonica coerente è stata ottenuta alla terza armonica (265 nm) del laser esterno. Dopo una prima caratterizzazione della sorgente, abbiamo introdotto un cristallo nonlineare per generare la seconda armonica del laser esterno e usare questa come seed. Attualmente il LEL di Elettra è in grado di produrre radiazione fino a 99.5 nm (la quarta armonica del seed) con la stessa stabilità della radiazione di sincrotrone (flutuazioni dell'ordine del %). Queste caratteristiche, insieme al numero di fotoni per impulso (~10^9) che supera di almeno due ordini di grandezza l'emissione di sincrotrone, permettono l'utilizzo della luce prodotta per esperimenti simili a quelli proposti per le sorgenti di quarta generazione. Riassumendo, la sorgente sviluppata durante la mia tesi è attualmente l'unica in grado di fornire luce coerente di durata variabile tra 100 fs e 1 ps con potenze dell'ordine del mega-Watt e polarizzazione variabile (lineare-circolare) in un ampia gamma spettrale nell'ultravioletto. Negli ultimi turni, questa radiazione è stata usata su due diversi tipi di esperimenti, uno in fase gassosa l'altro di stato solido. I risultati ottenuti dimostrano che la radiazione emessa può essere appetibile per gli utenti. Le prospettive sono estendere il range di lunghezze d'onda sotto i 100 nm e migliorare la tunabilità della sorgente.
XX Ciclo
1979

Experimental and theoretical work is presented on the control of wave packet dynamics in atomic and molecular systems. Using sequences of phase-locked optical pulses, the link between optical phase and quantum mechanical phase is explored in the Na atom and Naj dimer, representing a step towards the logical engineering of quantum states in more complicated systems. A novel apparatus, constructed to study and control the vibrational dynamics of vibrational wave packets on the Na ionic potential surface, is described in chapter 3. Theoretical simulations for proposed experiment are presented in chapter 4. Control is achieved by exploiting the phase-evolution of the constituent vibrational quantum states within the wave packet superposition. The phase relationship and the accumulated phase difference between the various components of the wave packet is determined, and a sequence of phase-locked optical pulses is employed to selectively enhance or depopulate specific vibrational states, or sets of vibrational states. The quantum state composition of the resulting wave packet, and the efficiency of the control scheme, is determined by calculating the multi-pulse response of the time-dependent vibrational state populations. In chapter 5, the quantum interference between Rydberg electron wave packets in the Na atom is investigated using pairs of phase-locked wave packets, allowing manipulation of the total orbital angular momentum of Na Rydberg atoms. Initially the wave packet is composed of a superposition of s and d Rydberg series. Exploitation of the difference between the quantum defects of the two series allows specific angular momentum compositions within the resultant wave packet to be engineered. Experimentally, this final quantum state distribution is analysed in the frequency domain using state selective field ionisation, and in the time domain using the optical Ramsey method. The theoretical calculations show how the phase difference between pairs of optical pulses is linked to the corresponding Rydberg frequency spectrum, therefore enabling the control of the quantum state composition of the wave packets.

Dans cette thèse, nous avons étudié la dynamique d'aimantation selon deux échelles de temps en utilisant la technique pompe-sonde magnéto-optique résolue en temps. A l'échelle de la picoseconde, la précession de l'aimantation est induite par des impulsions acoustiques dans des structures multicouches composées de deux couches ferromagnétique séparées par une couche métallique (Ni/Au/Py) avec différentes épaisseurs. La synchronisation de la précession des couches ferromagnétiques couplées a été observée. La modification de la précession de l'aimantation d'une couche de Ni est due l'interaction d'échange intercouche avec la couche Py. A l'échelle de 50fs, nous avons étudié la dynamique magnéto-optique cohérente, athermale, thermale et la relaxation des charges et des spins dans (Ni, Co et Fe) par impulsions de 11 fs dans un régime de faible perturbation. L'interaction spin-orbite et l'interaction d'échange jouent un rôle significatif dans la désaimantation ultrarapide
In this thesis, we have investigated the magnetization dynamics at picosecond and femtosecond time scale using time resolved magneto-optical pump probe technique. At picosecond time scale, the magnetization precession is induced by ultrafast acoustic pulses in a three layered structure with two ferromagnetic layers separated by varying thickness of metallic spacer layer (Ni/Au/Py). The magnetization precession dynamics of the Ni layer is modified due to the interlayer exchange interaction with the Py layer and the synchronized precession of the coupied ferromagnetic layers has been observed. At the timescale of 50fs, coherent magneto-optical, non-thermal, thermal and relaxation dynamics of charges and spins in ferromagnetic transition metals (Ni, Co and Fe) is studied by using 11fs optical pulses in a very low perturbation regime. The spin orbit interaction and exchange interaction play a significant role in the demagnetization of the ferromagnetic metals induced by femtosecond pulses

Les sources lasers femtosecondes sont utilisées dans grand nombre d’applications (industrielles, médicales, de recherche fondamentale) avec un besoin croissant d’impulsions très énergétiques à haut taux de répétition. Bien que la technologie Ti:Saphir fournisse des impulsions PetaWatt, son taux de répétition s’avère limité. Une alternative est l’utilisation de la technologie fibrée. Cependant, l’énergie extractible d’une seule fibre est intrinsèquement limitée.Une solution prometteuse est alors de réaliser une combinaison de fibres (jusqu’à plus de 10 000 fibres pour l’accélération de particules). La combinaison de fibres par méthode interférométrique (avec un record de 64 fibres combinées en régime continu) a prouvé qu’elle était un excellent candidat pour la combinaison d’un grand nombre de fibres.La collaboration XCAN entre l’Ecole Polytechnique et Thales, vise à réaliser un démonstrateur de combinaison cohérente de 61 fibres amplifiées en régime femtoseconde. Les travaux menés au cours de cette thèse s’inscrivent dans ce projet.Dans un premier temps, afin d’étudier les points durs inhérents au régime femtoseconde tout en s’affranchissant des difficultés liées à l’amplification, la méthode interférométique en régime femtoseconde a été étudiée sur un démonstrateur passif, c’est-à-dire sans amplification, de 19 fibres. Une fois la méthode de mise en phase validée celle-ci a pu être testée avec succès sur le démonstrateur avec amplification du projet XCAN.Nous présentons également les travaux menés afin d’augmenter un paramètre clé des systèmes de combinaison de faisceaux à savoir : l’efficacité de combinaison du système laser. Pour cela, nous avons réalisé une mise en forme de faisceaux issus des fibres de la tête optique. Cette mise en forme, gaussien vers super-gaussien, est réalisée à l’aide de deux réseaux de lames de phase dont nous présenterons le calcul des profils asphériques. Afin de valider expérimentalement nos simulations et après réalisation des lames de phase nous avons pu tester celles-ci sur le démonstrateur passif, démontrant une augmentation de 14 %.Les travaux présentés dans ce manuscrit présentent ainsi les premiers par vers la réalisation d’une nouvelle architecture laser massivement parallèle, capable de délivrer à la fois une haute puissance crête et une haute puissance moyenne
Femtosecond fiber sources are used in a large number of applications (industrial, medical, fundamental physics) with a growing need in high energy pulses at high repetion rate. Although Ti: Saphirre technology provides energies up to PetaWatt, its repetion rate is low (up to 1 Hz). An alternative is to use an amplified fiber. However, the extractable energy of a single fiber is intrinsically limited.A solution is then to combine several fibers (up to 10 000 fibers for particle acceleration). Coherent beam combining of fibers with an interferometric method (with a record of 64 fibers combined in the cw regime) has proven to be an excellent candidate to combine a large number of fibers.The XCAN project, a collaboration between l'Ecole polytechnique and Thales, aims to realize a demonstrator of 61 fibers coherently combined in the femtosecond regime.The works presented in this thesis are part of this project.In order to study the hard points inherent to the femtosecond regime and to free from the amplification issues, the interferometric method has been implemented on a passive demonstrator, meaning without amplification, of 19 fibers. Once the interferometric method validated, it has been succesfully tested on the amplified XCAN demonstrator.We present also the works done to increase a key parameter of beam combining systems : the combining efficiency. To do this, we have realized a beam shaping of the fiber array output beams. This beam shaping, gaussian to super-gaussian, is done with two arrays of phase plates. The aspherical profiles calculation is described. In order to validate our simulations we have tested the phase plates on the passive demonstrator by getting an increase of 14 %.The works presented in this manuscript are the first steps towards a new massively parallel laser architecture, able to provide both high peak power and high average power

Coherent pulse synthesis takes as its objective the piecewise assembly of a sequence of identical broadband pulses from two or more mutually-coherent sequences of narrowband pulses. The requirements for pulse synthesis are that the parent pulses share the same repetition frequency, are phase coherent and have low mutual timing jitter over the required observation time. The work carried out in this thesis explored the requirements for broadband coherent pulse synthesis between the multiple visible outputs of a synchronously pumped femtosecond optical parametric oscillator. A femtosecond Ti:sapphire laser was characterised and used to pump a PPKTP-based OPO that produced a number of second-harmonic and sum-frequency mixing outputs across the visible region. Using a novel lock-to-zero CEO stabilisation technique, broadband phase coherence was established between all the pulses on the optical bench, producing the broadest zero-offset frequency comb to date. Employing a common optical path for all the pulses provided common-mode rejection of noise, ensuring less than 150 attoseconds of timing jitter between the pulses over a 1 second observation window. The parent pulses were compressed and their relative delays altered in a quasi-common path prism delay line, allowing pulse synthesis at a desired reference plane.

The laser cooling of atoms to ultracold temperatures has propelled many groundbreaking advances in fundamental research and precision measurement, through such applications as quantum simulators and interferometric sensors. Laser cooling remains, however, highly species-selective, and techniques for its application to molecules are still in their infancy. This thesis broadly concerns the development of laser cooling schemes, based on sequences of coherent optical pulses, which can in principle be applied to a wide range of species. We describe a cooling scheme, in which a velocity-selective impulse analogous to that in Doppler cooling is generated by a light-pulse Ramsey interferometer, and present a proof-of-concept demonstration of the scheme using ultracold rubidium-85 atoms as a test-bed. We realise an interferometer for the atoms, as they are in free-fall after release from a magneto-optical trap, by inducing stimulated Raman transitions between their ground hyperfine states. We provide a comprehensive characterisation of these Raman light-pulse interferometer optics, where particular attention is paid to light shift effects. Raman pulses, and indeed coherent operations in any quantum control system, unavoidably suffer from systematic errors in the control field intensity and frequency, and these lead to reductions in pulse fidelity and readout contrast. In parallel to the work on interferometric cooling in this thesis, we report our preliminary investigations into composite pulses, whereby `naive' single pulses are replaced by sequences of rotations with tailored durations and phases, for improving pulse fidelity in the presence of inhomogeneities. We find that composite pulses can indeed be highly effective in our cold atom system, and propose that their application in such devices as interferometric sensors is a promising prospect.

Chemistry
Ph.D.
The SPECIFIC method a fast and accurate method for generating shaped femtosecond laser pulses. The femtosecond pulses are user specified from pulse parameters in the temporal domain. The measured spectral and recovered temporal phase and amplitudes from SEA TADPOLE are compared with the theoretical pulse profile from the user specified input. The SPECIFIC method has been shown to be a technique that can generate a diverse array of spectral/temporal phase and amplitude as well as polarization pulse shapes for numerous scientific applications. The spatio -temporal -spectral properties of focusing femtosecond laser pulses are studied for several pulse shapes that are important for non-linear spectroscopic studies. We have shown with scanning SEA TADPOLE that the spatio-spectral phase of focusing double pulse profile changes across the laterally across the beam profile. The spectral features of the sinusoidal spectral phase shaped pulse has been shown to tilt at with a changing angle away from the focus of the lens. Using spatio-spectral coupling, we have shown that multiple spatio-temporal foci can be generated along and perpendicular to the focusing direction of a femtosecond laser pulse. The spatial position of the spatio-temporal foci is controlled optically. Using sinusoidal spectral phase modulated pulse trains fragment ion production from Benzonitrile parent molecule can be controlled. A spectral transmission window perturbed the temporal pulse amplitudes resulting in fragment ion production dependant on spectral window position. The spectral window ion production was shown to also be dependant on temporal phase sequence.
Temple University--Theses

Pour un grand nombre d'applications, les sources laser impulsionnelles femtoseconde (fs) doivent fournir des puissances toujours plus importantes. En régime impulsionnel, on recherche d'une part une forte puissance crête par impulsion, et d'autre part une forte puissance moyenne, c'est à dire un taux de répétition élevé. Parmi les technologies existantes, les amplificateurs à fibre optique dopée ytterbium présentent de nombreux avantages pour l'obtention de fortes puissances moyennes, cependant le fort confinement des faisceaux dans la fibre sur de grandes longueurs d'interaction induit inévitablement des effets non-linéaires, et limite ainsi la puissance crête accessible. Nous avons étudié lors de cette thèse la combinaison cohérente d'impulsions fs appliquée aux systèmes fibrés. Ayant déjà fait ses preuves dans les régimes d'amplification continu et nanoseconde, la combinaison cohérente de faisceaux (dite combinaison spatiale) permet de diviser une seule et unique source en N voies indépendantes, disposées en parallèle et incluant chacune un amplificateur. Les faisceaux amplifiés sont ensuite recombinés en espace libre en un seul et unique faisceau, qui contient toute la puissance des N amplificateurs sans accumuler les effets non-linéaires. Cette architecture permet théoriquement de monter d'un facteur N le niveau de puissance crête issu des systèmes d'amplification fibrés. Au cours de cette thèse, nous avons démontré la compatibilité et l'efficacité de cette méthode en régime d'amplification fs avec deux amplificateurs, selon différents procédés. Les expériences démontrent d'excellentes efficacités de combinaison ainsi qu'une très bonne préservation des caractéristiques temporelles et spatiales initiales de la source. Les procédés de combinaison cohérente nécessitent cependant un accord de phase entre différents amplificateurs stable dans le temps, assuré en premier lieu par une boucle de rétroaction. Nous avons poursuivi notre étude en concevant une architecture totalement passive, permettant une implémentation plus simple d'un système de combinaison à deux faisceaux sans asservissement électronique. Enfin, une méthode passive de combinaison cohérente dans le domaine temporel est étudiée et caractérisée dans le domaine fs, et implémentée simultanément avec la méthode passive de combinaison spatiale proposée précédemment. Ces expériences démontrent la validité et la variété des concepts proposés, ainsi que leurs nombreuses perspectives pour les systèmes d'amplification fs fibrés.

[ES] Los sistemas de transmisión óptica no-coherente se emplean actualmente en las redes ópticas de corto alcance (< 80 km), como son las redes de ámbito metropolitano. La implementación más común en el estado del arte se basa en sistemas que emplean multiplexación por división en longitud de onda (WDM, wavelength division multiplexing) de cuatro longitudes de onda (¿) proporcionando un régimen binario de 100 Gbps (4¿×25 Gbps). En los últimos años, los sistemas de transmisión ópticos no-coherentes están evolucionando desde 100 Gbps a 400 Gbps (4¿×100 Gbps). Dado que este mercado comprende un gran número de sistemas, el coste es un parámetro importante que debe ser lo más bajo posible. El objetivo de esta tesis es investigar distintos aspectos del procesado de señal en general y, específicamente, investigar nuevas técnicas de procesado digital de señal (DSP, digital signal processing) que puedan ser utilizadas en sistemas de transmisión óptica no-coherentes empleando la modulación por amplitud de pulsos (PAM, pulse-amplitude modulation). Para que una técnica DSP sea interesante en el contexto de una red óptica WDM no-coherente, esta debe mitigar de manera efectiva al menos una de las tres limitaciones principales que afectan a estos sistemas: limitaciones de ancho de banda, limitaciones por dispersión cromática (CD), y el ruido. En esta tesis se proponen y examinan una serie de algoritmos cuyo su rendimiento es analizado mediante simulación y experimentalmente en laboratorio: - Feed-forward equalizer (FFE): este es el esquema de ecualización más común que se emplea principalmente en las transmisiones ópticas no-coherentes de alto régimen binario. Puede compensar grandes limitaciones en el ancho de banda. - Estimación de la secuencia de máxima verosimilitud (MLSE): el MLSE es un detector óptimo y, por lo tanto, proporciona las mejores prestaciones en detección cuando se abordan las limitaciones por CD y de ancho de banda. - Conformación geométrica de la constelación: en los esquemas de modulación de intensidad óptica multinivel, la distancia entre los niveles de amplitud puede ajustarse adecuadamente (de manera que no son equidistantes) a fin de aumentar la tolerancia de la señal frente al ruido. - Conformación probabilística: técnica diseñada específicamente para esquemas de modulación multinivel. Esta técnica ajusta la probabilidad de cada nivel de amplitud de modo que se incrementa la tolerancia al ruido óptico. - Señalización de respuesta parcial (PRS, partial signaling response): este es un enfoque basado en DSP donde una interferencia entre símbolos (ISI, inter-symbol interference) controlada es introducida intencionalmente de tal manera que la señal resultante requiere menos ancho de banda. La técnica PRS puede adaptarse para combatir también el efecto de CD. - Pre-énfasis digital (DPE, digital pre-emphasis): esta técnica consiste en aplicar el inverso de la función de transferencia del sistema a la señal en el transmisor, lo que reduce el impacto de las limitaciones de ancho de banda en el receptor. - Modulación con codificación Trellis (TCM, Trellis-coded modulation): esquema de modulación que combina elementos de corrección de errores (FEC, forward error correction) con técnicas de partición en conjuntos y modulación multidimensional para generar una señal más resistente al ruido. - Modulación multidimensional por partición en conjuntos: muy similar a TCM, pero sin ningún elemento FEC. Tiene menos ganancias que TCM en términos de tolerancia al ruido, pero no es tan sensible al ISI. Utilizando estas técnicas, esta tesis demuestra que es posible lograr una transmisión óptica con régimen binario de 100 Gbps/¿ empleando componentes de bajo coste. En esta tesis también demuestra regímenes binarios de más de 200 Gbps, lo que indica que la transmisión óptica no-coherente con modulación PAM puede ser una solución viable y eficiente en coste
[CAT] Actualment, s'utilitzen sistemes òptics no coherents en xarxes òptiques de curt abast ( < 80 km), com són les xarxes d'àmbit metropolità. La implementació més comuna que podem trobar en l'estat de l'art es correspon amb sistemes emplenant multiplexació per divisió en longitud d'ona (WDM, wavelength division multiplexing) de quatre longituds d'ona (¿) proporcionant un règim binari de 100 Gbps (4¿×25 Gbps). En els últims anys, els sistemes de transmissió òptica no-coherents han evolucionat des de 100 Gbps cap a 400 Gbps (100 Gbps/¿). Atès que el mercat de sistemes de curt abast compren un gran volum de dispositius òptics instal·lats, el cost unitari és molt important i ha de ser el més baix possible. L'objectiu d'aquesta tesi és analitzar aspectes del processament de senyal en general i, específicament, investigar noves tècniques de processament digital de senyal (DSP, digital signal processing) que puguen ser utilitzades en sistemes de transmissió òptica no-coherent que utilitzen la modulació per amplitud d'impulsos (PAM, pulse-amplitude modulation). Per tal que una tècnica DSP es considere interessant per a una xarxa òptica WDM no-coherent, aquesta ha de mitigar efectivament almenys una de les tres principals limitacions que afecten aquests sistemes: limitacions d'ample de banda, limitacions per dispersió cromàtica (CD), i el soroll. En aquesta tesi s'examinen una sèrie d'algoritmes, el seu rendiment s'analitza per simulació i experimentalment en laboratori: - Feed-forward equalizer (FFE): aquest és l'esquema d'equalització més comú i s'utilitza bàsicament en les transmissions òptiques no coherents d'alt règim binari. Pot compensar grans quantitats de limitacions d'ample de banda. - Estimació de la seqüència de probabilitat màxima (MLSE): el MLSE és un detector òptim i, per tant, proporciona el millor rendiment quan es tracta de limitacions d'ample de banda i de CD. - Conformació geomètrica de la constel·lació: en esquemes de modulació òptica d'intensitat multinivell es pot ajustar la distància entre els nivells d'amplitud (de manera que ja no són equidistants) per augmentar la tolerància del senyal al soroll. - Conformació probabilística: una tècnica dissenyada específicament per als esquemes de modulació multinivell; ajusta la probabilitat de cada nivell d'amplitud de manera que augmenta la tolerància al soroll òptic. - Senyalització de resposta parcial (PRS, partial signaling response): és un enfocament basat en DSP on la interferència entre símbols (ISI, inter-symbol interference) controlada s'introdueix intencionalment de manera que el senyal resultant requereix menys ample de banda. La tècnica PRS es pot adaptar per combatre els efectes del CD. - Pre-èmfasi digital (DPE, digital pre-emphasis): aquesta tècnica consisteix a aplicar la inversió de la funció de transferència del sistema a la senyal en el transmissor de manera que es redueix l'impacte de les limitacions d'ample de banda en la senyal en el receptor. - Modulació amb codificació Trellis (TCM, Trellis-coded modulation): esquema de modulació que combina els elements de correcció d'errors avançats (FEC, forward error correction) amb tècniques de partionament de conjunts i modulació multidimensional per generar un senyal més resistent al soroll. - Modulació multidimensional per partició en conjuntes: molt similar a TCM però sense elements FEC. Té guanys menors que TCM en termes de tolerància al soroll, però no és tan sensible a l'ISI. Mitjançant l'ús d'aquestes tècniques, aquesta tesi demostra que és possible aconseguir una transmissió òptica amb un règim binari de 100 Gbps/¿ utilitzant components de baix cost. Esta tesi també demostra règims binaris de més de 200 Gbps, el que indica que la tecnologia no-coherent amb modulació PAM és una solució viable i eficient en cost per a una nova generació de sistemes transceptors òptics WDM funcionant a 800 Gbps (4¿×200 G
[EN] Non-coherent optical transmission systems are currently employed in short-reach optical networks (reach shorter than 80 km), like metro networks. The most common implementation in the state-of-the-art is the four wavelength (¿) 100 Gbps (4¿×25 Gbps) wavelength division multiplexing (WDM) transceiver. In recent years non-coherent optical transmissions are evolving from 100 Gbps to 400 Gbps (4¿×100 Gbps). Since in the short-reach market the volume of optical devices being deployed is very large, the cost-per-unit of the devices is very important, and it should be as low as possible. The goal of this thesis is to investigate some general signal processing aspects and, specifically, digital signal processing (DSP) techniques required in non-coherent pulse-amplitude modulation (PAM) optical transmission, and also to investigate novel algorithms which could be applied to this application scenario. In order for a DSP technique to be considered an interesting solution for non-coherent WDM optical networks it has to effectively mitigate at least one of the three main impairments affecting such systems: bandwidth limitations, chromatic dispersion (CD) and noise (in optical or electrical domain). A series of algorithms are proposed and examined in this thesis, and their performance is analyzed by simulation and also experimentally in the laboratory: - Feed-forward equalization (FFE): this is the most common equalizer and it is basically employed in every high-speed non-coherent optical transmission. It can compensate high bandwidth limitations. - Maximum likelihood sequence estimation (MLSE): the MLSE is the optimum detector and thus provides the best performance when it comes to dealing with CD and bandwidth limitations. - Geometrical constellation shaping: in multilevel optical intensity modulation schemes the distance between amplitude levels can be adjusted (such that they are no longer equidistant) in order to increase the signal's tolerance to noise. - Probabilistic shaping: another technique designed specifically for multilevel modulation schemes; it adjusts the probability of each amplitude level such that the tolerance to optical noise is increased. - Partial response signaling (PRS): this is a DSP-based approach where a controlled inter-symbol interference (ISI) is intentionally introduced in such a way that the resulting signal requires less bandwidth. PRS can be customized to also mitigate CD impairment, effectively increasing transmission distances up to three times. - Digital pre-emphasis (DPE): this technique consists in applying the inverse of the transfer function of the system to the signal at the transmitter side which reduces the impact of bandwidth limitations on the signal at the receiver side. - Trellis-coded modulation (TCM): a modulation scheme that combines forward error correction (FEC) elements with set-partitioning techniques and multidimensional modulation to generate a signal that is more resistant to noise. - Multidimensional set-partitioned modulation: very similar with TCM but without any FEC elements. It has lower gains than TCM in terms of noise tolerance but is not so sensitive to ISI. By using the techniques enumerated above, this thesis demonstrates that is possible to achieve 100 Gbps/¿ optical transmission bitrate employing cost-effective components. Even more, bitrates higher than 200 Gbps are also demonstrated, indicating that non-coherent PAM is a viable cost-effective solution for next-generation 800 Gbps (4¿×200 Gbps) WDM transceivers.
Prodaniuc, C. (2019). Advanced Signal Processing for Pulse-Amplitude Modulation Optical Transmission Systems [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/117315
TESIS

ITC/USA 2015 Conference Proceedings / The Fifty-First Annual International Telemetering Conference and Technical Exhibition / October 26-29, 2015 / Bally's Hotel & Convention Center, Las Vegas, NV
SOQPSK-TG is a highly bandwidth-efficient constant-envelope modulation so that it has been applied in airspace telemetry widely. We compare four types of coherent detectors for SOQPSK-TG, which are optimal detector, pulse truncation (PT) detector, standard OQPSK detector and modified OQPSK detector. The simulation and analysis results indicate that PT detector has the advantages of low complexity and good performance, so it has more practicality value.

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Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Cette thèse s’inscrit dans le cadre du développement d’une source laser TW, de cadence élevée, stabilisée en phase, et délivrant des impulsions de quelques cycles optiques pour explorer la physique attoseconde. De telles impulsions contiennent seulement quelques oscillations de l’onde porteuse (durée de 5 fs à une longueur d’onde centrale de 800 nm) et ne sont pas directement disponibles à la sortie d’une source laser femtoseconde classique. Une technique de post-compression efficace pour obtenir de telles impulsions consiste à élargir le spectre des impulsions laser par automodulation de phase dans une fibre creuse remplie de gaz, puis à compenser la phase spectrale introduite avec des miroirs chirpés. Cette technique convient à des impulsions dont l’énergie est inférieure au millijoule. Au-delà, la transmission et la stabilité du compresseur chutent fortement à cause d'effets non linéaires tels que l'autofocalisation et l'ionisation. Pour comprimer des impulsions énergétiques et dont la phase de l’enveloppe est stabilisée par rapport à la porteuse (stabilisation de la CEP), il est possible de diviser l'impulsion initiale en plusieurs répliques d'énergie moindre et de réduire ainsi l'intensité crête en entrée de fibre. Le spectre de chaque réplique est alors élargi indépendamment. Dans le cadre de cette thèse, la combinaison cohérente passive d'impulsions de quelques cycles optiques issues d'une fibre creuse remplie de gaz est démontrée pour la première fois. L'utilisation de lames biréfringentes (calcite) dont l’orientation est soigneusement déterminée permet de générer et combiner des répliques avec une efficacité élevée. Ainsi, dans le cas d’une division en deux répliques, des impulsions stabilisées en phase (CEP), de durée 6 fs et d'énergie 0.6 mJ ont été générées de manière fiable et reproductible. L’étude détaillée de cette technique, aussi bien théorique qu’expérimentale, a permis de mettre en évidence les conditions requises pour générer des impulsions de quelques cycles optiques et présentant un bon contraste temporel. Plus précisément, la phase spectrale relative entre les répliques peut être mesurée à l'aide d'une méthode interférométrique permettant de quantifier les déphasages résiduels dus à la lame qui recombine les répliques, ainsi que ceux induits lors de la propagation dans la fibre par d'éventuels effets de modulation de phase croisée ou d'ionisation. Les effets qui affectent le processus de combinaison des répliques, tels que les modifications des états de polarisation des répliques ou bien les interactions non linéaires entre les répliques, sont analysés en détail. Une méthode est proposée pour minimiser ces effets, même dans le cas plus critique de la division et combinaison d'impulsions à quatre répliques
The framework of this thesis is the design and development of a TW-class, high-repetition rate, CEP-stabilized, few-cycle laser system devoted to attosecond physics. Few-cycle pulses includes only a few oscillations of the carrier wave (duration about 5 fs for 800nm central wavelength) and are not directly available at the output of typical femtosecond sources. One of the most popular techniques used for producing such pulses with high spatial quality is nonlinear spectral broadening in a gas-filled hollow-core fiber followed by temporal compression with chirped mirrors. However, as the input pulse energy approaches the milliJoule level, both the transmission and stability of hollow fiber compressors rapidly drop with the onset of self-focusing and ionization. A way of overcoming this limitation is to divide the input pulse into several lower energy replicas that can be subsequently recombined after independent spectral broadening in the fiber. In this thesis, the passive coherent combining of millijoule energy laser pulses down to few-cycle duration in a gas-filled hollow fiber is demonstrated for the first time. High combining efficiency is achieved by using carefully oriented calcite plates for temporal pulse division and recombination. Carrier-envelope phase (CEP)- stable, 6-fs, 800-nm pulses with more than 0.6 mJ energy were routinely generated in the case of twofold division and recombination. A detailed theoretical and experimental analysis of this temporal multiplexing technique is proposed to explain the conditions required for producing few-cycle pulses with high fidelity. In particular, an interferometric method for measuring the relative spectral phase between two replicas is demonstrated. This gives a measure of the phase mismatch in the combining plate, as well as that induced by eventual cross-phase modulation or ionization during propagation in the fiber. The effects degrading the combining process, as polarization change or nonlinear interactions between pulse replicas are analyzed in details. A method is proposed to overcome these limitations, even in the critical case of fourfold pulse division and combination

Orientadores: Michel Zamboni Rached, Julio Cesar Rodrigues Fernandes de Oliveira
Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de Computação
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Mestrado
Telecomunicações e Telemática
Mestre em Engenharia Elétrica

Abstract The development of a non-invasive glucose monitoring technique is very important because it would tremendously diminish the need to puncture the skin when taking blood samples and help diabetic patients in controlling their blood glucose levels and in treating Diabetes Mellitus. The focus of this thesis is on measuring the effect of glucose on the light scattering properties of a tissue-simulating phantom and biological tissues in vitro. Optical coherence tomography (OCT), the pulsed photoacoustic (PA) technique, and the time-of-flight (TOF) technique are used in the measurements and their capabilities for detecting changes in the scattering properties are evaluated and compared with each other. The theoretical background of the techniques, light propagation and PA wave generation are briefly explained. The glucose-induced changes in light scattering are also reviewed. The measurement results with the OCT and the PA technique from Intralipid, pig whole blood, and mouse skin tissue samples show that the glucose-induced changes are larger in the biological tissues than in the Intralipid phantom. The PA measurements show that although the PA signals are stronger at a wavelength of 532 nm than at 1064 nm, the glucose-induced change in the peak-to-peak value of the PA signal measured from pig whole blood is larger at a wavelength of 1064 nm than at 532 nm. The TOF measurements with a streak camera show that the scattering-related changes in the registered pulse shapes occur mainly in the rising part of the pulses. The utilization of fiber-optic measurement heads enabled the detection of back-scattered photons at different distances from the emitting fiber. Although all the techniques are able to detect changes induced by large glucose concentrations (0–5000 mg/dl) in Intralipid, the effect of glucose on the scattering properties of Intralipid is so weak that the techniques failed to detect changes with lower (50–500 mg/dl) concentrations. The measurements of biological samples with the PA technique and with the OCT also demonstrate capabilities to measure glucose concentrations in the physiologically relevant range (18–450 mg/dl) as well. The results compare well with earlier literature and also confirm some earlier findings.

Les centres de données deviennent de plus en plus importants, allant de petites fermes de serveurs distribuées à des grandes fermes dédiées à des tâches spécifiques. La diffusion de services "dans le nuage" conduit à une augmentation incessante de la demande de trafic dans les centres de données. Dans cette thèse, nous étudions l'évolution des réseaux dans les centres de données et proposons des solutions à court et à long terme pour leur intra-connexion physique. Aujourd'hui, la croissance de la demande de trafic met en lumière la nécessité urgente d’interfaces à grande vitesse capables de faire face à la bande passante exigeant de nouvelles applications. Ainsi, à court terme, nous proposons de nouveaux transpondeurs optiques à haut débit, mais à faible coût, permettant la transmission de 200 Gb /s utilisant des schémas de modulation en intensité et à détection directe. Plusieurs types de modulations d’impulsions en amplitude avancées sont explorés, tout en augmentant la vitesse à des débits symboles allant jusqu’à 100 GBd. La génération électrique à haute vitesse est réalisé grâce à un nouveau convertisseur analogique-numérique intégré, capable de doubler les vitesses des entrées et de générer des signaux à plusieurs niveaux d’amplitude. Cependant, le trafic continuera sa croissance. Les centres de données actuels reposent sur plusieurs niveaux de commutateurs électroniques pour construire un réseau d'interconnexion capable de supporter une telle grande quantité de trafic. Dans une telle architecture, la croissance du trafic est directement liée à une augmentation du nombre des composants du réseau, y-compris les commutateurs avec plus de ports, les interfaces et les câbles. Le coût et la consommation d'énergie qui peut être attendus à l'avenir est intenable, ce qui appelle à une réévaluation du réseau. Par conséquent, nous présentons ensuite un nouveau concept fondé sur la commutation de "slots" optiques (Burst Optical Slot Switching, i.e. BOSS) dans lequel les serveurs sont connectés via des nœuds BOSS à travers des anneaux de fibres multiplexé en longueur d'onde et en temps, et organisés dans une topologie en tore. Au cours de cette thèse, nous étudions la mise en œuvre des nœuds BOSS; en particulier, la matrice de commutation et les transpondeurs optiques. L'élément principal au sein de la matrice de commutation est le bloqueur de slots, qui est capable d'effacer n’importe quel paquet (slot) sur n’importe quelle longueur d'onde en quelques nanosecondes seulement. D'une part, nous explorons l'utilisation d'amplificateurs optiques à semi-conducteurs comme portes optiques à utiliser dans le bloqueur des slots, et étudier leur cascade. D'autre part, nous développons un bloqueur de slots intégré monolithiquement capable de gérer jusqu'à seize longueurs d'onde avec la diversité de polarisation. Ensuite, nous présentons plusieurs architectures de transpondeur et nous étudions leur performance. La signalisation des transpondeurs doit répondre à deux exigences principales: le fonctionnement en mode paquet et la résistance au filtrage serré. D'abord, nous utilisons des transpondeurs élastiques qui utilisent des modulations Nyquist N-QAM, et qui adaptent le format de modulation en fonction du nombre de nœuds à traverser. Ensuite, nous proposons l'utilisation du multiplexage par répartition orthogonale de la fréquence en cohérence optique (CO-OFDM). Avec une structure de paquet inhérente et leur grande adaptabilité fréquentielle, nous démontrons que les transpondeurs CO-OFDM offrent une capacité plus élevée et une meilleure portée que leurs homologues Nyquist. Finalement, nous comparons notre solution BOSS avec la topologie Clos replié utilisée aujourd'hui. Nous montrons que notre architecture BOSS nécessite 400 fois moins de transpondeurs et de câbles que les réseaux de commutation électronique d'aujourd'hui, ce qui ouvre la voie à des centres de données hautement évolutifs et durables
Data centers are becoming increasingly important and ubiquitous, ranging from large server farms dedicated to various tasks such as data processing, computing, data storage or the combination thereof, to small distributed server farms. The spread of cloud services is driving a relentless increase of traffic demand in datacenters, which is doubling every 12 to 15 months. Along this thesis we study the evolution of data center networks and present short- and long-term solutions for their physical intra-connection. Today, rapidly-growing traffic in data centers spotlights the urgent need for high-speed low-cost interfaces capable to cope with hungry-bandwidth demanding new applications. Thereby, in the short-term we propose novel high-datarate low-cost optical transceivers enabling up to 200 Gb/s transmission using intensity-modulation and direct-detection schemes. Several advanced pulse amplitude modulation schemes are explored while increasing speeds towards record symbol-rates, as high as 100 GBd. High-speed electrical signaling is enabled by an integrated selector-power digital-to- analog converter, capable of doubling input baud-rates while outputting advance multi-level pulse amplitude modulations. Notwithstanding, data centers’ global traffic will continue increasing incessantly. Current datacenters rely on high-radix all-electronic Ethernet switches to build an interconnecting network capable to pave with such vast amount of traffic. In such architecture, traffic growth directly relates to an increase of networking components, including switches with higher port-count, interfaces and cables. Unsustainable cost and energy consumption that can be expected in the future calls for a network reassessment. Therefore, we subsequently present a novel concept for intra-datacenter networks called burst optical slot switching (BOSS); in which servers are connected via BOSS nodes through wavelength- and time-division multiplexed fiber rings organized in a Torus topology. Along this thesis we investigate on the implementation of BOSS nodes; in particular, the switching fabric and the optical transceivers. The main element within the switching fabric is the slot blocker, which is capable of erasing any packet of any wavelength in a nanosecond time-scale. On the one hand, we explore the use of semiconductor optical amplifiers as means of gating element to be used within the slot blocker and study their cascadability. On the other hand we develop a monolithically integrated slot blocker capable of handling up to sixteen wavelength channels with dual-polarization diversity. Then we present several transceiver architectures and study their performances. Transceivers’ signaling needs to fulfill two main requirements: packet-mode operation, i.e. being capable of recovering few microsecond –long bursts; and resiliency to tight filtering, which occurs when cascading many nodes (e.g. up to 100). First we build packet-mode Nyquist-pulse-shaped N-QAM transceivers, which adapt the modulation format as a function of the number of nodes to traverse. Later we propose the use of coherent-optical orthogonal frequency division multiplexing (CO-OFDM). With inherent packet structure and high spectral tailoring capabilities, we demonstrate that CO-OFDM-based transceivers offer higher capacity and enhanced reach than its Nyquist counterpart. Finally, we compare our BOSS solution to today’s Folded Clos topology, and show that our BOSS architecture requires x400 fewer transponders and cables than today’s electronic switching networks, which paves the way to highly scalable and sustainable datacenters

Aims: Aims of this thesis are to evaluate the features of cerebrospinal fluid (CSF) pulse wave amplitude (pwa) of IIH by lumbar measurements of CSF pressure during intracranial pressure (ICP) monitoring, and to correlate the CSF pwa magnitude to the level of optic disc damage in patients suffering from IIH. Methods: The thesis consists of two parts. Part I assesses the CSF pwa value in IIH by short-term (1-hour) monitoring by lumbar puncture and compared with an age-matched control group. CSF pwa values were collected with the same method of analisys in age-matched controls and IIH patients. Part II assesses optic disc morphology in the same patients by using Optic Coerence Tomography and than describes Optic disc parameters associated with raised CSF pressure and raised CSF pwa values in IIH with and without papilledema groups. Results: We found that the 75% of IIH patients have raised CSF mean pwa as compared to control group. Of the 75% of IIH patients that presented raised CSF pwa during short-term monitoring, the 57.3% was suffering from IIHWP and the 42.7% was IIHWOP. OCT measurements collected in IIHWP patients, suffering from both raised CSF mean pressure and raised CSF mean pwa values, showed all abnormal neuroretinal parameters: rim area and thickness, cup volume and cup-to-disc ratio (C/D), retinal nerve fiber layer (RNFL). OCT measurements collected in IIHWOP patients suffering from raised CSF mean pressure, showed abnormal both rim area and thickness, cup volume and C/D, but rather normal RNFL. Furthermore, we found statistical significance when we correlated CSF mean pressure with RNFL (r=0.60, p=0.042) and CSF pwa with RNFL (r=0.77, p=0.028; Fig.3b) collected form all participants. No correlation was found for CSF opening pressure and RNFL (r=0.34; p=0.06) among patients. Conclusions: Our data show that, during CSF short-term monitoring by lumbar access, both CSF mean pressure and CSF mean pwa are increased in IIH group if compared to control group as expression of reduced intracranial compliance. Neuroretinal structures are more damaged in IIH patients suffering from both raised CSF mean pressure and CSF mean pwa. The lumbar CSFP pwa could be used to evaluate the intracranial compliance and the evolution of optic nerve head damage in IIH patients.

Nous avons construit des lasers qui ont servi a des experiences de melange a quatres ondes (reseau transitoire). Nous avons d'abord etudie la reponse non lineaire du troisieme ordre lorsque le milieu presente un continuum large d'etats (colorants). Le signal obtenu sur le cresyl-violet en film mince de colle cellulosique a ete analyse. La technique du reseau transitoire a ete ensuite utilisee pour caracteriser la dynamique de molecules dans un milieu poreux (vycor) et etudier la geometrie des connexions entre pores. Nous avons etudie la rotation de l'azobenzene en solution soit libre soit contenue a l'interieur de l'espace poreux. Ces experiences peuvent etre realisees sur des echelles allant de la femtoseconde a la seconde, permettant ainsi l'etude tant du temps de coherence que des proprietes de diffusion des materiaux

Dans cette thèse, nous proposons l'utilisation d'un nouveau type de fibre à cristal photonique, la fibre Kagomé à coeur creux, pour la livraison d'impulsions ultra-courtes en endoscopie non linéaire. Ces fibres permettent la livraison d'impulsions sans distorsion sur une large bande spectrale, avec un faible bruit de fond, grâce à la propagation dans le cœur creux. Nous avons résolu le problème de la résolution spatiale, à l'aide d'une microbille en silice, insérée dans le cœur de la fibre Kagomé. Nous avons développé un système d'imagerie compacte, qui utilise un tube piézo-électrique pour le balayage du faisceau, un système achromatiques de microlentilles et une fibre Kagomé double gaine, spécialement conçue pour l'endoscopie. Avec ce système, nous avons réussi à imager des tissus biologiques, à l'extrémité distale de la fibre (endoscopie), en utilisant des différentes techniques tels que TPEF, SHG et CARS, un résultat qui ne trouve pas d'égal dans la littérature actuelle. L'intégration dans une sonde portable (4,2 mm de diamètre) montre le potentiel de ce système pour de futures applications en endoscopie multimodale in-vivo
In this thesis, we propose the use of a novel type of photonic crystal fiber, the Kagomé lattice hollow core fiber, for the delivery of ultra-short pulses in nonlinear endoscopy. These fibers allow undistorted pulse delivery, over a broad transmission window, with minimum background signal generated in the fiber, thanks to the propagation in a hollow-core. We solved the problem of spatial resolution, by means of a silica micro-bead inserted in the Kagomé fiber large core. We have developed a miniature imaging system, based on a piezo-electric tube scanner, an achromatic micro-lenses assembly and a specifically designed Kagomé double-clad fiber. With this system we were able to image biological tissues, in endoscope modality, activating different contrasts such as TPEF, SHG and CARS, at the distal end of the fiber, a result which finds no equal in current literature. The integration in a portable probe (4.2 mm in diameter) shows the potential of this system for future in-vivo multimodal endoscopy

In this thesis, experimental studies are reported of ultrafast dynamics and third order optical nonlinear coefficients of carbon nanotubes, and time resolved coherent phonon dynamics of semiconductors and rare earth titanates. The thesis is divided into three parts. The first part presents (i) general introduction to theoretical background on nonlinear optical susceptibility and time resolved studies, and systems studied (chapter 1) and (ii) experimental techniques (chapter 2). The second part of the thesis deals with the measurements of third order nonlinear susceptibilities and ultrafast dynamics of single and double walled carbon nanotubes (chapter 3). The third part contains coherent phonon dynamics in semiconductors, Te (chapter 4), Bi2Te3 (chapter 5), and ZnTe (chapter 6) and spin-frustrated rare earth titanate insulators (chapter 7). Chapter 1: This chapter is a general introduction to the thesis. The chapter is divided into two parts: (i) light-matter interaction, and (ii) systems studied. Under light-matter interaction, we describe the required theoretical and conceptual background of nonlinear optical susceptibilities and time resolved carrier and phonon dynamics. In the next part, a brief summary of details of the systems studied, that include carbon nanotubes (single and double walled), semiconductors (Te, Bi2Te3 and ZnTe) and insulating spin-frustrated rare earth titanates (Gd2Ti2O7, Dy2Ti2O7 and Tb2Ti2O7), are presented. Chapter 2: Details of the ultrafast laser systems (femtosecond oscillator and amplifier), pulse width measurements and ultrafast experimental pump-probe and z-scan techniques, used in this thesis are given in this chapter. Chapter 3: Here the experimental results on the measurements of third order optical nonlinearity and ultrafast dynamics of single and double walled carbon nanotubes are presented. The chapter starts with a general overview of optical switching followed by known ultrafast dynamics and nonlinear studies on carbon nanotubes. In the next section, our theoretical modelling of nonlinear absorption and refraction in the limit of saturable absorption is described. The final two sections depict our results on single and double walled carbon nanotubes. These studies indicate that double walled carbon nanotubes are best candidates for ultrafast optical switching. Chapter 4: This chapter presents temperature and pump fluence dependent femtosecond time resolved reflectivity measurements on tellurium. The chapter starts with an overview of previous pump-probe reflectivity studies at room temperature on tellurium followed by our results. A totally symmetric A1 coherent phonon at 3.6 THz responsible for the oscillations in the reflectivity data is observed to be strongly positively chirped (i.e, phonon time period decreases at longer pump-probe delay times) with increasing photoexcited carrier density, more so at lower temperatures. We show for the first time that the temperature dependence of the coherent phonon frequency is anomalous (i.e, increasing with increasing temperature) at high photoexcited carrier density due to electron-phonon interaction. At the highest photoexcited carrier densities of ~ 1.4 x 1021cm-3 and the sample temperature of 3K, the lattice displacement of the coherent phonon mode is estimated to be as high as ~ 0.24 Å. Numerical simulations based on coupled effects of optical absorption and carrier diffusion reveal that the diffusion of carriers dominates the non-oscillatory electronic part of the time-resolved reflectivity. Finally, using the pump-probe experiments at low carrier density of 6 x 1018 cm-3, we separate the phonon anharmonicity to obtain the electron-phonon coupling contribution to the phonon frequency and linewidth. Chapter 5: This chapter begins with a introduction of previous ultrafast studies at room temperature on Bi2Te3 and then presents our results on the temperature dependent high pump fluence time resolved reflectivity measurements on Bi2Te3. The time resolved reflectivity data shows two coherently generated totally symmetric A1g modes at 1.85 THz and 3.6 THz at 296K which blue shift to 1.9 THz and 4.02 THz, respectively at 3K. At high photoexcited carrier density of ~ 1.7 x 1021cm-3, the phonon mode at 4.02 THz is two orders of magnitude higher positively chirped than the lower frequency mode at 1.9 THz. The chirp parameter, β is shown to vary inversely with temperature. The time evolution of these modes is studied using continuous wavelet transform of the time-resolved reflectivity data. The analysis shows that the build up time for the two coherent phonons is different. Chapter 6: This chapter starts with a general introduction on various as pects of ZnTe to be used in generation and detection of THz followed by our results on influence of carriers and sample temperature on coherent phonon and polariton generation in ZnTe. Combination of femtosecond Kerr, two photon absorption and impulsive stimulated Raman scattering experiments have been carried out to investigate the effect of pulse energy and crystal temperature on the generation of coherent polaritons and phonons in < 110 > cut ZnTe single crystals of three different resistivities. We demonstrate that the effect of two-photon induced free carriers on the creation of both the polaritons and phonons is largest at 4K where the free carrier lifetime is enhanced. Further, the temperature dependant impulsive stimulated Raman scattering on high and low purity ZnTe crystals allows us to unambiguously assign the phonon mode at 3.5 THz to the longitudinal acoustic mode at X-point in the Brillouin zone, LA(X) in contrast to the assignment as two-phonon process in earlier studies. Chapter 7: This chapter starts with an introduction on previous Raman studies on the pyrochlore systems accompanied by our results on the generation of coherent optical phonons in spin frustrated pyrochlore single crystals Dy2Ti2O7, Gd2Ti2O7 and Tb2Ti2O7 and their behavior as a function of sample temperature from 296K to 4K. At 4K, two coherent phonons are observed at 5.3 THz (5.0 THz) and ~ 9.3 THz (9.4 THz) for Dy2Ti2O7 (Gd2Ti2O7) whereas three coherent phonons are generated at ~ 4.8 THz, 8.6 THz and 9.6 THz for Tb2Ti2O7. In the case of spin-ice Dy2Ti2O7, a clear discontinuity is observed in the linewidths of both the coherent phonons as well as in the phase of low energy coherent phonon mode, indicating a subtle structural change as also suggested by Raman studies. In comparison, such changes are not seen in the coherent phonons of Gd2Ti2O7, and Tb2Ti2O7. Another important observation is the phase difference of ‘π’ between the modes in all the samples, thus suggesting that the driving forces behind the generation of these modes are different in nature unlike a purely impulsive or displacive mechanism. Chapter 8: This chapter summarizes our results reported in this thesis and gives future directions.

A detailed understanding of the corneal biomechanical response is an important prerequisite to understanding corneal diseases such as keratoconus and for placing the empirical equations used in refractive surgery on a physical basis. We have assembled a combined nonlinear optical microscopy (NLOM) and optical coherence microscopy (OCM) imaging system to simultaneously capture coregistered volumetric images of corneal morphology and biochemistry. Fudicial markers visible in the OCM volume enabled the calculation of strains for multiple depth layers in rabbit cornea. The results revealed a depth dependent strain distribution, with smaller strains in the anterior stroma and larger strains in the posterior stroma. The stress-strain curves can be grouped readily by depth into three groups: anterior (~20%), transitional mid (~40%), and posterior (~40%). Cross-sectional images of collagen lamellae, visible in NLOM, showed inhomogeneous collagen structure and its response to intraocular pressure along the anterior-posterior direction. The inhomogeneities correlate well with the noted heterogeneous corneal mechanical properties. The combined NLOM-OCM system can measure corneal microstructure and mechanical response uniquely, thus providing a microstructural understanding of corneal response to changes of collagen structure.

碩士
國立清華大學
光電工程研究所
102
In this thesis, an efficient light source for high resolution optical coherence tomography system was designed, constructed and studied. We optimized the characteristics of a supercontinuum light source generated in optical communication single-mode fiber pumped with noise-like pulses. Such kind of pulses can propagate over a long distance in optical fibers while maintaining the packet duration to generate broadband supercontinuum. The center wavelength of the generated supercontinuum is 1060 nm and the FWHM of the output spectrum can exceed 320 nm. We set up a free space Michelson interferometer with both axial and lateral scanning schemes as a prototype time-domain optical coherence tomography system. The experimentally measured point spread function indicates an axial resolution of <5 μm as compared to a theoretical resolution of <1.6 μm.

碩士
國立清華大學
物理系
103
In this thesis, we designed and studied a broadband light source for time-domain and spectral-domain optical coherence tomography (TD-OCT, SD-OCT). The supercontinuum (SC) was generated by noise-like pulses from an Yb-doped fiber laser in single mode fiber. The central wavelength of the generated SC is 1300 nm and the full width at half maximum can reach 365 nm. The free space and fiber-based OCT were constructed to examine the performance of the new broadband source. For TD-OCT, the experimentally measured point spread function (PSF) indicates an axial resolution of 2.3 μm as compared to a theoretical resolution of 2.1 μm. For free space SD-OCT, the experimentally measured PSF indicates an axial resolution of 2.8 μm as compared to a theoretical resolution of 2.7 μm. For fiber-based SD-OCT, due to the limited optical bandwidth of fiber components of the OCT system, the experimentally measured PSF indicates an axial resolution of 6.4 μm as compared to a theoretical resolution of 5.3 μm and the signal-to-noise ratio can reach 126.7 dB. Using the noise-like SC light source, speckle in the OCT images can be reduced significantly. In the regions of interest, the speckle signal-to-noise ratio (SNR) = 41.32 dB, contrast-to-noise ratio (CNR) = 2.86 dB and equivalent number of looks (ENL) = 3.91. In order to achieve real-time OCT, time-stretching technique was applied to generate the swept light source. Making use of dispersion-shifted fiber, we generated the scanning rate higher than 5 MHz from our noise-like SC light source.

博士
國立清華大學
光電工程研究所
104
Since the first demonstration of noise-like-pulse (NLP) operation in the ring cavity of an Er: doped fiber oscillator, there has been tremendous interests in this special regime of pulsed lasers. In this dissertation, we describe our work on generation and amplification of medium- and high-energy noise-like pulses with Yb-doped fibers. We also demonstrate supercontinuum (SC) generation techniques where NLPs serve as the pump. Theoretical aspects as well as discussions about physical mechanisms which make NLPs distinguishable from regular mode-locked pulses are also discussed. SC pumped by NLPs has been employed successfully in optical coherence tomography (OCT) systems. The advantages of such approach as well as the promising features of NLPs for such applications are presented. Beginning with a brief description of the cavity configurations that are typically used in fiber laser oscillators, we then focus our attention on ring-type cavities where nonlinear polarization evolution (NPE) is involved in pulsed operation. We show that both regular Gaussian pulses and noise-like pulses can be achieved in the same cavity by choosing proper cavity components and adjustment. We analyze and compare two popular cavity configurations: dispersion mapped cavity and all-normal-dispersion (ANDi) one. Simulation results based on coupled nonlinear Schrodinger equations are supported by experimental measurements. Second part of the dissertation is about supercontinuum (SC) generation. Here we analyze the possibilities of efficient SC generation by using standard silica fibers. It is shown that unique features of NLPs make them very useful for such purpose. That is, the central wavelength of the pump and zero-dispersion wavelength (ZDW) of SC generation media is not critical. We show that even if the pump wavelength is deep in the normal dispersion regime (for example, ~1 μm where ZDW=1.33 μm), SC can be efficiently generated. Simulations and experimental results of SC generation by NLPs using different single-mode fibers are presented. We discuss the optimal selection of fiber types and other characteristics to generate flat SC in spectral region above 1 μm. The pros and cons of using specialty fibers such as photonic crystal fibers pumped by NLPs will also be elaborated. In the third part of the dissertation we consider the application of noise-like pulses for selected applications. The SC spectrum scheme is flat with a bandwidth of 365 nm centered at 1320 nm. The light source is successfully employed in a time-domain OCT, achieving an axial resolution of 2.3 μm. High resolution fiber-based spectral-domain OCT imaging of bio-tissue (onion skin), comparable to that obtained using a commercial swept source, is also demonstrated.

碩士
國立中正大學
物理學系
85
There are many factors influence our OHD-RIKE experiment and make the Kerr signalvery noisy . We are wonder there are some physical inside , so how to contral these factors and how to get good Kerr signal is the point we want to discuss .

The dissertation will be focused on optical, coherently seeded ultrafast fiber amplifiers. Research work shall include building of experimental setups for characterization and spectrotemporal characterization of coherent supercontinuum, as well as building of fiber amplifier setups and their characterization. Specifically, thulium-doped, and synchronized thulium-doped and erbium-doped amplifiers will be investigated, under optical seeding from a common, coherent supercontinuum signal. Research will be therefore conducted using laser sources covering wavelengths around 1560 nm and between 1800 nm and 2100 nm, which correspond to gain bands of erbium-doped and thulium-doped fiber amplifiers. In particular, the latter – amplifiers based on thulium doped active fibers is especially interesting because these devices dispose of the widest gain band among the three main fiber amplifiers (doped with either of Yb3+, Er3+ or Tm3+ ions). It is planned, that during the work, advantage would be taken from seeding one or two amplifiers from a common, spectrally broad, and coherent seed signal. This would specifically include addressing of entire amplification bandwidth of every seeded amplifier, as well as intrinsic synchronization and assuring of the same CEO (carrier-envelope offset) of pulses amplified in each of the seeded amplifiers. This opens entirely new possibilities in generation and synthesis of ultrashort light pulses in all-fiber systems. The first part of dissertation shall include description of measurements and detailed analysis of results obtained in spectro-temporal characterization of group delay in coherent supercontinuum light pulses in all-normal dispersion photonic crystal fibers. This part of work is aimed at experimental verification of temporal structure of spectrally broad pulses and selecting the most suitable fiber for generation of seed signal for seeding of ultrafast amplifiers. Selection criteria shall include dispersion and nonlinear properties, as well as birefringence and feasibility to obtain linearly polarized and spectrally broad seed pulses. In the second part of work, discussion shall be focused on experimental results of broadband seeding of thulium doped fiber amplifiers. The research will include work on setups built using polarization maintaining optical fibers and fiber components. Research methodology will be based on measurements of group delay characteristics using frequency resolved optical gating (FROG) and on measurements of noise characteristics of amplifiers, while the general aim of work will be identification of the compression point, i.e. the shortest obtainable output pulse duration in the investigated systems. Various factors influencing the compression point will be discussed, including seed pulse characteristics, spectral width of the output pulse, uncompensated third-order dispersion. The third part of dissertation shall be dedicated to discussion of results obtained with seeding of thulium-doped fiber amplifiers and erbium-doped amplifiers with a common, broadband and coherent seed signal. Only polarization maintaining fiber-based setups will be investigated. In addition to characterization of group delay profiles and noise performance of the output pulses, the work includes an attempt of nonlinear mixing of the two output pulses from each of the amplifiers and generation of sum frequency signal. The discussion thus includes results of work on group delay matching between the output pules, as well as their spectro-temporal characteristics and measurements of noise characteristics of the obtained sum frequency signal.
Rozprawa będzie dotyczyć ultraszybkich wzmacniaczy światłowodowych zasiewanych koherentnym sygnałem szerokopasmowym. Badania obejmą budowę układów eksperymentalnych do charakteryzacji oraz charakteryzację spektralno-czasową sygnału szerokopasmowego w postaci źródła czasowo-spójnego superkontinuum, budowę układów wzmacniaczy światłowodowych oraz ich charakteryzację. W szczególności badane będą światłowodowe wzmacniacze tulowe, oraz układy wzmacniaczy tulowych i erbowych zasiewanych ze wspólnego źródła koherentnego sygnału szerokopasmowego. Prace będą zatem prowadzone przy wykorzystaniu źródeł laserowych na zakres długości fali wokół 1560 nm oraz pomiędzy 1800 nm i 2100 nm, które odpowiadają pasmom wzmocnienia światłowodowych wzmacniaczy erbowych oraz tulowych. W szczególności ta druga grupa – wzmacniaczy wykorzystujących światłowody domieszkowane jonami Tm3+ jest szczególnie interesująca, ze względu na najszersze dysponowane pasmo wzmocnienia wśród trzech głównych wzmacniaczy światłowodowych (domieszkowanych odpowiednio jonami Yb3+, Erb3+ lub Tm3+). W trakcie badań, wykorzystane mają być zalety oczekiwane w przypadku zasiewania jednego lub dwóch wzmacniaczy wspólnym sygnałem szerokopasmowym. Należy tu wymienić adresowanie całego pasma wzmocnienia każdego wzmacniacza w układzie, możliwość automatycznej synchronizacji dwóch różnych wzmacniaczy przy jednoczesnym zapewnieniu tego samego przesunięcia obwiednia-nośna we wszystkich zasiewanych wzmacniaczach. Otwiera to ciekawe możliwości w generacji i syntezie ultrakrótkich impulsów światła w układach całkowicie światłowodowych. W pierwszej części pracy zawarty zostanie opis pomiarów i szczegółowej analizy wyników w zakresie charakteryzacji spektralno-czasowej impulsów przewidzianych jako sygnał zasiewający. Badane będą charakterystyki opóźnienia grupowego impulsów koherentnego superkontinuum w światłowodach fotonicznych o charakterystyce dyspersji chromatycznej całkowicie w zakresie normalnym. Celem badań będzie eksperymentalne potwierdzenie struktury czasowej impulsu o szerokim widmie oraz wybór światłowodu najlepszego do zastosowania w generacji sygnału zasiewającego dla ultraszybkich wzmacniaczy światłowodowych. Kryterium wyboru będzie uwzględniało właściwości dyspersyjne i nieliniowe, a także dwójłomność i możliwość uzyskania szerokopasmowego impulsu zasiewającego o polaryzacji liniowej. 2 W drugiej części pracy zawarte zostaną wyniki badań zasiewania szerokopasmowego światłowodowych wzmacniaczy tulowych. Badania będą dotyczyły układów eksperymentalnych zbudowanych ze światłowodów oraz komponentów światłowodowych utrzymujących polaryzację. Metodologia badań będzie zawierała pomiary charakterystyk opóźnienia grupowego w układzie frequency-resolved optical gating, pomiary charakterystyk szumowych wzmacniaczy, a ogólnym celem badań będzie wskazanie punktu kompresji, czyli najkrótszego osiągalnego czasu trwania impulsu wyjściowego w badanym układzie. Dyskusja obejmie wpływ na punkt kompresji czynników tj. charakterystyka impulsu początkowego, pasmo wzmocnionego impulsu, nieskompensowana dyspersja trzeciego rzędu. W trzeciej części pracy omówione będą wyniki badań układów wzmacniaczy tulowych oraz erbowych zasiewanych wspólnym spójnym sygnałem szerokopasmowym. Wyniki badań zawarte w dwóch poprzednich częściach pracy oraz komplikacja układu dwóch wzmacniaczy sprawia, że podjęta zostanie próba budowy układu z wykorzystaniem wyłącznie światłowodów i komponentów światłowodowych utrzymujących polaryzację. Ponad badania charakterystyk opóźnienia grupowego i właściwości szumowych w tej części pracy podjęta została próba mieszania nieliniowego impulsów wyjściowych każdego z obydwu wzmacniaczy i generacji sygnału o częstości sumacyjnej. Omówione zostały zatem wyniki prac związane z kontrolą opóźnienia grupowego pomiędzy impulsami oraz z ich charakteryzacją czasowo-spektralną, a także z pomiarami charakterystyki szumowej sygnału o częstości sumacyjnej.

Nonlinear optical microscopy serves as a great tool for biomedical imaging due to its high resolution, deep penetration, inherent three dimensional optical sectioning capabilities and superior performance in scattering media. Conventional nonlinear optical microscopy techniques, e.g. two photon fluorescence and second harmonic generation, are based on detecting a small light signal emitted at a new wavelength that is well separated from the excitation light. However, there are also many other nonlinear processes, such as two-photon absorption and self-phase modulation, that do not generate light at new wavelengths and that have not been extensively explored for imaging. This dissertation extends the accessible mechanisms for contrast to the later nonlinear optical processes by combining femtosecond laser pulse shaping and homodyne detection. We developed a rapid pulse shaper with a relatively simple and compact instrument design that modifies the spectrum of individual laser pulses from an 80 MHz mode-locked laser. The pulse shaper enables simultaneous two-photon absorption and self-phase modulation imaging of various nanoparticles in-vitro with high sensitivity. We also applied this imaging technique to study the nonlinear optical response in graphene. Because our technology detects the nonlinear signature encoded within the laser pulse itself, we achieve intrinsic contrast of biological and non-biological samples in highly scattering media. These capabilities have significant implications in biomedical imaging and nanophotonics.

Dissertation