Generally, for single-mode connectors, the recommended insertion loss is below 0. Insertion loss, also known as attenuation, is the loss of optical power that occurs when light passes through a fiber optic connector. It is caused by factors such as misalignment, air gaps, and imperfections in the connector components. Losses in the optical fiber can be categorified. Design and validate fiber-optic links in seconds. Add each MUX or DEMUX on the path. Therefore. 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.