For singlemode fiber, the loss is about 0. 5 dB per km for 1310 nm sources, 0. 5 dB/km at either wavelength for outside plant max per EIA/TIA 568)This roughly translates into a loss of 0. 1. 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. In addition to length, events that cause reflections. All Singlemode fibers work very similarly in either wavelength—that is, you don't need to buy fiber based on wavelength, one fiber fits all. Essentially, the guided mode from the first fiber (the input) creates some amplitude profile in the second fiber, which may be somewhat displaced, for example, due to an imperfect splice.