**4. Discussion**

From our analysis in Sections 2 and 3, to achieve the best symmetry and higher efficiency of combating fiber nonlinearity, different Raman schemes have to be considered for different span lengths. A table (Table 1) summarizing the span symmetry with Raman pumping schemes at different span lengths is demonstrated below. It is shown that for the short span length of 25 km, the first-order bidirectional Raman pumping was sufficient to achieve 97% signal power symmetry, but the length of 25 km was very short for OPC-based application (e.g., long-haul transmission systems). In addition, for this short length, the symmetry changes will be relatively small when using different Raman schemes, and bidirectional Raman pumping will introduce significant RIN-replated penalty. For the span length of around 50 km, optimized dual-order Raman pumping is required to achieve 97% span symmetry. However, for longer span length, bidirectional second-order Raman pumping would be needed as the signal gain can be generated from the start of the span. Thus, the scheme based on a random fiber laser with bidirectional second-order pumping without introducing RIN penalty was demonstrated to achieve 97% span symmetry at 62 km. Further extending the span length to 100 km, higher order bidirectional pumping would be required, but given the RIN penalty introduced from conventional Raman pumps, we had to stick to RIN-penalty-free bidirectional pumping based on a random fiber laser. In this case, the optimum span symmetry dropped to 72% at 100 km. Alternatively, we could break the 100 km into two 50 km spans, and then the problem became how to leverage the loss between the two spans. We used the EDF with Raman pumps to account for the loss between spans and improved the span symmetry from 72% (single span) to 93% (two spans).

**Table 1.** Summary of the best span symmetry with corresponding Raman schemes at different span lengths.

