  Noiseless optical components are critical for applications ranging from
metrology to quantum communication. Here we characterize several commercial
telecom C-band fiber components for parasitic noise using a tunable laser. We
observe the spectral signature of trace concentrations of erbium in all devices
from the underlying optical crystals including YVO4, LiNbO3, TeO2 and AMTIR
glass. Due to the long erbium lifetime, these signals are challenging to
mitigate at the single photon level in the telecom range, and suggests the need
for higher purity optical crystals.
