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Thanks to colorless operation and cost-effectiveness, an incoherent-light-injected Fabry-Perot laser diode (FP-LD) is considered as a promising optical transmitter for wavelengthdivision- multiplexed (WDM) passive optical networks (PONs). However, the wide spectral linewidth of the transmitter output, which comes from wide linewidth of injected seed light to offer a sufficient signal-to-noise ratio and colorless operation, makes the transmitter output vulnerable to fiber dispersion. Hence, we propose and demonstrate a return-to-zero (RZ) transmitter based on incoherent-light-injected FP-LDs which offers the improved tolerance to fiber dispersion for wide-linewidth transmitter output. For incoherent light, RZ signals have the same spectral linewidth as non-return-to-zero (NRZ) signals but they have wider timing margin between adjacent bits, and thus offer better dispersion tolerance than NRZ. For the generation of RZ signals in cost-sensitive WDM-PON systems based on incoherent-lightinjected FP-LDs, we simply add electrical NRZ signals to a clock signal and then apply them to the FP-LD for direct modulation. The nonlinear light-output versus current (LI) curve of the FP-LD converts the applied electrical signals into optical RZ signals. Our experimental demonstration for 1.25-Gb/s upstream transmission shows -50% improvement of dispersion tolerance compared to NRZ signals. The back-to-back receiver sensitivity is also improved by 1.4 dB by using the proposed transmitter. Thus, we successfully transmit 1.25-Gb/s RZ signals over 45- km standard single-mode fiber without dispersion compensation. This is achieved at a low signal-to-Rayleigh-crosstalk ratio of 13.8 dB, thanks to the incoherence and wide spectral linewidth of the output light.