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This paper reviews and extends the work done on photodarkening by our project consortium and present our latest results on bleaching and photodarkening mitigation in fiber lasers. We shows the need for a standard set-up to avoid underestimation of photodarkening equilibrium losses and we suggest photodarkening losses scale with the square of Yb doping level. Investigation on visible light emission suggest an interplay of visible light with the Yb excited level. Finally we present an extensive investigation of photobleaching, both as post-irradiation and as simultaneous bleaching. We show evidence photobleaching can effectively mitigate the impact of photodarkening on laser performance when highly-doped Al-silicate fibers are used.

In this study, detailed investigation of photodarkening in Yb - doped aluminosilicate fibers is reported. The cooperative luminescence loss influenced by photodarkening is measured simultaneously with 633 nm probe loss. The quadratic dependence of photodarkening and cooperative luminescence versus dopant concentration was observed. The results indicated lower cooperative luminescence loss at 513 nm than expected and provided new pieces of information to the photodarkening mechanism description.

A setup for core-pumping of Yb-doped optical fibers has been developed to induce photodarkening and benchmark their suitability as a long-life active medium in high power fiber lasers for industrial applications. The measurements setup, its reliability and preliminary tests on PD affected and PD free fibers are here presented. Repeatability of measurements has also been carried out.

Micro-channels were fabricated in sodalime glass through imprinting, and then joined to a glass slab by thermal assisted direct bonding (TADB). These techniques are simple and low cost, suitable for mass production of micro-fluidic devices. The joined samples were characterized before and after TADB by optical microscopy, profilometer, SEM, shear strength test and Vickers hardness test. The integrity of channels is maintained also after the TADB. The interface between the two glass slabs was found to be without impurities, bubbles and cracks. The bonding strength was also measured to be 31.94MPa.

TeO 2 -ZnF 2 -PbO-Nb 2 O 5 based fluorotellurite glasses were synthesized and studied for the first time for laser applications. The property characterizations including XRD and thermal analysis as well as optical properties measurement were performed. It is demonstrated that this fluorotellurite system has good glass formation ability; and increasing the ZnF 2 concentration to 30 mol% can significantly increase the thermal stability of the glass. Adding ZnF 2 also reduced remarkably the hydroxyl (OH) concentration of the glass resulting in lower optical absorption in the infrared region, which is crucial for infrared laser applications. In addition, the glass absorption cut-off edge near 400 nm blue-shifted with increasing ZnF 2 addition.

RF sputtering of an amorphous photosensitive target at different deposition temperature and time conditions was performed in order to obtain thin film planar waveguides for telecom applications. Silica glass was used as a substrate, the chosen target was for the first time a photosensitive SiO 2 :GeO 2 :Na 2 O glass and the process was carried out under Ar atmosphere. The so-obtained thin films were tested in terms of composition, microstructure and optical properties. A planar waveguide was obtained at room temperature, with a refractive index of 1.5707 at a measurement wavelength of 1553 nm, with respect to a refractive index value of the substrate of 1.45.

We report the application of the direct writing technique to tellurite glasses of three different compositions and the achievement of the first examples, to our knowledge, of directly written channel waveguides in tellurite glasses.

Er/Yb-doped multicomponent SiO 2 -GeO 2 -B 2 O 3 -Na 2 O glasses with different GeO 2 , B 2 O 3 and Na 2 O contents were studied. The effect of glass component on the properties of Er ions in terms of absorption cross-section, fluorescence lifetime τ m and quantum efficiency was investigated with the aim of obtaining efficient active waveguide.

New germanosilicate glasses with high photosensitivity were fabricated and studied. Addition of sodium produced an increase of 6-fold Ge ions and the appearance of a UV absorption band at 285 nm in the as cast glasses. Laser exposure produced bleaching of such band, when present. Further studies are needed to correlate the two phenomena and find out the best composition in terms of photosensitivity. A new tin sodium silicate glass was also fabricated: glass fibers were produced and high temperature stable gratings were obtained by irradiation with a UV laser.