Le développement technologique dans le domaine des télécommunications, ainsi que des systèmes de détection, a accru ces dernières années la nécessité de signaux de référence présentant une très haute pureté spectrale. L'augmentation des débits, la saturation des bandes de fréquence ainsi que les performances imposées pour la détection radar ont ouvert la voie à la génération micro-onde par l'optique. Ces références de fréquence sont souvent issues d'oscillateurs optoélectroniques (OEO). Ces oscillateurs intègrent un élément de stockage de l'énergie au travers de résonateurs ou de longues lignes à retard fibrées afin d'augmenter leur facteur qualité et permettant ainsi d'atteindre des performances supérieures aux signaux multipliés à partir de sources basses fréquences ou directement à partir d'oscillateurs micro-ondes à résonateur diélectrique (DRO). Une topologie originale d'oscillateurs optoélectroniques a été proposée à la fin des années 1990 par une équipe américaine : il s'agit de remplacer le résonateur passif nécessitant un verrouillage du laser sur ce dernier par un résonateur actif, intégrant un amplificateur optique. Ce résonateur actif, un laser à blocage de modes, permet un couplage entre l'oscillation optique du laser et l'oscillation optoélectronique. On parle alors d'oscillateur optoélectronique couplé (COEO). Les performances du COEO sont étroitement liées à la pureté spectrale du signal issu du laser à blocage de modes. Ce travail de thèse traite de l'étude et de l'optimisation de ces systèmes. Une étude approfondie sur le bruit dans les amplificateurs optiques a tout d'abord été menée afin de déterminer quel type d'amplificateur choisir pour le COEO et sous quelles conditions l'amplification optique apporte un bruit de phase minimal. Ensuite, un COEO à 10 GHz a été réalisé, présentant un très faible bruit de phase atteignant - 132 dBc/Hz à 10 kHz de la porteuse. Un modèle a par ailleurs été implémenté, permettant de déterminer a posteriori l'efficacité du couplage et ainsi la bande de verrouillage entre l'oscillation optoélectronique et le laser à blocage de modes. Ce couplage interne dépend fortement de la dynamique du système. Cependant, les différents effets non linéaires qui ont lieu dans l'amplificateur à semiconducteur et les fibres ne permettent pas d'obtenir un modèle analytique. Un modèle itératif a alors été proposé afin d'obtenir les propriétés de l'enveloppe complexe lentement variable du peigne de fréquence généré en sortie du laser dont la photodétection conduit à la puissance RF générée par le COEO. Le COEO génère un peigne de fréquence suffisamment large pour produire des harmoniques RF supérieurs à la fréquence de répétition du laser à blocage de modes, si les modes longitudinaux espacés de plusieurs intervalles spectraux libres (ISL) sont en phase. Le modèle itératif développé permet, à partir des paramètres expérimentaux de déterminer le spectre optique ainsi que la distribution de phase à l'intérieur de celui-ci. Il est possible alors d'augmenter la puissance d'une harmonique en sortie de la photodiode par un ajout d'éléments dispersifs. Cette multiplication de fréquence permet la génération de signaux à haute pureté spectrale en bande millimétrique. Une démonstration expérimentale à 90 GHz a été proposée, basée sur un COEO fonctionnant à 30 GHz. Ces résultats sont prometteurs et une intégration du COEO dans un boîtier thermalisé ainsi qu'une gestion plus fine de la dispersion des fibres peut permettre des améliorations significatives sur le bruit de phase du système. / The important rise of telecommunication systems in the past decades, together with the sensitivity improvement of radar systems, has increased the necessity for high spectral purity frequency references at high frequencies. The saturation of classical microwave bandwidths motivated the search of frequency references at higher frequencies, such as K-band. Frequency multiplication from highly stable sources, such as quartz sources, is limited by the increase of the noise floor, which is often prohibitive at millimeter wave frequencies. On the contrary, microwave generation using optics becomes a very efficient technique in this frequency range. Indeed, passive optical resonators or delay lines feature a high Q factor which can be used to stabilize the microwave frequency. The best phase noise performance is today obtained with long delay line oscillators. However, a spurious mode suppression technique has to be implemented in this type of OEOs. The use of an active optical resonator is a third solution, which avoids any locking technique between the laser and the passive resonator. The first architecture of this type has been proposed at the end of the 1990's. In such a system, a mode-locked laser is coupled to a microwave oscillator (COEO). COEO phase noise performances are strongly dependent on the spectral purity of the mode locked laser signal. This thesis work focus on the study and the optimization of this system. Optical amplifiers noise is firstly investigated, in order to determine the optimal conditions to minimize their phase noise contribution to the COEO. A 10 GHz SOA based COEO has been realized and features a low phase noise level reaching - 132 dBc/Hz at 10 kHz from the carrier. An analytical model has also been developed to obtain the locking range of the coupled oscillations. This frequency range is strongly dependent on the coupling efficiency between optical oscillation and the optoelectronic oscillation. This parameter cannot be calculated analytically and an iterative model has been proposed to determine the amplitude and phase of the optical spectrum. Therefore, one can calculate the RF power on the photodiode, on which the coupling efficiency is depending. Since COEO features a large optical frequency comb where each tooth of the comb is phase locked thanks to the mode locked laser, harmonic generation from COEO is possible. Wide frequency comb from high frequency COEO allow millimeter wave generation. The iterative model developed in this work enable to determine the RF power of one specified harmonic from experimental parameters. Harmonic selection can also be performed through the management of the chromatic dispersion. Such frequency multiplication has been implemented to generate a high purity 90 GHz signal from a 30 GHz COEO.These results are promising and an integration of the system in a thermalized box is under process.

A true linearized interferometric intensity modulator for pulsed light has been proposed and experimentally presented in this thesis. This has been achieved by introducing a mode-locked laser into one of the arms of a Mach-Zehnder interferometer and injection-locking it to the input light (which is pulsed and periodic). By modulating the injection-locked laser, and combining its output light with the light from the other arm of interferometer in quadrature, one can achieve true linearized intensity modulator. This linearity comes from the arcsine phase response of the injection-locked mode-locked laser (as suggested by steady-state solution of Adler's equation) when it is being modulated. Mode-locked lasers are fabricated using a novel AlGaInAs-InP material system. By using the BCB for planarization and minimizing the metal pad size and directly modulating the laser, we have achieved very effective fundamental hybrid mode-locking at the repetition rate of ~ 23 GHz. This laser also provided the short pulses of 860 fs and 280 fs timing jitter integrated from 1 Hz- 100 MHz. The linearized intensity modulator has been built by using two identical two-section mode-locked lasers with the same length, one as the slave laser in one of the arms of the Mach-Zehnder interferometer injection-locked to the other one as the master which is the input light to the modulator. A low V? of 8.5 mV is achieved from this modulator. Also the current of the gain section or the voltage of the saturable absorber section of the slave laser has been used to apply the modulation signal. A spur free dynamic range of 70 dB.Hz2/3 is achieved when modulating the modulator through the saturable absorber. Modulating the saturable absorber provides a reduced third-order intermodulation tone with respect to modulating the gain. This is simply because of the unwanted amplitude modulation created when modulating the gain section current. Finally an improved design is proposed and demonstrated to improve the modulator performance. This is achieved by introducing a third section to the laser. Using the impurity free vacancy disordering technique the photoluminescence peak of this section is blue-shifted selectively and therefore there would not be any absorption in that passive section. By applying the modulation signal to this passive section rather than applying it to the gain section or saturable absorber section, the amplitude and phase modulation could be decoupled. The experimental results have presented here and an almost six-fold reduction in V? and 5 dB improvement in spur free dynamic range have been achieved. The proposed and demonstrated configuration as an analog optical link has the potential to increase the performance and resolution of photonic analog-to-digital converters.

The construction and characterization of ultra-low noise semiconductor-based mode-locked lasers as frequency comb sources with multi-gigahertz combline-to-combline spacing is studied in this dissertation. Several different systems were built and characterized. The first of these systems includes a novel mode-locking mechanism based on phase modulation and periodic spectral filtering. This mode-locked laser design uses the same intra-cavity elements for both mode-locking and frequency stabilization to an intra-cavity, 1,000 Finesse, Fabry-Pérot Etalon (FPE). On a separate effort, a mode-locked laser based on a Slab-Coupled Optical Waveguide Amplifier (SCOWA) was built. This system generates a pulse-train with residual timing jitter of

This dissertation details work on high repetition rate semiconductor mode-locked lasers. The qualities of stable pulse trains and stable optical frequency content are the focus of the work performed. First, applications of such lasers are reviewed with particular attention to applications only realizable with laser performance such as presented in this dissertation. Sources of timing jitter are also reviewed, as are techniques by which the timing jitter of a 10 GHz optical pulse train may be measured. Experimental results begin with an exploration of the consequences on the timing and amplitude jitter of the phase noise of an RF source used for mode-locking. These results lead to an ultralow timing jitter source, with 30 fs of timing jitter (1 Hz to 5 GHz, extrapolated). The focus of the work then shifts to generating a stabilized optical frequency comb. The first technique to generating the frequency comb is through optical injection. It is shown that not only can injection locking stabilize a mode-locked laser to the injection seed, but linewidth narrowing, timing jitter reduction and suppression of superfluous optical supermodes of a harmonically mode-locked laser also result. A scheme by which optical injection locking can be maintained long term is also proposed. Results on using an intracavity etalon for supermode suppression and optical frequency stabilization then follow. An etalon-based actively mode-locked laser is shown to have a timing jitter of only 20 fs (1Hz-5 GHz, extrapolated), optical linewidths below 10 kHz and optical frequency instabilities less than 400 kHz. By adding dispersion compensating fiber, the optical spectrum was broadened to 2 THz and 800 fs duration pulses were obtained. By using the etalon-based actively mode-locked laser as a basis, a completely self-contained frequency stabilized coupled optoelectronic oscillator was built and characterized. By simultaneously stabilizing the optical frequencies and the pulse repetition rate to the etalon, a 10 GHz comb source centered at 1550 nm was realized. This system maintains the high quality performance of the actively mode-locked laser while significantly reducing the size weight and power consumption of the system. This system also has the potential for outperforming the actively mode-locked laser by increasing the finesse and stability of the intracavity etalon. The final chapter of this dissertation outlines the future work on the etalon-based coupled optoelectronic oscillator, including the incorporation of a higher finesse, more stable etalon and active phase noise suppression of the RF signal. Two appendices give details on phase noise measurements that incorporate carrier suppression and the noise model for the coupled optoelectronic oscillator.

Mid-infrared (IR) lasers (2-12 μm) have found tremendous applications in medical surgeries, spectroscopy, remote sensing, etc. Nowadays, mid-IR emissions are usually generated from semiconductor lasers, gas lasers, and solid-state lasers based on nonlinear wavelength conversion. However, they usually have disadvantages including poor beam quality, low efficiency, and complicated configurations. Mid-IR fiber lasers have the advantages of excellent beam quality, high efficiency, inherent simplicity, compactness, and outstanding heat-dissipating capability, and have attracted significant interest in recent years. In this dissertation, I have studied and investigated Q-switched and mode-locked fiber lasers in the mid-IR wavelength region. My dissertation includes six chapters: In Chapter 1, I review the background of mid-IR lasers and address my motivation on the research of mid-IR fiber lasers; In Chapter 2, I present the experimental results of microsecond and nanosecond Er³⁺-doped and Ho³⁺-doped fiber lasers in the 3 μm wavelength region Q-switched by Fe²⁺:ZnSe and graphene saturable absorbers. In Chapter 3, Q-switched 3 μm laser fiber amplifiers are investigated experimentally and theoretically and their power scaling are discussed. In Chapter 4, a graphene mode-locked Er³⁺-doped fiber lasers at 2.8 μm with a pulse width < 50 ps is presented. In Chapter 5, extending the spectral range of mid-IR fiber lasers by use of nonlinear wavelength conversion is addressed and discussed. I have proposed 10-watt-level 3-5 μm Raman lasers using tellurite fibers as the nonlinear gain medium and pumped by our Er³⁺-doped fiber lasers at 2.8 μm. In the last chapter, the prospect of mid-IR fiber laser is addressed and further research work is discussed.

A novel dispersion-managed breathing-mode semiconductor mode-locked ring laser is developed. The "breathing-mode" designation derives from the fact that intracavity pulses are alternately stretched and compressed as they circulate around the ring resonator. The pulses are stretched before entering the semiconductor gain medium to minimize the detrimental strong integrating self-phase modulation and to enable efficient pulse amplification. Subsequently compressed pulses facilitate bleaching the semiconductor saturable absorber. The intracavity pulse compression ratio is higher than 50. Down chirping when compared to up chirping allows broader mode-locked spectra and shorter pulse generation owing to temporal and spectral semiconductor gain dynamics. Pulses as short as 185 fs, with a peak power of ~230 w, and a focused intensity of ~4.6 gw/cm2 are generated by linear down chirp compensation and characterized by shg-frog method. To our knowledge, this is the highest peak power and the shortest pulse generation from an electrically pumped all-semiconductor system. The very good agreement between the simulated and the measured results verifies our understanding and ability to control the physical mechanisms involved in the pulse shaping within the ring cavity. Application trends such as continuum generation via a photonic crystal fiber, two-photon fluorescence imaging, and ultrafast pulse source for pump-probe experiments are demonstrated.

Harmonically mode-locked semiconductor lasers with external ring cavities offer high repetition rate pulse trains while maintaining low optical linewidth via long cavity storage times. Single frequency injection locking generates widely-spaced and tunable frequency combs from these harmonically mode-locked lasers, while stabilizing the optical frequencies. The output is stabilized long-term with the help of a feedback loop utilizing either a novel technique based on Pound-Drever-Hall stabilization or by polarization spectroscopy. Error signals of both techniques are simulated and compared to experimentally obtained signals. Frequency combs spaced by 2.5 GHz and ~10 GHz are generated, with demonstrated optical sidemode suppression of unwanted modes of 36 dB, as well as RF supermode noise suppression of 14 dB for longer than 1 hour. In addition to the injection locking of actively harmonically mode-locked lasers, the injection locking technique for regeneratively mode-locked lasers, or Coupled OptoElectronic Oscillators (COEOs), is also demonstrated and characterized extensively.

Diese Arbeit beschäftigt sich mit der Carrier-Envelope Phasendrift modengekoppelter Laser sowie Maßnahmen zu deren aktiver und passiver Stabilisierung. Das restliche Phasenrauschen wird in verschiedenen Messaufbauten im Hinblick auf physikalische Ursachen und mögliche Optimierungen untersucht. Dazu werden einführend verschiedene Interferometeranordnungen zur Messung der CEP Drift von Ti:Saphir Oszillatoren systematisch auf ihr Eigenrauschen hin untersucht. In einem Vergleichsexperiment wird die Überlegenheit kompakter Interferometeraufbauten demonstriert. Zusätzlich wird eine einfache Anordnung vorgestellt, welche die Bestimmung der Phase eines spektralen Interferenzmusters. Das analoge Verfahren wird zur Einzelschussanalyse der CEP Drift von Verstärkerimpulsen verwendet. Hiermit werden erstmals schnelle Rauschbeiträge aufgedeckt. Ergänzend wird ein Konzept zur orthogonalen Kontrolle der CEP Drift mithilfe einer speziellen Kompensatoreinheit diskutiert. Der Kompensator besteht aus zwei Keilprismen, hergestellt aus unterschiedlichen optischen Materialien, die als Einheit verschoben werden. Durch geeignete Wahl der Materialien werden Effekte auf die Gruppenlaufzeit oder deren Dispersion gleichzeitig eliminiert. Darüber hinaus wird ebenfalls erstmalig ein lineares Messverfahren demonstriert, das die CEP Drift auch für ps-Oszillatoren erschließt. Zur linearen Detektion wird die spektrale Interferenz aufeinander folgender Impulse eines Impulszuges aufgelöst, die mit einem Ringresonator überlagert werden. Abschließend wird ein neues Verfahren präsentiert, das einen Impulszug generiert, dessen Einzelimpulse eine identische Feldstruktur aufweisen. Dazu das Messsignal direkt an ein externes Rückstellelement übergeben ohne eine zusätzliche Regelschleife zu benötigen. Auf diese Weise können Bandbreitenprobleme der Regelschleife vermieden werden. Da Rückstellelement und Laser voneinander getrennt sind, bleibt der Laser in seinem Betrieb ungestört. / This work discusses the carrier-envelope phase drift of mode-locked lasers as well as techniques for its active and passive stabilization. In order to reveal the physical origin of the drift, the investigations focus on the analysis of residual phase noise. From the analysis, potential improvements are developed. For this purpose, in a first experimental approach, different interferometer configurations for CEP drift detection of Ti:sapphire oscillators are compared. Comparative studies clearly reveal the superiority of compact interferometer set-ups in terms of noise. In a second series of experiments, a simple assembly is introduced for the direct extraction of the phase from spectral interference patterns. The analog method is demonstrated with single-shot measurements and utilized for stabilization of the CEP drift of an amplifier system, thereby enabling monitoring of additional fast noise contributions for the first time. Moreover, a concept for orthogonal control of the CEP drift by a specially designed compensator assembly is discussed. This assembly consists of two thin wedge prisms made from different optical materials. By choosing an appropriate material group delay and its dispersion are eliminated synchronously. Furthermore, a linear method is presented that provides access to the CEP drift of ps-oscillators for the first time. The newly introduced much more general linear approach relies on resolving the spectral interference of subsequent pulses from a pulse train, which are superimposed utilizing a ring resonator. Finally, a technique is demonstrated that generates a pulse train consisting of single pulses with identical field structure. Particularly, the heterodyne signal is directly fed forward to the external feedback element, replacing the classical servo loop. Therefore, servo bandwidth limitations are eliminated. Since feedback element and laser oscillator are decoupled, the laser performance is not corrupted by side effects from the feedback.

Short optical pulses and engineered nonlinear media is a powerful combination. Mode locked pulses exhibit high peak powers and short pulse duration and the engineered ferro-electric KTiOPO4 facilitates several different nonlinear processes. In this work we investigate the use of structured, second-order materials for generation, characterization and frequency conversion of short optical pulses. By cascading second harmonic generation and difference frequency generation the optical Kerr effect was emulated and two different Nd-based laser cavities were mode locked by the cascaded Kerr lensing effect. In one of the cavities 2.8 ps short pulses were generated and a strong pulse shortening took place through the interplay of the cavity design and the group velocity mismatch in the nonlinear crystal. The other laser had a hybrid mode locking scheme with active electro-optic modulation and passive cascaded Kerr lensing incorporated in a single partially poled KTP crystal. The long pulses from the active modulation were shortened when the passive mode locking started and 6.9 ps short pulses were generated. High-efficiency frequency conversion is not a trivial task in periodically poled materials for short pulses due to the large group velocity mismatch. Optimization of parameters such as the focussing condition and the crystal temperature allowed us to demonstrate 64% conversion efficiency by frequency doubling the fs pulses from a Yb:KYW laser in a single pass configuration. Quasi phase matching also offers new possibilities for nonlinear interactions. We demonstrated that it is possible to simultaneously utilize several phase matched second harmonic interactions, resulting in a dual-polarization second harmonic beam. Short pulse duration of the fundamental wave is a key parameter in the novel method that we demonstrated for characterization of the nonlinearity of periodically poled crystals. The method utilizes the group velocity mismatch between the two polarizations in a type II second harmonic generation configuration. The domain walls of PPKTP exhibit second order nonlinearities that are forbidden in the bulk material. This we used in a single shot frequency resolved optical gating arrangement. The spectral resolution came from Čerenkov phase matching, a non-collinear phase matching scheme that exhibits a substantial angular dispersion. The second harmonic light was imaged upon a CCD camera and with the spectral distribution on one axis and the temporal autocorrelation on the other. From this image we retrieved the full temporal profile of the fundamental pulse, as well as the phase. The spectral dispersion provided by the Čerenkov phase matching was large enough to characterize optical pulses as long as ~200 fs in a compact setup. The Čerenkov frequency resolved optical gating method samples a thin stripe of the beam, i.e. the area close to the domain wall. This provides the means for high spatial resolution measurements of the spectral-temporal characteristics of ultrafast optical fields. / QC 20100831

Die vorliegende Dissertation behandelt das Zusammenspiel von effizienten aktiven Lasermedien und neuartigen sättigbaren Absorbern, welche auf den Kohlenstoff-Nanostrukturen Graphen und den einwandigen Kohlenstoff Nanoröhren (SWCNTs) basieren. Die aktiven Lasermedien decken den Spektralbereich von 1,0 Mikrometer bis 2,1 Mikrometer ab, d.h. eine ganze Oktave, und nutzen die laseraktiven Ionen des Ytterbiums, Chroms und Thuliums. In dieser Arbeit werden die auf Graphen und SWCNT basierenden sättigbaren Absorber hinsichtlich ihres einer Anregung folgenden Relaxationsverhaltens, ihrer von der Fluenz abhängigen Transmission und ihres Sättigungs- verhaltens bei hohen Fluenzen untersucht. Eine vorangestellte Einführung der optischen Eigenschaften von Graphen und SWCNTs wird gegeben und die Modelle zur Beschreibung realer Proben werden aus theoretischen Modellvorstellungen hergeleitet. Die sättigbaren Absorber basierend auf Graphen und SWCNTs werden untereinander und mit klassischen halbleitenden sättigbaren Absorbern verglichen. Diese Arbeit zeigt ferner die Erzeugung ultrakurzer Pulse verschiedener Laser mit diesen neuartigen sättigbaren Absorbern basierend auf Kohlenstoff Nanostrukturen. Die erhaltenen Pulse werden mittels Spektrometrie, Autokorrelation, Radiofrequenz- und partiell FROG-Messungen charakterisiert, und der zugrunde liegende Pulsformungsmechanismus, sowie die Stabilität gegen das Güteschalten werden diskutiert. / This thesis addresses the interplay of highly efficient active laser media and novel saturable absorbers based on the carbon nanostructures graphene and single-walled carbon nanotubes (SWCNTs). The active laser media cover the spectral region from 1.0 micron up to 2.1 micron, i.e. a whole octave, and apply ytterbium, chromium and thulium as active lasing ions. Within this work, the saturable absorbers based on SWCNTs and graphene are characterized with respect to their relaxation behaviour after excitation, and with respect to their fluence-dependent transmission and saturation. A precedent introduction of the general optical properties of graphene and SWCNTs is presented as well and the models to describe real samples experimentally are deduced from theoretical model conceptions. The saturable absorbers based on graphene and SWCNTs are compared to each other and to classical semiconducting saturable absorbers. This thesis further presents the generation of ultrashort laser pulses applying these novel carbon nanostructure based saturable absorbers in different lasers. The obtained pulses are characterized by spectrometry, autocorrelation, radio-frequency measurements and partially by FROG measurements. Additionally, the underlying pulse formation process and the Q-switching stability are discussed.