Review of All-Optical Modulation Format Conversion for 8QAM Signals Based on Nonlinear Effects

This paper reviews the recent all-optical modulation format conversion (MFC) schemes for the 8-ary quadrature amplitude modulation (8QAM) signal based on nonlinear effects arising from the χ
⁽²⁾
and χ
⁽³⁾
nonlinear mediums including the high nonlinear fiber (HNLF), semiconductor optical amplifier (SOA) and periodically-poled lithium niobate (PPLN). The optical signals with different modulation formats are allocated to different types of optical networks according to the transmission distance, cost, energy consumption, receiving devices complexity, sizes of optical networks and other factors. The constellations of the common 8QAM signal have many different distribution types. It can be aggregated by some simple modulated signals and the inverse conversion can also be performed in the optical domain. The investigation of MFC about 8QAM signals is meaningful at the network intermediate nodes, which are used to connect to different optical networks, for example, the short-reach and long-haul optical networks.