Such devices can be sensitive to: modal distribution, wavelength, contamination, vibration, temperature, damage due to power bursts, may cause back reflections, may cause signal dispersion etc. A Variable Optical Attenuator (VOA) is a controllable device used to reduce the optical power traveling through a fiber or free-space optical path. Think of it as a permanent filter. Once it is installed in your link, the amount of signal reduction stays exactly the same. The attenuator circuit will allow a known source of power to be reduced by a predetermined factor, which is usually expressed as decibels. Unlike fixed attenuators (which have a set value like 5dB or 10dB), a VOA allows for continuous or incremental adjustment of attenuation, typically ranging from 1dB to 30dB or even 60dB.