A bubble appears as a dark circular or oval void within the splice region. Cause: Contamination on one or both fiber end-faces. This guide reveals the secrets to fusion splicing with little fluff—just proven, straightforward techniques refined from years of work in the field. The guide provides the complete workflow, covering safety precautions, tool selection, fiber preparation, fusion operation, quality control, and. The fusion splicer flags every kind of problem with its own visual signature, but the troubleshooting is the same: identify the defect, find the root cause, fix it, and re-splice. I hope it will help you a little.