  Tapered optical fibers with a nanofiber waist are widely used tools for
efficient coupling of light to photonic devices or quantum emitters via the
nanofiber's evanescent field. In order to ensure well-controlled coupling, the
phase and polarization of the nanofiber guided light field have to be stable.
Here, we show that in typical tapered optical fibers these quantities exhibit
high-frequency thermal fluctuations. They originate from high-Q torsional
oscillations that opto-mechanically couple to the nanofiber-guided light. We
present a simple ab-initio theoretical model that quantitatively explains the
torsional mode spectrum and that can be used to design tapered optical fibers
with tailored mechanical properties.
