For singlemode fiber, the loss is about 0. 5 dB per km for 1310 nm sources, 0. 1 dB per 600 (200m) feet for 1310 nm . 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. When testing fiber optic cabling, determining acceptable loss is crucial. Contractors often install, terminate, and certify cabling without knowing the client's specific requirements. Cable loss (dB) = cable length (km) × attenuation coefficient (dB/km). While some loss is expected, excessive or unexpected loss can lead to poor performance, network downtime, and signal failure. Light attenuates as it travels through glass, scatters at connection points, and bends around corners.