  We present experimental data on message transmission in a free-space optical
(FSO) link at an eye-safe wavelength, using a testbed consisting of one sender
and two receiver terminals, where the latter two are a legitimate receiver and
an eavesdropper. The testbed allows us to emulate a typical scenario of
physical-layer (PHY) security such as satellite-to-ground laser communications.
We estimate information-theoretic metrics including secrecy rate, secrecy
outage probability, and expected code lengths for given secrecy criteria based
on observed channel statistics. We then discuss operation principles of secure
message transmission under realistic fading conditions, and provide a guideline
on a multi-layer security architecture by combining PHY security and
upper-layer (algorithmic) security.
