  The ALFA laser subsystem uses a 4 watt continuous wave laser beam to produce
an artificial guide star in the mesospheric sodium layer as a reference for
wavefront sensing. In this article we describe the system design, focusing on
the layout of the beam relay system. It consists of seven mirrors, four of
which are motor-controlled in closed loop operation accounting for turbulences
inside the dome and flexure of the main telescope.
  The control system features several computers which are located close to
analysis and control units. The distribution of the tasks and their interaction
is presented, as well as the graphical user interface used to operate the
complete system. This is followed by a discussion of the aircraft detection
system ALIENS. This system shuts off the laser beam when an object passes close
to the outgoing laser.
