If the second fiber has higher backscatter than the first, the OTDR can measure apparent gain (negative loss) at the splice. It is impossible -- a passive splice cannot amplify light -- but it appears in the trace because of the backscatter. To be able to judge whether a fiber optic cable plant is good, one does a insertion loss test with a light source and power meter and compares that to an estimate of what is a reasonable loss for that cable plant. The estimate, called a "loss budget" is calculated using typical component losses for. A high loss on a fusion splice can mean that the fusion of the two fibers may not have properly occurred and you have a weak slice that could fail pre-maturely. I feel like the correct answer here is “optical design”. Fiber engineers will design a build and account for losses. You want low splice loss because signal loss can weaken communication and reliability. Understanding its causes and solutions is critical for reliable fiber optic installations.
