Atomic layer deposition (ALD) was used to fabricate an ytterbium (Yb)-doped silica fiber in combination with the
conventional modified chemical vapor deposition (MCVD) method. An MCVD soot-preform with a porous layer of SiO<sub>2</sub>
doped with GeO<sub>2</sub> was coated with layers of Yb<sub>2</sub>O<sub>3</sub> and Al<sub>2</sub>O<sub>3</sub> prior to sintering, using the ALD method. ALD is a surface
controlled CVD-type process enabling thin film deposition over large substrates with good thickness control, uniformity
and high conformality. A materials analysis study showed that the dopants successfully penetrated the full thickness of
320 μm of the soot layer. Preliminary preform and fiber experiments on refractive index profiles, background losses,
lifetime and the characteristic gain-loss curve were performed demonstrating the potential of this method for fabricating
Yb-doped fibers with high concentration of dopants.
We have recently shown that the photodarkening (PD) resistivity in Yb/Al-doped fibers can be greatly improved by adding cerium (CE) to the core glass composition. We are now further investigating the laser performance and logn term stability of Yb/Ce/Al fibers in high inversion applications such as 980nm lasers and amplifiers. Our objective is to study the limitations of Yb/Ce/Al fibers and elaborate on their potential of becoming the next generation of fibers for high power fiber lasers and applifiers.
We present an overview of quenching processes in Yb-doped lasers. Experiments made on Yb:YAG crystals and
Yb-doped fiber lasers show that induced losses appear upon UV-irradiation through a charge-transfer process.
The valence stability of the Yb ion is believed to be the key issue for the quenching processes in Yb-doped high
power laser systems.
In this paper we present how charge transfer processes influences the induced optical losses (photodarkening)
in ytterbium doped fiber lasers. The location of the charge transfer absorption band is strongly composition
dependent and is correlated to the valence stability of the ytterbium ion in the silicate glass matrix. An improved
photodarkening performance can in general be observed for a charge-transfer band shifted to shorter
wavelengths, although other routes are also possible to reduce photodarkening. Other parameters that affect the
laser performance, such as absorption and emission cross section, must also be considered.
A strong charge-transfer band at UV-wavelengths is found to play a major role for the observed induced optical losses (photodarkening) in ytterbium doped high-power fiber lasers. This is correlated to the valence stability of the ytterbium ion in the silicate glass matrix, which we believe is the origin of photodarkening. We have performed UV-irradiation experiments on ytterbium-doped preform samples and accelerated photodarkening experiments on Yb-doped fibers, by using 915 nm high power diodes. Our results show that photodarkening can be reduced, to low levels, either by preparing the preform glass in a reducing atmosphere or by hydrogen loading the fiber in a pressure chamber at room temperature.