Optical parametric frequency conversion based on quasi phase matching (QPM) in nonlinear optical crystals is a powerful technique for generating coherent radiation in wavelength ranges spanning from the mid-infrared (mid-IR) to the blue, displaying low thermal load and high efficiency.This thesis shows how QPM in one- (1D) or two-dimensional (2D) lattices can be employed to engineer novel devices for parametric downconversion in the IR, af-fording freedom in designing both spectral and angular properties of the parametric output. Experimental demonstrations of parametric devices are supported by theoreti-cal modelling of the nonlinear conversion processes.In particular, broadband parametric downconversion has been investigated in 1D QPM lattices, through degenerate downconversion close to the point of zero group-velocity dispersion. Ultra-broadband optical parametric generation (OPG) of 185 THz bandwidth (at 10 dB), spanning more than one octave from 1.1 to 3.7 μm, has been achieved in periodically poled 1 mol% MgO-doped near-stoichiometric LiTaO3 (MgSLT) of 25 μm QPM period, pumped at 860 nm. Such broadband gain is of high interest for ultrashort optical pulse amplification, with applications in high harmonic generation, ultrafast spectroscopy and laser ablation. Furthermore, the det-rimental impact of parasitic upconversion, creating dips in the OPG spectrum, has been investigated. By altering the pump pulse duration, energy can be backconverted to create peaks at the involved OPG wavelengths, offering a possible tool to enhance broadband parametric gain spectra.The engineering of the angular properties of a parametric output benefits greatly from 2D QPM, which is investigated in this thesis by the specific example of hexagonally poled MgSLT. It is demonstrated how two OPG processes, supported by a single 2D QPM device, can exhibit angularly and spectrally degenerate signals (idlers). This degeneracy results in a coherent coupling between the two OPG pro-cesses and a spectrally degenerate twin-beam output in the mid-IR (near IR). 2D QPM devices exhibiting such coherently coupled downconversion processes can find applications as compact sources of entangled photon-pairs. This thesis further illus-trates the design freedom of 2D QPM through the demonstration of a device support-ing multiple parametric processes, thus generating multiple beams from the mid-IR to the blue spectral regions. / <p>QC 20131204</p>

Bragg reflection waveguides (BRW) or one-dimensional photonic bandgap structures have been demonstrated for phase-matching chi(2) nonlinearities in AlxGa1-xAs. The method exploits strong modal dispersion of a Bragg mode and total internal reflection modes co-propagating inside the waveguide. It is shown that phase-matching is attained among the lowest order modes of interacting harmonics, which allows maximizing the utilization of harmonics powers for nonlinear interactions. As our first demonstration, we report second-harmonic generation (SHG) of a 2-ps telecommunication pump in a 2.4 mm long slab BRW. The conversion efficiency is estimated as 2.0 %/W.cm^2 with a generated SH power of 729 nW. This efficiency has been considerably improved by introducing lateral confinement of optical modes in ridge structures. Characterizations denote that efficiency of SHG in ridge BRWs can increase by over an order of magnitude in comparison to that of the slab device. Also, we report continuous-wave SHG in BRWs. Using a telecommunication pump with a power of 98 mW, the continuous-wave SH power of 23 nW is measured in a 2.0 mm long device. Significant enhancements of chi(2) interactions is obtained in the modified design of matching-layer enhanced BRW (ML-BRW). For the first time, we report type-II SHG in ML-BRW, where the second-harmonic power of 60 µW is measured for a pump power of 3.3 mW in a 2.2 mm long sample. Also, we demonstrate the existence of type-0 SHG, where both pump and SH signal have an identical TM polarization state. It is shown that the efficiency of the type-0 process is comparable to type-I and type-II processes with the phase-matching wavelengths of all three interactions lying within a spectral window as small as 17 nm. ML-BRW is further reported for sum-frequency and difference-frequency generations. For applications requiring high pump power, a generalized ML-BRW design is proposed and demonstrated. The proposed structure offsets the destructive effects of third-order nonlinearities on chi(2) processes when high power harmonics are involved. This is carried out through incorporation of larger bandgap materials by using high aluminum content AlxGa1-xAs layers without undermining the nonlinear conversion efficiency. Theoretical investigations of BRWs as integrated sources of photon-pairs with frequency correlation properties are discussed. It is shown that the versatile dispersion properties in BRWs enables generation of telecommunication anti-correlated photon-pairs with bandwidth tunablity between 1 nm and 450 nm.

The topic of this thesis is the development of devices for telecom applications based on poled optical fibers. The focus is on a specific function, optical switching/modulation. Some of the most important results are summarized below. Optical switching at telecom wavelengths (1.55 μm) is demonstrated in an all-fiber switch based on a fiber with internal electrodes. The fiber is made electro-optically active with a thermal poling process in which a strong electric field is recorded in the glass at a temperature of 255 °C. After poling, the fiber is put in one arm of a Mach-Zehnder interferometer and by applying a voltage across the two electrodes the refractive index is modulated and the optical signal switched from one output port to the other. A switching voltage of 190 V at 1550 nm was achieved, which to the best of our knowledge is the lowest value reported. By carefully matching the lengths of the fibers in the two arms of the interferometer the optical bandwidth could be made as large as 20 nm. The extinction ratio, determined by the power ratio in the two arms, was 30 dB and the highest modulation frequency was 30 MHz. Poled fibers were packaged to increase the thermal and mechanical stability and to make handling easier. 40 Gb/s transmission test through the device showed no bit-error-rate performance degradation. Protection switching of a 10 Gb/s signal is also demonstrated. The depletion region in a poled fiber was found to be wedge-shaped and very wide, 13 μm and completely overlapped with the core. In a time-resolved poling experiment the recorded electric field was measured. The sign of the field changed after ~20 min, when the depletion region passed through the core, which led to the conclusion that an electric field is present also outside of the depletion region. A ring laser was constructed with an erbium doped fiber as the gain medium. A fiber modulator was placed inside the cavity and when a small RF signal, with a frequency matched to the cavity ground frequency, was applied to the modulator the laser was modelocked. The output pulse train contained pulses of sub ns duration and is the first demonstration of mode-locking using poled fibers. A sampled grating with 16 channels spaced by 50 GHz was inserted into the cavity. The fiber modulator had optical bandwidth of 7 nm with center wavelength that depends on the applied voltage. By applying of 10 – 210 V to the modulator it was possible to tune the laser to 11 of the 16 channels for a total tuning range of over 4 nm. A scheme to deposit 1 μm thin silver electrodes inside the holes of an optical fiber is demonstrated together with a new method of creating periodic electrodes by periodically ablating the silver film electrodes. The periodic electrodes are used to create a quasi-phase matched (QPM) nonlinearity in a fibers which is showed in a proof of principle experiment. / QC 20101015

Nonlinear optical frequency conversion is one of the most important key techniques in order to obtain lasers with wavelengths targeted for specific applications. In order to realize efficient and tailored lasers, the quasi-phase-matching (QPM) approach using periodically-poled ferroelectric crystals is getting increasingly important. Also understanding of damage mechanisms in nonlinear materials is necessary to be able to design reliable and well working lasers. This is especially true for high power application lasers, which is a rapidly growing field, where the damage problem normally is the ultimate limiting factor. In this thesis work, several promising novel ferroelectric materials have been investigated for nonlinear optical applications and the emphasis has been put on QPM devices consisting of periodically-poled structures. The materials were selected from three different types of ferroelectric materials: 1) MgO-doped stoichiometric LiNbO3 (MgO:SLN) and LiTaO3 (MgO:SLT), and non-doped stoichiometric LiTaO3 (SLT), 2) KTiOPO4 (KTP) and its isomorphs RbTiOPO4 (RTP), and 3) KNbO3 (KN). The focus in our investigations have been put on the spontaneous polarization switching phenomena, optimization of the periodic poling conditions, and the photochromic optical damage properties which were characterized by the help of blue light-induced infrared absorption (BLIIRA) measurements. With electrical studies of the spontaneous polarization switching, we were able to determine quantitatively, and compare, the coercive field values of different materials by applying triangularly shaped electric fields. We found that the values of the coercive fields depended on the increase rate of the applied electric field. The coercive field of KN was the lowest (less than 0.5 kV/mm) followed by the ones of KTP, SLT, and MgO:SLT (1.5 to 2.5 kV/mm). MgO:SLN, and RTP had relatively high coercive fields, approximately 5.0 to 6.0 kV/mm, respectively. Based on the domain switching characteristics we found, we successfully fabricated periodically-poled devices in all of the investigated materials with 30 μm periodicities and sample thickness of 1 mm. Blue light-induced infrared absorption (BLIIRA) has been characterized for unpoled bulk and periodically-poled samples using a high-sensitivity, thermal-lens spectroscopy technique. SLT showed a large photorefraction effect and the BLIIRA signal could not be properly measured because of the large distortion of the probe beam. The rise and relaxation time of BLIIRA, after switching the blue light on and off was in a time span of 10 to 30 sec except for KTP and its isomorphs, which needed minutes to hours in order to saturate at a fixed value. KN and MgO:SLN showed the lowest susceptibility to the induced absorption. Periodic poling slightly increased the susceptibility of KTP, MgO:SLT, and KN. Relatively high thresholds were observed in MgO:SLT and KN. By increasing the peak-power intensity of the blue light, the induced absorption for MgO:SLN, KTP and KN saturated at a constant value while that of MgO:SLT increase in a constant fashion. This trend is critical issue for the device reliability at high-power applications. / QC 20100830

This thesis examines second-order optical nonlinear wave mixing processes in domain-disordered quasi-phase matching waveguides and evaluates their potential use in compact, monolithically integrated wavelength conversion devices. The devices are based on a GaAs/AlGaAs superlattice-core waveguide structure with an improved design over previous generations. Quantum-well intermixing by ion-implantation is used to create the quasi-phase matching gratings in which the nonlinear susceptibility is periodically suppressed. Photoluminescence experiments showed a large band gap energy blue shift around 70 nm after intermixing. Measured two-photon absorption coefficients showed a significant polarization dependence and suppression of up to 80% after intermixing. Similar polarization dependencies and suppression were observed in three-photon absorption and nonlinear refraction. Advanced modeling of second-harmonic generation showed reductions of over 50% in efficiency due to linear losses alone. Self-phase modulation was found to be the dominant parasitic nonlinear effect on the conversion efficiency, with reductions of over 60%. Simulations of group velocity mismatch showed modest reductions in efficiency of less than 10%. Experiments on second-harmonic generation showed improvements in efficiency over previous generations due to low linear loss and improved intermixing. The improvements permitted demonstration of continuous wave second-harmonic generation for the first time in such structures with output power exceeding 1 µW. Also, Type-II phase matching was demonstrated for the first time. Saturation was observed as the power was increased, which, as predicted, was the result of self-phase modulation when using 2 ps pulses. By using 20 ps pulses instead, saturation effects were avoided. Thermo-optically induced bistability was observed in continuous wave experiments. Difference frequency generation was demonstrated with wavelengths from the optical C-band being converted to the L- and U-bands with continuous waves. Conversion for Type-I phase matching was demonstrated over 20 nm with signal and idler wavelengths being separated by over 100 nm. Type-II phase matched conversion was also observed. Using the experimental data for analysis, self-pumped conversion devices were found to require external amplification to reach practical output powers. Threshold pump powers for optical parametric oscillators were calculated to be impractically large. Proposed improvements to the device design are predicted to allow more practical operation of integrated conversion devices based on quasi-phase matching superlattice waveguides.

Neste trabalho estudamos uma nova classe de materiais ópticos não-lineares, os cristais orgânicos, que possuem potencial para muitas aplicação em dispositivos. Determinamos diferentes propriedades ópticas lineares e não-lineares de três cristais orgânicos: a l-alanina, a l-treonina e a l-lisina. Esses cristais de aminoácido foram escolhidos por apresentarem propriedades de geração de segundo harmônico (GSH) e por serem materiais nunca estudados sob o ponto de vista da óptica não-linear. Estudamos algumas das propriedades ópticas lineares fundamentais desses cristais biaxiais transparentes, pois deles dependem os fenômenos não-lineares sobre os quais são feitas intensas pesquisas. Determinamos as propriedades lineares como os espectros de absorção, os índices de refração, os eixo ópticos, velocidade de propagação da luz num meio anisotrópico e os limiares de dano por radiação. Posteriormente, determinamos as condições de casamento de fase, a eficiência de GSH, e a auto-modulação de fase. A GSH e a auto-modulação de fase pertencem, respectivamente, a processos não-lineares de segunda e de terceira-ordem. Para o estudo de muitas dessas propriedades foram desenvolvidas novas técnicas experimentais. Uma dessas novas técnicas, a varredura-Z oscilante, teve sensibilidade para determinar, pela primeira vez, o índice de refração não oscilante, vez, o índice de refração não linear desses cristais orgânicos. Além disso, como a todos os processos de caracterização envolveram a preparação de amostras, tivemos a oportunidade de criar procedimentos padrões para manipulação e utilização desses novos materiais. / In this work we studied a new class of nonlinear organic crystals that are potential candidates for devices application. We determined several linear and nonlinear optical properties of three organic crystals: l-alanine, l-threonine and l-lysine. These aminoacid. These aminoacid because their nonlinearities were never studied before. We characterized some of the fundamental linear optical properties of these biaxial transparent crystals, because they have influence on nonlinear phenomena that attracted a lot of research. We determined linear properties like absorption spectra, indices of refraction, optical axes, the light speed propagation in anisotropy media and the optical damage threshold. Subsequently, we determined the phase-matching condition for SHG, efficiency of the SHG and self-phase modulation. The SHG and self-phase modulation belongs, respectively, to the second-order and third-order nonlinear processes. To study several of these properties we had to develop some new experimental techniques. One of them, the oscillatory Z-scan, allows enough sensibility to determine, for the first time, the nonlinear refraction index of these organic crystals. Besides, as all these optical characterization involve the preparation of the samples, we had the opportunity to establishing standard procedures for manipulation of these new materials.

The Quantum Cascade Laser (QCL)-based terahertz-over-fibre (ToF) concept combines the strength of QCLs as ultra-wide bandwidth, high speed data sources, with the mature optical fibre technology. In this thesis, for the first time, by fusing multiple technologies, digitally selected, electronically-switchable ToF concept is experimentally demonstrated. Furthermore, the digital mode selection principle and electronic tuning mechanism provided by novel aperiodic distributed feedback (ADFB) multi-band filters are presented. For the development of electronically tunable ADFB lasers, a range of bound-to-continuum and chirped superlattice terahertz (THz) QCLs are measured across the frequency range 2.9 – 4.5 THz. The availability of these active materials allowed rapid assessments of the optimum design parameters for subsequent measurements. First, a range of photonic lattice-engineered lasers operating at 4.4 THz are characterized and key design parameters identified. Following this initial development, full electrical and spectral characterization of ADFB lasers operating at 2.9 THz are presented. The novelty of this work lies in the first-ever successful demonstration of discretely tunable QCLs, operating at six distinct THz frequencies. The ADFB technology was experimentally applied using various device geometries and gain dynamics. Toward this aim, results are presented for a Y coupled QCL architecture, showing that complex on-chip signal manipulation can be extended into the THz regime. In addition, it is demonstrated that ADFB technology provides broadband multi-channel optical filtering for the entire gain bandwidth. It is shown that discrete, purely electronic, tuning of simultaneous dual colour output can be achieved. Multi band optical filter functions derived from ADFB gratings possess highly nonlinear dispersion across the filter bandwidth and are found to modify the gain-induced, driving current-dependent continuous mode tuning. This thesis, therefore, presents a systematic experimental analysis of the dispersion engineered continuous fine-tuning in THz QCLs. In the final two chapters, the thesis presents, for the first time, transmission of tunable THz signals over standard single-mode optical fibre by up converting 2.9 THz QCL radiation via intra-cavity nonlinear mixing with an optical fibre-injected near-infrared (NIR) carrier in the 1.3 µm band. Discrete and continuous tuning technologies, as developed in chapters 3 – 5, are now successfully transferred to THz sidebands on the NIR carrier, extracted via a butt coupled single mode fibre and recorded using an optical spectrum analyzer. The major novel outcome of this thesis is the first demonstration of electronically tunable phase-matched points in a THz plasmon waveguide. The key breakthrough is the experimental confirmation of the photonic band-gap engineering of group velocity of THz signals – as both ‘fast’ and ‘slow’ switchable side bands are observed. Such novel nonlinear up-conversion of spectrally flexible THz signals may open up new possibilities for ultrafast THz telecom frameworks.

Le développement de sources moyen infrarouge compactes et accordables dans les gammes de transmission de l’atmosphère présente un intérêt majeur dans les secteurs de la défense et de la sécurité. Les sources paramétriques à quasi-accord de phase en configuration guidée sont prometteuses pour gagner en compacité puisque l’on réduit la puissance de pompe nécessaire par rapport aux sources « massives ». Le premier axe de la thèse consiste à étudier des guides d’onde en arséniure de gallium périodiquement orientés (OP-GaAs) adaptés à un pompage par laser fibré et à des puissances relativement élevées. Le second vise à étudier de façon novatrice la possibilité d’intégrer dans un composant monolithique une diode laser en matériaux antimoniures avec un convertisseur de fréquence en antimoniure de gallium (GaSb). L’enjeu dans les deux cas est de réduire au maximum les pertes à la propagation dans ces guides d’onde pour exploiter pleinement leurs propriétés non-linéaires.Ce travail de thèse a permis de modéliser des structures de guides d’onde ambitieuses pour réduire les pertes, de développer les briques technologiques nécessaires à la fabrication de guides d’onde OP-semi-conducteur faibles pertes et de faire de premières caractérisations de ces composants dans le moyen-infrarouge. Les performances de guides d’onde GaAs ruban enterrés ou non ont pu être comparées, donnant une réduction des pertes d’un facteur trois avec des rubans enterrés. Plusieurs générations de guides d’onde GaSb ont vu le jour, et montrent des performances à l’état de l’art des structures en GaAs. En conséquence, diverses solutions ont été explorées pour intégrer une diode laser en matériaux antimoniures avec le guide d’onde convertisseur de fréquence. / The development of compact and tunable mid-infrared laser sources in the atmospheric transmission windows presents a major interest for several security and defense applications. Quasi-phase-matched parametric sources in guided wave configuration are promising solutions to enhance compactness, because of the reduction in pump power requirements with respect to bulk devices.The first axis of this thesis consists in studying orientation-patterned gallium arsenide (OP-GaAs) waveguides, adapted to fiber laser pumping and to relatively high pump power. The second axis is devoted to the original idea of integrating an antimonide based laser diode with a gallium antimonide (GaSb) frequency converter in a monolithic component. The goal in both cases is to minimize propagation losses in those waveguides to exploit the whole potential of their non-linear properties.This work led to model ambitious low-loss waveguides structures, to develop the technological fabrication steps necessary for OP-semiconductor waveguides manufacturing, and to characterize these components in the mid-infrared. The first buried ridge GaAs waveguide structure has been compared to the ridge one, giving a reduction of a factor three in the propagation losses. Several generations of GaSb waveguides have come forward, with constant losses improvement and reach GaAs state-of-the-art performances. Lastly, multiple solutions have been explored in order to integrate an antimonide-based laser diode with the frequency converter waveguide.

Les applications de spectroscopie, en particulier l’analyse de gaz à effet de serre, de composés organiques volatils ou autres polluants atmosphériques motivent le développement d’instrumentations spécifiques. L’étude présentée ici vise à proposer de nouvelles sources aptes à caractériser la composition d’un milieu gazeux, liquide ou solide. Les raies d’absorption optique de la plupart des gaz à détecter sont particulièrement fortes dans l’infrarouge moyen (en particulier entre 3 et 5 µm). Pour adresser cette plage spectrale, l’optique non linéaire propose de nombreuses solutions. Les sources rapportées ici sont des oscillateurs paramétriques optiques (OPO) dont la spécificité repose sur l’utilisation de cristaux non linéaires à quasi-accord de phase apériodique. Ces cristaux présentent de larges bandes de gain intrinsèques. Les travaux présentés permettent une étude du comportement de telles sources, absentes de la littérature en régime d’impulsions picosecondes. Des caractéristiques propres à l’utilisation des cristaux apériodiques sont rapportées. Un outil permettant une observation spectrale dynamique en régime picoseconde est proposé puis utilisé pour l’étude du démarrage de nos OPO. Ces sources large bande sont ensuite associées à des filtres spectraux rapides placés dans la cavité. Deux types de filtres sont utilisés. D’abord l’association d’un réseau de diffraction en configuration Littrow avec un déflecteur rapide. Ensuite l’insertion dans la cavité d’un réseau de Bragg en volume chirpé, placé sur une platine de translation, pour tirer profit de la condition de pompage synchrone. Ces deux solutions nous permettent d'obtenir des dispositifs largement et rapidement accordables en longueur d'onde. L’utilisation de telles sources pour des applications de détection de gaz est démontrée. / Spectroscopy applications related to greenhouse gases or other atmospheric pollutants, involve the development of a large range of specific tools. The aim of the work presented here is to develop new devices to characterize the composition of gas, liquid, or solid media. Most of the species to be detected show particularly strong optical absorption lines in the mid-infrared region (in particular around 3 to 5 µm). To address this specific spectral range, nonlinear optics provide a wide range of solutions. The sources reported here are optical parametric oscillators (OPO) based on aperiodically poled nonlinear crystals. Such crystals offer broad gain bandwidths. The work presented here contains a study of those sources, not reported so far in the literature for picosecond regime. Behaviors that are specific to the use of aperiodic crystals are reported. A useful tool allowing dynamical spectral studies is proposed, and implemented to investigate buildup regime of the picosecond OPO. Those sources are then associated with rapid spectral filters inserted in the cavity. Two different approaches are developed. The first is based on the association of a diffraction grating in Littrow configuration with a deflecting device. The second takes advantage of the synchronous pumping scheme, with an intracavity chirped volume Bragg grating mounted on a translation stage. Fast and wide wavelength tuning is demonstrated with those devices. Gas detection applications are also demonstrated.

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