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Direct detection systems with advanced modulation schemes are of great importance in metropolitan networks, because of their low cost and low power requirements. In particular, PAM-4 has attracted considerable attention, but has significant transmission distance limitations in the C-band. To extend its reach, we used a dual drive Mach-Zehnder modulator to generate a chromatic dispersion (CD) pre-compensated signal with an extra (j-1) multiplication to align the optical carrier and the modulated optical signal; by doing so, we achieved successful 128 Gbit/s transmission over an 80 km SSMF link, the longest reported reach of single lane 100 Gbit/s PAM-4 signals over DCF-free links. Synchronized bandwidth pre-compensation was also used, to reduce the influence of bandwidth-limitations.

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We experimentally demonstrate the generation and transmission of a single-lane 180  Gbit/s (90 GBaud) four-level pulse-amplitude modulation (PAM-4) signal in an intensity-modulation direct-detection system with a 7.5 GHz 3 dB bandwidth. The generated signal is transmitted over a 2 km standard single-mode fiber with, to the best of our knowledge, the highest reported net data rate in the C-band: 150  Gbit/s. A net data rate of 168  Gbit/s is also reachable with 1 km reach. The PAM-4 and duobinary (DB) PAM-4 modulation schemes are compared; the obtained results show that DB-PAM-4 significantly outperforms PAM-4 in the considered strong bandwidth-constrained system. Both a feed-forward equalizer and a maximum-likelihood sequence estimator are investigated for data recovery.

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