Compared with the optical communication system using intensity modulation with direct detection (IM-DD), the
coherent optical system enhances the sensitivity and enables access to all the optical characteristics. In this paper, a novel
scheme of coherent QAM transmission system based on heterodyne optical detection is experimentally demonstrated. In
the proposed scheme, the intermediate frequency (IF) carrier modulation at double-sideband suppressed carrier (DSB-SC)
mode at the transmitter side enables demodulation of QAM signal with heterodyne detection. Furthermore, the receiver
is simplified by avoiding the use of high frequency broadband microwave devices due to the employment of IF carrier
modulation. The performance of the proposed coherent QAM system is analyzed theoretically based on numerical
simulation. The simulation results show that the improved structure of the proposed system can effectively depress the
phase noise induced by laser. An experiment of 4-QAM transmission exploiting the proposed coherent heterodyne
system is presented to justify the principle.
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