The active core diameter in silica preforms can be significantly increased by the deposition of ytterbium (Yb) and the most important codopant aluminum (Al) in the gas phase through the high-temperature evaporation of an Yb chelate compound and Al chloride in the Modified Chemical Vapor Deposition (MCVD) process. Here, we report on systematic investigations of the incorporation of Yb and Al into silica by gas phase doping technique. Preforms and fibers were prepared in a wide range of Yb and Al concentrations. The samples were characterized concerning the radial distribution of the refractive index and dopant concentrations, the efficiency of the deposition, and the absorption and emission properties in the NIR region. First laser experiments have demonstrated a slope efficiency of 80%, which is comparable to fibers made by MCVD/ solution doping and powder sinter technology.
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.