Traceback equalization for non-uniformly synthesized optical QAM signals

Optical quadrature-amplitude modulation (QAM) enables high-spectral- efficiency transmission, where we can generate high-order optical multilevel signals, combined from multiple low-order multilevel signals (like binary streams). This signal combining approach is useful because it is difficult to directly generate multilevel signals at high symbol rate beyond ∼10 Gbaud. Electrical combining is one of the possible ways, where electrical multilevel signals are generated combining binary sequences first, and then IQ modulator is driven with the signals. Another one is optical synthesis approach, where optical signals are combined in optical domain by using integrated serial or parallel modulators to create higher order multilevel signals [1][2].