  We report a series of experimental, analytical and numerical studies
demonstrating strong circular dichroism in helically twisted hollow-core
single-ring photonic crystal fiber (SR-PCF), formed by spinning the preform
during fiber drawing. In the SR-PCFs studied, the hollow core is surrounded by
a single ring of non-touching capillaries. Coupling between these capillaries
results in the formation of helical Bloch modes carrying orbital angular
momentum. In the twisted fiber, strong circular birefringence appears in the
ring, so that when a core mode with a certain circular polarization state (say
LC) phase-matches to the ring, the other (RC) is strongly dephased. If in
addition the orbital angular momentum is the same in core and ring, and the
polarization states are non-orthogonal (e.g., slightly elliptical), the LC core
mode will experience high loss while the RC mode is efficiently transmitted.
The result is a single-circular-polarization SR-PCF that acts as a circular
polarizer over a certain wavelength range. Such fibers have many potential
applications, for example, for generating circularly polarized light in
gas-filled SR-PCF and realizing polarizing elements in the deep and vacuum
ultraviolet.
