Michelson interferometer advantages

A Michelson interferometer (MI) can be considered as a Young-interferometer with perfect reflectors at the ends of both branches. Compared to the MZI and YI the MI has the advantage that the sensing region is passed twice. Because for monitoring the reflected light a second Y- junction is... 

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... 

Fourier transform infrared (FTIR) spectroscopy has been extensively developed over the past decade and provides a number of advantages. The main part of FTIR spectrophotometer is the Michelson interferometer. Radiation containing all IR wavelengths (e.g., 4000-400 cm 1) is emitted by source of infrared radiation (Globar) and is split into two beams. One beam is of fixed length, and the other is of variable length (movable mirror). 

There are two types of spectrometers that one can use to generate such spectra.12 One uses a monochromator to evaluate each frequency in turn. The second uses a Michelson interferometer to examine all frequencies simultaneously, and then a Fourier transform to display the spectrum. The advantage of the latter approach is its greater sensitivity, and the speed with which it can produce a spectrum. 

Michelson interferometers also have the advantage of being much more tolerant to misadjustment of the sample arrangement, since they have circular Jacquinot stops, compared to the straight small entrance slits of grating spectrometers (Hirschfeld, 1977 b). 

Between the source and the detector is put either monochromators used in dispersive instruments or interferometers used in Fourier transform infrared (FT-IR) instruments. In a dispersive instrument the intensity at each wavenumber is measured one by one in sequence and only a small spectral range falls on the detector at any one time. In a FT-IR instrument the intensities of all the wavenumbers are measured simultaneously by the detector. Fourier transform infrared spectroscopy offers some advantages compared to dispersive instruments, namely (i) higher signal-to-noise ratios for spectra obtained under conditions of equal measurement time, and (ii) higher accuracy in frequency for spectra recorded over a wide range of frequencies. Therefore we will give below a brief picture of the principle of FT-IR spectroscopy, based on a Michelson interferometer (Fig. 2). 

For many applications, there may be some advantage in employing phase modulation 54,85) instead of the usual amphtude modulation. In the latter technique the path of the radiation from the source to the detector is blocked and opened periodically by a chopper (cf. Fig. 20 and Section 4.3). For phase modulation, the chopper is removed from the spectrometer and the fixed mirror of the Michelson interferometer is moved back and forth about its mean position with a certain frequency. In contrast to the interference modulation (see Section 4.2), the amplitude of the mirror motion is small, being a quarter of the wavelength of the light. For the analogue Fourier transform or interference modulation, the amplitude of the mirror has to have many wavelengths in order to achieve a reasonable resolution... 

Summarizing the results of our discussion of the practice of Fourier transform spectroscopy, we start with the presumption that the equipment for most routine spectroscopic investigations consists of a Fourier spectrometer with a Michelson interferometer and a digital computer. In other words, the advantages of the lamellar grating used as a two-beam interferometer, and of phase modulation, for example, have been utilized only for certain special applications in the extreme far-infrared. All commercial Fourier spectrometers are available with a computer attached, which in most cases not only performs the Fourier transform but is also programmed to control the instrument. Commercial instruments have a remote switch for the selection of the different spectral ranges, and the filters and beams... 

Recently, first experimental results were published which have been obtained with a Michelson interferometer especially designed by J. Cast and L. Genzel > for reflection studies on small solid samples by means of asymmetric or amplitude Fourier spectroscopy. The main advantage of the optical layout is that sample and reference mirror are located at focal points which do not take part in the motion to produce the path diflerence in the interferometer. Therefore, these foci can be placed inside a cryostat that allows the sample to be cooled. Another recent development in this field is concerned with the difficulty that, when studying the reflectivity of solids, the determination of the phase depends strongly on the exact positioning of the sample mirror instead of the background mirror. This is... 

It should be noted further that an increase in resolution is easily achieved in this case by increasing the maximum path difference and the scanning time. The power flux is not influenced by an increase of Smax- However, there will be an increase in noise, as we shall see later. An increase in resolution means for a grating instrument a reduction of slit width and hence, a reduction of the power flux, which is proportional to the square of the slit width [see Eq. (5.12)]. It also seems worth mentioning that the Jacquinot or throughput advantage exists not only in the Michelson interferometer but also in other instruments, e.g. a Fabry-Perot interferometer. 

The other principal advantage which applies to Fourier transform spectroscopy is the multiplex or "Fellgett" advantage 21,64) n yas P. Fellgett who first pointed out that there is an advantage when the data in all elements of a spectrum are obtained simultaneously instead of being measured for each element successively. In Fourier transform spectroscopy, the radiation in the Michelson interferometer is not separated into spectral elements. The interferogram contains... 

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