  This paper considers secure communication over free-space optical (FSO) links
suffering from turbulence-induced fading. In particular, we study the average
secrecy capacity of applying physical layer security to achieve security in an
FSO communication system with binary-level on-off keying intensity modulation
and threshold detection, a widely used system in FSO communication. We first
define the instantaneous secrecy capacity for our system and then provide a
lower bound of it, which is achieved based on a simple design principle for
practical applications and yields a simpler equation. We next derive a general
expression of the average secrecy capacity and the corresponding lower bound
based on bit error probabilities. Finally, numerical analysis is conducted for
the special case of the correlated log-normal fading model to show the impacts
of fading and channel correlation on the average secrecy capacity.
