  We describe an easy-to-implement technique that allows for a complete
experimental characterization of sub-wavelength diameter tapered optical
fibers. Our method is based on a direct and fast numerical analysis of the
backward Brillouin spectrum measured using a highly sensitive single-ended
heterodyne coherent detection. It can be performed in situ without any
manipulation nor optical alignment of optical microfibers. Sensitivity as high
as a few nanometer for fiber diameter ranging from 500 nm to 1.2 micron is
reported. This new method may help for the design and characterization of
optical fiber tapers widely used in many applications such as optical sensing,
atom trapping, quantum optics, and plasmonics.
