  Stimulated Brillouin scattering (SBS) has many applications, for example, in
sensing, microwave photonics and signal processing. Here we report the first
experimental study of SBS in chiral photonic crystal fiber (PCF), which
displays optical activity and robustly maintains circular polarization states
against external perturbations. As a result, circularly polarized pump light is
cleanly back-scattered into a Stokes signal with the orthogonal circular
polarization state, as is required by angular momentum conservation. By
comparison, untwisted PCF generates a Stokes signal with an unpredictable
polarization state, owing to its high sensitivity to external perturbations. We
use chiral PCF to realize a circularly polarized continuous-wave Brillouin
laser. The results pave the way to a new generation of stable circularly
polarized SBS systems with applications in quantum manipulation, optical
tweezers, optical gyroscopes and fiber sensors.
