Nonlinear crosstalk and compensation in QDPASK optical communication systems

Nonlinear crosstalk in the quaternary differential-phase amplitude-shift-keying (QDPASK) modulation format is analyzed. Significant crosstalk penalty is measured on a QDPASK signal with a 20-Gb/s aggregate capacity for nonlinear phase shifts of 0.17/spl pi/ rad and above. Two different compensation techniques are demonstrated based on either prechirping or postchirping of the optical signal, increasing the nonlinear tolerance by 6.1 and 4.5 dB, respectively.