  While it has been shown that backscattering induced phase noise can be
suppressed by adopting acoustic-optic-modulators (AOMs) at the local and remote
sites to break the frequency symmetry in both directions. However, this issue
can not be avoided for conventional fiber-optic multiple-access coherent
optical phase dissemination in which the interference of the signal light with
the Rayleigh backscattered light will probably destroy the coherence of the
stabilized optical signal. We suppress the backscattering effect by locally
breaking the frequency symmetry at the extraction point by inserting an
additional AOM. Here, we theoretically analyze and experimentally demonstrate
an add-drop one more AOM approach for suppressing the Rayleigh backscattering
within the fiber link. Near-complete suppression of backscattering noise is
experimentally confirmed through the measurement the elimination of a common
interference term of the signal light and the Rayleigh backscattered light. The
results demonstrate that the Rayleigh backscattering light has a limited effect
compared to the residual delay-limited fiber phase noise on the system's
performance. Our results also provide new evidence that it is possible to
largely suppress Rayleigh and other backscattering noise within a long optical
fiber link, where the accumulated phase noise could be large, by using
frequency symmetry breaking at each access node to achieve robust
multiple-access coherent optical phase propagation in spite of scatters or
defects.
