  We observe an interference pattern using a simple fiber-optic interferometer
consisting of an electrical spectrum analyzer and a narrowband light source,
which is commonly employed for observing the Brillouin gain spectrum. This
interference pattern expands well beyond the frequency range corresponding to
the Brillouin frequency shift in silica fibers (approximately 11 GHz at 1550
nm). Using both silica single-mode and polymer optical sensing fibers, we then
experimentally prove that the distinctive noise in a self-heterodyne-based
Brillouin measurement with an unoptimized polarization state originates from
the interference between the reference light and the Fresnel-reflected light.
This noise can be almost completely suppressed by employing a delay line that
is longer than the coherence length of the light source and by artificially
applying a high loss near the open end of the sensing fiber.
