This thesis discusses a new scheme for generating radiation in the mid infrared spectral region, especially the 3.5-5 µm range. The scheme uses established Nd3+-lasers at 1.06 µm and down conversion in nonlinear optical crystals. The down conversion is made by two optical parametric oscillators (OPO) in series. The second OPO is a classical OPO using a zink germanium phosphide (ZGP) crystal. ZGP is the best nonlinear material available for the 4-8 µm spectral range, but it is absorbing below 2 µm. The new development presented in this thesis is the OPO used to convert the 1.06 µm laser radiation to a suitable OPO pump near 2 µm. The OPO uses a type I quasi phase-matched crystal, which accesses high nonlinearities and avoids walk-off. The problem with type I OPOs close to degeneracy is the broad bandwidth of the generated radiation, which reduces the efficiency of a second OPO. This has been solved with a spectrally selective cavity using a volume Bragg grating output coupler. Unlike other bandwidth limiting schemes this introduces no intracavity losses and thus efficient OPO operation is achievable. Narrow linewidth (~0.5 nm) OPO operation has been achieved with periodically poled LiNbO3 (PPLN) and periodically poled KTiOPO4 (PPKTP) while locking the signal wavelength at 2008 nm and simultaneously generating an idler at 2264 nm. A high average power PPLN OPO with 36 % conversion efficiency and 47 % slope efficiency is reported. Operation very close to degeneracy at 2128 nm with the narrowband signal and idler peaks separated by 0.6 nm was demonstrated in a PPKTP OPO. Both the signal at 2008 nm and the combined signal and idler around 2128 nm from the PPKTP OPOs have been used to show efficient pumping of a ZGP OPO. The maximum conversion efficiency from 1 µm to the mid-IR demonstrated is 7 % with a slope efficiency of 10 %. This is not quite as high as what has been presented by other authors, but the experiments reported here have not shown the optimum efficiency of the new scheme. Relatively simple improvements are expected to give a significant increase in conversion efficiency. / QC 20101108

No description available.

<p>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.</p><p>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.</p><p>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.</p><p>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.</p><p>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.</p><p>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.</p>

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.

This thesis describes the use of counterpropagating pulse trains to quasi-phase-match high-harmonic generation (HHG). Two novel techniques for generating trains of ultrafast pulses are described and demonstrated. The first method makes use of a birefringent crystal array to split a single pulse into a sequence of pulses. The second method makes use of the time-varying polarisation of a chirped pulse passed through a multiple-order wave plate to generate a train of pulses by the addition of a polariser. It is demonstrated that this second technique can be used to make pulse trains with non-uniform pulse separation by using an acousto-optic programmable dispersive filter to manipulate the higher-order dispersion encountered by the chirped pulse. The crystal array method is used to demonstrate quasi-phase-matching of HHG in a gas-filled capillary, using one and two counterpropagating pulses. Enhancements of up to 60% of the intensity of the 27th harmonic of the 800,nm driving laser light are observed. Information on the spatial and dynamic properties of the HHG process is obtained from measurements of the coherence length in the capillary. Simulations of HHG in a capillary waveguide have been performed. These agree well with the results of the quasi-phase-matching experiments. The effect of mode-beating on the generation process in a capillary and its use as a quasi-phase-matching mechanism are investigated.

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.

Nous avons développé un oscillateur paramétrique optique simplement résonant (SRO) basé sur un cristal non linéaire de niobate de lithium dopé 5%-MgO et périodiquement polarisé (ppMgCLN). Il est pompé à 1064 nm par une diode laser en cavité étendue balayable continûment de 1050 à 1070 nm injectant un amplificateur Yb-fibré de puissance 10 W. Il génère une onde idler comprise entre 3 et 4 µm et une onde signal entre 1450 et 1650 nm. La cavité SRO est asservie sur le pic de transmission d'une cavité Fabry-Perot de grande finesse. Nous avons alors pu démontrer un balayage mono-fréquence sans saut de mode de l'onde idler sur 500 GHz. Cette large accordabilité continue pourrait être utilisée pour la spectroscopie haute résolution multi-espèces dans le moyen infrarouge. Par ailleurs, nous avons revisité la théorie ondes planes du SRO, dont les solutions analytiques ont été données pour la première fois en 1969 par Kreuzer sous la forme d'une équation transcendante, en utilisant une méthode perturbative très puissante qui tient compte de la déplétion de la pompe. Nous avons pu ainsi déterminer les relations d'entrée-sortie du SRO sous la forme de relations explicites très simples, montrant que les puissances de sortie sont proportionnelles à la racine cubique de la puissance pompe. / We developed a singly-resonant optical parametric oscillator (SRO) based on a nonlinear crystal of 5%-ppMgCLN congruent lithium niobate chip and pumped at 1064 nm by an extended cavity diode laser widely tuneable from 1050 to 1070 nm injecting a 10 W Yb-fiber amplifier. It generates an idler wave between 3 and 4 µm and a signal wave between 1450 and 1650 nm. The SRO cavity is stabilized to the top of a Fabry-Perot transmission fringe. We then demonstrated a mode-hop-free idler tuning range of 500 GHz. This broad continuous tunability could be used for multi-species high resolution spectroscopy in the mid-infrared. Moreover, we have revisited the plane waves SRO theory, whose analytical solutions were given for the first time in 1969 by Kreuzer in the form of a transcendental equation, using a very powerful perturbative method which takes into account the depletion of the pump. We were able to determine the input-output relations of SRO in the form of very simple explicit relationships, showing that the output powers are proportional to the cubic root of the pump power.

Ferroelectric crystals are commonly used in nonlinear optics for frequency conversion of laser radiation. The quasi-phase matching (QPM) approach uses a periodically modulated nonlinearity that can be achieved by periodically inverting domains in ferroelectric crystals and allows versatile and efficient frequency conversion in the whole transparency region of the material. KTiOPO4 (KTP) is one of the most attractive ferroelectric non-linear optical material for periodic domain-inversion engineering due to its excellent non-linearity, high resistance for photorefractive damage, and its relatively low coercive field. A periodic structure of reversed domains can be created in the crystal by lithographic patterning with subsequent electric field poling. The performance of the periodically poled KTP crystals (PPKTP) as frequency converters rely directly upon the poling quality. Therefore, characterization methods that lead to a deeper understanding of the polarization switching process are of utmost importance. In this work, several techniques have been used and developed to study domain structure in KTP, both in-situ and ex-situ. The results obtained have been utilized to characterize different aspects of the polarization switching processes in KTP, both for patterned and unpatterned samples. It has also been demonstrated that it is possible to fabricate sub-micrometer (sub-μm) PPKTP for novel optical devices. Lithographic processes based on e-beam lithography and deep UV-laser lithography have been developed and proven useful to pattern sub- μm pitches, where the later has been the most convenient method. A poling method based on a periodical modulation of the K-stoichiometry has been developed, and it has resulted in a sub-μm domain grating with a period of 720 nm for a 1 mm thick KTP crystal. To the best of our knowledge, this is the largest domain aspect-ratio achieved for a bulk ferroelectric crystal. The sub-micrometer PPKTP samples have been used for demonstration of 6:th and 7:th QPM order backward second-harmonic generation with continuous wave laser excitation, as well as a demonstration of narrow wavelength electrically-adjustable Bragg reflectivity. / QC 20100930

This thesis focuses on two topics: thermal limitations of volume Bragg gratings (VBGs) employed as laser-cavity mirrors and formation of color centers in KTiOPO4 and its isomorphs. To explore the mechanisms of the thermal limitations of VBGs in high power lasers, I designed and constructed a diode-pumped, solid‑state laser with a VBG as cavity mirror that had a significantly higher absorption than what is typical. Thereby I could study the limiting thermal effects by using only moderate intra-cavity power. Additionally, I designed a computer model to numerically investigate the thermal effects in VBGs. Both the experiments and the simulations showed that the laser became successively more unstable when the power was increased. Absorption of the reflected laser beam causes broadening of the grating spectrum accompanied by decreasing diffraction efficiency. The reduced reflectivity leads to a leakage of the radiation through the grating. Moreover, the simulations showed that this increased instability was due to a reshaping of the intensity distribution profile inside the grating, which, in turn, leads to a sharp reduction of the diffraction efficiency. High-intensity visible radiation induces color centers in KTiOPO4, which can lead to severe decrease in the performance of the crystal and can cause catastrophic breakdown. The formation of color centers was investigated by measuring picosecond, blue-light induced infrared absorption (BLIIRA) in periodically-poled KTiOPO4, Rb:KTiOPO4, RbTiOPO4, KTiOAsO4 and RbTiOAsO4 through thermal lens spectroscopy using a common-path interferometer. This setup is capable of measuring absorption as low as 10-5 cm-1. The dependence of the BLIIRA signal on blue light average power and intensity as well as on the crystal temperature was studied. The results show the presence of at least two different types of color centers. A higher level of remnant absorption was observed in the phosphates compared to that of the arsenates. The largest portion of the induced absorption is attributed to photo-generated electrons and holes being self-trapped in the proximity to the Ti4+ and O2- ions, respectively, forming polaron color centers. Stabilization of these centers is aided by the presence of mobile alkali metal vacancies in the crystal. / Denna avhandling fokuserar på både volymbraggitters (VBGs) termiska begränsningar, i tillämpning som speglar i laserkaviteter, och på bildandet av färgcentra i KTiOPO4 och isomorfa kristaller. För att undersöka de termiska effekterna i VBGer som medför begränsningar på högeffektlasrar utfördes både experiment och simuleringar. För experimenten konstruerades en diod-pumpad Yb:KYW laser med ett VBG som har betydligt högre absorption än vad som är typiskt. Därmed kunde de termiska effekterna studeras vid måttliga intrakavitetseffekter. Simuleringarna bestod av två delmodeller; gitterstrukturen modelerades med överföringsmatriser och värmeflödet med en tredimensionell modell baserad på finita elementmetoden. Både experimenten och simuleringarna visade att en laser blir successivt mer instabil när den optiska effekten ökar. Absorptionen av laserstrålen i VBGt förändrade dess spektrala egenskaper, vilket i sin tur påverkade laserns stabilitet och prestanda. De huvudsakliga effekterna var en breddning av gittrets spektrum med en minskad reflektans. Simuleringarna visade även att den ökade instabiliteten berodde på en förändring av strålningens intensitetsfördelning inuti gittret, vilket accelererade reduktionen av gittrets reflekterande förmåga. I termer av den effekt som faller in mot gittret, har lasern en tydlig övre effektgräns. När den gränsen har uppnåtts leder vidare ökning av pumpeffekten i huvudsak till ökat läckage genom volymbraggittret, i stället för till ökad uteffekt hos laserstrålen. Kortvågigt synlig ljus av hög intensitet inducerar färgcentra i KTiOPO4, vilket kan leda till kraftigt reducerad transparens och kan orsaka permanent skada i kristallen. För att undersöka skapandet av dessa färgcentra mättes den termiska lins som uppstår vid blå-ljus-inducerad infraröd absorption (Eng: blue-light induced infrared absorption = BLIIRA) inducerad av blåa laserpulser vid en våglängd av 398 nm och vid pulslängder i storlek av pikosekunder i periodiskt‑polad KTiOPO4, Rb:KTiOPO4, RbTiOPO4, KTiOAsO4 och RbTiOAsO4. Den termiska linsen mättes med en metod kallad gemensam-vägsträcka-interferometer (Eng: common-path interferometer), en metod känslig nog för att mäta absorption så låg som 10-5 cm-1. Dessutom undersöktes hur nivån av BLIIRA beror på medeleffekten och intensiteten hos den blåa laserstrålen samt på kristalltemperaturen. Resultaten visar att det bildas minst två typer av färgcentra med olika livslängder. Vidare observerades en högre grad av långsamt avklingande absorption i fosfaterna jämfört med arsenaterna. Den största delen av den inducerade absorptionen tillskrivs fotogenererade elektroner och hål som ”självfångas” i närheten av Ti4+ respektive O2- joner, och bildar färgcentra av polaron karaktär. Stabilisering av dessa centra underlättas av lättrörliga alkalivakanser i kristallerna. / <p>QC 20150922</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.