A transmitter module is disclosed, where the transmitter module includes optical sources, an optical multiplexer to multiple optical signals each output from optical sources, an optical detector to detect an output of the optical multiplexer, and a controller. The controller alternately superposes a dither signal on a bias current provided to the optical source, and the bias current is controlled based on a magnitude of the dither signal, which is detected by a configuration similar to the lock-in amplifier.