Physical-layer security of a binary data sequence transmitted with bessel-Gaussian beams over an optical wiretap channel

When an eavesdropper performs an optical beam-splitting attack in a free-space optical communications channel, it is referred to as an optical wiretap channel, which is an extension of Wyner's wiretap channel model. Even though physical-layer security can be compromised, it is possible to exploit the noisy and degraded channel conditions experienced by the eavesdropper to obtain positive secrecy capacity even when a shared secret key is not used. In our previous work, we found that employing Bessel-Gaussian beams can help to improve physical-layer security and provide higher secrecy capacity over that of Laguerre-Gaussian beams in a turbulent free-space optical communications channel. In this companion paper, we conducted an experiment exclusively with Bessel-Gaussian beams onto which we encoded a pseudorandom binary sequence to emulate data transmission over this optical wiretap channel. Bit-error rate curves for the intended receiver and the eavesdropper were calculated from which estimates of secrecy capacity were derived. We found that the bit-error rate curves for the eavesdropper were consistently worse than those of the intended receiver under several turbulence conditions and that further evidence of an error floor even when the eavesdropper uses an optical amplifier is promising for secure communications.