An optical amplifier for determining the attenuation of a length of optical fiber (7) to which it is adapted to be connected comprising means (5) to receive an input signal from the optical fiber (7), means (6) to output an amplified signal to the optical fiber (7), a gain medium (2) and a pump means (3) arranged to generate the amplified signal from the input signal, the amplifier (1) further comprising means (13) to measure the optical power of the signal leaving the gain medium and means (14) to measure the optical power of the signal backscattered from the optical fiber (7), wherein the pump means (3) is controlled by a control means (4), the control means (4) being adapted to modulate the pump means (3) with a determination signal, the control means (4) being adapted to change the frequency of the determination signal over a plurality of determination frequencies while keeping the amplitude of the determination signal constant, the control means (4) further being adapted to calculate the attenuation of the length of the optical fiber (7) from the length of the optical fiber (7), the effective refractive index of the fiber (7), and the power of the signal from the gain medium and the power of the backscattered signal measured at each of the plurality of determination frequencies.