The interferometer is the core of every Fourier-transform spectrometer. Today's FT spectrometers use a variety of interferometer designs. However, they are all still based on the simple, yet historically most important, Michelson interferometer.
In this chapter, the operating principle of the Michelson interferometer for FT spectroscopy is discussed. It is the objective of this chapter to provide a thorough physical understanding of how a spectrum is generated in an FT spectrometer. Enough mathematics is used to aid comprehension. First, a qualitative overview is provided, which is followed by a more detailed explanation starting with the wave description of light. Then, the factors that limit the output spectral resolution are explored, and finally, the interferogram processing techniques sometimes necessary to obtain accurate spectra are briefly discussed.
Figure 3.1 shows a schematic of a Michelson's interferometer. It consists of a beamsplitter and two plane mirrors that are perpendicular to each other. One of the plane mirrors, M2, moves linearly in the direction shown by the arrow. As light enters the interferometer, it is amplitude divided at the beamsplitter. Approximately one-half of the light is transmitted and the other half is reflected. The transmitted and reflected beams are then reflected at mirrors M2 and M1, respectively. The beams are then recombined at the beamsplitter and detected by a photodetector.
Under first consideration is light from a monochromatic source. When is equal to , the two beams travel the same distance from the point they leave the beamsplitter to the point where they recombine at the beamsplitter. Being in-phase, they interfere constructively (Fig. 3.2) and the detector sees a maximum intensity.