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We propose a delay-based multicast overlay scheme to superimpose a multicast differential phase-shift keying (DPSK) modulated signal on a point-to-point downstream inverse-return-to-zero (IRZ) modulated signal in a wavelength-division-multiplexed passive optical network (WDM-PON). By adjusting the synchronization of the DPSK and the IRZ modulation on the downstream carrier, simple and flexible multicast control could be realized. We have successfully demonstrated the proposed scheme for three different traffics, namely 10-Gb/s IRZ downstream point-to-point data, 10-Gb/s DPSK downstream multicast data, and 10-Gb/s non-return-to-zero (NRZ) upstream re-modulated data, respectively. An error floor of 10-4 is observed for the multicast data when it is disabled. The effect of timing misalignment on the downstream DPSK/IRZ orthogonal modulation and the upstream re-modulation is analyzed. We also study the case when the data rates of the multicast and downstream point-to-point data are different.