64/128-QAM Half-Cycle Subcarrier Modulation for Short-Reach Optical Communications

The half-cycle subcarrier modulation (SCM) with 64/128-quadrature-amplitude modulation (QAM) format in short-reach optical communications system is experimentally demonstrated. Without utilizing erbium-doped fiber amplifier, we have demonstrated the transmission of single-polarization 64/128-QAM half-cycle SCM using cost-effective directly modulated laser and also transmission of dual-polarization signal using a pair of external modulators. The experiment is enabled by spectral-efficient overlap frequency-domain-equalization and polarization division multiplexing channel estimation based on a pair of time-interleaved training sequences. Utilizing an electrical bandwidth of only 3 GHz, the bit-error rate (BER) of the 32.5-Gb/s dual-polarization 64-QAM format after 20-km fiber transmission is under 3.8 × 10^-3, and BER of the 38-Gb/s dual-polarization 128-QAM format is under 2.4 × 10^-2.