an optical amplifier comprises an active fiber , a pump unit spaced from the active fiber and adapted to give a nominal , continuous pump power in an operational state , and a pump fiber adapted to transfer optical pump power from the pump unit to the active fiber . moreover , in a safety state , the pump unit is adapted to give a pulsed pump power whose mean power is lower than a prescribed safety limit . a method of preventing emission of optical power exceeding a prescribed safety limit on interruption of an optical fiber which transfers pump power from a pump unit to an active fiber , comprises changing the mean power of the pump power in response to a signal received from the active fiber so that the mean power assumes a value below said safety limit if said signal is not received , and assumes a nominal value if said signal is received .