| 摘要 |
A 2 n -QAM (e.g. 16-QAM) optical modulator 10 comprising cascaded I-Q modulators 22, 38. The first I-Q modulator 22 applies 2 n-2 (e.g. 4) QAM to an optical signal, having a constellation diagram with the 2 n-2 constellation points located in quadrant I. The second I-Q modulator 38 subsequently applies a quaternary phase-shift keying (QPSK) modulation scheme to the optical signal, thereby rotating the constellation points of the 2 n-2 -QAM modulation scheme to quadrants II, III and IV, to produce a 2 n -QAM modulation constellation diagram. The rotation causes the 2 n -QAM modulator to inherently apply four quadrant differential encoding to the optical signal. A method of 2 n -QAM optical modulation is also provided and optical signal transmission apparatus comprising the 2 n -QAM optical modulator. |
