The fabrication of high quality custom-designed photonic crystal and microstructured fibres is a central to the research in the Division. The goal is to remain at the forefront of developing novel types of PCFs, and to provide a short path from the first idea of a new experiment to the realization of the often unique fibre required for the experiment. The main procedure used is "stack-and-draw", which relies on manual assembly of glass capillaries and rods into an appropriate preform stack whose structure corresponds approximately to the desired fibre structure and which is then drawn to fibre in one or two steps. By tuning process parameters such as temperature, preform feed rate and drawing speed, as well as the pressure inside the preform, the size of the air-holes and their spacing can be controlled. We have two drawing towers for silica capillaries and fibres, and one for drawing of soft-glass. All three are located in a brand-new cleanroom in the new MPL building complex.
The stack and draw process
The fabrication of high quality custom-designed PCF is central to the research of the Division. We use the "stack-and-draw" procedure, which relies on manual assembly of glass capillaries and rods into an appropriate preform stack whose structure corresponds approximately to the desired fibre structure. After inserting the preform stack into a glass tube and fusing during the drawing process, one obtains a microstructured preform or "cane". The final step in PCF fabrication involves drawing the cane into fibre with the desired dimensions, such as cladding-lattice pitch and the outer fiber diameter. By tuning process parameters such as temperature, preform feed rate and drawing speed, as well as the pressure inside the preform, the size of the air-holes and their regularity can be controlled. As for standard fibers, the fabricated PCF is coated with a polymer jacket for improved mechanical strength.
Silica glass has excellent transparency in the visible and near IR, as well as good drawing properties and excellent mechanical strength. As a result it is the material most commonly used to form PCF. We routinely fabricate a wide range of different PCF structures for use in many different projects throughout the Division. The technology permits a wide range of different structures to be realised, from periodic lattices of air channels to nanoweb structures and more complicated architectures. High quality hollow-core PCF is essential for the projects on gas-laser devices, optical sensors and particle guidance. Solid-core structures include dual-nanoweb fibre, twisted PCF and small-core PCFs for use in passive optoacoustic mode-locking of fibre lasers. Division members who require special PCFs for their experiments are encouraged to participate in their fabrication and design. The figure shows examples of structures made at MPL.
Chlorine treatment to reduce contaminants
The step-index preforms used for conventional optical fibres can be drawn directly to fibre without exposing the core to the atmosphere, so that contamination from, for example, water does not occur. In the case of PCF, however, the stacking procedure involves exposure to the atmosphere, which introduces contaminants. We have explored reducing contamination by treating the primary and/or secondary preform with chlorine and/or oxygen gas while heating it to 900°C. When treated with chlorine gas, the preforms produced fibres with a clear reduction in the water-related absorption peak at 1380 nm (the first vibrational overtone of the OH-bond). We also investigated the influence of the dryness of the gas used for pressurising the air-hole microstructure during fibre drawing. We found that reducing the water content of the gas from <2 ppm to <0.5 ppm approximately halved the absorption loss at 1380 nm, even when the preforms were not treated with chlorine before fibre drawing [Frosz (2016)].
Already the very first hollow-core PCFs demonstrated in 1999 had a complex microstructure consisting of several (~10) layers of air-holes in the cladding. In the case of the photonic bandgap fibres it is known that the confinement loss decreases for each layer added to the cladding. In recent years, however, it has become increasingly clear that in many scientifically and technically interesting cases, it is sufficient to have a single ring of hollow glass capillaries surrounding the air-core, as long as the cladding capillaries are thin enough compared to the wavelength. In close collaboration with the Russell division, we have explored several aspects of these new types of single-ring hollow-core fibres. First, we used a tube with a hexagonal inner bore to precisely position six capillaries 60° apart at the tube vertices (advantageous compared to a circular tube) [Edavalath (2015)]. Second, we demonstrated theoretically and experimentally that this fibre design can provide endlessly single-mode guidance even though the core is hollow, provided the capillary-to-core diameter ratio is close to 0.682 [Uebel (2016)]. This value ensures phase-matching between LP11, which is the most critical higher-order mode (HOM) in the core, and the fundamental mode in the cladding capillaries, thereby eliminating it from the core, as proven by prism-assisted side-coupling [Trabold (2014)]. Third, we have demonstrated the twisting of these types of fibre directly during fibre drawing, which provides another route to optimizing the suppression of unwanted HOMs [Günendi (2016)]. Finally, we have derived a simple analytical expression to estimate the bend-radius at which single-ring hollow-core PCF acquires high bend-loss, and demonstrated its validity experimentally [M. H. Frosz, P. Roth, M. C. Günendi, and P. St.J. Russell, "Analytical formulation for the bend-loss in single-ring hollow-core photonic crystal fibers," Phot. Res., in review (2017)].
We have an on-going collaboration with Dwayne Miller’s group at the Max Planck Institute for the Structure and Dynamics of Matter (Hamburg) on developing hollow-core fibres for delivering light at 3 µm for scar-free surgery. Together with the Max Planck Institute for Biological Cybernetics in Tübingen we are investigating the feasibility of collecting weak fluorescent signals through a PCF inserted into a brain while simultaneously mapping the brain function using functional MRI. Researchers from the Karlsruhe Institute of Technology are using their synchrotron to transmit x-rays through PCF to compare their transmission properties with those of the simple tapered glass capillaries currently used for focusing, as well as measuring the surface roughness inside PCFs fabricated by different methods. For a collaboration with researchers at the University of Twente, we designed and fabricated high numerical aperture PCFs (see below) for use in high-resolution endoscopic imaging by wavefront shaping at the fibre output [Amitonova (2016)].