The performance of the Four-level Manchester coding (4-MC) and Four-level Modified Manchester (4-MM) technique were assessed at 20 Gb/s and 40 Gb/s bitrates using the first-order and second-order optical equalizers. The results indicate that applying the second-order optical equalizer, at 20 Gb/s, the transmission distance of 4-MC and 4-MM increases by 25 km and 27 km of standard single-mode fiber (SSMF), respectively, whereas at the bitrate of 40 Gb/s, the 4-MC and 4-MM transmission distance were extended by 6.5 km and 7.5 km of SSMF, respectively. Thus, with second-order optical equalizer the fiber dispersion can be adequately compensated and subsequently improved the overall BER performance. Hence, the proposed technique could serve as a substitute to the next-generation short-reach optical transmission links.