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We propose and demonstrate a novel colorless optical transmitter based on all-optical wavelength conversion using a reflective semiconductor optical amplifier (RSOA) for upstream transmission in wavelength-division-multiplexed passive optical networks. The proposed colorless optical transmitter for the optical network unit is composed of an electro-absorption modulated laser (EML), an optical coupler, and an RSOA. Through cross-gain modulation in the RSOA, the upstream data from the EML pump light are imposed onto a continuous-wave probe light provided from the central office (CO). An optical delay interferometer at the CO tailors the chirp of the upstream signal to improve the bandwidth of the system and dispersion tolerance. The proposed upstream optical transmitter is based on the fast gain recovery of the RSOA governed by carrier-carrier scattering and carrier-phonon interactions. Thus, it can potentially operate at >; 10 Gb/s. Two separate wavelength bands are allocated, one for the pump signals and the other for the probe signals. Therefore, the proposed transmitter operates in a colorless manner since the EML can have any arbitrary wavelength within the pump band. We demonstrate the transmission of a 10.7-Gb/s upstream signal generated by the proposed scheme in a single-fiber loopback-configured network. The conditions for colorless operation of the proposed transmitter are investigated through experiment.