Negative loss means the fiber under test is measuring less loss than what was recorded when the reference measurement was performed. 09 dB, the following warning is given on the CertiFiber Pro: A negative loss is often referred to as a gainer. This should not be possible on a passive link, yet your CertiFiber Pro is reporting just that! The most common cause is setting a reference through a. 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 this guide, we will show you how to interpret. All single mode fibers work very similarly at any wavelength, and if your fiber optic components are properly constructed using quality materials and good technique, then the insertion loss value for any given fiber optic connector when tested on a 1310 or 1550 Should be very similar.