the present invention relates to the quality monitoring of optical signals , which have different symbol rates and generated by different modulation schemes , used for example in a wavelength division multiplexing network . the apparatus according to the invention includes an optical splitter for outputting the input optical signal to a first optical route and a second optical route , an optical coupler for coupling a optical signal from the first optical route with a optical signal from the second optical route , a delay unit provided on the first optical route , and a phase shift unit provided on the first optical route or the second optical route .