Complex air-structured optical fibre drawn from a 3D-printed preform KEVIN COOK 1 , JOHN CANNING 1* , SERGIO LEON-SAVAL 2 , ZANE REID 1,+ , MD ARAFAT HOSSAIN 1 , J ADE-EDOUARD COMATTI 1 , YANHUA LUO 3 AND GANG-DING PENG 3 1 interdisciplinary Photonics Laboratories (iPL), School of Chemistry, The University of Sydney, NSW 2006, Australia; 2 School of Physics, The University of Sydney, NSW 2006, Australia; 3 National Fibre Facility, School of Electrical Engineering and Telecommunications, University of New South Wales, Sydney, NSW 2052, Australia; + Currently with Precision 3D Printing (P3P), Unit 2, 28 Anvil Road, Silverdale, Auckland, New Zealand. *Corresponding author: john.canning@sydney.edu.au A structured optical fibre is drawn from a 3D-printed structured preform. Preforms containing a single ring of holes around the core are fabricated using filament made from a modified butadiene polymer. More broadly, 3D printers capable of processing soft glasses, silica and other materials are likely to come on line in the not-so-distant future. 3D printing of optical preforms signals a new milestone in optical fibre manufacture. In recent years, there has been an explosion of interest in 3D printing technologies and there is now a vast range of 3D printing methods that are finding new applications every day in a whole host of areas from science and engineering to medicine and the arts [1]. They are set to revolutionise manufacturing. Fused deposition modelling (FDM) is one of the most commonly used techniques, being the first and simplest demonstration of 3D printing in the late 1980s [2]. It is especially suited for instrument prototype casings such as those required for novel smartphone spectrometers [3]. In FDM, a polymer is fed through a heated nozzle which melts the polymer - essentially a fancy thermal glue gun using a finite extrusion nozzle with programmable xyz positioning capability. Optical polymerisation methods using both light emitting diodes (LEDs) and lasers offer higher resolution and are increasingly popular; however, they do not have the same material range given the specific absorption and monomer polymerization requirements. Selective laser sintering (SLS) is another popular technique where a high power laser is utilised to fuse not only plastic, but glass, metal or ceramic particles or powders into 3D objects [4]. A 3D profile is achieved by scanning the laser on the horizontal plane layer-by-layer, lowering the printing bed between each layer. All these methods have their particular rate limiting steps determined by the consolidation process details and the need for full xyz scanning. More recently, holographic imaging using projectors promises significantly accelerated production of the 3D printed object by removing the requirement for xy scanning [5]. FDM, SLS and other related variants are experiencing tremendous growth world-wide, particularly for rapid prototyping and manufacturing. 3D printing is also being applied to cellular assembly processing of biomedical interest [6]. 3D printing technologies are reaching most fields, including more recently photonics. For example, a company called Luxexcel is already producing high transparency Fresnel lenses by printing with polymethylmethacrylate (PMMA) filament [7]. Most interesting, is recent work demonstrating direct- print short, solid plastic fibre [8] and “light pipes” [9] using transparent polymers, the first 3D printed waveguides. Here, we explore harnessing 3D printing for optical fibre fabrication. We see it as revolutionising the manufacture of all optical fibre fabrication, including glass optical fibres as 3D glass printing comes on line [10,11]. Whilst there is debate as to the merits of the technology for conventional step-index optical fibres and how dopant distributions might be 3D printed, one breed of optical fibre that could particularly stand to benefit immediately are so-called structured (both microstructured and nanostructured) optical fibres. The fabrication of these fibres is limited to methods that are often suitable for specific materials. The stack-and-draw technique, where capillaries are manually assembled into a hexagonal structure prior to drawing [12], is the most popular method but is obviously limited to hexagonally packed periodic structures, giving rise to so-called photonic crystal fibres. More complex structures such as Fresnel [13,14] and related fractal fibres [15] cannot be easily constructed by assembling capillaries and have required other methods including drilling [12], extrusion [16,17] and injection molding [17]. As can be imagined, these methods are currently all time-consuming and 3D printing offers an immediately competitive advantage, and certainly for polymer fibres where 3D printing manufacturing is most mature. In this paper, an alternative approach to making structured fibres is explored by utilising a 3D printer to design and print a structured preform that is then drawn to fibre. As proof of principle, the more mature FDM printing method is chosen to print a preform using a transparent thermosetting polymer that is subsequently drawn to fibre. An arbitrary fibre geometry consisting of a solid core surrounded by 6 air holes is chosen to assess the 3D-printed preform approach. The material used is a commercially available 3D printing filament consisting of a propriety polystyrene mixture containing styrene-butadiene-copolymer and polystyrene, labelled here as SBP. SBP can offer several significant advantages when compared to other polymer filaments: it is transparent, it has a high degree of flexibility and it does not suffer from discoloration under stress or when bent. A low- cost, commercially-available thermosetting 3D printer is used to print the fibre preform with this filament. The preform can be subsequently drawn to fibre with relative ease, without the need for pre-annealing or hole-pressurisation, maintaining its cross-sectional features after being drawn to fibre form. We also show that preform transparency can be improved by annealing which removes the scattering component arising from the printed interfacial layers and bubbles trapped during printing. The aim of this paper is to demonstrate the first 3D printed structured optical fibres, choosing to focus on existing polymer based 3D printing as proof of concept. As new materials such as soft glasses and silica glasses become accessible to 3D printing, the revolution will eventually encompass all fibre technologies. In the meantime, structured polymer fibres have significant potential in their own right within a range of application areas. For example, chirped graded Fresnel fibre designs could find applications in LANs where step-index fibres are traditionally used [18]. Microstructured polymer optical fibre also shows strong potential in biomedical applications due to polymer compatibility with organic materials. Basic structured fibres with only a small number of holes offers cheap, biocompatible attenuation sensors for applications such as orthodontic pain monitoring within the human organism [19]. The air-structured preform was designed on Inventor computer aided design (CAD) software. Fig. 1 shows a depiction of the preform shape and a photo of the 3D printed preform end. The length of the preform was chosen to be L = 10 cm, although given the volume workspace of the 3D printer used (a Deltasine “Redback-2”), lengths of up to L = 39.5 cm can be achieved. The preform diameter of ϕ = 1.6 cm was chosen in order to be compatible with the polymer fibre draw tower and furnace. The solid core of the preform is surrounded by 6 air holes each with diameter ϕhole = 0.2 cm and evenly separated along the periphery. A low-cost, commercially-available 3D thermal printer was used to print the preform. The Redback-2 is a delta design model which is ideally-suited for printing circular structures compared to traditional xyz printers. The SBP polymer filament has a higher degree of transparency (specified > 90%, loss α < 1.2 dB) when compared to the more commonly used acrylonitrile butadiene styrene (ABS) polymer. The extruder and bed temperatures of the 3D printer were set at Tex = 210 °C and Tbed = 95 °C respectively. The preform used in the