Forward, Bidirectional and Higher-Order Raman Amplification

Distributed Raman amplification can be achieved by optical pumping at either end of the fiber. In copumped Raman configuration, the pump is launched at the front end and copropagates with the optical signal along the transmission span. In the counterpumped architecture that is widely deployed, the optical pump and signal launch at the opposite ends. Finally, Raman pumping at both ends of the transmission span characterizes the bidirectional scheme. The latter term is also used for optical links that support two-way signal traffic and often leads to confusion. To avoid this ambiguity, we will designate optical transmission as unidirectional or bidirectional, independently from any amplification considerations. Bidirectional network functionality is supported by a separate fiber thet carries signal traffic propagating in the opposite direction. In contrast, bidirectional transmission can be used to realize two-way traffic within a single fiber line: counterpropagating signal traffic is launched and received at the opposite ends of the optical link. A bidirectionally pumped fiber span, however, almost exclusively supports unidirectional signal traffic.