“Splicing a single fiber containing four cores should be interesting.”
A single cable has one core. Special connectors are used to ‘splice’ cables.
https://www.nict.go.jp/en/press/2021/07/12-1.html
The press release for the current experiment is at:
https://www.nict.go.jp/en/press/2022/05/30-1.html
While connectors could be used, I thought that fusion splicing was still the main technique used in splicing cables, such as after a dig up.