  A Fabry-Perot cavity polarimeter, installed in 2003 at HERA for the second
phase of its operation, is described. The cavity polarimeter was designed to
measure the longitudinal polarisation of the HERA electron beam with high
precision for each electron bunch spaced with a time interval of 96ns. Within
the cavity the laser intensity was routinely enhanced up to a few kW from its
original value of 0.7W in a stable and controllable way. By interacting such a
high intensity laser beam with the HERA electron beam it is possible to measure
its polarisation with a relative statistical precision of 2% per bunch per
minute. Detailed systematic studies have also been performed resulting in a
systematic uncertainty of 1%.
