Experimental Characterization of Optical Fiber Bundles for Free-Space Optical Communication in High-Altitude Platforms

Optical fiber bundles (FBs) can offer significant benefits in the design of free-space optical communication systems. Until now, their primary use has been for imaging of difficult-to-access areas in medical and chemical applications. This work explores the optical and communication properties of FBs as light guiders to connect multiple optical heads to a central laser communication terminal in high altitude platforms, specifically, on the properties of FBs as part of a receiver module. We characterize the optical properties of a 1.829m-long FB from Schott AG, designed for image guiding of visible wavelengths, both at 850 nm and at 1550 nm. A 1.25 Gbit/s data transmission using on-off keying modulation at 1550 nm is demonstrated with bit error ratio (BER) measurements. To evaluate the impact of atmospheric turbulence, a spatial light modulator is used to simulate an air-to-air optical link between two aircrafts at an altitude of 10 km, separated by 20 km. Including transmission losses, to achieve a BER of 10^-6, the FB introduces a link budget penalty of 6.55 dB in the absence of turbulence and 7.94 dB under turbulent conditions, compared to an optical link without it. Future improvements for FB performances in the infrared C-band are expected through optimized material selection.