20 Msymbol/s, 128 QAM Coherent Optical Transmission over 500 km Using Heterodyne Detection with Frequency-stabilized Laser

Abstract
We have successfully transmitted a 20 Msymbol/s, 128 QAM coherent signal over 500 km using heterodyne detection with a frequency-stabilized fiber laser and an offset-locking technique. Error free transmission (7 bit/symbol) was achieved.