Novel gain monitoring scheme in Remotely-Pumped EDF/DRA hybrid inline amplifier is proposed using Optical Time Domain Reflectometer (OTDR). Signal degradation due to cross gain modulation (XGM) caused by an OTDR pulse in the distributed Raman amplifier (DRA) section and remotely-pumped EDF (RP-EDF) unit is analyzed theoretically. The required conditions for suppressing of XGM in the DRA section are derived. We propose the directional bypass configuration to realize OTDR measurement without XGM in the EDF unit. Transmission experiments using the RP-EDF/DRA hybrid inline amplifier demonstrate the absence of transmission impairement induced by OTDR. An analysis of the OTDR trace for each gain medium is also discussed. The theoretical analysis agrees well with the experimental result.

We have clarified both theoretically and experimentally the basic performance of distributed Raman amplification (DRA) transmission systems in trunk networks with DSF or SMF spans with limited pump power where the pump power is limited by practical considerations. The gain and noise characteristics of a fiber span with splice loss are accurately determined by employing three approximation models. A novel pumping scheme called band enhanced pumping (BEP) is proposed that improves the DRA gain and optical SNR (OSNR) by 1.5 and 0.55 dB, respectively, compared with those of a conventional pumping scheme, under typical system conditions. We show that a DRA system with a DSF span has OSNRs that are 2.1 and 2.9 dB higher than those of a system with an SMF span at limited pump powers of 200 and 400 mW, respectively, as typical examples.