Interferometer designs

In the mid-IR, routine infrared spectroscopy nowadays almost exclusively uses Fourier-transform (FT) spectrometers. This principle is a standard method in modem analytical chemistry45. Although some efforts have been made to design ultra-compact FT-IR spectrometers for use under real-world conditions, standard systems are still too bulky for many applications. A new approach is the use of micro-fabrication techniques. As an example for this technology, a miniature single-pass Fourier transform spectrometer integrated on a 10 x 5 cm optical bench has been demonstrated to be feasible. Based upon a classical Michelson interferometer design, all... 

Figure 6 shows the interferometer designed for strong fields. 

For FT-Raman spectrometers, an equivalent one-point calibration is more reliable because interferometers are less prone to mechanical errors. Nearly all interferometer designs include a well-defined reference wavelength (often a He-Ne laser at 632.8 nm), which is used to control data acquisition. In addition, observed FT frequencies are calculated from a large number of individual measurements, so minor mechanical jitter and random timing errors are averaged out. Provided the laser and reference frequencies are known accurately, an observed FT-Raman frequency is quite accurate, and the one-point calibration is usually adequate. 

The basic component of most Fourier Transform Infrared spectrometers is the Michel son interferometer. This is not the only interferometer used in FT-IR, but it is employed more often than other designs. A treatment of many other interferometer designs is available. The Michel son interferometer in a Fourier Transform Infrared spectrometer replaces the monochromator in a dispersive instrument, although the functions cannot be correlated. A monochomator divides a continuous bandwidth into its component frequencies, whereas an interferometer produces interference patterns of the bandwidth in a precise and regulated manner. It should be noted that this type of interferometer is not restricted to the infrared region and its use can be extended to the visible and millimeter regions of the electromagnetic spectrum. 

Figure 1. Basic interferometer design with source optics and detector optics.

Figure 2 shows the interference fringes produced by the interferometer as the mirror moves along its track. The independent variable in this plot is optical path difference rather than mirror position. The use of optical path difference simplifies the discussion when more exotic interferometer designs employing multiple moving mirrors, or multipass optics are used. 

The basis of Fourier transform infrared (FT-IR) spectroscopy is the two-beam interferometer, designed by Michelson in 1891 [28]. and shown... 

Multiple interferometer types exist in modern spectrometers the reader is referred to Candler (1951), Jamieson et al. (1963), Steel (1983), and Strobel and Heineman (1989) for a more exhaustive description of the optical configurations for these devices. The classical interferometer design is represented by the Michelson interferometer as shown in Fig. 3. A MM is displaced linearly at minute distances to yield an interference pattern (or interferogram) as a series of sine waves when the interference pattern is observed from a specific field of view (Fig. 7A). 

Figure 5.21. Interferometer designed by Kauppinen et al. (Reproduced from [23], by permission of the Optical Society of America copyright 1995.)...

Figure 5.24. Field-widened interferometer designed by Bouchareine and Connes [29]. The movable rear-silvered mirror is translated in a direction perpendicular to that of a conventional Michelson interferometer, so that the OPD is increased by increasing the distance traveled through the prism. (Originally reproduced from [29], by permission of the author.)...

Figure 5.25. Refractively scanned interferometer designed by Doyle. (Originally reproduced by permission of the Analect Instruments Division of Laser Precision Corporation.)...

It has been established that an interferometer design first proposed by Martin and Puplett [2] is the most effective FT-IR spectrometer configuration for far-infrared measurements, particularly in the wavenumber region lower than 150 cm [3],... 

There are a number of interferometer designs used by FTIR manufacturers. The oldest and perhaps the most common type of interferometer in use today is the Michelson interferometer. It is named after Albert Abraham Michelson (1852-1931) who first built his interferometer in the 1880s [1] and went on to win a Nobel Prize in Physics for the discoveries he made with it. The optical design of a Michelson interferometer is shown in Figure 2.2. Even if your FTIR does not have a Michelson interferometer in it, the following discussion will be relevant because the basics of interferometer operation are similar for all interferometer types. 

Interferometer design and implementation has evolved to the point where all manufacturers of F,T. spectrometers have built-in sufficient rigidity and sufficient isolation from external perturbations such that their instruments can now operate reliably in a laboratory environment over extended periods. 

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