We will report on recent advances in fabrication of large volume silica based, doped fiber preform materials synthesized via powder-based processes.
Recently, there has been increased interest for power scaling in fiber based laser applications that requires large core volumes with excellent homogeneity in refractive indices, but also chemical variety (in terms of high dopant concentrations, different dopants). A structural fiber variety requires dedicated large volume core material of reproducible and tailorable chemical composition.
Established technologies such as modified chemical vapor deposition (MCVD) or crucible melting rely on complex thermal processing, and are limited in accessible chemistries, dopant concentration, achievable functionalities, and in case of MCVD in achievable core sizes. The current process development thus targets to overcome such draw-backs by including novel approaches to enable extreme material combinations, enhanced reactivity, or novel functions.
We demonstrate the dramatic progress in Yb-doped spun tapered double-clad fiber amplifiers delivering up to 550 W of average power with single mode spatial profile and 50 ps pulses at 20 MHz repetition rate. The special geometrical architecture of the fiber enables the direct amplification of short pulses from tens of mW to hundreds of watt levels in a single amplification stage, leading towards the realization of a compact and highly efficient picosecond fiber-based laser system with excellent output spatial and temporal characteristics.
We present new designs of optical silica fibers tailored for sensing applications. Highly versatile MCVD technology allows co-doping of fused silica in a large variety of elements and the deposition of differently doped layer structures. Co-doping of Ge/B- or Ge/Al enables tuning the refractive index and thus adapting the mode field of the fibers. Ge/Ce co-doping allows tuning the photosensitive properties for Fiber Bragg grating inscription. Combining Ge- and Al-doping modifies the fiber properties for improved distributed sensing applications. We report realization of the designs and present first results of fiber properties.
Work is supported by German BMBF contract 03RU1U071J.
Applying the highly versatile and flexible MCVD technology at Leibniz-IPHT two new designs for optical sensing fibers were realized by co-doping of fused silica. For FBG sensing a Ge/B co-doped fiber with a mode field diameter adapted to standard single mode telecom fibers was prepared. The influence of boron on the attenuation at the inscription wavelength 1550 nm is visible. For distributed Brillouin sensing applications a preform with lateral separated germanium and aluminum doped regions and nearly step-index characteristic was fabricated by the MCVD in combination with the solution doping technique. Theoretical analysis of the acoustic properties and Brillouin spectrum have been shown, that this design is a potential candidate for strain and temperature discrimination. Because of the high temperatures during the preparation processes the radial refractive index and dopant concentration profiles of both fiber designs are influenced by diffusion.
We have demonstrated for the first time, to the best of our knowledge, the successful direct amplification of a cylindrical-vector beams with axially symmetric polarization and doughnut-shaped intensity profile in picosecond MOPA system based on a double-clad ytterbium-doped spun tapered fiber with a ring-shaped active core. The output radially polarized beam with absolute contrast between bright and dark zones carries 10 ps pulses at 1030 nm with a 14.5 W average power level, 91 kW peak power and 0.97 μJ pulse energy.
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.
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