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We investigate the effects of back-reflection in wavelength-division-multiplexing passive optical networks having an injected seed light for color-free operation of an optical network unit. The back-reflection-induced penalty is reduced as the linewidth of the seed light is increased. The wavelength-locked Fabry-Perot laser diode to an injected spectrum-sliced amplified spontaneous emission light shows a high robustness on the back-reflections, while a coherent seed light exhibits a high-reflection-induced penalty. We also estimate the maximum allowable fiber loss according to the linewidth of the seed light.