Transmission of a frequency channel through a long-haul optical fiber communications link

Finding ways to communicate precise frequency and time through an optical fiber network has become a critical issue. In dark optical fibers, Rayleigh scattering may impose a limit on duplex transport that can be reduced by appropriately modulating the signal. New limits appear in communication networks in which neighboring communication channels can impair the performance of a frequency communication channel. We review the physical phenomena that can potentially impair a frequency communication channel. These include linear impairments - amplified spontaneous emission (ASE) noise, chromatic dispersion, and polarization mode dispersion. These also include nonlinear scattering impairments - Rayleigh scattering, Brillouin scattering, and Raman scattering. Finally, these include impairments due to the Kerr nonlinearity - self-phase modulation, cross-phase modulation, and four-wave mixing. We show that ASE noise imposes a lower limit on the frequency signal of ~1 nW, while both Brillouin scattering and self-phase modulation impose an upper limit of ~1 mW. Finally, we heuristically examine the effect of cross-phase modulation and show that it can lead to a fractional frequency uncertainty ~6.5×10^-16 after 1 s.