Joining Forces: Doctoral Research at BUAS and University of Bern


Dereje Etissa, a PhD student from Ethiopia, is currently working on a research project aimed at minimizing fiber scattering losses. Parts of his doctoral thesis on the material improvement of fiber optics are carried out jointly by BUAS and the University of Bern.

Parts of his doctoral thesis on the material improvement of fiber optics are carried out jointly by BUAS and the University of Bern. Dereje Etissa received his Bachelor in physics from the University of Addis Ababa and after first years of work experience as a physicist in the Ministry of Education in Ethiopia he continued his studies at the Institute of Radiation Physics at the University of Stuttgart. There he received his Master of Science in Sensor Technology and in Physics. After the successful completion of his Master degrees in Stuttgart, Dereje Etissa took up a research position at EMPA in Switzerland, working in the field of exhaust gas soot particle research, before he was offered a PhD scholarship at the Institute of Applied Physics of the University of Bern.

Production of doped granualted optical fiber core material using CO2-laser at the Institute of Applied Physics, University of Bern. Photo: IAP Bern

Rare-earth activated optical fibers
Dereje Etissa is currently conducting his doctoral research in the framework of an ongoing project at the Institute of Applied Physics that is aimed at studying and improving the production of rare-earth activated microstructured optical fibers by the granulated silica method for laser applications. The results of his present research will form the basis for a broader project in the same field that will lead to fibers with large cores for fiber lasers and amplifiers. Rare-earth (e.g. neodymium, erbium or ytterbium) doped optical fibers are the active elements for fiber lasers and amplifiers; they are incorporated in their trivalent ionic form into the glass matrix of the fiber core where they act as active laser media. The core of a fiber is the region in which the light is guided, i.e. it is responsible for the waveguiding effect. The non-uniformities and material impurities of the core are responsible for the fiber losses through scattering and absorption. Two different methods are used for the production of the doped materials: i) granulated oxides of the different species are mixed, put into adequate silica tubes and directly drawn to fibers; ii) the sol-gel method is used to produce a porous glass that is already doped with the desired dopants, densified and then milled to a granulate. Method ii) leads to improvements in homeogeneity and hence to less scattering losses.

Refractive index profile of Yb+3, Al+3, P+5 doped optical fiber. Figure: IAP Bern

Minimizing fiber scattering losses
Dereje Etissa has chosen the sol-gel method because it is very attractive if one wants to draw fibers with complicated optical fiber geometries including multicore or microstructured fibers. In both methods, after the inclusion of dopants into a glass matrix, milling and melting is applied to obtain homogenous core material. This helps to avoid fluctuation of refractive index and scattering losses. Typically a fiber is then drawn by appropriately filling a glass tube with the previously produced granulated silica (doped silica for the core, undoped for the cladding and empty capillaries for the holes) and evacuating the preform while heating it in the drawing tower's furnace. The optical properties of the fabricated fiber will be characterized by different analytical techniques (energy dispersive x-ray, electron probe microanalyses, x-ray diffraction analysis and refractive index profilometry). In this work, particular emphasis is given to the minimization of fiber scattering losses. First findings of Dereje Etissa’s research have been presented at this year’s «Photonics Europe» conference in Brussels, Belgium. Dereje Etissa does not know yet where his research will take him, but he does know that his passion for physics will guide his way wherever he goes – in Switzerland, Europe, or, indeed, Ethiopia.