  Optical waveguide segments based on geometrically unbound photonic crystal
fibers (PCF) designs could be exploited as building blocks to realize
miniaturized complex devices which implement advanced photonic operations.
Here, we show how to fabricate optical waveguide segments with PCF designs by
direct high-resolution 3D printing and how the combination of these segments
can realise complex photonic devices. We demonstrate the unprecedented
precision and flexibility of our method by fabricating the first-ever fiber
polarising beam splitter based on PCFs. The device was directly printed in one
step on the end-face of a standard single-mode fiber and was 210~$\mu$m-long,
offering broadband operation in the optical telecommunications C-band. Our
approach harnesses the potential of high-resolution 3D printing and of PCF
designs paving the way for the development of novel miniaturised complex
photonic systems, which will positively impact and advance optical
telecommunications, sensor technology, and biomedical devices.
