Interferometric spectrometer

The discussion in the present section concerns only dispersive optical spectroscopy. We shall not treat the resolution characteristic of a Fourier interferometric spectrometer, which is determined by the optical path difference scanned and the apodizing function used. Nevertheless, these... 

The key innovation in passive dispersive (as opposed to interferometric) spectrometers introduced recently is the use of a programmable digital spatial light modulator (SLM) to encode spectral information. The most advanced version of SLM currently is a reflective digital micromirror array (DMA). The feasibihty of DMA-based multiplexed spectrometers has been experimentally demonstrated recently. ... 

When the spectral characteristics of the source itself are of primary interest, dispersive or ftir spectrometers are readily adapted to emission spectroscopy. Commercial instruments usually have a port that can accept an input beam without disturbing the usual source optics. Infrared emission spectroscopy at ambient or only moderately elevated temperatures has the advantage that no sample preparation is necessary. It is particularly applicable to opaque and highly scattering samples, anodized and painted surfaces, polymer films, and atmospheric species (135). The Voyager interferometric spectrometer (IRIS) spectra from the outer planets demonstrated the analytical capabilities of ftir emission spectroscopy. As an example of industrial... 

Martin, A. E. in Durig, J. R. (ed.) Vibrational Spectra and Structure, Vol. 8 Infrared Interferometric Spectrometers, Amsterdam Elsevier 1980... 

Voyager with an IR interferometric spectrometer (IRIS) on it [96, 97] examined Titan s atmospheric chemistry. Six simple HCs (C2H2, CjHg, C3H4,... 

A. E. Martin, Infrared interferometric spectrometers, in Vibrational Spectra and Structure, J. R. Durig, Ed., Elsevier, Amsterdam, The Netherlands, 1980. 

Martin, D. H. (1982). Polarizing (Martin-Puplett) interferometric spectrometers for the near- and submillimeter spectra. In Infrared and Millimeter Waves, Vol. 6, Systems and Components, ed. K. J. Button, Chapter 2. New York Academic Press. 

There are two light sources involved, a white light and a laser source. The white light uses the same moving mirror and therefore makes up a second interferometric system within the spectrometer. When the moving mirror and the fixed mirror of this secondary interferometer are equidistant, a centerburst is produced which is... 

Commercial Fourier transform spectrometers operating at moderate resolution (1cm-1) require fractions of seconds to complete a scan of the interferometric mirror (scans may only take tens of milliseconds if only low spectral resolution is required). A new strategy must now be used to study the... 

Historical Development. In this category of FT spectrometer the complete time-evolution of the IR transient is digitized whilst the interferometric mirror is held stationary at each sampling point. The transient can be initiated repeatedly and signal averaged to achieve an adequate SNR. The... 

Essentially two types of IR spectrometer are commercially available dispersive or interferometric. The former is the more familiar to most chemists. The source of continuous IR radiation is dispersed, by means of a prism or grating, and the spectrum is scanned within the required limits of frequency the slower the scanning speed, the better the resolution. 

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. 

Fig. 1. Schematic representation of the main features of the interferometric surface forces apparatus. Crossed mica sheets (1) are glued onto semi-cylinder optically polished silica discs (2). One of the discs is attached to a piezoelectric crystal tube (3), and another to the force measuring double cantilever spring (4). White light passes through the window positioned in the bottom of the apparatus and reflects between two silver mirrors. Constructive interference occurs for some wavelengths and the fringes of equal chromatic order are passed through the upper silver mirror to the spectrometer where they can be viewed and their wavelengths determined. Adapted from Ref. [9]. 1996, with permission from Elsevier.

Ly is a property of the radiation source, it is discussed in Sec. 3.3.1. Gy depends on the type of spectrometer, which may be dispersive or interferometric, and r is the overall transmission factor of the entire instrument. 

Improvements in the single side-band performance of a mixer-based receiver can be made by filtering the unwanted side band before it is down-converted in the mixer. Such a scheme, which is described in detail by Goldsmith (1982) is based on interferrometric techniques. We will not discuss single side-band filtering any further, except to note that it is a particularly apposite demonstration of the use of optical techniques to process the radiation in the spectrometer. We will discuss the use of interferometric techniques in Section IX as a means to realize a reflection mode spectrometer. These few examples indicate the flexibility of application of optical techniques to problems of instrument design in the FIR. 

The dimensions of the sample are important in determining the performance of the spectrometer because the sample can extend over several wavelengths in several dimensions, at least in principle, which enhances interferometric effects within the sample. Neglecting losses in the sample for the moment, we note that if the sample is an integral number of half-wavelengths thick, it functions like a Fabry-Perot. In order to understand this, we will sketch a derivation that takes into account the index of refraction of the dielectric material and reflection from the sample-air interfaces. First, note that the optical phase difference across the sample is nkt, where n is the index of refraction and t is the thickness. The resonance condition for such a slab is given by Eq. (44) with kt replaced by nkt, namely,... 

Interferometric Raman spectroscopy Interferometric Raman Spectroscopy is a measurement technique that utilizes time-domain or space-domain measurements of electromagnetic radiation or other type of radiation for collecting Raman spectra based on the coherence of a radiative source. An example is a Fourier transform (FT) Raman spectrometer. 

The properties of the dual-film electrode were characterized by in situ Fourier transform infrared (FTIR) reflection absorption spectroscopy [3]. The FTIR spectrometer used was a Shimadzu FTIR-8100M equipped with a wide-band mercury cadmium teluride (MCT) detector cooled with liquid nitrogen. In situ FTIR measurements were carried out in a spectroelectro-chemical cell in which the dual-film electrode was pushed against an IR transparent silicon window to form a thin layer of solution. A total of 100 interferometric scans was accumulated with the electrode polarized at a given potential. The potential was then shifted to the cathodic side, and a new spectrum with the same number of scans was assembled. The reference electrode used in this experiment was an Ag I AgCl I saturated KCl electrode. The IR spectra are represented as AR/R in the normalized form, where AR=R-R(E ), and R and R(E ) are the reflected intensity measured at a desired potential and a base potential, respectively. 

Optical methods of spectrometer calibration include laser interferometric methods and Moire fringe counting techniques. Since such methods depend on the accurately known wavelength of light (for example, 6328.1983 A for a He-Ne laser at 293 K under standard conditions), they are independent of any assumptions made in the iron foil technique and, thus, are intrinsically more reliable. Moreover, such methods do not require the acquisition of57 Co Mdssbauer sources and reference materials for iron absorber studies and may thus be attractive... 

These instruments (Figure 10.7) can be divided into two categories the Fourier transform spectrometers, which undertake a simultaneous analysis of the whole spectral region from interferometric measurements, and numerous specialized analysers for the second category. Dispersive-type spectrometers are also used for the near-IR. 

The first step is to generate a Sky Map to be fed to the subsequent modules in the Sky Generator Module and the corresponding photon noise, computed in the Sky Photon Noise Module. In parallel, given the parameters defined by the user for the instrument, an interferometric MV-map is created at the v-Map Generator Module from the position of the two telescopes. The FTS Drive module calculates the spectrometer scan parameters. Once a MV-map and the scan parameters are defined, the instrument beam is calculated at the Beam Generator Module. The sky map and the beam are then combined to recreate the observed sky map. 

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