Extrusion of multimaterial preforms is also possible when the billet is formed by a stack of different materials 18.Įxtrusion dies are typically CNC machined, and stainless steel is the most common die material. In this case, the extruded preform is heated by the tower furnace and pulled to reduced diameter 17. This technique was shown to be successful in producing high-quality MOFs from soft-glass 14 (such as lead-silicate, tellurite, bismuth, fluoride, chalcogenide, phosphate) and polymer (such as PMMA 15, 16).įor soft glasses, the billet extrusion rig has also been combined with fibre drawing by directly placing the rig on the top of a fiber drawing tower. The extrusion die comprises an initial section with holes to feed the material to be extruded and a posterior segment that has solid features for blocking the material flow in pre-defined regions, allowing the extrusion of a preform with a holey pattern. Billet extrusion involves the preparation of a billet from the chosen optical material, heating the billet to reduce its viscosity (to typically 10 8-10 10 dPa ⋅s 13) and, with the help of a ram, forcing the materials through a die with the desired pattern 14. Soft glass and polymer preforms have also been prepared via billet extrusion, a direct and straightforward way to obtain structures with elaborate designs. An alternative is casting the fiber material in a pre-designed mold, a procedure used to form plastic 11 and glass fibers 12. Like the stacking technique, drilling is limited to circular holes. Polymer MOFs have been manufactured by directly drilling the holes in the plastic rod 6, a technique that has also been applied with glasses 9, 10. Soft-glass MOFs can also be produced with this procedure but with extra complexity due to the initial need to produce the tubes 8. This is a convenient and versatile procedure when tubes are widely available commercially such as for silica and, also, for some borosilicate glasses (e.g. Silica MOFs are typically made via the stack-and-draw technique 1 where millimeter thick capillaries are manually stacked, forming the desired structure. MOF preforms with their characteristic air holes array, on the other hand, have been produced with different techniques. Standard optical fibers rely on vapor deposition methods to produce low loss preforms. Different approaches have been used to produce the macroscopic preform. In all cases, optical fibers are usually drawn in a multiple-stage process whose primary step is the manufacture of an enlarged version of the fiber, the preform. In the early 2000´s microstructured polymer optical fibers were developed 6 extending the application of conventional polymer fibers. While most traditional fibers and MOFs are made of silica due to their remarkable optical and physical properties, fibers can also be made of polymers and non-silica glasses. Simpler structures provide low loss transmittance via inhibited coupling 4 or anti-resonance 5. Complex cladding designs allow the guidance via photonic bandgap. While fibers with a solid core and a holey cladding with a lower refractive index guide by total internal reflection as traditional optical fibers, hollow-core fibers (HCFs) enabled new guiding mechanisms. Conventional optical fibers, on the other hand, have a small core/cladding index contrast, usually below 1%. Chromatic dispersion, modal area, cladding evanescent field, birefringence, and non-linearity, for example, can be very dependent on the specific holes distribution - size, shape, position 1, 2. The presence of wavelength-scale structures with high index contrast (fiber material to air) opened the possibility to extensively control the optical properties of the fiber. of Bath (UK) and his research team, the development of photonic crystal fibers (PCF), or microstructured optical fibers (MOF), expanded and revolutionized the whole field of guided optics 1, 2, 3. Optical fibers had a significant development at the end of the 1990s when structures with an internal microstructure cross-section were proposed and developed. The data traffic is doubling every two years what represents a 1000-fold increase in just 20 years. Today there are hundreds of millions of kilometers of optical fibers installed around the planet. Optical fibers revolutionized the way we communicate, being responsible for most of the actual global data traffic.
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