  Rayleigh scattering generates intensity noise close to an optical carrier
that propagates in a single-mode optical fiber. This noise degrades the
performance of optoelectronic oscillators and RF-photonic links. When using a
broad linewidth laser, we previously found that the intensity noise power
scales linearly with optical power and fiber length, which is consistent with
guided entropy mode Rayleigh scattering (GEMRS), a third order nonlinear
scattering process, in the spontaneous limit. In this work, we show that this
behavior changes significantly with the use of a narrow linewidth laser. Using
a narrow linewidth laser, we measured the bandwidth of the intensity noise
plateau to be 10 kHz. We found that the scattered noise power scales
superlinearly with fiber length up to lengths of 10 km in the frequency range
of 500 Hz to 10 kHz, while it scales linearly in the frequency range of 10 Hz
to 100 Hz. These results suggest that the Rayleigh-scattering-induced intensity
noise cannot be explained by third-order nonlinear scattering in the
spontaneous limit, as previously hypothesized.
