2 × 10 Gbit/s quaternary intensity modulation generation using an optical QPSK modulator

Two 2x10 Gbit/s quaternary intensity modulation signals (4-IM) can be generated using quadrature amplitude modulation (QAM), with unequal modulation amplitudes in two orthogonal quadratures. Two 10 Gbit/s NRZ ASK signals and a QPSK modulator allow to generate 4-IM with the same bandwidth as an NRZ-ASK signal. Measured sensitivity at a BER of 10-9 and chromatic dispersion (CD) tolerance are -21.2 dBm and ~ +130 ps/nm, respectively. Two duobinary 10 Gbit/s data streams and a QPSK modulator allow to generate a 9-point QAM signal, with the same bandwidth as a duobinary signal. A stub filter with a frequency response dip at 5 GHz was used to generate the duobinary signals. Detected as a 4-IM, this scheme features a sensitivity and a CD tolerance of -21 dBm and ~ +140 ps/nm, respectively. A polarization and phase insensitive direct detection receiver with a single photodiode has been used to detect all generated QAM signals as 4-IM signals.