  We report on the optical characterization of an ultra-high diffraction
efficiency grating in 1st order Littrow configuration. The apparatus used was
an optical cavity built from the grating under investigation and an additional
high reflection mirror. Measurement of the cavity finesse provided precise
information about the grating's diffraction efficiency and its optical loss. We
measured a finesse of 1580 from which we deduced a diffraction efficiency of
(99.635$\pm$0.016)% and an overall optical loss due to scattering and
absorption of just 0.185 %.
  Such high quality gratings, including the tool used for their
characterization, might apply for future gravitational wave detectors. For
example the demonstrated cavity itself presents an all-reflective, low-loss
Fabry-Perot resonator that might replace conventional arm cavities in advanced
high power Michelson interferometers.
