  We experimentally demonstrate the phase relations of 3-port gratings by
investigating 3-port coupled Fabry-Perot cavities. Two different gratings which
have the same 1st order diffraction efficiency but differ substantially in
their 2nd order diffraction efficiency have been designed and manufactured.
Using the gratings as couplers to Fabry-Perot cavities we could validate the
results of an earlier theoretical description of the phases at a three port
grating.
