Hollow core fibers (HCFs) have found extensive use for biological sensing applications, with areas such as Raman spectroscopy being of key interest for deployment of these fibers due to their low Raman background and broad transmission spectral windows. However, fabrication of these fibers is currently based on capillary stacking of hollow tubes, a complex, time-consuming and therefore costly process that limits their potential for use in practical devices.
Glass billet extrusion, an alternative to the capillary stacking process, presents a potential pathway to reducing the high fabrication cost of HCFs. Extrusion is a process in which a glass billet is heated to its softening point and is then forced through a metallic die containing the inverse of the desired structure. This is a one-step, automated process that requires no manual stacking to obtain the desired glass preform.
Initial work using extrusion for HCF fabrication has resulted in fibers with large variations in the uniformity of the core walls, leading to increases in the optical loss of these fibers over the theoretical predictions. Here we present work on improvements to the extrusion process of these fibers, aiming to both reduce the loss of the fibers. Several iterations of the die exit geometry are shown to affect the geometry and uniformity of the preform structure, which translates into changes in the final fiber geometry and therefore the transmission properties of the HCFs. The fabrication methods used here show strong potential for improved guidance properties in future generations of HCFs.