Analytical solution of transmission performance improvement in fiber spans with forward Raman gain and its application to repeaterless systems

A simple analytical expression for the optical signal-to-noise ratio (SNR) benefit expected from adding forward Raman gain to backward Raman gain in a fiber transmission span is derived. This result is then applied to modify the stimulated Brillouin scattering (SBS) threshold for the case of a signal experiencing forward Raman gain. These concepts are applied to demonstrate repeaterless transmission of 10 and 40 Gb/s data rates over standard single mode fiber (SSMF).