Transmission Span Optimization in Fiber Systems With Cavity and Random Distributed Feedback Ultralong Raman Laser Amplification

Optimal design of a Raman-amplified periodic cell depends strongly on the constraints set by overall system requirements. The best possible tradeoff in terms of cost-effectiveness, nonlinearity, pumping efficiency, optical signal-to-noise ratio (OSNR), and tolerance to relative intensity noise transfer varies depending on characteristics, such as total transmission length, desirable number of spans or power budget requirements. Here, we perform a thorough experimental and theoretical study of the best performing configurations for second-order amplification over a broad range of applications, with span lengths ranging from 10 to 150 km. Results highlight the presence of different regimes in which different contributions to noise become dominant. We find that ultralong Raman fiber laser cells with high forward pumping are to be preferred in unrepeatered transmission applications, whereas in long-haul transmission a shorter cell based on random distributed feedback lasing can be a better option as it enables low relative intensity noise transfer with a higher output OSNR at the cost of a reduced pumping efficiency.