The present invention can operate a reflective semiconductor optical amplifier at ultrahigh speed using a polar return-to-zero (RZ) modulation method, and operate a reflective semiconductor optical amplifier (RSOA) whose modulation bandwidth is limited at ultrahigh speed by generating signals vertically symmetrical using a newly suggested polar RZ signal generator when generating an amplitude modulation signal at a transmission end. The present invention can overcome the problem that a modulation speed cannot be increased to 10 Gb/s or above due to signal distortion by inter-symbol-interference when generating an ultrahigh speed amplitude modulation signal using an RSOA of low price having a very narrow modulation bandwidth in an RSOA-based optical network. Also, the present invention has an effect of receiving the generated amplitude modulation signal through a direct detection receiver which is cost-effective and simple, and further has an effect of enabling ultrahigh speed operation of the RSOA-based WDM PON.