Fiber Raman amplification using the transmission line is a promising technology to increase the repeater distance as well as the capacity of the communication systems. Because of the growing importance of fiber Raman amplification, it is desired to predict the magnitude and shape of the Raman gain spectrum from the doping level and refractive index profiles of different fiber designs. This thesis develops a method to predict the Raman gain coefficients and spectra for a pure silica core fiber and two different types of GeO2-doped silica fibers given their index profiles. An essential feature of the model is the inclusion of the variation in Raman gain coefficient over the mode field due to the variation in the Ge concentration across the fiber core. The calculated Raman gain coefficients were compared with measurements of the peak Raman gain on a step-index GeO2-doped fiber and with published measurements from various sources. Agreement between the calculated and measured peak gain for the step-index fiber was excellent. There was qualitative agreement with published measurements but there were significant differences between the calculated and published gain coefficients, which are not understood. Part of the work sought a way of predicting Raman gain coefficients from a standard gain curve given only the fiber type and the effective area. This approach appears promising for moderately-doped fibers with the proper choice of effective area. / Master of Science

The recent increased availability of additional wavelengths in the telecommunications window of about 1300-1600 nm has generated an interest in new optical materials and devices that can operate outside the normally used regions of 840 nm, 1310 nm, and 1550 nm. Specifically, methods to amplify fiber optical data transmission in the regions where there is limited or no existing methods to achieve amplification is of interest in the chemistry and photonic communities. Raman gain is one method that has been proposed to passively amplify optical data transmission through a distributed process. Amplification is obtained through a nonlinear light scattering process where an optical wave is amplified at the expense of a higher frequency pump wave. Multiple wavelengths can be evenly amplified simultaneously in a desired region by specific selection of one or more pump wavelengths. Herein, the synthesis and characterization of new hybrid inorganic/organic sol-gels and monomers capable of producing broad wavelength Raman scattering over a spectral range of 1200-1670 nm are presented. The synthetic methodology developed facilitates the systematic approach to produce sol-gel derivatives with functional groups known to be strongly Raman scattering. Additionally, a method to synthesize and characterize a large number of different compounds using a combinatorial approach was demonstrated. Thio based derivatives of sulfonyldiphenol, isopropylidenediphenol, and triallyloxy triazine were synthesized in addition to thio derivatives of poly(hydroxystyrene). Micro-Raman spectra of the hybrid sol-gels, thiobased derivatives, and IR spectra of select sol-gel monomers were obtained.

Interest in Raman amplification has undergone a revival due to the rapidly increasing bandwidth requirements for communications transmission, both for long haul and local area networks, and recent developments in the telecom fiber industry and diode laser technology. In contrast to rare earth doped fiber amplifiers, for which the range of wavelengths is fixed and limited, Raman gain bandwidths are larger and the operating wavelength is fixed only by the pump wavelength and the bandwidth of the Raman active medium. In this context, glasses are the material of choice for this application due to their relatively broad spectral response, and ability of making them into optical fiber. This dissertation summarizes findings on different oxide-based glasses that have been synthesized and characterized for their potential application as Raman gain media. Two main glass families were investigated: phosphate-based glass matrices for broadband Raman gain application and TeO2-based glasses for high Raman gain amplification. A phosphate network was preferred for the broadband application since the phosphate Raman active modes can provide amplification above 1000 cm-1, whilst TeO2-based glasses were selected for the high gain application due to their enhanced nonlinearities and polarizabilities among the other oxide-based network formers. The results summarized in this dissertation show that phosphate-based glasses can provide Raman amplification bandwidths of up to 40 THz, an improvement of almost 5 times the bandwidth of SiO2. On the other hand, tellurite-based glasses appear to be promising candidates for high gain discrete Raman applications, providing peak Raman gain coefficients of up to 50 times higher than SiO2, at 1064 nm. Although, visible spontaneous Raman scattering cross-section measurement is the most frequently used tool for estimating the strength and spectral distribution of Raman gain in materials, especially glasses, there are some issues that one needs to be aware when conducting these measurements near the absorption band edge of the material. This led to the detection of an inherent frequency-dispersion in the Raman susceptibility and a resonant enhancement phenomenon when measurements were conducted near the absorption edge of the material.

The need for more bandwidth in communications has stimulated the search for new fiberizable materials with properties superior to fused silica which is the current state-of-the-art. One of the key properties is Raman gain by which a pump beam amplifies a signal beam of longer wavelength. An apparatus capable of directly measuring the spectral dependence and absolute magnitude of the material Raman gain coefficient using nonlinear optics techniques has been built. Using radiation from a 1064 nm Nd:YAG laser as the pump and from a tunable Optical Parametric Generator and Amplifier as the signal, the Raman gain spectrum was measured for different families of glass samples with millimeter thickness. A number of glass families were investigated. Tellurites with added oxides of tungsten, niobium, and thallium produced the largest Raman gain coefficients of any oxide family reported to date, typically 30-50 times higher than that of fused silica. On the other hand, phosphate families were found with spectrally broad Raman gain response, 5 times broader than fused silica and flat to [plus or minus] dB over the full spectral range in some compositions. Although the chalcogenides were found to photodamage easily, coefficients 50 - 80 times that of fused silica were measured. Finally, a numerical study was undertaken to predict the theoretical performance and noise properties of tellurite fibers for communications. Included in the computer modeling were linear loss; the interaction among multiple pumps and signals; forward and/or backward propagating pump beams; forward, backward and double Rayleigh scattering; noise properties of amplifiers; excess noise, etc. This led to a comparison of the optical signal-to-noise characteristics for Raman gain in a tellurite versus a silica fiber.

Novel technologies capable of generating wavelengths not accessible with typical laser gain media have been among the primary drivers of the field of nonlinear optics. Here, we are interested in the linear and nonlinear properties of liquids beyond the visible spectrum, motivated in part by their use as core materials in optical fibers. Given their dispersion, nonlinearities, transparency, and ability to be mixed, liquids show potential for exploiting in-fiber nonlinear phenomena for developing the new generation of low cost, size, weight, and power wavelength-agile fiber-laser sources. For the design, modeling, and experimental realization of these liquid-core fiber laser sources, proper knowledge of dispersion and Raman gain coefficients is necessary. However, the data for the liquids in the near-IR spectrum are sparse, with most reported values being in the visible and only for commonly used solvents. In this thesis, we report a Rayleigh interferometry-based refractometer to characterize the refractive index of 26 solvents relative to standard materials at seven different wavelengths (543.5, 632.8, 780, 973, 1064, 1550, and 1970 nm) at a temperature of ~ 21.3±0.6 °C. The corresponding Sellmeier equations fitted to our data for each liquid are given and compared with previously published literature; percent transmittance data for each liquid are also provided. Furthermore, we use a well-known technique for obtaining the relative total differential Raman cross-section of eight selected solvents at 532 nm. By measuring and analyzing the solvents' spontaneous Raman emission, we obtain their depolarization ratios, linewidth, and calculate their Raman gain coefficients. With knowledge of the electronic resonance and frequency dependence of the total differential cross-section, extrapolations were used to provide values for the total differential cross-section and gain coefficient at 1064 nm.

Dans cette thèse, nous nous intéressons aux propriétés des fluctuations de la lumière diffusée par un nuage d'atomes froids, que ce soit les variations temporelles de l'intensité ou les fluctuations spectrales du champ électrique dans le régime de diffusion simple ou multiple de la lumière. Bien que notre analyse soit réalisée sur un système passif, l'ajout de gain dans le système peut conduire à l'obtention d'un laser aléatoire dont l'étude des corrélations temporelles de l'intensité émise peut permettre une étude détaillée de ses propriétés de cohérence.La première étape de cette caractérisation est l'étude du bruit de fréquence de lasers conventionnels. La mesure est réalisée grâce à un discriminateur de fréquence, pouvant être une cavité Fabry-Pérot ou une transition atomique, utilisé pour convertir le bruit de fréquence en bruit d'intensité mesuré. Un modèle simple est présenté montrant que, alors que les résultats obtenus pour la cavité ou la transition atomique soient identiques à faibles fréquences de Fourier, de nouvelles structures apparaissent à hautes fréquences, permettant de réaliser de la spectroscopie de bruit en analysant les fluctuations de la lumière transmise.Les propriétés de cohérence peuvent aussi être étudiées grâce à la fonction de corrélation g(2) de l'intensité, offrant un accès à la statistique des photons de la lumière émise. Nous mesurons cette fonction dans un milieu passif en expansion balistique en contrôlant finement le régime de diffusion de la lumière. Nous analysons en détails l'évolution du contraste, la perte de cohérence ainsi que le changement de forme de g(2) dans le régime de diffusion multiple. Ces résultats sont combinés à des études numériques et analytiques pour mettre en évidence le rôle de la diffusion multiple dans les changements de la fonction g(2). Cette mesure est la première réalisation expérimentale de spectroscopie des ondes diffuses sur un nuage d'atomes froids en mouvement balistique.La caractérisation de la cohérence temporelle d'un laser aléatoire passe par l'étude de la fonction g(2) sur un milieu actif sous le seuil d'émission. Nous implémentons alors un schéma de gain Raman hyperfin, combinant efficacement gain et diffusion. Nous présentons les premiers tests de la quantification du gain dans le nuage par spectroscopie pompe-sonde, montrant l'apparition d'une fenêtre de transparence électromagnétiquement induite. Enfin, par une méthode hérérodyne, nous sommes en mesure d'accéder au spectre optique de la lumière diffusée en présence de gain. / In this thesis, we are interested in studying the properties of the fluctuations of the light scattered by a cloud of cold atoms, namely temporal fluctuations of the intensity or spectral fluctuations of the electric field in the single or multiple scattering of light. Although our analysis is focused on a passive medium, gain can be added in the system leading to a random laser whose the study of the temporal correlations of the emitted intensity allows to better characterize its coherence properties.The first step towards this characterization is the study of the frequency noise power spectral density of conventional lasers. This measurement is made using a frequency discriminator, being a Fabry-Pérot cavity or an atomic transition, used to convert frequency noise into measurable intensity noise. A simple model is developed showing that, while results obtained with the Fabry-Perot cavity and the atomic transition are the same at low Fourier-frequency, new features appear at high Fourier-frequency showing the influence of the atoms in the noise conversion, allowing to perform spectroscopic measurements by analyzing the intensity fluctuations of the transmitted light.Coherence properties can also be studied with the correlation function g(2) of the intensity, giving access to the photon statistics of the emitted light. We measure this function in a passive medium ballistically expanding while controlling the regime of scattering of light. We analyze in detail the evolution of contrast, the loss of coherence and the change of shape in the multiple scattering regime. Those results are combined with numerical and analytical studies showing the role of multiple scattering in the changes of the g(2)-function. This measurement is the first experimental demonstration of diffusing wave spectroscopy on cold atoms in ballistic motion.The characterization of the temporal coherence of a random laser requires the study of the g(2)-function in an active medium below threshold. We implement a scheme based on hyperfine Raman gain, combining effectively gain and scattering. We present our first results to quantify the amount of gain in the cloud with pump-probe spectroscopy, showing the appearance of an electromagnetically induced transparency window. Finally, based on a heterodyne method, we are able to access the optical spectrum of the scattered light in presence of gain.