Abstract
We experimentally demonstrate a 56-Gb/s vestigial sideband (VSB) modified duobinary (MDB) 4-level pulse amplitude modulation (PAM4) signal transmission over an 80-km single-mode fiber (SMF) in an intensity-modulation and direct-detection (IM/DD) system without optical dispersion compensation. Compared to the conventional PAM4 signal without pulse shaping, the MDB PAM4 signal exhibits a higher spectral efficiency (SE) due to the modified duobinary precoding. Moreover, if VSB modulation is employed to mitigate the chromatic dispersion (CD)-induced power fading, the VSB MDB PAM4 signal suffers less image interference resulting from VSB filtering and direct detection (DD), thanks to its lower components at the carrier frequency. A proof-of-concept experiment is performed, in which the performances of the conventional VSB PAM4 signal, the VSB Nyquist PAM4 signal, and the VSB MDB PAM4 signal are investigated and compared comprehensively. The experimental results show that only the VSB MDB PAM4 signal can reach the bit error ratio (BER) below the 20% soft-decision forward error correction (SD-FEC) threshold of $2.4\times 10^{-\mathrm {2}}$ at the 56-Gb/s rate after the 80-km SMF transmission, due to the mitigated image interference.
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