Abstract: A filament for use as a feedstock in three-dimensional printing of an optical device has a strand-like structure for continuous feeding into a printing nozzle of a three-dimensional printer. The strand-like structure includes one or more of elongated side-by-side core/cladding sections each having an optically transmissive inner core surrounded by a lower-index optically transmissive outer cladding for corresponding light guiding in the optical device. In embodiments, the core may be a solid material or an air core, and in the case of solid material may include scintillation material or other enhancements. Other variations and specifics are disclosed.