Increasing the transmission distance is an attractive technological option for lowering the cost of loopback-configured coherent wavelength-division-multiplexed passive optical networks implemented by using reflective semiconductor optical amplifiers (RSOAs). In this regard, it is necessary to increase the fiber launch power of the seed light provided from the central office. However, the stimulated Brillouin scattering (SBS) limits this power to less than 6 dBm. It is well-known that the frequency-dithering technique increases the SBS threshold effectively, allowing us to launch a higher-power continuouswave light. However, the linewidth of the seed laser (caused by the frequency dither) makes the carrier phase estimation (CPE) very difficult at the coherent receiver. To solve this problem, we propose a dither-frequency tuning technique in this letter. We set the dither frequency to be the integer times the inverse of the round-trip propagation delay. Then, we can make the frequency dither of the upstream signal unseen at the receiver since it becomes in phase with the frequency dither of the local oscillator. Thus, the CPE performance would not be limited by the frequency dither, but by the natural linewidth of the seed laser. The efficacy of the proposed technique is experimentally demonstrated over a 60-km loopback link using a 28-Gb/s quadrature phase-shift keying signal generated from RSOA.