Comparison of forward- and backward-propagating optical-fiber-induced noise for application to optical fiber frequency transfer

Current schemes for the photonic transfer of radio frequencies rely on bidirectional active feedback in which the optical signal must propagate in both directions through a single optical fiber. This requirement is not readily compatible with existing optical fiber networks; hence, it is important to develop alternate means of suppressing optical-fiber-induced noise. Previously, we experimentally characterized an optical-fiber-length-dependent noise source that contributes to the phase noise of the radio frequency signal, and we demonstrated that it can be mitigated by frequency dithering the laser. However, we have not developed an adequate model to describe the noise source. In this work, we compared the experimentally-measured forward- and backward-propagating optical-intensity noise spectra. We found that the power of the backward-propagating noise is over 40 dB higher than the power of the forward-propagating noise. We also found that the forward-propagating noise scales faster with respect to optical-fiber length than does the backward-propagating noise. These results will aid the development of a complete theory to describe the optical-fiber-length-dependent noise source.