Close-up inside view of fiber being fusion spliced together. Fusion splice machines use an electric arc discharge to melt and fuse the fibers together. The splicer holds the fibers in place and adjusts them so that their cores are perfectly aligned. The electric arc heats the fibers to a high temperature, causing them to melt and join together. The short flash you see before the splice itself is the machine burning off any dust or microscopic shards prior to the burn.
This video shows a single strand of glass being fused however the industry has largely moved to “ribbon” fiber where 12 strands are glued together (it looks like a ribbon, hence the name). The process is generally the same only the electrodes arc across all 12 strands at one time.
This video shows what a ribbon fusion splice looks like: