A transmitter for transmitting an optical signal in an optical communication system includes a plurality of light sources configured to output optical signals; a plurality of first optical couplers configured to multiplex the optical signals, which are output from the plurality of light sources, to generate a first optical signal, and output the first optical signal through a first output port and a second output port of each of the plurality of first optical couplers; a first monitoring unit configured to monitor the first optical signal which is output through the second output port of each of the plurality of first optical couplers; and a controller configured to control an optical output of each of the plurality of light sources on the basis of a result of the monitoring.