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. 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. Q: How is fibre optic loss measured? A: Fibre optic loss is typically measured using an Optical Loss Test. Fiber optic loss, also known as optical attenuation, refers to the light loss between the transmitter and receiver. While some loss is expected, excessive or unexpected loss can lead to poor performance, network downtime, and signal failure. Recognizing what constitutes too much loss is essential.