Interferometer-based spectrometers

In principle, an interferometer-based spectrometer has several basic advantages over a classical dispersive instrument ... 

The most important component of an FTIR spectrometer is an interferometer based on the original design by Michelson in 1891, as shown in Figure 3.11. 

FTIR has become more portable in recent years due in part to the miniaturisation of the interferometer. However, FTIR instruments have only recently been reduced in size to the same extent as UV-Vis or NIR devices. D P Instruments has developed the Model 102 portable interferometer-based FTIR spectrometer (Figure 7.6) for use in remote sensing applications. Weighing less than 7 kg, it can run off batteries or a mains supply. A PC is built into the case along with the FTIR module. The spectral range is 625-5000cm with a resolution of 4 cm... 

Figure 2.34 Schematic of a Michelson interferometer-based FTIR spectrometer.

An open-path spectrometer is designed according to the features of dispersive and interferometer-based instruments with the exception that the sample is located remotely from the instrument and the light source is either sunlight or laser power. The basic configuration for sampling in the open path design is shown in Fig. 4. 

In both the continuous-scan and step-scan spectrometers, the extremely precise positioning of the movable mirror is ensured by adopting an efficient feedback mechanism such as the above-mentioned dynamic alignment. Further, some spectrometers are constructed so as to isolate the interferometer base from shocks, including earthquakes. It should be kept in mind that, even if such spectrometers are in use, shocks, vibrations, and tilts given inadvertently to the spectrometer may still have an adverse effect on the results of measurements. 

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... 

Fourier-transform infrared (FTIR) spectrometers encode infrared wavenumbers by moving a mirror in a Michelson interferometer which results in a unique, path-dependent pattern of interference for each light wavelength in the IR beam. FTIRs have come to totally dominate the IR market and are the means by which most of the work described in this review was accomplished. Only for some special applications (modulation spectra and time-dependence studies) are dispersive-based (scanning monochromator or tuned laser) spectrometers still used. The advantages of the FTIR approach are that the entire spectral region of interest can... 

Figure 10.11—Optical arrangement of a Fourier transform IR spectrometer, a) A 90c Michelson interferometer including the details of the beam splitter (expanded view) b) optical diagram of a single beam spectrometer (based on a Nicolet model). A weak intensity HeNe laser (632.8 nm) is used as an internal standard to measure precisely the position of the moving mirror using an interference method (a simple sinusoidal interferogram caused by the laser is produced within the device). According to the Nyquist theorem, at least two points per period are needed to calculate the wavelength within the given spectrum.

Fourier transform (FT) IR spectroscopy is one of several nondispersive optical spectroscopies based on interferometry. A two-beam interferometer first proposed by Michelson is the basis of most modern FT-IR spectrometers, as exemplified by the schematic of the Bruker Equinox 55 spectrometer (Bruker Optik, Ettlingen, Germany) in Fig. 2. Simply described, the interferometer comprises a beam splitter and two mirrors. A collimated beam of IR energy is split at the beam splitter into equal halves. Half of the energy travels through the beam splitter to one of the mirrors, which is positioned at a fixed distance away from the beam splitter. The reflected beam travels perpendicular to the incident beam to a moving mirror. IR radiation reflects off the fixed and moving mirrors and recombines at the beam splitter. The recombined IR beam projects from the interferometer towards the detector on an optical path perpendicular to the source beam. 

A modification of an interferometrically-based system, which was first described by Dohi and Suzuki (24), is known as a selectively-modulated interferometric dispersive spectrometer, this system is a hybrid in that a rotating grating (a dispersive element) is used to limit the number of wavelengths which can interfere at any one time in a modified Michelson interferometer. 

The application of FTIR in chemistry, its unique features, and the relevant instrumentation are well documented [34,35], In brief, an FUR spectrometer is based on a Michelson interferometer that provides a spectrum in the time domain which is Fourier-transformed by a computer to a spectrum in the frequency domain. The sample can be scanned repeatedly, and the accumulated spectra can be averaged, thus producing a representative IR spectrum of a very high signal to noise ratio. This enables the measurement of samples containing a very low concentra-... 

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