This thesis contains the results of research on the spectral control of lasers, specifically, the thermal limitations of volume Bragg gratings (VBGs), employed as laser-cavity end mirrors. The investigations consisted of both experiments and numerical simulations. For the experiments, a diode-pumped Yb:KYW laser with a VBG that had an absorption coefficient of 2.8% cm-1, in the 1 μm spectral region, was constructed. The computer model comprised of a transfer-matrix model and a three-dimensional, finite element model, working together. The absorption of the reflected laser beam changed the reflecting properties of the VBG, which affected the laser’s stability and other performances. The primary effect was a broadening of the grating spectrum accompanied by decreased diffraction efficiency. The reduced reflectivity lead to a leakage of the radiation through the grating during lasing. Both the experiments and the simulations showed that the laser became successively more unstable when the power was increased. Also, the simulations showed that this increased sensitivity was due to a reshaping of the intensity distribution profile inside the grating, which, in turn, lead to a sharp reduction of the diffraction efficiency. For circulating powers above this limit, the laser output rolled off and the power was instead leaking out through the VBG. Furthermore, the simulations also showed that the power limit was highly dependent on the length of the employed VBG. For instance, a 2 mm long VBG could withstand approximately 9.5 times higher incident power than a 10 mm long one could. Also, it was found that the limit, expressed in the terms of the incident power, related approximately linearly to the size of the beam radius. / Denna licentiatavhandling innehåller forskningsresultat som behandlar spektral kontroll av lasrar, specifikt genom volymbraggitters (VBG) termiska begränsningar, då de används som speglar i laserkaviteter. Undersökningarna bestod av både experiment och simuleringar. För experimenten konstruerades en diod-pumpad Yb:KYW laser med ett VBG som hade en absorptionskoefficient på 2,8% cm-1, i våglängdsområdet kring en mikrometer. Datormodellen innefattade en modell av gitterstrukturen baserad på överföringsmatriser och en tredimensionell modell baserad på finita elementmetoden, för beräkning av värme-fördelningen. Absorption av den reflekterade laserstrålen ändrade volymbraggittrets reflekterande egenskaper, vilket i sin tur påverkade laserns stabilitet och dess prestanda. De huvudsakliga effekterna var en breddning av gittrets spektra och en minskad diffraktionsverkningsgrad. Både experimenten och simuleringarna visade att en laser blir successivt mer instabil när den optiska effekten ökar. Simuleringarna visade även att den ökade känsligheten berodde på en förändring av intensitetsfördelningen inuti gittret, vilket accelererade reduktionen av gittrets reflekterande förmåga. Uttryckt i mot gittret infallande effekt, har lasern en tydlig övre effektgräns. När den gränsen har uppnåtts leder vidare ökning av pumpeffekten i huvudsak till ökat läckage genom volymbraggittret, och inte till ökad uteffekt hos laserstrålen. Vidare visade simuleringarna också att en lasers effektgräns är starkt beroende av längden på det använda volymbraggittret. Till exempel tålde ett 2 mm långt gitter cirka 9,5 gånger högre infallande effekt än ett 10 mm långt gitter. Dessutom visade det sig att effektgränsen, uttryckt i infallande effekt, berodde approximativt linjärt på strålradiens storlek. / <p>QC 20130507</p>

Recent developments in holographic recording of volume Bragg gratings (VBGs) in photo-thermo-refractive (PTR) glass have demonstrated their utility as components in high power laser systems for spectral narrowing, transverse mode control, beam combining, and pulse stretching/compression. VBG structures are capable of diffracting incident light into a single diffraction order with high efficiency given the Bragg condition is met. The Bragg condition depends on both the wavelength and angle of the incident light making VBGs useful for filtering and manipulating both the wavelength and angular spectrum of a source. This dissertation expands upon previous research in PTR VBGs by investigating multiplexed VBGs and their applications in laser systems. Multiplexing involves the integration of several VBGs into the same volume of PTR glass. This process enables the fabrication of splitting and combining elements which have been used for high power beam combining with significantly reduced complexity as compared to other combining schemes. Several configurations of multiplexed beam combiners were demonstrated for both spectral and coherent combining systems with high power results yielding a combined power of 420 W with 96% efficiency. Multiplexing was also used to produce unique phase structures within VBGs. This effect was exploited to create extremely narrowband spectral filters called moire Bragg gratings. The technical challenges of producing moire gratings in bulk glass have revealed new insights into the use of PTR glass as a recording medium and produced devices capable of narrowband filtering of only 15 pm in the near infrared. Experiments were performed using such devices as intra-cavity laser elements for longitudinal mode selection. Investigations have also been made into increasing the level of multiplexing possible within PTR glass. These explorations included scaling the number of beam combining channels, fabrication of integrated multi-notch filters, and generated several other potentially interesting devices for future research. The summation of this work indicates a promising future for multiplexed VBGs in PTR glass.

No description available.

No description available.

<p> As the fundamental basic building blocks of photonic circuits, optical waveguide structures play important roles in modem telecommunication and sensing systems. Various structures ranging from the dielectric waveguide utilizing the total internal reflection (TIR) to the more advanced structures based on the surface plasmon polaritions (SPPs) are widely investigated and studied in industrial and research areas. With the fast development of fabrication technologies, more and more complicated structures are predicated to emerge as the requirement of highly integrated photonic circuits. Modeling and simulation methods, as efficient as well as excellent cost performance tools comparing to costly facilities and time-consuming fabrication procedures, are demanded to explore and design the devices and circuits before their finalization. </p> <P> This thesis reports a series of techniques to model two dimensional waveguide structures, including the conventional planar and surface plasmon polariton waveguides. This thesis contains both the methods and their applications to model and investigate the mode and propagation characteristics including the guided waves and the radiative waves. The methods include mode solvers based on fmite difference method (FDM) and complex mode matching method (CMMM), furnished with perfect matching layer (PML) for both guided and radiation modes. Based on the developed techniques, solutions of design of Bragg gratings with deep corrugations are presented; also various surface plasmon polariton (SPPS) grating structures are investigated. </p> / Thesis / Master of Applied Science (MASc)

The higher-order modes (HOMs) of an optical fiber has been demonstrated as a new dimension to transmitting signals with the development of mode-division multiplexing (MDM) technique. This dissertation aims to explore the HOMs as an extra degree of freedom for device innovation. In particular, with femtosecond (FS) laser point-by-point (PbP) inscription technique which opens up a unique possibility to explore the HOMs for device innovation, we design, fabricate, and characterize novel-structured fiber Bragg gratings (FBGs) written in the step-index two-mode fibers. We also develop a numerical model for the PbP gratings which has the potential for inverse design problem. Chapter 2 begins with a general framework of MDM with adaptive wavefront shaping in few-mode fibers (FMFs) and multimode fibers (MMFs), followed by two examples in slightly more detail. The fabrication setup and an short overview of the FS laser system will also be covered. In Chapter 3, we show the design, fabrication, and characterization of off-axis Bragg gratings in a step-index two-mode fiber (TMF). Through measuring the transmission and reflection spectra along with the associated reflected mode intensity profiles under different input polarization, we experimentally investigate the off-axis TM-FBGs (FBGs in a TMF) with multiple characteristics reported for the first time to our best knowledge. To highlight, we report the laser-induced birefringence exhibits strong offset dependence, the reflectivity heavily depends on the offset and polarization, and particularly the mode pattern can be controlled solely through polarization. The design and characterization of cross-axis TM-FBGs are presented in Chapter 4. Specifically, these gratings show six primary reflection peaks, which are identified through mode-decomposition based on the intensity profiles through nonlinear optimization problem. We also show in this chapter the development of a numerical model for the general PbP gratings, implementation of this model into standard coupled-wave analysis shows reasonable agreement to the experimental findings. In Chapter 5, discussions and suggestions for future studies are given. / Doctor of Philosophy / The higher-order modes (HOMs) of an optical fiber has been demonstrated as a new space for signal transmission, in the ``mode space'' one can use the modes as distinct multiplexing channel and therefore increase the data capacity of a single fiber. This work aims to explore if the the higher-order modes can also add some extra degree of freedom for device innovation. In particular, we use femtosecond (FS) laser point-by-point (PbP) technique for device fabrication, since the structural change induced by this fabrication methods is highly localized, typically ranging from a few hundred nanometers to a few micrometers. Hence this particular fabrication technique offers a unique possibility of exploiting the HOMs for device innovation. In this work, we fabricate, and characterize fiber Bragg gratings (FBGs) with novel structural designs written within the step-index two-mode fibers, with multiple characteristics reported for the first time as far as we know. We also develop a numerical model for the PbP gratings which has the potential for inverse design problem.

Structural health monitoring (SHM) has become a strong national interest because of the need of reliable and accurate damage detection methods for aerospace, civil and mechanical engineering infrastructure. Health monitoring of these structures usually requires the sensors to have such features as large area coverage, maintenance free or minimum maintenance, ultra-low cost per measurement point, and capability of operation in harsh environments. Fiber Bragg grating (FBG) has attracted considerable interest for this application because of its compactness, electromagnetic immunity, and excellent multiplexing capability. Several FBG multiplexing techniques have been developed to increase the multiplexing number and further reduce the unit cost. To the author's best knowledge, the current demonstrated maximum multiplexing number are 800 FBG sensors in a single array using optical frequency domain reflectometry (OFDR), whose maximum fiber span is limited by the coherence length of light source. In this work, we proposed and demonstrated a wavelength-scanning time-division multiplexing (WSTDM) of 1000 ultra-weak FBGs for distributed temperature sensing. In comparison with the OFDR method, the WSTDM method distinguishes the sensors by different time delays, and its maximum operation distance, which is limited by the transmission loss of the fiber, can be as high as tens of kilometers. The strong multiplexing capability and low crosstalk of the ultra-weak FBG sensors was investigated through both theoretical analysis and experiment. An automated FBG fabrication system was developed for fast FBG fabrication. With this WSTDM method, we multiplexed 1000 ultra-weak FBGs for distributed temperature sensing. Besides the demonstrated temperature measurement, the reported method can also be applied to measure other parameters, such as strain, pressure. / Ph. D.

Essa dissertação de mestrado é dedicada ao estudo, desenvolvimento e caracterização da resposta dinâmica de transdutores de deformação mecânica e de temperatura baseados em grades de Bragg inscritas em fibras ópticas (FBG). Como não se encontram referências diretas na literatura da área sobre a caracterização da resposta dinâmica de sensores de temperatura e deformação a FBG, percebeu-se a importância de realizar um trabalho com ênfase nesse aspecto. O desenvolvimento de transdutores FBG de deformação e de temperatura com resposta dinâmica rápida é de grande importância, já que as grades de Bragg são afetadas, de maneira intrínseca, simultaneamente pelas duas grandezas e que existe um amplo leque de aplicações em que uma delas, ou ambas, variam rapidamente. Nesses casos, diferenças em termos de tempos de resposta entre os sensores utilizados para medição de deformação e os usados para compensação de temperatura podem acarretar em erros significativos nas medidas, o que pode até inviabilizar o uso dos sensores a grades de Bragg em tais aplicações. Este trabalho apresenta um compêndio da teoria da tecnologia das FBG para aplicação em sensores ópticos de deformação e de temperatura. Buscando contribuir para o domínio das técnicas de fabricação de sensores a FBG no Brasil, também são apresentados os resultados experimentais da utilização de grades de Bragg inscritas em fibras ópticas fabricadas no laboratório da Subdivisão de Sensores (EFO-S), da Divisão de Fotônica do Instituto de Estudos Avançados (IEAv) do Comando-Geral de Tecnologia Aeroespacial (CTA), como elementos sensores em ensaios de deformação e de temperatura. Os resultados de caracterizações, tanto de FBG produzidas no IEAv quanto de sensores de deformação e de temperatura comerciais das empresas Micron Optics e Fiber Sensing, foram obtidos por meio de três interrogadores diferentes e são comparados e apresentados neste trabalho. Em uma análise realizada dos resultados apresentados de sensores a FBG, utilizados para medição de temperatura e de deformação mecânica em alguns arranjos experimentais, foram observados comportamentos dinâmicos não lineares que apresentam-se como um obstáculos a serem vencidos para a viabilização da aplicação desta técnica à medição desses parâmetros quando os mesmos são rapidamente variáveis. / This dissertation is devoted to the study, development and characterization of the dynamic response of mechanical strain and temperature transducers based on Bragg gratings inscribed in optical fibers (FBG). Since there are no direct references in the literature on the characterization of the dynamic response of FBG based temperature and strain sensors, it was realized the importance of conducting a study with emphasis on this aspect. The development of FBG strain and temperature transducers with fast dynamic response is of great importance, since the Bragg gratings are intrinsically and simultaneously affected by the two quantities and there is a wide range of applications in which one or both quantities vary rapidly. In such cases, differences in response times between the sensors used for measurement of strain and used for temperature compensation can result in significant errors, which may even preclude the use of FBG sensors in such applications. This work presents a compendium of the theory related to FBG technology for application in optical strain and temperature sensors. Seeking to master the techniques of manufacture of the FBG sensors in Brazil, experimental results from the use of FBG manufactured in the laboratory of Sensors Subdivision (EFO-S), of Photonics Division of Institute of Advanced Studies (IEAv) of General Command for Aerospace Technology (CTA) as sensor elements in strain and temperature trials are also presented. The results of both characterizations, of FBG produced in IEAv and strain and temperature commercial sensors, produced by Micron Optics and Fiber Sensing manufacturers, were obtained from three different interrogators and are compared and presented in this paper. In an analysis of the results of FBG sensors used for measuring temperature and mechanical deformation in some experimental arrangements, nonlinear dynamic behaviors were observed. Such behaviors present themselves as an obstacle to be overcome to achieve the feasibility of applying this technique to measurement of these parameters when they are rapidly changing.

Essa dissertação de mestrado é dedicada ao estudo, desenvolvimento e caracterização da resposta dinâmica de transdutores de deformação mecânica e de temperatura baseados em grades de Bragg inscritas em fibras ópticas (FBG). Como não se encontram referências diretas na literatura da área sobre a caracterização da resposta dinâmica de sensores de temperatura e deformação a FBG, percebeu-se a importância de realizar um trabalho com ênfase nesse aspecto. O desenvolvimento de transdutores FBG de deformação e de temperatura com resposta dinâmica rápida é de grande importância, já que as grades de Bragg são afetadas, de maneira intrínseca, simultaneamente pelas duas grandezas e que existe um amplo leque de aplicações em que uma delas, ou ambas, variam rapidamente. Nesses casos, diferenças em termos de tempos de resposta entre os sensores utilizados para medição de deformação e os usados para compensação de temperatura podem acarretar em erros significativos nas medidas, o que pode até inviabilizar o uso dos sensores a grades de Bragg em tais aplicações. Este trabalho apresenta um compêndio da teoria da tecnologia das FBG para aplicação em sensores ópticos de deformação e de temperatura. Buscando contribuir para o domínio das técnicas de fabricação de sensores a FBG no Brasil, também são apresentados os resultados experimentais da utilização de grades de Bragg inscritas em fibras ópticas fabricadas no laboratório da Subdivisão de Sensores (EFO-S), da Divisão de Fotônica do Instituto de Estudos Avançados (IEAv) do Comando-Geral de Tecnologia Aeroespacial (CTA), como elementos sensores em ensaios de deformação e de temperatura. Os resultados de caracterizações, tanto de FBG produzidas no IEAv quanto de sensores de deformação e de temperatura comerciais das empresas Micron Optics e Fiber Sensing, foram obtidos por meio de três interrogadores diferentes e são comparados e apresentados neste trabalho. Em uma análise realizada dos resultados apresentados de sensores a FBG, utilizados para medição de temperatura e de deformação mecânica em alguns arranjos experimentais, foram observados comportamentos dinâmicos não lineares que apresentam-se como um obstáculos a serem vencidos para a viabilização da aplicação desta técnica à medição desses parâmetros quando os mesmos são rapidamente variáveis. / This dissertation is devoted to the study, development and characterization of the dynamic response of mechanical strain and temperature transducers based on Bragg gratings inscribed in optical fibers (FBG). Since there are no direct references in the literature on the characterization of the dynamic response of FBG based temperature and strain sensors, it was realized the importance of conducting a study with emphasis on this aspect. The development of FBG strain and temperature transducers with fast dynamic response is of great importance, since the Bragg gratings are intrinsically and simultaneously affected by the two quantities and there is a wide range of applications in which one or both quantities vary rapidly. In such cases, differences in response times between the sensors used for measurement of strain and used for temperature compensation can result in significant errors, which may even preclude the use of FBG sensors in such applications. This work presents a compendium of the theory related to FBG technology for application in optical strain and temperature sensors. Seeking to master the techniques of manufacture of the FBG sensors in Brazil, experimental results from the use of FBG manufactured in the laboratory of Sensors Subdivision (EFO-S), of Photonics Division of Institute of Advanced Studies (IEAv) of General Command for Aerospace Technology (CTA) as sensor elements in strain and temperature trials are also presented. The results of both characterizations, of FBG produced in IEAv and strain and temperature commercial sensors, produced by Micron Optics and Fiber Sensing manufacturers, were obtained from three different interrogators and are compared and presented in this paper. In an analysis of the results of FBG sensors used for measuring temperature and mechanical deformation in some experimental arrangements, nonlinear dynamic behaviors were observed. Such behaviors present themselves as an obstacle to be overcome to achieve the feasibility of applying this technique to measurement of these parameters when they are rapidly changing.

[pt] Nesse trabalho, descreve-se o desenvolvimento de uma calibração do sistema de interrogação dos espectros das redes de Bragg baseando-se em uma simulação numérica adotada de testes experimentais. O objetivo é calibrar a técnica de demodulação óptica utilizando dois filtros de transmissão fixos, procurando-se principalmente a posição verdadeira dos sensores. O espectro refletido de uma rede tem uma forte dependência com o espectro da luz incidente que resultará em uma deformação no espectro refletido da rede, gerando um desvio no sensor detectado. A metodologia é gerar uma perturbação na fonte de luz e analisar a leitura do espectro da rede. Esse desvio gerado na leitura espectral é associado à potência óptica lida nos fotodetectores com a posição do comprimento de onda do sensor; as leituras indicadas são proporcionais ao grau de superposição entre os espectros do sensor e dos filtros. A calibração é enfatizada em simulações comparando-se com resultados experimentais e será recuperado principalmente o espectro deformado da rede por um espectro indicado que fornecerá a informação correta da posição do sensor. A simulação conclui em uma recuperação do espectro deformado, diminuindo incertezas de medição e da posição do sensor comparando-se com medidas experimentais proporcionando bons resultados. / [en] The development of a calibration system for Fiber Bragg Grating sensors based on two fixed spectral filters has been described basing in adopted numeric simulation of experimental tests. The objective is to gage the technique of optical demodulação using two fixed transmission filters, being sought mainly the true position spectral of sensors. The reflected spectrum of a Bragg Grating has a strong dependence with the spectrum of the incident light that will result in a deformation to the reflected spectrum, generating a deviation in the sensor detected. The methodology will be to generate a disturbance in the light source and to analyze the reading of the spectrum Bragg Grating. That deviation in the position is associated to the potency optical work in the photodetectors with the position of wavelength sensor; the suitable readings in the photodetectors are proportional to the overlap degree among the spectra of the sensor and filters. The calibration will be emphasized in simulations being compared with experimental results, and to recover mainly the deformed spectrum of Bragg Grating sensor for a suitable spectrum that will give the correct information at the position of sensor. The simulation ends in a recovery of the deformed spectrum, reducing measurement uncertainties and of the position of sensor, being compared with experimental results.