Free-space optical communications links have the perpetual challenge of coupling light from free-space to a detector or fiber for subsequent detection. It is especially challenging to couple light from free-space into single-mode fiber (SMF) in the presence of atmospheric tilt due to its small acceptance angle; however, SMF coupling is desirable because of the availability of extremely sensitive digital coherent receivers developed by the fiber-telecom industry. In this work, we experimentally compare three-mode and single-mode coupling after propagating through 1.6 km of free-space with and without the use of a fast-steering mirror (FSM) control loop to mitigate atmospherically induced tilt. Here, the 3-mode fiber is a 3-mode photonic lantern multiplexer (PLM) that passively couples light into three SMF outputs. With the FSM control loop active, coupling into the PLM and the SMF yielded nearly identical coupling efficiencies, as expected. Experimental results with the FSM control loop off show that coupling from free-space to PLM increases the average power received, and mitigates the negative impacts of tilt-induced fading relative to coupling directly to SMF.
D. J. Geisler, T. M. Yarnall, G. Lund, C. M. Schieler, M. L. Stevens, N. K. Fontaine, B. S. Robinson, and S. A. Hamilton, "Experimental comparison of 3-mode and single-mode coupling over a 1.6-km free-space link," Proc. SPIE 10524, Free-Space Laser Communication and Atmospheric Propagation XXX, 105240H (Presented at SPIE LASE: January 29, 2018; Published: 15 February 2018); https://doi.org/10.1117/12.2294039.
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