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Third-order intermodulation (IM3) is a very important issue as a degradation factor of system performance in the range of high input signal power. In this paper, the effect of IM3 from a dual-electrode Mach-Zehnder modulator (DEMZM) and a photodetector (PD) is analyzed for optical single-sideband (OSSB) and optical double-sideband (ODSB) signals incorporating fiber dispersion. In addition, the optimum input signal power and the signal-to-noise-and-distortion ratio (SNDR) for the two cases are also investigated to optimize the performance of the entire system. In the case of OSSB signals, the fundamental components are robust against fiber dispersion, whereas their IM3 components are still sensitive to fiber dispersion. Subsequently, the SNDR for OSSB signals fluctuated to within 6 dB in the relatively high input power range due to fiber dispersion. In the case of ODSB signals, both powers of the fundamental and IM3 components are attenuated. However, the power attenuation of IM3 due to fiber dispersion is significantly faster than that of the fundamental. Thus, the SNDR for ODSB signals is improved as fiber dispersion increases until the power of IM3 is greater than that of the additive noise level.