Pilot-tone-based WDM monitoring technique for DPSK systems

To evaluate the effectiveness of the pilot-tone-based monitoring technique in differential phase-shift keying (DPSK) systems, we measure the mutual interactions between the pilot tones and DPSK signals and compare the results with those of on-off keying (OOK) signals. The results show that, by using the DPSK format instead of OOK, it is possible to increase the number of wavelength-division-multiplexed channels that can be monitored simultaneously by using pilot tones without using optical demultiplexers by more than one hundred times.