Optical back propagation for fiber optic networks with hybrid EDFA Raman amplification.

We have investigated an optical back propagation (OBP) method to compensate for propagation impairments in fiber optic networks with lumped Erbium doped fiber amplifier (EDFA) and/or distributed Raman amplification. An OBP module consists of an optical phase conjugator (OPC), optical amplifiers and dispersion varying fibers (DVFs). We derived a semi-analytical expression that calculates the dispersion profile of DVF. The OBP module acts as a nonlinear filter that fully compensates for the nonlinear distortions due to signal propagation in a transmission fiber, and is applicable for fiber optic networks with reconfigurable optical add-drop multiplexers (ROADMs). We studied a wavelength division multiplexing (WDM) network with 3000 km transmission distance and 64-quadrature amplitude modulation (QAM) modulation. OBP brings 5.8 dB, 5.9 dB and 6.1 dB Q-factor gains over linear compensation for systems with full EDFA amplification, hybrid EDFA/Raman amplification, and full Raman amplification, respectively. In contrast, digital back propagation (DBP) or OPC-only systems provide only 0.8 ~ 1.5 dB Q-factor gains.