How does a laser-wire work?

A laser-wire uses a high power laser (1 MW in our PETRA setup, 1 GW in our ATF setup) to shoot short laser pulses on a particle beam.
A movable mirror is used to "scan" the beam: each laser pulse is deflected by the mirror with a different angle.
When the pulse hits a bunch of electrons compton photons are produced. As the pulses have been deflected with different angles by the scanner mirror, the pulse will hit the electron bunch at different vertical position. Some pulses will even be too high or too low and "miss" the electron bunch.
A calorimeter is installed downstream after a bend in the electrons trajectory. For each electron bunch this calorimeter will measure the total energy of the compton photons produced.
This distribution of the energy measured by the calorimeter as a function of the deflection angle given by the scanner mirror (i.e. the vertical position of the laser pulse) gives the beam profile.
From the beam profile, it is possible to exctract the size of the electron beam.