Patent ID: 8004752

Claim:
An optical transmission system comprising: an optical transmitter transmitting a WDM optical signal including a plurality of optical signals with different wavelengths; a multi-stage optical amplifier receiving the WDM optical signal transmitted by the optical transmitter, amplifying the received WDM optical signal and outputting the amplified WDM optical signal; and an optical receiver receiving the amplified WDM optical signal output from the multi-stage optical amplifier, the multi-stage optical amplifier including a first stage optical amplifier amplifying the WDM optical signal received by the multi-stage optical amplifier, a level controller controlling a power level of the WDM optical signal amplified by the first stage optical amplifier, a second stage optical amplifier amplifying the WDM optical signal of which level is controlled by the level controller, and an optical coupler splitting the WDM optical signal amplified by the second stage optical amplifier into a monitor optical signal and the amplified WDM optical signal to be output from the multi-stage optical amplifier.