Optimisation of Quantum Trajectories Driven by Strong-field Waveforms

Quasi-free field-driven electron trajectories are a key element of strong-field dynamics. Upon recollision with the parent ion, the energy transferred from the field to the electron may be released as attosecond duration XUV emission in the process of high harmonic generation (HHG). The conventional sinusoidal driver fields set limitations on the maximum value of this energy transfer, and it has been predicted that this limit can be significantly exceeded by an appropriately ramped-up cycleshape. Here, we present an experimental realization of such cycle-shaped waveforms and demonstrate control of the HHG process on the single-atom quantum level via attosecond steering of the electron trajectories. With our optimized optical cycles, we boost the field-ionization launching the electron trajectories, increase the subsequent field-to-electron energy transfer, and reduce the trajectory duration. We demonstrate, in realistic experimental conditions, two orders of magnitude enhancement of the generated XUV flux together with an increased spectral cutoff. This application, which is only one example of what can be achieved with cycle-shaped high-field light-waves, has farreaching implications for attosecond spectroscopy and molecular self-probing

Carrier-envelope phase control for the advancement of attosecond pulse generation

When the optical pulses emitted by a laser become so short in time that they encompass only a few cycles of the carrier wave, the phase between carrier and envelope becomes a crucial parameter. The ability to control this carrier-envelope phase (CEP) is elemental to experiments probing the fastest processes in the microcosm, occurring on the time-scale of attoseconds. More than a decade into the attosecond era, the limitations of the established CEP stabilisation technique have begun to curtail experimental progress. First, increasingly complex experiments require many hours of uninterrupted operation at the same waveform. Second, the pulses used in experiments are approaching the single-cycle boundary, calling for ever-decreasing CEP noise. With the conventional stabilisation technique, already these two requirements cannot be fulfilled simultaneously. Ultimately, the low efficiency of the underlying nonlinear processes can only be compensated by driver lasers at a higher repetition rate than available at present. In order to advance attosecond pulse generation, novel approaches to CEP control thus face a threefold challenge that outlines this thesis: To simultaneously provide low CEP noise and long-term operation to present-day few-cycle lasers and amplifiers, and to investigate CEP control capability in high average power sources that are currently under development. This thesis describes the adaptation of cavity-external CEP stabilisation for use with few-cycle pulses. The intrinsic limitations of the conventional feed-back technique are lifted. A laser oscillator is demonstrated to maintain record-low CEP noise for tens of hours of operation free from phase discontinuities. In addition, a modification of the technique is presented that further enhances the applicability to amplified systems. Two routes are investigated to achieve CEP control in system architectures that represent potential megahertz repetition rate driver sources. In combination with temporal pulse compression, a thin-disk laser is shown to yield few-cycle pulses. Experiments are presented that provide the groundwork towards the first CEP-stabilised thin-disk oscillator. The second approach targets the seed oscillator of a fibre chirped-pulse amplifier. The CEP noise properties of different amplification regimes are examined. Intensity enhancement of the output pulses in a passive resonator is shown to benefit greatly even from a coarse lock of the CEP slip rate. For few-cycle pulse energy to reach the millijoule level and above, amplification and temporal compression will remain indispensable in the foreseeable future. Maintaining CEP stability across such stages is crucial, irrespective of the technology employed. Cavity-external CEP control is demonstrated to enable more than 24 hours of constant-CEP operation in chirped-pulse amplifiers. Furthermore, a novel actuator is introduced that, in conjunction with a fast means of measuring the CEP, is able to provide phase correction of the amplified waveform up to several kilohertz bandwidth. The result is a train of millijoule-level pulses with residual CEP noise comparable to that of state-of-the-art nanojoule oscillators. Eventually, an experiment is presented to examine the influence of different types of hollow-core fibre-based temporal compression on the CEP. The findings shed new light on the origin of adverse effects introduced by this technique, and point out ways towards effective compensation.Wenn die von einem Laser emittierten Lichtpulse so kurz werden, dass ihre Dauer nur noch wenige Schwingungszyklen des elektrischen Feldes umfasst, kommt der Phase zwischen Trägerwelle und Einhüllender (CEP) eine entscheidende Rolle zu. Ihre Regelung ist essentiell für jene Experimente, die die schnellsten Prozesse in der Natur auf der Zeitskala von Attosekunden ausloten. Mehr als zehn Jahre nach Beginn der Attosekunden-Ära ist die etablierte Methode der CEP-Regelung zum Hindernis für experimentelle Fortschritte geworden. Einerseits erfordern immer komplexere Experimente, dass das elektrische Feld der Pulse über viele Stunden konstant bleibt. Andererseits zeichnet sich eine Entwicklung der Pulsdauer zu immer kürzerer Dauer in Richtung eines einzigen Zyklus ab, was eine steigende Präzision der Regelung erfordert. Die gleichzeitige Erfüllung schon dieser beiden Anforderungen ist mit der konventionellen Methode nicht zu erreichen. Schlussendlich kann die niedrige Effizienz der zugrunde liegenden nichtlinearen Prozesse nur die Verwendung von Lasersystemen mit deutlich erhöhter Wiederholrate ausgeglichen werden. Um die Erzeugung von Attosekunden-Pulsen voranzutreiben, müssen neue Ansätze zur CEP-Regelung einer dreifachen Herausforderung gerecht werden, die dieser Dissertation ihren Rahmen gibt: Einerseits hohe Präzision und andererseits hohe Langzeittauglichkeit zur Verfügung zu stellen, und überdies neue Wege zur CEP-Regelung von derzeit in Entwicklung befindlichen Laserquellen mit hoher Durchschnittsleistung aufzuzeigen. Diese Dissertation beschreibt die Anpassung einer alternativen Methode der CEP-Regelung auf Pulse mit einer Dauer von wenigen Zyklen. Die intrinsischen Beschränkungen der konventionellen Technik werden damit behoben. Der solchermaßen stabilisierte Oszillator bietet geringstes CEP-Rauschen über mehrere zehn Stunden Laufzeit ohne Phasensprünge. Zusätzlich wird eine Abwandlung der Methode beschreiben, die deren Anwendbarkeit für Verstärkersysteme erweitert. Die CEP-Regelung in Systemarchitekturen für hohe Durchschnittsleistungen wird an zwei Lasersystemen untersucht, die exemplarisch für potentielle Attosekunden-Quellen mit Megahertz-Wiederholrate stehen. Es wird gezeigt, dass ein Scheibenlaser in Kombination mit zeitlicher Pulskompression genutzt werden kann, um Pulse in der Größenordnung von 10 fs zu erzeugen. Erste Experimente zu deren CEP-Stabilisierung ebnen den Weg für den ersten CEP-stabilen Scheibenlaser. Der zweite Ansatz betrifft die CEP-Regelung eines Oszillator-Verstärker-Systems. Das CEP-Rauschverhalten verschiedener Faserverstärker wird untersucht. Es wird gezeigt, dass die Überhöhung des Pulszugs in einem passiven Resonator auch von einer groben Stabilisierung der CEP-Änderungsrate deutlich profitiert. Um Pulse von wenigen Zyklen Dauer auf eine Energie von Millijoule und darüber hinaus zu bringen, wird Verstärkung und zeitliche Kompression auf absehbare Zeit unverzichtbar bleiben. Unabhängig von der hierzu gewählten Technologie ist es von entscheidender Bedeutung, den Einfluss dieser Prozesse auf die CEP gering zu halten. Die Verwendung eines mit der alternativen CEP-Regelung ausgestatteten Oszillators zur zeitlich gestreckten Verstärkung wird beschrieben, was in hochenergetischen Pulsen mit über 24 Stunden konstanter Wellenform resultiert. Alsdann wird ein neuartiger CEP-Aktuator beschrieben, der in Kombination mit einer schnellen Messmethode die CEP-Korrektur eines jeden Pulses bei einer Bandbreite von mehreren Kilohertz leistet. Das Resultat ist ein Pulszug auf Millijoule-Niveau, dessen CEP-Rauschen mit dem eines Nanojoule-Oszillators vergleichbar ist. Abschließend wird ein Experiment vorgestellt, mit dem der Einfluss von Hohlfaser-Kompression auf die CEP untersucht wird. Die Ergebnisse werfen neues Licht auf den Ursprung zusätzlichen Rauschens in solchen Aufbauten, und zeigen Wege zu dessen Vermeidung auf

Chirped-pulse phase-sensitive optical time domain reflectometry

El mundo actual funciona gracias a las grandes infraestructuras que dotan de energía y transporte seguros a sus ciudadanos. Dichas infraestructuras (presas, diques, gaseoductos, oleoductos, puentes, líneas de ferrocarril, carreteras…) típicamente presentan grandes dimensiones y es especialmente difícil monitorizar su buen funcionamiento y su salud estructural además de protegerlas de posibles amenazas. Los sensores distribuidos de fibra óptica son una solución fiable y rentable para esta problemática, ya que permiten medir vibraciones, deformaciones y temperatura a lo largo de todos los puntos de una fibra óptica estándar de comunicaciones. Los sensores de fibra óptica basados en scattering Rayleigh son particularmente útiles cuando las medidas deben ser realizadas en tiempo real, como por ejemplo en la detección y caracterización de vibraciones. En esta tesis, se ha realizado un estudio acerca de distintas soluciones y alternativas a las limitaciones de la tecnología OTDR. Se ha propuesto una nueva técnica, derivada de ésta, que ofrece unas prestaciones que superan notablemente a las de los sistemas OTDR tradicionales. Para ello, en primer lugar, se ha procedido a realizar un estudio en profundidad de los fundamentos y el estado del arte de las técnicas de monitorización basadas en Reflectometría Óptica en el Dominio del Tiempo (OTDR, por sus siglas en inglés) y, en particular, sobre la implementación sensible a la fase, también conocida como OTDR. Se ha estudiado la limitación en rango y resolución de los sistemas OTDR principalmente asociada a la aparición de efectos no lineales como la inestabilidad de modulación. Actualmente, un OTDR tradicional presenta una resolución máxima del orden de los 10 metros para un rango de medida del orden de pocas decenas de km (si no se aplica ningún tipo de técnica de amplificación distribuida). Además de estudiar esta limitación y a qué es debida, se han propuesto dos técnicas para mitigar los efectos perjudiciales de la MI. En primer lugar, se ha realizado un estudio del efecto de la forma de los pulsos ópticos empleados en el sensor en la traza retrodispersada en un OTDR. Se ha podido comprobar cómo los pulsos triangulares o gaussianos presentan mayor robustez que los pulsos rectangulares, tradicionalmente empleados, frente a la MI. En segundo lugar, se ha propuesto una técnica basada en el concepto de Amplificación de Pulsos Chirpeados (CPA, por sus siglas en inglés), que ha permitido desarrollar un OTDR con resoluciones milimétricas. Hasta el momento ningún OTDR había podido llegar a tales resoluciones, lo que abre un nuevo abanico de aplicaciones a la tecnología OTDR donde se requiera alta resolución espacial en la medida. También se ha estudiado la otra gran limitación de este tipo de sensores: su comportamiento no lineal ante una perturbación. Actualmente, salvo que se implementen técnicas de recuperación de fase o barridos en longitud de onda que implican más complejidad, coste y tiempo de medida, no es posible realizar medidas cuantificables de temperatura o deformaciones. Del mismo modo, tampoco se pueden realizar medidas acústicas reales. En este trabajo, en primer lugar, se propone emplear la técnica de Reconstrucción de Fase empleando Diferenciación Óptica Ultrarápida (PROUD, por sus siglas en inglés) para recuperar el campo complejo de una señal OTDR. Con esta medida, el sensor pasaría a comportarse de forma lineal sin la complejidad intrínseca de los métodos tradicionales de detección de fase. En segundo lugar, y de aquí viene el nombre de esta tesis doctoral, se propone el uso de pulsos chirpeados en los sensores OTDR. La nueva técnica llamada Chirped-Pulse OTDR, ha permitido la medida de forma lineal de cambios de temperatura y deformaciones, en un único disparo y sin la necesidad de realizar barridos en frecuencia o implementar detección coherente. A lo largo de este trabajo, se han alcanzado resoluciones de 0.5mK/4n y se ha demostrado la posibilidad de hacer medidas acústicas reales. También se han estudiado las limitaciones de esta técnica y propuesto varias soluciones. Se ha demostrado que el ruido de fase del láser empleado en el sistema, puede ser mitigado con esta nueva técnica. Además, se ha propuesto el uso de amplificación distribuida basada en scattering Raman estimulado para alcanzar rangos de medida mayores, hasta 75 km con una resolución espacial de 10 m

Low Noise And Low Repetition Rate Semiconductor-based Mode-locked Lasers

The topic of this dissertation is the development of low repetition rate and low noise semiconductor-based laser sources with a focus on linearly chirped pulse laser sources. In the past decade chirped optical pulses have found a plethora of applications such as photonic analogto-digital conversion, optical coherence tomography, laser ranging, etc. This dissertation analyzes the aforementioned applications of linearly chirped pulses and their technical requirements, as well as the performance of previously demonstrated chirped pulse laser sources. Moreover, the focus is shifted to a specific application of the linearly chirped pulses, timestretched photonic analog-to-digital conversion (TS ADC). The challenges of surpassing the speeds of current electronic converters are discussed, while the need for low noise linearly chirped pulse lasers becomes apparent for the realization of TS ADC. The experimental research addresses the topic of low noise chirped pulse generation in three distinct ways. First, a chirped pulse (Theta) laser with an intra-cavity Fabry-Pérot etalon and a long-term referencing mechanism is developed that results in the reduction of the pulse-topulse energy noise. Noise suppression of \u3e 15 times is demonstrated. Moreover, an optical frequency comb with spacing equal to the repetition rate (≈100 MHz) is generated using the etalon, resulting in the first reported demonstration of a system operating in the sub-GHz regime based on semiconductor gain. The path for the development of the Theta laser was laid by the precise characterization of the etalon used in this laser cavity design. A narrow linewidth laser is used in conjunction with an acousto-optic modulator externally swept for measuring the etalon\u27s iv free spectral range with a sub-Hz precision, or 10 parts per billion. Furthermore, the measurement of the etalon long-term drift and birefringence lead to the development of a modified intra-cavity Hänsch-Couillaud locking mechanism for the Theta laser. Moreover, an external feed-forward system was demonstrated that aimed at increasing the temporal/spectral uniformity of the optical pulses. A complete characterization of the system is demonstrated. On a different series of experiments, the pulses emitted by an ultra-low noise but high repetition rate mode-locked laser were demultiplexed resulting in a low repetition rate pulse train. Experimental investigation of the noise properties of the laser proved that they are preserved during the demultiplexing process. The noise of the electrical gate used in this experiment is also investigated which led into the development of a more profound understanding of the electrical noise of periodical pulses and a mechanism of measuring their noise. The appendices in this dissertation provide additional material used for the realization of the main research focus of the dissertation. Measurements of the group delay of the etalon used in the Theta laser are presented in order to demonstrate the limiting factors for the development of this cavity design. The description of a balancing routine is presented, that was used for expanding the dynamic range of intra-cavity active variable delay. At last, the appendix presents the calculations regarding the contribution of various parameters in the limitations of analog-todigital conversion

Neutral gas plasma interactions in space plasma

A sounding rocket experiment, CRIT-II, involving the injection of shaped-charge barium in ionospheric plasma was conducted on May 7, 1989, to investigate Alfven\u27s critical ionization velocity (CIV) hypothesis in space. The CRIT-II main payload was instrumented to make in situ measurements within the neutral barium beam. Among the detectors, UNH provided three energetic particle detectors and two photometers. The data from these detectors are presented. The typical features of the CIV effect were observed including plasma density enhancement, energy and momentum loss of a fast ion beam, excitation of plasma waves, and electron heating. It was found by optical observations that about 4% of the neutral barium was ionized. We believe that about one half of these barium ions were created by electron impact ionization--a CIV mechanism. The cross section for collisions between the barium atoms and the ionospheric oxygen ions was also calculated, assuming that the other half of ionizing barium ions were mainly generated by charge exchange, and found to be in the range from 1 \times \ 10\sp{-17} cm\sp{-2} at a velocity of 4 km/s to 1 \times \ 10\sp{-15} cm\sp{-2} at a velocity of 20 km/s. We also confirmed that the early observed ions were originally from the collisionally accelerated neutral oxygen which charge exchanges with the local oxygen ions. The early stage of electron heating was confirmed to be the result of lower hybrid instabilities excited by the precursor ion beam, using our quasi-linear model calculation. However, the wave spectrum during the passage of main streaming barium was found to be inconsistent with the lower hybrid instabilities proposed by current CIV theories. This could be the main reason for a relatively low ionization yield that one otherwise would expect from CRIT-II. A multi-fluid model of the wave dispersion relation for an unmagnetized beam with finite width in a magnetized plasma was also derived. We found that the nonuniform beam density effect could be the main driver which altered the plasma wave spectrum from the typical lower hybrid waves. A quasi-DC electric field model based on the momentum coupling between an ionizing barium beam and an ionospheric plasma was developed. We found that CIV is a self-limiting ionization process in a conical type of neutral beam, which may have caused the low ionization yield in most of the shaped-charge CIV experiments