The “signal footprint” of a transmission refers to how much a signal diverges from transmitter to receiver. For example, when an RF signal is sent from a satellite to the ground, the signal footprint can spread out from the 1-meter transmitter to the size of a continent. Not only does a large footprint cause a signal to lose power, but it also opens the signal up to interception.
An FSO signal has low divergence from transmitter to receiver because its source is a laser. Laser beams have small beam widths which stay small for the entirety of transmission due to the collimated nature of the light. To intercept a laser beam transmission, an eavesdropper would have to be in the path of the beam which would immediately let the receiver know that the signal is being listened to (see Interception Resistance). This makes FSO transmissions much harder to intercept, decreasing its security risk.
For example, a LEO-to-ground FSO link has a laser beam footprint of 100 m. This is much smaller than a continent-sized RF footprint, making the signal much more difficult to intercept. The same is also true about satellite-to-satellite communication. [1]