Littrow-mounted diffraction grating cavity.

Grating resonators are not characterized by a single cavity length, and thus the cavity-mode spacing cannot simply be obtained from the standing-wave pattern. This problem is studied in a Littrow grating cavity with geometric ray tracing and a result of the scalar diffraction theory for the phase of a plane wave diffracted at a grating. The round-trip phase in the cavity is considered, and it is shown that a grating cavity may be modeled by a tilted-mirror Fabry-Perot cavity. The tilt magnitude depends linearly on the wavelength deviation from the resonant Littrow wavelength, and the mirror separation, L(C), is equal to the grating-cavity length at the center of the aperture. The model shows that the cavity axial mode separation may be determined from the standard expression, c/2L(C). An effect of finesse decrease and mode broadening, which are linearly dependent on wavelength deviation from the central Littrow wavelength, are predicted. A passive grating cavity was experimentally studied with an interferometric method and a tunable laser to demonstrate the discussed hypotheses.