Centralized-Light-Source Two-Way PAM8/PAM4 FSO Communications With Parallel Optical Injection Locking Operation

For the first time up to our knowledge, a centralized-light-source two-way eight-level pulse amplitude modulation (PAM8)/four-level pulse amplitude modulation (PAM4) free-space optical (FSO) communication with parallel optical injection-locked vertical-cavity surface-emitting laser (VCSEL) transmitter is practically demonstrated. With the assistance of parallel optical injection locking, injection locking in the polarization sideband causes a simultaneous generation of the free-running orthogonal polarization sideband to form a centralized-light-source scheme. With the centralized light source, additional light source or wavelength reuse component, such as Fabry-Perot laser diode or reflective semiconductor optical amplifier, is not needed at the receiver side. It is very attractive because it enables the receiver side to share the light source remotely located at the transmitter side. Based on the centralized-light-source scheme and parallel optical injection-locking operation, the injection-locked parallel polarization sideband is used for downlink transmission, and the induced free-running orthogonal polarization sideband is transmitted and used as the optical carrier for uplink transmission. Over a 200-m free-space link, good bit-error-rate (BER) performance, the qualified PAM8, and PAM4 eye diagrams are obtained for two-way FSO communications. This proposed centralized-light-source two-way FSO communication provides a practical choice for two-way high transmission capacities and considerably develops the scenario characterized by parallel optical injection locking.