2014, Kobelke, J., Bierlich, J., Wondraczek, K., Aichele, C., Pan, Z., Unger, S., Schuster, K., Bartelt, H.

All-solid microstructured optical fibers (MOF) allow the realization of very flexible optical waveguide designs. They are prepared by stacking of doped silica rods or canes in complex arrangements. Typical dopants in silica matrices are germanium and phosphorus to increase the refractive index (RI), or boron and fluorine to decrease the RI. However, the direct interface contact of stacking elements often causes interrelated chemical reactions or evaporation during thermal processing. The obtained fiber structures after the final drawing step thus tend to deviate from the targeted structure risking degrading their favored optical functionality. Dopant profiles and design parameters(e.g., the RI homogeneity of the cladding) are controlled by the combination of diffusion and equilibrium conditions of evaporation reactions. We show simulation results of diffusion and thermal dissociation in germanium and fluorine doped silica rod arrangements according to the monitored geometrical disturbances in stretched canes or drawn fibers. The paper indicates geometrical limits of dopant structures in sub-μm-level depending on the dopant concentration and the thermal conditions during the drawing process. The presented results thus enable an optimized planning of the preform parameters avoiding unwanted alterations in dopant concentration profiles or in design parameters encountered during the drawing process.

2013, André, R.M., Silva, S.O., Becker, M., Schuster, K., Rothardt, M., Bartelt, H., Marques, M.B., Frazão, O.

Suspended core fiber tapers with different cross sections (with diameters from 70 μm to 120 μm) are produced by filament heating. Before obtaining the taper, the spectral behavior of the suspended core fiber is a multimode interference structure. When the taper is made, an intermodal interference between a few modes is observed. This effect is clearly visible for low taper core dimensions. Since the core and cladding do not collapse, two taper regions exist, one in the core and the other in the cladding. The cladding taper does not affect the light transmission, only the core is reduced to a microtaper. The spectral response of the microtaper based-suspended core fiber is similar to a beat of two interferometers. The strain is applied to the microtaper, and with the reduction in the transverse area, an increase in sensitivity is observed. When the taper is immersed in a liquid with a different index of refraction or subjected to temperature variations, no spectral change occurs.

2013, Favero, F.C., Becker, M., Spittel, R., Rothhardt, M., Kobelke, J., Bartelt, H.

We report on a fast and sensitive temperature sensor using a micro-structured or photonic crystal fiber interferometer with a high germanium doped fiber core. The wavelength sensitivity for temperature variation was as high as δλ/δT= 78 pm/ C up to 500 C, which was 6 times more sensitive than the fiber Bragg grating temperature sensitivity of δλ/δT= 13 pm/ C at 1550 nm. The sensor device was investigated concerning the sensitivity characteristics and response time.

2014, Schmälzlin, E., Moralejo, B., Rutowska, M., Monreal-Ibero, A., Sandin, C., Tarcea, N., Popp, J., Roth, M.M.

Until now, spatially resolved Raman Spectroscopy has required to scan a sample under investigation in a time-consuming step-by-step procedure. Here, we present a technique that allows the capture of an entire Raman image with only one single exposure. The Raman scattering arising from the sample was collected with a fiber-coupled high-performance astronomy spectrograph. The probe head consisting of an array of 20 × 20 multimode fibers was linked to the camera port of a microscope. To demonstrate the high potential of this new concept, Raman images of reference samples were recorded. Entire chemical maps were received without the need for a scanning procedure.

2014, Leich, M., Fiebrandt, J., Schwuchow, A., Jetschke, S., Unger, S., Jäger, M., Rothhardt, M., Bartelt, H.

We demonstrate a distributed measurement technique to observe temperature changes along pumped Yb-doped fibers. This technique is based on an array of fiber Bragg gratings acting as a temperature sensor line. The Bragg gratings are inscribed directly into the Yb-doped fiber core using high-intensity ultrashort laser pulses and an interferometric setup. We studied the temperature evolution in differently co-doped Yb fibers during optical pumping and identified different effects contributing to the observed temperature increase. We found that preloading of fibers with hydrogen supports the formation of Yb2+ during UV irradiation and has a large impact on fiber temperature during pumping. The proposed technique can be applied to investigate the homogeneity of pump absorption in active fibers and to support spatially resolved photodarkening measurements.