Experimental Demonstration of Using a Low-Divergence Pin-Like Beam in a 2-Gbit/s FSO Link With Limited Receiver Aperture Sizes and Variable Link Lengths Under Turbulence Effects

In FSO communication links employing conventional Gaussian beams, power loss can become an issue due to the divergence nature of a Gaussian beam. This issue becomes more severe when the Rx aperture is relatively small and/or the link length varies. Additionally, atmospheric turbulence can further degrade the link performance by causing beam wandering and scintillation. In this paper, we use a “pin-like” beam to mitigate the adverse effects of limited aperture, variable link length, and turbulence in an FSO communication link. The beam rapidly contracts upon transmission and keeps a small size over a relatively long distance. Experimental results show the pin-like beam's diameter converges from 7 mm to3.6 dB compared to a Gaussian beam at different distances (0.6 m and 0.9 m) and with different Rx aperture diameters (1 mm and 3 mm), in the presence of turbulence effects emulated by a phase plate of r0= 1 mm. This improvement translates to reduced BER and outage probability in a 2-Gbit/s OOK link. As an example, under a 3-mm-diameter Rx aperture, using the pin-like beam achieves error-free transmission in 40000 symbols for 100 random turbulence realizations, while the Gaussian beam has a 3% outage probability (BER > 3.8e-3). We further simulate a 1.5-km FSO link to compare the pin-like beam with the regular Gaussian beam and focused Gaussian beams with different focal lengths. Results reveal that, while a focused Gaussian beam may have low turbulence-induced power loss at its focal point, the pin-like beam delivers consistent performance across the link.