Deep-ultraviolet to mid-infrared supercontinuum generated in solid-core ZBLAN photonic crystal fibre Xin Jiang 1 * , Nicolas Y. Joly 1,2 , Martin A. Finger 1 , Fehim Babic 1 , Gordon K. L. Wong 1 , John C. Travers 1 and Philip St. J. Russell 1,2 Silica-based photonic crystal fibre has proven highly successful for supercontinuum generation, with smooth and flat spectral power densities. However, fused silica glass suffers from strong material absorption in the mid-infrared (>2,500 nm), as well as ultraviolet-related optical damage (solarization), which limits performance and lifetime in the ultraviolet (<380 nm). Supercontinuum generation in silica photonic crystal fibre is therefore only possible between these limits. A number of alternative glasses have been used to extend the mid-infrared performance, including chalcogenides, fluorides and heavy- metal oxides, but none has extended the ultraviolet performance. Here, we describe the successful fabrication (using the stack-and-draw technique) of a ZBLAN photonic crystal fibre with a high air-filling fraction, a small solid core, nanoscale features and near-perfect structure. We also report its use in the generation of ultrabroadband, long-term stable, supercontinua spanning more than three octaves in the spectral range 200–2,500 nm. T he physics of supercontinuum generation, having been studied for more than four decades 1 , is now well understood. Detailed studies of nonlinear dynamics in optical fibres have led to several breakthroughs in extending and improving the quality of supercontinuum light sources 2 . The central role of the group velocity dispersion in controlling these dynamics means that solid-core silica–air photonic crystal fibre (PCF), the dispersion properties of which can be extensively engineered by varying the microstructure 3 , has become the dominant medium not only for supercontinuum generation 4 , but arguably for nonlinear fibre optics in general 5 . The main limitations of current solid-core PCF-based supercon- tinuum sources are material absorption (which in fused-silica glass climbs rapidly in the infrared, limiting spectral broadening to wave- lengths below 2.5 μm) and solarization (which reduces the lifetime of silica fibres operating with wavelengths less than ∼380 nm) 6 . For example, although Stark and colleagues reported superconti- nuum generation down to a record 280 nm in a tapered silica solid-core PCF 7 , ultraviolet-generated defect centres in the glass caused the performance to degrade after even short periods of oper- ation. For these reasons, stable long-term deep-ultraviolet supercon- tinuum generation has not yet been successfully demonstrated in solid-core silica PCF. Among the existing non-silica glasses, zirconium fluoride-based (ZrF 4 > 50 mol%) ZBLAN (ZrF 4 –BaF 2 –LaF 3 –AlF 3 –NaF) glass is transparent from 0.2 to 7.8 μm (see, for example, Fig. 1f) and has been viewed as an attractive material for optical devices from the deep-ultraviolet to the mid-infrared 8,9 . Since its discovery in 1975 8 , ZBLAN has been regarded as a promising replacement for fused silica in telecommunications, suggesting that data trans- mission could be shifted to longer wavelengths where its attenuation is intrinsically much lower (less than 0.01 dB km –1 at 2.5 μm) than in fused silica (0.185 dB km –1 at 1.55 μm) 9,10 . Lack of effective methods for eliminating impurities (such as transition metals, oxy-fluorides and water), together with a steep viscosity–temperature characteristic, have made the drawing of high-quality ZBLAN fibres very difficult 9 , even for conventional step-index structures. Nevertheless, it is also believed that, if carefully synthesized, ZBLAN glass can have extremely low water absorption, unlike common heavy-metal oxide or chalcogenide glasses 11 , making it ideal for the generation of multi-octave-wide supercontinua over its entire transmission window. Previous ZBLAN glass fibres have mainly been restricted to all- solid step-index geometries. The narrow temperature range (<10 °C, compared to ∼300 °C for silica) over which the glass has suitable vis- cosity and is stable against devitrification 12,13 has created the percep- tion that the drawing of ZBLAN microstructured fibres is extremely difficult, if not impossible. Yet another difficulty is low heat-transfer efficiency; because ZBLAN glasses are transparent in the infrared, radiative heat transfer over small distances from a heating element to the fibre preform is inefficient 9 . In addition, the thermal conduc- tivity of ZBLAN is much lower than that of silica, so a specially designed drawing furnace must be used 9 . The only previous work on microstructured ZBLAN fibre used extrusion to produce a structure with a large (∼100 μm) core surrounded by one ring of hollow channels 14 . Before the present work, ZBLAN-based supercontinuum spectra have been generated only in step-index fibres over the wavelength range from the visible to the mid-infrared, at very high powers 15–19 . No significant conversion to visible or ultraviolet wavelengths has been possible, however, because of the unsuitable dispersion of these large-core all-solid fibres, whose relatively low effective non- linearity makes it necessary to use long fibre lengths, further limiting the transmission window to the range 300 nm to 4.5 μm (ref. 20). Further broadening into the mid-infrared region from 1.4 to 13.3 μm has recently been achieved using mid-infrared laser pulses at 6.3 μm and a large-core step-index As 2 Se 3 chalcogenide glass fibre 21 . In this Article, we describe a simple supercontinuum system based on a 4-cm-long, small-solid-core ZBLAN PCF with high axial uniformity. By controlling the dispersion through appropriate fibre design we generate several supercontinuum spectra, some of which extend down to 200 nm and others up to 2,500 nm, using a relatively low power and compact laser operating at 1,042 nm. The 1 Max Planck Institute for the Science of Light, Guenther-Scharowsky Strasse 1, Bau 24, Erlangen 91058, Germany. 2 Department of Physics, University of Erlangen-Nuremberg, Guenther-Scharowsky Strasse 1, Bau 24, Erlangen 91058, Germany. *e-mail: xin.jiang@mpl.mpg.de ARTICLES PUBLISHED ONLINE: 19 JANUARY 2015 | DOI: 10.1038/NPHOTON.2014.320 NATURE PHOTONICS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturephotonics 1 © 2015 Macmillan Publishers Limited. All rights reserved