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Optical interferometers have been used extensively in many applied fields of science including chemistry and physics. Conventional interferometers have provided one, if not more of the following advantages over conventional dispersive spectrometers: (1) multiplex advantage (Fellgett's advantage) (2) high frequency precision (Connes' advantage) (3) high optical etendue' or throughput (Jacquinot advantage) There are many different types of optical interferometers available on the market today (e.g. Fizeau, Fabry-Perot, etc.,) but perhaps the most important, at least historically, is the Michelson interferometer. Most Fourier transform infrared spectrometers today are based on this design. The Michelson-type interferometer, which employs a beamsplitter and a moving mirror can be thought of as an amplitude splitting device. In other words, the original beam from the source is split in half by a beamspliiter with each half traversing one "arm" of the interferometer. One arm contains a movable mirror which modulates the interference fringe pattern generated as the two halves (i.e. each arm) are recombined by the beamsplitter. Since the movable mirror is translated with time, the interferogram is effectively recorded as a function of time. This temporal interferogram can be further related back to the optical path difference between the two arms of the interferometer. Stroke and Funkhouser proposed a Fourier transform spectrometer which created a spatial interferogram as opposed to the temporal interferogram produced by conventional Michelson-type interferometers. Some have referred to this device as a holographic spectrometer or source doubling interferometer. These devices can be thought of as wavefront splitting devices analogous to Young' double slit experiment or the Fresnel biprism. The primary source is split into two coherent secondary sources which will exhibit constructive and destructive interference in the plane. An appropiate Fourier transform of this interferogram will reconstruct the cosine frequency components of the primary source. There are many devices that make use of a spatial interferogram. Presently, in our laboratory, work is underway to develop a stationary Hadamard transform (HT) interferometer that utilizes a liquid crystal optical shutter array to encode a spatial interferogram created by a Fizeau interferometer design. Thus, the HT stationary interferometer becomes a no moving parts spectrometer.
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