Acceptable dB loss for fiber depends on the component you're measuring: a single mated connector pair should lose no more than 0. 75 dB, a fusion splice should stay under 0. Q: How is fibre optic loss measured? A: Fibre optic loss is typically measured using an Optical Loss Test. 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 lower the dB loss, the higher the quality of the signal, and the farther it can travel without significant degradation.