  The response of Michelson interferometers to weak plane gravitational waves
is computed at one order of accuracy beyond the eikonal equation. The
modulation of the electromagnetic field amplitude and polarisation are taken
into account by solving the transport equations of geometrical optics with
boundary conditions adapted to laser interferometry. An explicit formula for
the interferometer output power is derived, comprising perturbations of the
optical path length, frequency, amplitude and polarisation.
