An apparatus for transferring monitor signals, which apparatus receives a primary
signal, and a secondary
signal for monitoring a
wavelength, and outputs the primary
signal and the modulated secondary signal. The apparatus includes an
optical coupler to which the primary signal is input, a first optical branching filter optically connected to a signal output terminal, a first
optical amplifier optically connected in series between the
optical coupler and the first optical branching filter, and a first optical band-pass filter optically connected in series between the
optical coupler and the first optical branching filter, but optically connected in parallel with the first
optical amplifier. The primary signal is input into the optical coupler and output from the signal output terminal through the first
optical amplifier and the first optical branching filter. The primary signal is divided into two parts by the first optical branching filter, and one of the two parts is input again into the first optical
amplifier through the first optical band-pass filter and the optical coupler. The apparatus makes it possible to generate a monitor signal without employing a
laser module.