Thesis (MSc (Physics))--University of Stellenbosch, 2009. / In the presented study a diode laser was characterised and used for spec- troscopy, measuring the resonance lines of atomic rubidium. The characteristics of diode lasers and external cavity diode lasers (ECDL) for the purposes of ab- sorption spectroscopy were investigated and an experimental setup for tunable diode laser spectroscopy using an ECDL was developed. In external cavity diode lasers, the advantages of low cost, small size and e ciency of a diode laser is combined with tunability and a narrow frequency bandwidth. The ECDL was applied in experimental setups for absorption spectroscopy and saturated ab- sorption spectroscopy. Measurement of the absorption of atomic rubidium's D2 line near 780 nm is discussed. The Doppler broadened, as well as the Doppler free spectrum of the ne and hyper ne structure of the D2 line were measured and is discussed. Finer control of the ECDL's stability and frequency, using a servo circuit, were investigated and tested. An overview is given of laser cool- ing and trapping of neutral rubidium atoms, which is the main application the ECDL were developed for.

Nesta dissertação, propomos um método que combina a técnica de varredura-Z e a luz branca supercontínua gerada em uma fibra fotônica para caracterizar o espectro de não-linearidades de amostras. O espectro não-linear fornece informações cruciais a respeito da característica, da origem e do potencial da não-linearidade para aplicação em dispositivos. Neste sentido, a luz branca supercontínua aliada à técnica de varredura-Z vem permitir uma rápida determinação de espectros numa banda larga. Neste trabalho, a luz branca foi gerada numa fibra fotônica comercial bombeada por pulsos de femtossegundo de um oscilador laser de Ti:safira. Tanto os lasers de femtosegundo quanto as fibras fotônicas estão se tornando cada vez mais acessíveis. Ademais, a técnica de varredura-Z é um método bastante conhecido para a determinação de não-linearidades devido à sua simplicidade e precisão. Dessa forma, aliamos essas vantagens para fazer um aparato simples, compacto e de baixo custo. Em geral, a determinação do espectro de não-linearidades necessita de sistemas lasers complexos que possam sintonizar o comprimento de onda. O método de varredura-Z de luz branca desenvolvido aqui foi usado para medir o espectro não-linear do corante disperse red 13 (DR13) em metanol. Esse corante possui um efeito não-linear absortivo (absorção saturada) bem caracterizado, sendo um bom material não apenas para calibração, mas também para testar nosso sistema experimental. Os resultados experimentais comprovaram a viabilidade do sistema para caracterização de amostras com efeitos não-lineares absortivos intensos. / In this dissertation, we proposed a method that combines the Z-scan technique and the white-light continuum generated by a photonic crystal fiber to characterize the nonlinear spectrum of materials. The nonlinear spectrum provides crucial information on the characteristic, the origin and the potential of the nonlinearity for devices application. In this sense, the white-light continuum allied to the Z-scan technique allows fast spectra determination in a broad spectral range. In this work, the white light was generated in a commercial photonic crystal fiber pumped by femtosecond pulses from a Ti:sapphire laser oscillator. Both the femtosecond lasers and the photonic crystal fibers are becoming more accessible. In addition, the Z-scan technique is a well-known method for nonlinearities determination due to its simplicity and precision. In this way, we merged these advantages to make a simple, compact and low-cost apparatus for studying nonlinear samples. In general, the nonlinear spectrum determination requires complex laser systems which are able to tune the wavelength. The white-light Z-scan method we developed here was used to measure the nonlinear spectrum of disperse red 13 (DR13) dye in methanol. This dye presents a well-characterized absorcive nonlinear effect (saturable absorption), being a good material not only to calibrate, but also to test our experimental setup. The experimental results confirmed the viability of the setup for characterizing samples with strong nonlinear absorption effect.

Nesta dissertação, propomos um método que combina a técnica de varredura-Z e a luz branca supercontínua gerada em uma fibra fotônica para caracterizar o espectro de não-linearidades de amostras. O espectro não-linear fornece informações cruciais a respeito da característica, da origem e do potencial da não-linearidade para aplicação em dispositivos. Neste sentido, a luz branca supercontínua aliada à técnica de varredura-Z vem permitir uma rápida determinação de espectros numa banda larga. Neste trabalho, a luz branca foi gerada numa fibra fotônica comercial bombeada por pulsos de femtossegundo de um oscilador laser de Ti:safira. Tanto os lasers de femtosegundo quanto as fibras fotônicas estão se tornando cada vez mais acessíveis. Ademais, a técnica de varredura-Z é um método bastante conhecido para a determinação de não-linearidades devido à sua simplicidade e precisão. Dessa forma, aliamos essas vantagens para fazer um aparato simples, compacto e de baixo custo. Em geral, a determinação do espectro de não-linearidades necessita de sistemas lasers complexos que possam sintonizar o comprimento de onda. O método de varredura-Z de luz branca desenvolvido aqui foi usado para medir o espectro não-linear do corante disperse red 13 (DR13) em metanol. Esse corante possui um efeito não-linear absortivo (absorção saturada) bem caracterizado, sendo um bom material não apenas para calibração, mas também para testar nosso sistema experimental. Os resultados experimentais comprovaram a viabilidade do sistema para caracterização de amostras com efeitos não-lineares absortivos intensos. / In this dissertation, we proposed a method that combines the Z-scan technique and the white-light continuum generated by a photonic crystal fiber to characterize the nonlinear spectrum of materials. The nonlinear spectrum provides crucial information on the characteristic, the origin and the potential of the nonlinearity for devices application. In this sense, the white-light continuum allied to the Z-scan technique allows fast spectra determination in a broad spectral range. In this work, the white light was generated in a commercial photonic crystal fiber pumped by femtosecond pulses from a Ti:sapphire laser oscillator. Both the femtosecond lasers and the photonic crystal fibers are becoming more accessible. In addition, the Z-scan technique is a well-known method for nonlinearities determination due to its simplicity and precision. In this way, we merged these advantages to make a simple, compact and low-cost apparatus for studying nonlinear samples. In general, the nonlinear spectrum determination requires complex laser systems which are able to tune the wavelength. The white-light Z-scan method we developed here was used to measure the nonlinear spectrum of disperse red 13 (DR13) dye in methanol. This dye presents a well-characterized absorcive nonlinear effect (saturable absorption), being a good material not only to calibrate, but also to test our experimental setup. The experimental results confirmed the viability of the setup for characterizing samples with strong nonlinear absorption effect.

The main aim of the thesis is a frequency stabilization of the DFB semiconductor laser diodes. The temperature stability of the laser diode chip, the stability and the noise of the injection current and the backward reflections are the crucial parameters which affects the frequency stability. These influences are described and the resolution is proposed. The theory of the external methods of the frequency stabilization and the comparison of these methods is presented. One method was choosed and this method was realized for 760 and 1540 nm wavelength laser diodes. In this method was used the frequency stabilization based on the linear absorption to the spectral lines of the gases. The diploma work is closed by the measured results of the frequency stability of the used laser diodes and by the comparison of level of stability achieved by the other methods.

Um dos primeiros requisitos para utilização de lasers de diodo em espectroscopia é sua estabilização térmica. Neste trabalho desenvolvemos um sistema de controle de temperatura para laser de diodo que permite estabilização melhor que 0.01&#176C. O controle é feito através de um sistema sensor e um elemento Peltier como atuador. Lasers somente estabilizados em temperatura (estabilização primária) foram utilizados para realização de espectroscopia de vapores atômicos de Rb e Cs. / One of the first requisites for utilization of diode lasers in spectroscopy is its thermal stabilization. In this work, we develop a diode laser temperature controlling system, which yields stabilization better than 0.01&#176C. The controlling is obtained by means of a sensor system and a Peltier element as actuator. Lasers stabilized in temperature only (primary stabilizations) were utilized in the spectroscopy of atomic vapors of Rb and Cs.

Optical frequency standards are necessary tools for accurate measurement of time and length. In practice these standards are stabilised laser systems locked to a known frequency reference. These references are typically the resonant frequencies of the atoms of an absorption medium that have been theoretically calculated to a high degree of accuracy. This thesis describes a combination of experimental and theoretical research performed on modulation transfer spectroscopy (MTS)--a technique used to frequency stabilise a laser in order to produce an accurate frequency reference--with emphasis placed on developing techniques and procedures to overcome the limitations found in existing MTS stabilised laser systems. The focus of the thesis is to generate a highly accurate frequency reference by researching the system parameters that will increase the signal to noise ratio and improve the accuracy of the reference through refinement of the signal structure. The early theoretical interpretation of MTS was effectively a low absorption approximation that occurs at low pressures. This approximation ignores the depletion of beam energy through absorption and is a distinct limitation of the theoretical model in its ability to accurately predict the influence of a range of system parameters on signal strength and structure. To overcome this limitation a 3-D (or volumetric) analysis was developed and is presented here for the first time. This volumetric model is a measure of two depleted beams interacting collinearly in an absorbing medium of iodine and is described to accurately predict the signal maximum as a function of pressure for all wavelengths. This model was found to be more accurate in predicting the influence of system parameters on the signal strength and structure, including that of pump beam intensity, pressure, saturation parameter, cell length and modulation parameters. The volumetric model is a novel approach to MTS theory but is more complex computationally than the traditional low pressure model and therefore more difficult to implement in many situations. To overcome this problem a hybrid model was developed as a combination of the low pressure and volumetric models. The comparison between the rigorous volume model and the hybrid model indicate that there is a deviation in the signal strength at high pressures. However, the agreement was very good in the pressure regimes that are commonly used to realise actual frequency references. Comparison of the hybrid model to experimental data was performed over a range of different wavelengths (532 nm, 543.5 nm, 612 nm and 633 nm) and found to be in close agreement. This gives confidence in the model to accurately predict signal strength and structure in any situation. Three mechanisms have been identified that limit the accuracy of frequency references due to the creation of residual amplitude modulation (RAM) where it shifts the frequency of the reference. The influence of RAM is included in the hybrid model as a ratio of the amplitude modulated and frequency modulated components of the saturating beam. These RAM production mechanisms result from the modulation of the saturating beam, the overlap of the beams in the medium, and the differential absorption of the sidebands in the medium. While the first mechanism has been previously reported the latter two are discussed here in detail for the first time. RAM generated by the modulators used (acousto-optic or electro-optic modulators) was typically of the order of 10% to 12%, depending on the excursion of the created sidebands. RAM generated by an asymmetric beam overlap with the modulators used was found to be as large as 30%. A combination of these two independent mechanisms can be used to provide a "RAM-free" state of the system by using one to cancel the effects of the other. The third RAM generation process--medium induced RAM--is difficult to remove but through a careful combination of absorption related parameters--namely, pump intensity, cell length, pressure and detector phase--the effects of RAM can be removed, leading to a distortion free MTS signal. Further investigation into the predictions provided by the hybrid model shows that there is a complex relationship between cell length and the optimum pressure required for maximum signal strength, such that longer cell lengths will not necessarily improve the signal strength. This is contrary to conventional thinking and is important in the MTS design process to reduce unnecessary costs and improve the signal to noise ratio and frequency accuracy. Optimisation of frequency stabilised laser systems using MTS are generally performed using trial and error. Comparison of these optimum parameter values to those predicted by the hybrid model show that for popular wavelengths such as 532 nm they are similar. In addition, the hybrid model is able to predict the frequency shifts that arise within the system parameters used and has shown that existing systems being used at 532 nm, 633 nm and 778 nm could improve their signal to noise ratio and accuracy through a variation in the parameters. A methodology based on the hybrid model is presented that can be used to calculate the optimum parameters for maximum signal strength and a "RAM-free" state for any wavelength. This systematic approach can therefore be used to guide the design of actual frequency stabilised laser systems prior to and during the design process.

Um dos primeiros requisitos para utilização de lasers de diodo em espectroscopia é sua estabilização térmica. Neste trabalho desenvolvemos um sistema de controle de temperatura para laser de diodo que permite estabilização melhor que 0.01&#176C. O controle é feito através de um sistema sensor e um elemento Peltier como atuador. Lasers somente estabilizados em temperatura (estabilização primária) foram utilizados para realização de espectroscopia de vapores atômicos de Rb e Cs. / One of the first requisites for utilization of diode lasers in spectroscopy is its thermal stabilization. In this work, we develop a diode laser temperature controlling system, which yields stabilization better than 0.01&#176C. The controlling is obtained by means of a sensor system and a Peltier element as actuator. Lasers stabilized in temperature only (primary stabilizations) were utilized in the spectroscopy of atomic vapors of Rb and Cs.

科学研究費補助金 研究種目:一般研究(B) 課題番号:05452196 研究代表者:後藤 俊夫 研究期間:1993-1994年度