Sönke Pilz

Analysis of phase contrast imaging of terahertz phonon-polaritons

We analyze three different techniques, namely, interferometry, polarization gating, and the Talbot effect to record two-dimensional images of propagating coherent terahertz waveforms in LiNbO3. Polarization gating yields significant improvements in the accuracy of waveform reproduction and sensitivity compared to Talbot imaging, which has been extensively used to date.

A beam monitor detector based on doped silica and optical fibres

A beam monitor detector prototype based on doped silica fibres coupled to optical fibres has been designed, constructed and tested, mainly for accelerators used in medical applications. Scintillation light produced by Ce and Sb doped silica fibres moving across the beam has been measured, giving information on beam position, shape and intensity. Mostly based on commercial components, the detector is easy to install, to operate and no electronic components are located near the beam. Tests have been performed with a 2 MeV proton pulsed beam at an average current of 0.8 μA. The response characteristics of Sb doped silica fibres have been studied for the first time.

Time-resolved coherent imaging of a THz multilayer response

We report on time-resolved coherent imaging of terahertz (THz) single-cycle pulses after reflection from or transmission through different types of multilayer systems. The multilayers are integrated into a single solid-state platform that permits THz generation, manipulation, and detection. The multilayer systems supplement the polaritonic toolkit of integrated functional THz devices.

Atomic-scale imaging of dopant atoms and clusters in Yb-doped optical fibers

Fabrication of Ytterbium-doped active fibers with different designs, compositions and high Yb concentration has attracted an intense interest. For making highly Yb-doped fibers, co-dopants like phosphorous (P) and aluminum (Al) are also employed in order to modify refractive index and increase Yb solubility, avoiding clusters and phase segregations. Indeed, Yb-clustering results in quenching effects and increased propagation losses due to energy transfer between clustered ions. Therefore, the chemical composition and phase homogeneity of the fiber core have key influences on the performance of an active fiber. However, conventional fabrication techniques such as MCVD (modified chemical vapor deposition) and OVD (outside vapor deposition) are approaching the limit. In this contribution, we have developed an approach for fabrication of such active fibres based on granulated silica derived from the sol-gel process. The advantage of this method is the fabrication of active fibers with high dopant contents and homogeneity. Here, using high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) in atomic scale, we report the direct, nano-scale and atomic-resolution observation of individual Yb dopant and co-dopant (i.e. Al, P) atoms for different fabricated fibers. The chemical mapping from STEM-EDX shows an extremely homogeneous distribution of the dopants and co-dopants in nano-scale for our fabrication protocol. However in atomic resolution, we also identified the possible Yb clusters in the range of 10 atoms within the core structure. The size, structure, and distribution of these clusters are determined with an Yb-atom detection efficiency of almost 100% by STEM.

Rare earth doped optical fiber fabrication by standard and sol-gel derived granulated oxides

We present our progress in the production of ytterbium (Yb) doped optical fibers fabricated by two variants of the granulated aluminophosphosilicate method. We show advantages and disadvantages of mixing rare earth and aluminophosphosilicate granulated oxides directly (variant 1) or by using the sol-gel method to produce doped granulate material (variant 2). For both methods we studied the effects of varying the dopant concentrations and of introducing iterative melting and milling procedures. In particular, the sol-gel based method eases the inclusion of P2O5 and thus, in combination with Al2O3, higher dopant concentration of Yb and Er are possible. Sintering the sol-gel material at high temperature eliminated bubbles in the core. We fabricated optical fibers that, piecewise, between individual strong scatterers, exhibited attenuation losses as low as 0.35dB/m. For our comparative study we determined volume percentage and distribution of chemical elements in the fabricated fiber glasses by the analytical technique of Energy-Dispersive X-ray, Electro Probe Microanalysis and the degree of crystallization by X-Ray Diffraction analysis. Furthermore we measured fluctuations of the refractive index profile and scattering losses of the fiber core.

Design and realisation of leakage channel fibres by the powder-in-tube method

The applications of fibre lasers demand for increasing power. Limits are set by various nonlinear effects. Leakage channel fibres (LCF) are one approach to this problem. With this type of fibre, most nonlinear effects can, in principle, be mitigated simultaneously by increasing the mode field area and by maintaining the single mode regime. For its implementation, we propose to use the powder-in-tube preform technique. While the microstructure consists of commercial pure silica rods, the surrounding is filled with index-raised aluminum-doped silica oxide granulate. For the fabrication of the latter, we tested two different methods. For the first one, the oxide precursors were mixed in pure powder form. In the other method, the material was produced with the helps of the sol-gel process, where the mixing takes place in liquid phase, thus resulting in an expected improved homogeneity. Prior to the fabrication of a prototype, their feasibility has been tested with the help of a finite-difference method simulation tool (Lumerical MODE Solutions). Two such fibres have been fabricated according to this results. The influence of the granulate mixing method and of the grain size on the homogeneity in refractive index has been tested. Although the produced fibres do not yet show the desired performance, the produced prototypes prove that LCFs can indeed be realised with this approach.

Progress in the fabrication of optical fibers by the sol-gel-based granulated silica method

Novel special optical fibers nowadays can take advantage of several new preform production techniques. During the last years we have devoted our attention to the granulated silica method. It is one of the variants of the powder-in-tube technique and potentially offers a high degree of freedom regarding the usable dopants, the maximum possible dopant concentration, the homogeneity of the dopants, the geometry and minimal refractive index contrast. We developed and refined an approach for the production of doped granulated silica material based on the sol-gel process. Here, we present material analysis results of an ytterbium (Yb) doped, aluminum (Al) and phosphorous (P) co-doped glass on the basis of our sol-gel glass based granulated silica method as well as first measurements of two LMA fibers obtained from this material. For the material analysis we used advanced analysis techniques, such as HAADF-STEM and STEM-EDX spectroscopy to determine the composition of the material and the distribution of the dopants and the codopants. The chemical mapping of the STEM-EDX shows an extremely homogeneous distribution of the dopants and co-dopants in nano-scale. Based on self-made LMA fibers, we measured the refractive index contrast of the sol-gelbased granulated silica derived core compared to the pure silica cladding. In addition we quantified optical characteristics such as the emission and absorption spectrum. The measured upper state lifetime of the optical active dopant ytterbium was 0.99ms, which in turn confirms the homogeneous distribution of the Yb atoms. The propagation losses were determined to be 0.2dB/m at 633nm and 0.02414dB/m at1550nm.

Powder Process for Fabrication of Rare Earth-Doped Fibers for Lasers and Amplifiers

The use of powder-based technologies for the production of rare earth (RE)doped fibers and preforms is discussed. Although these technologies cannot compete with vapor-based technologies such as modified chemical vapor deposition (MCVD) with respect to purity of the silica material obtained, they offer a high degree of versatility with respect to the material composition and the obtainable topology of microstructured fibers. The production of core rods starting from powder technologies and the powder-in-tube method are discussed. The challenges when using powder-based technologies lie in obtaining homogeneously doped and co-doped material as well as avoiding scattering by ion clusters. To reach a homogeneous distribution of dopants, the use of the sol-gel technology is discussed. Especially the incorporation of aluminum (Al) and phosphorus (P) to enhance the solubility of the rare earth activators as well as to control the index raise is found to be considerably eased. Considerations from materials science point of view are made and serve as guidelines to understand the process. In this context, extremely precise characterization techniques such as wavelength dispersive x-ray fluorescence (WDXRF), scanning transmission electron microscopy with high-angle annular dark-field (STEM-HAADF), and differential thermal analysis (DTA) are discussed in order to mature the tuning of glass composition and drawing process. The thermodynamic properties of the doped glass powders discussed here could be crucial in assessing the thermal stability of the glass, required cooling rate, and its susceptibility to temperature changes during vitrification, devitrification, and fiber drawing steps.

Spectroscopy of a heated Yb-doped optical fiber with high aluminum content

The generation and amplification at wavelengths longer than 1100 nm is not straightforward when using Yb-doped optical fibers, since light emission of ytterbium occurs preferentially in the region of 1020 nm - 1100 nm with a maximum at 1030 nm. One well known approach is to heat the Yb-doped fiber up to temperatures above 100 °C. This increases the re-absorption in the lower emission band and also enhances at the same time the emission at longer wavelengths. Consequently, heating allows to extend the spectral gain-region of Yb-doped fibers by at least 60 nm up to 1160 nm. However, the drawback of this method is that it results in a shorter durability of the fiber, since heating damages the polymer-coating. Moreover, such a laser has a reduced overall efficiency, due to heating, isolation and heat removal issues. It has been reported, that at the presence of an aluminosilca host (silica doped with Al) efficient laser activity at around 1150 nm can be achieved by heating the Yb-doped fiber to only 60 °C. In this work we investigate the spectroscopy of a heated Yb-doped fiber with a high aluminum concentration. The fiber is drawn in our in-house fiber drawing tower. The preforms are produced by the sol-gel-based granulated silica method which allows us to vary the aluminum as well as the ytterbium concentrations within a large range. The fiber is investigated with respect to their spectroscopic data as well as their lasing performance.

Multi-stage ytterbium fiber-amplifier seeded by a gain-switched laser diode

We demonstrated all-fiber amplification of 11 ps pulses from a gain-switched laser diode at 1064 nm. The diode was driven at a repetition rate of 40 MHz and delivered 13