  Laser interferometer response to a plane gravitational wave on the Minkowski
background is given. The derivation does not assume any particular gauge within
a class compatible with almost Minkowskian coordinates that preserve a plane
wave form of the solutions. Consequently all ten modes of the metric are taken
into an account. The final result, the time of flight of a laser signal
exchanged between freely moving observers, is expressed in the form of
integrals of the metric perturbations taken along the undisturbed trajectories.
The result can be applied in any metric theory o gravity; it is valid in the
first order of the perturbation and does not assume the long wavelength
approximation. The obtained response is shown to define an observable, i.e. it
is gauge--invariant with respect to the assumed class of gauge transformations
