Enhancement of spectral efficiency FSO links using linear polarizer-based MLPolSK detection

This paper proposes the enhancement of spectral efficiency (SE) by linear polarizer based multilevel linear polarization shift keying (MLPolSK) detection in free-space optical (FSO) communication. At the transmitter end, in order to improve SE of FSO links, the various linear state of polarizations (SOPs) with an equal magnitude-spacing of x- or y-polarization are modulated for MLPolSK signal. At the receiver end, the cascaded polarization-independent semiconductor optical amplifier (SOA) and linear polarizer are introduced to detect MLPolSK signal. Polarization-independent SOA is applied to effectively mitigate the turbulence induced scintillation effect by the gain saturation without the distortion of polarization. Linear polarizer with high extinction ratio (ER) is used to transform various SOPs into the signal with an equal intensity-spacing of x- or y-polarization. Besides, the amplified spontaneous emission (ASE) noises are reduced by the polarization of ASE noises by linear polarizer. Finally, MLPolSK is detected by a single photodiode (PD) and decided by multilevel-thresholdings. In experiments, the performance of the proposed technique is evaluated under turbulence channels, which are emulated using Mach-Zehnder modulator (MZM)-based fading simulator. Experimental results show that the SE is improved up to 3 bit/s/Hz under the effective scintillation mitigation using the proposed technique.