The NCLas project concentrated mainly on the investigation, characterisation and provisioning of the matrix glass nanoparticles as well as the laser-active nanocrystals Ti3+:sapphire and Pr3+:yttria.We were able to observe the formation of YPO4 NCs from the initially present Y2O3 NCs and the phosphate matrix glass during sintering and glass melting experiments. We were following up on this encouraging result and focused on this material system. Firstly, we successfully demonstrated that this phase change from Y2O3 to YPO4 can also be achieved while drawing a fiber on the draw tower and keeping the laser active ion in the new crystalline phase. Interestingly, the unique Pr3+ luminescence was ascribed to Pr3+ into the YPO4 host lattice. Neither Pr3+ emission from Y2O3 nor from the glass was detected, indicating that all Y2O3 was turned into YPO4. The next important idea was start out with the more stable YPO4 NCs in order to avoid the phase changes and demonstrate the NC survival during the fiber drawing process. Ytterbium was chosen as alternative ion for future laser experiments.The new NCs required also the consideration of new matrix glasses to allow refractive index matching. The work package WP4 was again the most active and most collaborative effort during RP3. The temperature stability of the new actively doped YPO4 NCs together with selected matrix glasses were investigated in series of sintering experiments using an oven and a flux inductor. Subsequently, preforms were prepared with a powder mix of matrix glass particles and active NCs (ANCs) in a suitable cladding tube, i.e. using the glass powder doping process. The pre-treatment of the powder mix was investigated using different compaction processes of pressing and pre-sintering like cold and hot isostatic pressing (CIP & HIP) as well as advanced temperature cycles directly at the draw tower. This resulted in the successful demonstration of the survival of the NCs during a fiber drawing process using glass powder doping and it was shown using both above mentioned phosphate matrix glasses. We researched the fluorescence properties of the YPO4:Yb3+ NCs to better understand their spectra, lifetimes, and quenching mechanisms also with the final goal to determine the most suitable Yb3+ doping level in the NCs for highest laser gain. The fluorescence was also compared to the lifetimes in fibers with surviving NCs. Due to the improvements of optical fibres, we were able to estimate the background losses for the first time, which are now dominated by scattering losses. Unfortunately, they are approximately 20dB/cm and too high to demonstrate any net gain. Nevertheless, we conducted an amplification experiment to establish the relative gain coefficient (pumped vs. unpumped) and achieved a relatively high value of 1dB/cm. While this value is much smaller than the loss coefficient, it shows the potential if these high losses can be avoided.
Thus, future work would need to concentrate on the reduction of scattering loss by employing smaller NCs and improving the refractive index matching between the NCs and the matrix glass by synthesizing an adequate phosphate glass. While there is still much research to be conducted, we were able to demonstrate that NC-doped fibres using the glass powder doping is a valid technology to add new functionalities to optical fibres.
