**5. Conclusions**

We present, for the first time, the use of half-open cavity random DFB Raman laser amplification with combinations of different FBGs to extend the bandwidth of the mid-link OPC system beyond the standard C band. We performed gain profile optimization in a 60 km standard SMF span utilizing a 6 THz bandwidth with total of 60 WDM channels. With a fine optimization of FBGs and pump powers, we achieved for the first time a record

5.9% average asymmetry in a 192–198 THz band using single wavelength 1366 nm Raman laser pump only in an OPC system. We also achieved 2.1% asymmetry performance for a single channel, which is the lowest asymmetry achieved to date in a 60 km standard SMF span length. The results show the potential for further improvement by utilizing 6 THz band using DWDM with a 25 GHz or even 12.5 GHz spacing that would increase the total transmission capacity of an OPC system by an order of magnitude.

**Author Contributions:** P.R., G.R.M. and M.T. proposed the concept and initiated the study. P.R. and G.R.M. carried out numerical simulations and conducted analytical calculations. M.T. supervised the studies. The paper was written by and P.R., G.R.M. and M.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by a UK Engineering and Physical Sciences Research Council (EPSRC) Grant (ARGON), a Royal Society International Exchange Grant (IEC\NSFC\211244), the Aston Impact Fund, and a Polish Ministry of Science and Higher Education Grant (12300051).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Original data is available at Aston Research Explorer (https://doi.org/ 10.17036/researchdata.aston.ac.uk.00000575).

**Acknowledgments:** Not applicable.

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