140-Gb/s 20-km Transmission of PAM-4 Signal at 1.3 $\mu \text{m}$ for Short Reach Communications

We experimentally demonstrated 140-Gb/s transmission over 20-km standard single-mode fiber employing pulse-amplitude modulation (PAM)-4 and direct detection (DD) at 1.3 μm. DD faster than Nyquist is employed to compensate for channel impairments. The optimal length of taps of decision directed least mean square and the optimal tap coefficient for digital postfilter are investigated. A receiver sensitivity of -5.5 dBm at bit error rate of 3.8× 10
^-3
is realized for 140-Gb/s PAM-4 signal after 20-km transmission. To the best of our knowledge, this is the highest reported baud rate (70 GBd) of direct detected PAM-4 signal and the highest per channel bit rate with single polarization and DD for short reach communications.