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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.

Distributed fibre optic sensing (DFOS) is essential for structural health monitoring (SHM) of strain changes induced during the lifetime of a structure. Among different DFOS systems, the Brillouin Optical Time Domain Reflectometry (BOTDR) takes the advantages of obtaining full frequency spectrum to provide strain and temperature information along the optic fibre. The key parameters of distributed fibre optic sensors, spatial and frequency resolution, are strongly linked with the time-frequency (T-F) localisation in the system in three parts: pulse, hardware design and optical fibre. T-F localization is fundamentally important for the communication system, whereas in this study the importance of the T-F localisation to the spatial and frequency resolution, repeatability and the measurement speed are introduced in BOTDR. In this dissertation, the development of DFOS is first introduced, including both traditional methods and new developed designs. The literature review shows the signal to noise ratio (SNR) of BOTDR can be improved by investigating its T-F localisation. In the hardware design, in order to improve the T-F localisation in hardware architecture, a Short-Time Fourier Transform-Brillouin Optical Time-Domain Reflectometry (STFT-BOTDR), which implements STFT over the full frequency spectrum to measure the distributed temperature and strain along the optic fibre, is applied so that the conventional frequency sweeping method can be replaced for high resolution and fast speed measurement, providing new research advances in dynamic distributed sensing. The STFT based BOTDR has better T-F localisation, which in turn provides an opportunity for off-line post signal processing that is more adaptable for fast speed measurements. The spatial and frequency resolution of dynamic BOTDR sensing is limited by the Signal to Noise Ratio (SNR) and the T-F localization of the input pulse shape. The T-F localized input pulse shape can enhance the SNR and the spatial and frequency resolution in STFT-BOTDR. In this study, simulation and experiments of T-F localized different pulses shapes are conducted to examine the limitation of the system resolution. The result indicates that a rectangular pulse should be selected to optimize the spatial resolution and a Lorentzian pulse could be chosen to optimize the frequency resolution, while a Gaussian shape pulse can be used in general applications for its balanced performance in both spatial and frequency resolution. Meanwhile, T-F localization is used for pulse T-F localisation optimisation. A set of Kaiser-Bessel functions is used to simulate different pulse shapes and to compare their parameters in terms of T-F localisation and their Brillouin scattering spectrum. A method using an iterative filtering algorithm to achieve the optimised pulse in terms of T-F localisation is introduced to converge the Effective-pulse Width (TEW) in the time-domain and Effective-pulse Linewidth (FEL) in the frequency domain to identify the fundamental limitations. The optimised pulse can be fitted with a 7th order Gaussian (super-Gaussian) shape and it offers the best experimental performance compared to a Rectangular pulse. The sensitivity of a sensor to strain or temperature variations due to distributed Brillouin scattering is closely related to the power distribution on the Brillouin scattering spectrum which is related to the property of the optic fibre. The performance of a highly nonlinear fibre that can generate a higher Brillouin scattering signal is compared to that of a standard single mode fibre. The results show that much higher SNR of the Brillouin scattering spectrum and smaller frequency uncertainties in the sensing measurement can be achieved by using a highly nonlinear fibre for comparable launched powers. With a measurement speed of 4 Hz, the frequency uncertainty can be 0.43 MHz, corresponding to 10 με in strain or 0.43°C in temperature uncertainty for the tested highly nonlinear fibre. In contrast, for a standard single mode fibre, the value would increase to about 1.02 MHz (25 με or 1.02°C), demonstrating the advantage of the tested highly nonlinear fibre for distributed strain/temperature sensing. Results show that, by using a small effective area highly nonlinear fibre, the strain or temperature resolution can be improved because it generates stronger Brillouin scattering signal with high SNR and high Q factor spectrum, both of which determine the optimal averaging time in a single measurement. In general, the STFT-BOTDR can achieve 1 m spatial resolution, 10 με frequency resolution on a 10 km fibre with measurement speed at about 2.5 kHz.

Cette thèse présente des travaux portant sur l'instabilité modulationnelle (IM) dans des fibres optiques dotées à la fois d'une forte non linéarité et d'un fort coefficient d'absorption. Une analyse comparative des performances de plusieurs grandes classes de fibres (de verres silice et non silice) sur leur aptitude à générer des bandes latérales d'IM avec un minimum de puissance de pompe, et sur la plus courte distance possible, est effectuée. Les fibres de verres non silice de type Chalcogénure ou Tellure apparaissent à première vue comme étant les plus performantes, mais un examen attentif révèle que leurs spectres sont altérés par un phénomène de dérive en fréquence des bandes latérales d'IM, qui est provoquée par une forte absorption du laser de pompe. Nous développons alors une méthode qui permet de supprimer la dérive en fréquence des bandes latérales d'IM dans les fibres à fort coefficient d'absorption. Cette méthode, que nous avons baptisé "méthode du réservoir de photons", consiste à créer au sein de la fibre, par un ajustement approprié des paramètres de dispersion d'ordre deux et quatre, un réservoir de photons qui alimente in situ le processus d'IM en lui fournissant continuellement l'équivalent de la quantité de photons détruits par l'absorption matérielle au cours de la propagation. L'efficacité de la méthode du réservoir de photons est démontrée sur des processus d'IM aussi bien en configuration scalaire que vectorielle. Cette démonstration marque un progrès décisif vers des dispositifs de génération de fréquences optiques qui seront extrêmement précis et stables. / This thesis considers modulational instability (MI) in optical fibers that have both a strong non-linearity and strong absorption coefficient. We carry out a comparative analysis of the performance of several major classes of silica- and non-silica glass fibers, on their ability to generate MI sidebands with a minimum of pump power, and over the shortest distance possible. Chalcogenide glass fibers appear at first sight as being the most competitive, but a careful examination reveals that their spectra are altered by a phenomenon of frequency drift of the MI sidebands, caused by a strong depletion of the pump. We have then developed a method which allows to suppress frequency drifts in MI processes in fibers having strong absorption parameters. The fundamental idea of this method, that we called "the photon reservoir method", is to create in the fiber, by an appropriate adjustment of the second-order and fourthorder dispersion coefficients, a photon reservoir which supplies (in situ) the MI process by continually providing the equivalent of the amount of photons destroyed by absorption during the propagation. We have demonstrated the effectiveness of the method of photon reservoir on MI processes, in scalarand vector configurations, using glass fibers endowed with extremely high nonlinear parameters. This method constitutes a decisive step forward in the development of highly competitive devices for optical frequency generation.