Circular polarization shift-keying and amplitude shift-keying hybrid modulation method based on orbital angular momentum division multiplexing

摘要

Circular polarization shift-keying (CPolSK) modulation maps binary digital signals to the two circular polarization states of the optical carrier, which can be applied to the modulation-demodulation of free space optical communication systems. This paper constructs a new optical communication system that uses vortex beams with different orbital angular momentum (OAM) modes as the optical carriers to multiplex CPolSK modulated signals. And with limited OAM modes, we combine amplitude shift-keying (ASK) modulation and CPolSK modulation to construct a modulation and demodulation system which is similar to quadrature amplitude modulation. In the receiver, a designed reference beam is used to interfere with the signal beam. Then the hybrid modulation signals can be easily converted into the intensity changes that can be directly detected for demodulation. The numerical simulation results show that this modulation system can effectively expand the communication capacity of CPolSK modulation system. And after combining ASK and CPolSK, the system can get better communication performance. It shows a wide prospect in the field of free space optical communication.