The work of this thesis is focused on two research lines: the first on the development of high power fibre lasers, the second on the development of bioresorbable, inorganic optical devices. The common aspect of these two lines is the use of phosphate glasses as a base material for the fabrication of specialty optical fibres. Phosphate glasses are extremely interesting materials in virtue of the unique combination of their properties. They have been widely exploited in laser science as active gain media, due to the high solubility of rare earths in the glass matrix, and to the high absorption/emission cross section. Calcium phosphate glasses, on the other hand, have been studied as promising biomaterials due to their solubility in aqueous media, and to the ability of being safely reabsorbed by the human body. The thesis starts with a literature review on the use of phosphate glasses in the fabrication of optical fibres and lasers. The properties of rare earth doped glasses are reviewed and a detailed description of the quenching phenomena in doped glasses reported and compared to the major results obtained in the literature. The study of the literature reveals how the issues of heat dissipation, thermal expansion and mechanical stability are still relevant problems in the field of high power lasers. These issues were studied in the course of the thesis. Results obtained on the development of a Nd3+- doped phosphate cane laser are reported in Chapter 5. Cane lasers have the same core/cladding structure that is typical of an optical fibre, but present a much larger diameter. This allows an increased mechanical stability of the device, combined with the easy cooling and good beam quality that are typical for a fibre laser. The development of a first prototype of a phosphate cane laser required the fabrication of a suitable glass (namely CL) that is featured by an exceptional matching of thermo-mechanical properties between core and cladding. A core glass composition (CL1:Nd) and a cladding glass composition (CL1) that present a difference in the glass transition temperature of only 8°C and identical coefficient of thermal expansion were fabricated ad-hoc for this scope. The materials were fully characterized and used for the fabrication of a cane with a diameter of 800μm. Power scaling experiments, performed on a 60mm-long section of the cane, show laser emission at 1054mm, with a maximum output power of 2.5W and a slope efficiency of 44% with respect to the absorbed power. Another issue that emerged from the literature and from the studies of fibre/cane lasers is the interest in developing new fibres with complex geometry. Chapter 4 of this thesis describes work carried on this topic, focusing on the critical step of fabricating and assembling a fibre preform. With the aim to develop rapidly and effectively optical fibre preforms with a wide range of geometries, a project for the in-house development of an extrusion facility in Politecnico di Torino was kick-started. A first prototype of the facility is available, and preliminary results on the extrusion of phosphate glasses are presented. The second part of the thesis is dedicated to the development of resorbable optical materials. An overview on the use of calcium phosphate glasses in the biomedical field is given, with particular interest in the use of glass fibres in biomedical applications. Subsequently, the results obtained on the use of resorbable glasses in biophotonics are reported. The idea at the basis of this research is to combine in a single device the two main field of application of phosphate glasses: the biomedical field and the optical one. This becomes particularly interesting as it enables fabricating multifunctional optical devices, which are of interest in optical sensing and photo-therapy. In particular, the bioresorbability minimizes the impact of the therapies, eliminating the need of removal surgery. Chapter 4 reports a detailed description of the design, fabrication and characterization of transparent calcium phosphate glasses. The materials show a window of transparency ranging from 240 to 2600nm, therefore are able to guide light in the near UV region, and the refractive index can be tailored according to the composition. The glasses proved to be stable against devitrification and suitable for fibre drawing. Single material fibres were fabricated and proved to be soluble in aqueous media, in simulated physiological conditions. Once the fabrication of the material is complete, resorbable glasses were used for the fabrication of single and multi-mode optical fibres. Step index fibres were fabricated using the rod in tube technique and the attenuation loss was measured by cut-back method. The fibres showed values of attenuation loss between 5 and 15dB/m in the visible region and from 2 to 5dB/m in the near infra-red. These values are from one to two orders of magnitude lower than those reported in literature for other resorbable optical devices. These results paved the way towards the application of such fibres for the inscription of fibre Bragg-gratings and for the use in time-domain diffuse optics experiments. Preliminary results on these topics are presented in Chapter 6. Finally, resorbable hollow fibres were fabricated by drawing a tube-shaped preform. These fibres were used for obtaining a controlled release of a photosensitive drug, that could be activated by the light guided trough the same fibre. Experiments on the controlled release of drugs are still ongoing, and involved the development of a silanization method for phosphate glasses, in order to increase the release time of drugs.
Advanced application of phosphate glass optical fibres in photonics and biophotonics / CECI GINISTRELLI, Edoardo. - (2018 Mar 16).
|Titolo:||Advanced application of phosphate glass optical fibres in photonics and biophotonics|
|Data di pubblicazione:||16-mar-2018|
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