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8th Scientific Meeting

At our last Scientific Meeting of 2013, we have Dr Jens Kobelke and Dr Jörg Bierlich from IPHT - Jena, Germany, with 2 talks about Microstructured Optical Fibers for sensor applications. See you all there!
When Dec 11, 2013
from 02:00 to 03:30
Where Room 005 Mathematics Department FCUP
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Author: Doctor Jens Kobelke
Title: "Doped Microstructured Fibers for nonlinear and sensor applications
Abstract:

The doping or substitution of the typical matrix material silica in microstructured fibers (MOFs) enables the improvement of their optical performance and functionality. Cross sectional views of three MOFs discussed here are exemplarily shown in Figure 1.

MOF with MCG core

Ge-doped seven holes hollow core fiber

Ge-doped triangular hollow core fiber

Fig. 1 Micrographs of MOFs with MCG core (left), Germanium doped seven holes core (middle) and Germanium doped triangular core hollow  fiber (right)

Holey cladding MOFs with extremely high Germanium doped silica (up to 36 mol% GeO2) and high index multi-component glass (MCG) in the core center were prepared for nonlinear applications. In case of MOF with MCG core (see Figure 1/left), the MCG has the molar composition 70SiO2-20Al2O3-10La2O3. Stack-and-Draw technique was applied for preform preparation. The radial GeO2 profile of the core rod was graded to manage the large viscosity and thermal expansion differences between core material and silica holey cladding. In the case of MCG the inner core rod was overcladded with a small silica clad [1].  The fibers show an extended broadband supercontinuum generation (500 nm -2200 nm) using fiber lengths in the meter range.

The introduction of moderate concentrations of refractive index increasing dopants such as Germanium in the core allows adapting the mode field distributions in holey core fibers. Figure 1/middle shows a MOF with a holey doped core, consisting of seven holes. The concentration of GeO2 corresponds to a refractive index difference of Dn= 0.01. The hole pitch ratio is about 0.4 in the core and 0.7 in the cladding. The difference of the hole pitch ratios is caused by material based viscosity differences during the fiber drawing. The MOF shows an intense overlap of the evanescent field due to the lower air fraction of the doped hollow core. The fiber was used for chemical sensing. Hydrocarbon detection is possible in the wavelength range 1150 nm – 1500 nm using overtone of C-H stretching vibrations.

A further discussed fiber design is a hollow core fiber with triplicated Germanium doped triangular cores (Figure 1/right). The cores are uniformly doped with 5 mol% GeOand 1 mol% P2O5. A special preparation technique was used. The core material was doped in 4 layers by MCVD in a substrate tube of Heraeus Suprasil F300. The deposited tube was grinded and polished in azimuthal positions over 120°.  The triangular shaped tube was fitted with a pressure control system and drawn to a capillary fiber.  The hole diameter was controlled by overpresssure to be about 20 µm. The formation of the three Germanium doped cores was adjusted by the fiber drawing parameters. The shift of the the outer cross-sectional profile shifts from triangular to circular shape by application of high drawing temperature. Contrary, the circular deposited germanium doped layer of the triangular preform tends to form triangular cores by non-radial flow components surrounding a circular central capillary.

Author: Doctor Jörg Bierlich
Title: "Preparation and Characterisation of Optical Fiber Tips for Nanoscopic and Sensor Applications


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