**4. Conclusions**

In this paper, we demonstrate the experimental and numerical characterisation and optimisation for representative optical amplifiers, including an EDFA, a discrete Raman amplifier, a hybrid Raman/EDFA, first-order only, and second-order (dual-order) distributed Raman amplifiers, with a 600 Gb/s PM-64QAM transceiver (11-channel WDM grid) over a 75.6 km SMF. Our stand-alone characterisation results demonstrate that the second-order Raman scheme had flatter signal power profiles along the fibre, the lowest ASE noise level, and the highest OSNR. However, in the experimental transmission test, the first-order distributed Raman amplifier gave the best overall transmission overall performance. In the linear regime, the improvement introduced by higher-order pumping was not apparent, and therefore the first-order scheme showed similar performance to the second-order Raman scheme. However, because of the lower average signal power, the first-order scheme showed significantly superior transmission performance in the nonlinear regime in comparison with the second-order scheme. Therefore, the simpler first-order scheme gave the optimum balance between the linear noise and fibre nonlinearities in a single-span system with a high-data-rate transceiver. In addition, both distributed schemes demonstrated better BERs than the hybrid and discrete schemes. As expected, the hybrid Raman/EDFA scheme showed better performance than discrete schemes. Due to the extra 7.5 km Raman gain fibre, the discrete Raman scheme performed worst among all the amplification schemes considered.

**Author Contributions:** M.T. and M.A.I. proposed the concept and initiated the study; M.T., P.R., T.T.N. and M.A.I. carried out numerical simulations; M.T., M.A.I. and L.K. performed the experiments; M.T., M.A.I., P.R. and T.T.N. conducted analytical calculations; I.D.P., P.H. and W.F. supervised the studies. The paper was written by all authors. All authors have read and agreed to the published version of the manuscript.

**Funding:** UK Engineering and Physical Sciences Research Council (EPSRC) Grant EP/S003436/1 (PHOS), EP/V000969/1 (ARGON), Polish Ministry of Science and Higher Education Grant 12300060, and National Natural Science Foundation of China Grant 61975027.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We acknowledge industrial support from Lumentum (UK), II-VI, and Sterlite. The original data for this work is available at https://doi.org/10.17036/researchdata.aston.ac.uk.00 000522. We would like to thank Son Thai Le from Nokia Bell Lab, US, for their insightful discussions.

**Conflicts of Interest:** The authors declare no conflict of interest.
