We experimentally investigate the roles of electrical and optical equalizers in the 28-Gb/s wavelength-division-multiplexed passive optical network implemented by using reflective semiconductor optical amplifiers (RSOAs) and direct-detection receivers. For the generation of 28-Gb/s signal, we directly modulate an RSOA in the polar return-to-zero (RZ) N-ary pulse-amplitude-modulation (PAM) format. The results show that the optical equalization is capable of compensating the waveform distortions caused by both the bandwidth-limitation of RSOA and fiber's chromatic dispersion without deteriorating the signal-to-noise ratio (SNR). On the other hand, the electrical equalization is effective for the compensation of the bandwidth-limitation of RSOA only at the cost of SNR. The results also show that the electrical equalization is helpful to compensate for the remaining waveform distortions after the optical equalization. By using both equalizers, we demonstrate the transmission of the 28-Gb/s signals in polar RZ-PAM-N formats (N = 2, 3, and 4) over 20 km of standard single-mode fiber.