Multilevel signaling for increasing the reach of 10 Gb/s lightwave systems

TLDR

These signal types concentrate power near the optical carrier to reduce chromatic‑dispersion‑induced phase distortion. The study investigates 10 Gb/s polybinary, AM‑PSK, M‑ary ASK, and polyquaternary signaling schemes in 1550‑nm single‑mode fiber systems. The authors analyze modulator chirp, optimal 4‑ary ASK level spacing, and PM‑AM noise conversion due to laser linewidth. Higher‑order polybinary signals do not improve dispersion tolerance over duobinary; 4‑ary ASK extends dispersion‑limited reach to 225 km experimentally (350 km simulated) but degrades receiver sensitivity by ~8 dB, and requires laser linewidth <1 MHz to limit PM‑AM RIN over 200‑300 km.

Abstract

Polybinary, optical amplitude modulated phase shift keying (AM-PSK) polybinary, M-ary amplitude shift keying (ASK), and polyquaternary signaling schemes operating at 10 Gb/s are investigated in 1550-nm lightwave systems operating over standard, single-mode fiber. The premise for exploring these signal types is that they concentrate power at frequencies closer to the optical carrier where phase distortion of the optical field from chromatic dispersion is less severe. Issues such as modulator chirp, optimal level spacing in a 4-ary ASK signal, and phase modulated to amplitude modulated (PM-AM) noise conversion from a nonzero laser linewidth are studied. It is found that higher order polybinary signals do not offer an improvement in dispersion tolerance over a duobinary signal. 4-ary ASK is demonstrated to increase the dispersion-limited distance to 225 km experimentally and 350 km through simulation, but at the expense of a /spl sim/8 dB degradation in receiver sensitivity due to the increased number of levels and the signal dependence of signal-spontaneous beat noise. Furthermore, the linewidth requirement for a 4-ary ASK signal is less than 1 MHz in order to minimize the impact of PM-AM relative intensity noise (RIN) when transmitting over 200-300 km.