Structured material combined HMO-silica fibers: preparation, optical and mechanical behavior

We report about preparation technique and characterization of structured fibers composed of HMO core glasses and silica cladding. Two processes as material preparation techniques have been developed based on glasses prepared by melting of SAL (e.g. 70SiO2-20Al2O3-10La2O3) glasses and the reactive powder sintering (REPUSIL) method. The melted glasses have been characterized by dilatometrical methods to find Tg values of 827-875°C and expansion coefficients between 4.3 and 7.0×10-6 K-1. The latter is one order of magnitude higher than the expansion coefficient of pure silica glass. Structured fibers (SAL core, silica cladding) were fabricated following the Rod-in-Tube (RIT) and Granulate-in-Tube (GIT) process. The HMO glasses were chosen due du their high lanthanum content and the expected high nonlinearity, suitable for nonlinear applications (e.g. supercontinuum sources). The partial substitution of lanthanum by other rare earth elements (e.g. Ytterbium) allows the preparation of fibers with extremely high rare earth concentration up to 5 mol% Yb2O3. The concentration of alumina in the HMO glasses as "solubilizer" for lanthanide was adjusted to about 20 mol%. So we overcame the concentration limits of rare earth doping of MCVD (maximum ca. 2 mol% RE2O3). Nevertheless, the investigated HMO glasses show their limits by integration in structured silica based fibers: Optical losses are typically in the dB/m range, best value of this work is about 600 dB/km. The mechanical stability of fibers is influenced by mechanical strain caused by the high thermal expansion of the core material and the lower network bonding stability of the HMO glasses, but partially compensated by the silica cladding.