Patent ID: 8320040

Claim:
An optical transmission system comprising: an optical transmitter that transmits a wavelength division multiplexed (WDM) optical signal including a plurality of optical signals with different wavelengths; a multi-stage optical amplifier that receives the WDM optical signal transmitted by the optical transmitter, amplifies the received WDM optical signal and outputs the amplified WDM optical signal; and an optical receiver that receives the amplified WDM optical signal output from the multi-stage optical amplifier, wherein the multi-stage optical amplifier includes a first optical splitter that splits the received WDM optical signal into a first monitor optical signal and a first transmission WDM optical signal, a first stage optical amplifier that amplifies the first transmission WDM optical signal, a second optical splitter that splits the first transmission WDM optical signal amplified by the first stage optical amplifier into a second monitor optical signal and a second transmission WDM optical signal, a level controller that controls a power level of the second transmission WDM optical signal, a third optical splitter that splits the second transmission WDM optical signal of which level is controlled by the level controller into a third monitor optical signal and a third transmission WDM optical signal, a second stage optical amplifier that amplifies the third transmission WDM optical signal, and a fourth optical splitter that splits the third transmission WDM optical signal amplified by the second stage optical amplifier into a fourth monitor optical signal and the amplified WDM optical signal.