Theoretical limit of repeater spacing in an optical transmission line utilizing Raman amplification

The limit of repeater spacing is investigated theoretically for optical transmission lines utilizing stimulated Raman scattering to amplify the signal light. Achievable repeater spacing is numerically estimated on the basis of coupled power equations and measured fiber characteristics for various signal wavelengths and relative index differences. Three types of transmission line configurations are considered, i.e., utilization of forward only, backward only, and bidirectional amplification. In the third case, a transmission distance of more than 400 km is predicted for an input signal light power of 100 μW, a signal wavelength of 1.57 μm, and a pump power of 0.5 W using a fiber with a relative index difference of 1 percent for signal light amplification.