The list of antonyms for ‘amplify’ includes words like ‘decrease’, ‘weaken’, ‘diminish’, ‘reduce’, etc. So, if you were looking for something to do the opposite of fibre optic amplify a signal, you might be tempted to look for something along the lines of a ‘diminisher’ or ‘anti-amplifier’— and if you asked for help looking for those things, you’d probably get some weird looks thrown your way. Fortunately, that’s not the case because what you’re looking for is actually called an ‘Attenuator’.
Attenuators are the effective opposite of amplifiers; they reduce the power connector of a signal without appreciably distorting the waveform and provides loss instead of gain. For optical signals, there’s the fiber optic attenuator, useful for when an optical signal is too strong to work with things like multi-wavelength systems or optical receivers with certain input specifications. Fiber optic attenuators are also common in fire optic communications applications where they test power level margins by temporarily adding a calibrated amount of signal loss, or by being installed permanently to properly match transmitter and receiver levels.