Smoke, mirrors and the GBU - 12

Yesterday I read two articles on laser assisted
bombing which were quite satisfying but still
managed to leave me wondering about the technology
and the possibility of employing countermeasures
during weapon delivery.
Without giving away any "state secrets" perhaps
someone on rec.aviation,military could dispell
my idle thoughts on future possibilities?

In description of F15 strikes in Afghanistan
one author described instances where cloud cover
prevented the AC from lasing ground targets.
Numerical recognition codes required by the
F15 mounted laser and its bomb were relayed
by pilot voice (encrypted?) to well trained
FACs on the ground who punched the codes into their
man portable units. The GBU-12s were dropped
and completed flight under control of the FACs
using battery powered lasers aimed at their
targets, usually armour.

Question - how many times per second is the
recognition code transmitted on the beam or
is it continuous? When the laser is reflected
off of the target how widely dispersed does
the beam become and what happens to information
encoded in the laser beam? (If I am not
understanding the process, please explain.)
Assuming that countermeasures
are designed for a particularly costly and
sensitive military asset, what would be the
approximate configuration of kit designed to
acquire the beam, decode the signal, and turn
on countermeasures to make the bomb miss?
....because of course you cannot stop F15s from
flying or their bombs from being dropped.

Thankyou.