We report on the use of an ultranarrow (similar to 0.01 nm) spectrum-sliced incoherent light source for transmission of 10-Gb/s nonreturn-to-zero signals over 20-km dispersion-uncompensated standard single-mode fiber and 0.2-nm-bandwidth optical band-pass filter. A wideband amplified spontaneous emission is first generated using an erbium-doped fiber amplifier and then spectrum-sliced by an ultranarrow fiber Fabry-Perot filter (3-dB bandwidth: similar to 0.006 nm). The spectrum-sliced light is intensity-smoothed by using a gain-saturated reflective semiconductor optical amplifier and then modulated at 10.7 or 12.5 Gb/s, assuming forward error correction (FEC) with 7% or 25% overheads, respectively. Thanks to the narrow linewidth of the source, we are able to retain the intensity smoothing after the transmission, achieving uncorrected bit-error ratios better than 10(-3) and 3 x 10(-3) at 10.7 and 12.5 Gb/s, respectively. We discuss the applicability of the proposed light source to wavelength-division-multiplexed passive optical networks and the choice of FEC codes for the proposed scheme.