Abstract: A Sagnac interferometer can include a beamsplitter arranged to receive an input beam of light of a design wavelength, to split the input beam of light into first and second beams that counter propagate around an optical path, and to recombine the first and second beams into an output beam of light. The optical path can include at least one diffraction grating that is arranged to satisfy an effective Littrow geometry.

Abstract: A Sagnac interferometer can include a beamsplitter arranged to receive an input beam of light of a design wavelength, to split the input beam of light into first and second beams that counter propagate around an optical path, and to recombine the first and second beams into an output beam of light. The optical path can include at least one diffraction grating that is arranged to satisfy an effective Littrow geometry.

Abstract: Systems and methods are disclosed for a modified Sagnac interferometer having a plurality of gratings that can be reflective or transmissive. The gratings allow measurement of wavelength spectra in counter-circulating beams of the interferometer. In one embodiment, diffraction geometries at each pair of neighboring gratings are configured so that diffractive and angular contributions reinforce each other at the second of the pair of gratings. In one embodiment, diffraction geometries at the gratings are configured so that the exiting beams of the interferometer satisfy the crossing condition wherein the exiting beams are on the opposite sides of a reference beam axis for a design wavelength input beam. Also disclosed are techniques for restoring the reinforcement and/or crossing conditions when these conditions are not otherwise met.

Abstract: A Fourier transform spectrometer based on a modified Sagnac interferometer is described. The instrument uses one or more gratings as dispersive elements to greatly improve the spectral resolution, and in-line optics to match the beam size in the interferometer to optimize the efficiency. The resolution can approach about 0.6 GHz at about 632.8 nm (?/??=8.4×105). A CCD or photodiode array is used as the detector and the fast Fourier transform of the fringe pattern is performed by a computer. Some embodiments employ no mechanically moving parts; therefore, they are compact and easy to align. Practical applications include remote sensing, process monitoring, and basic research. The fast response time of the detector also allows real time spectral analysis.