A: For singlemode fiber, loss should be under 0. Q: Why is my fiber showing 10 dB loss?At TREND Networks, we are frequently asked how much loss is allowed when conducting testing on fiber optic cabling. Unfortunately, it is not a simple answer and depends on several factors. So how do you determine acceptable loss? When testing fiber optic cabling, determining acceptable loss is. Fiber loss can be also called fiber optic attenuation or attenuation loss, which measures the amount of light loss between input and output. There are various causes of fiber optic loss, such as absorption/scattering of light energy by fiber material, bending loss, connector loss, etc.