Gratings,—diffraction

Diffraction gratings may be made by a holographic process, but blaze characteristics cannot be controlled and their efficiency is low in the infrared. They are mostly used for low-order work in the visible and near-ultraviolet. 

Question. A diffraction grating has a ruled area that is 10.40 cm wide, has 600.0 grooves per millimetre and is blazed at an angle of 45.00°. 

A further problem of simple gratings is that the incident light normally is diffracted into quite a few orders, so that, in general, the efficiency of diffraction into any individual order m is small. Also, the diffraction envelope is broadest and varying the least in intensity for the specular angle 6i = dm, where, however, the chromatic dispersion is zero. This means that most of the incident intensity would be channelled into the least useful zero-order diffraction. 

The resolution or chromatic resolving power of a grating describes its ability to separate adjacent spectral lines. Resolution is generally defined as 

First-order ditfracted iaeam at e to undeviated tight 

The positions of the diffraction maxima from a transmission grating illuminated by monochromatic light normal to the surface is given by  

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The first requirement is a source of infrared radiation that emits all frequencies of the spectral range being studied. This polychromatic beam is analyzed by a monochromator, formerly a system of prisms, today diffraction gratings. The movement of the monochromator causes the spectrum from the source to scan across an exit slit onto the detector. This kind of spectrometer in which the range of wavelengths is swept as a function of time and monochromator movement is called the dispersive type. 

Figure B2.1.1 Femtosecond light source based on an amplified titanium-sapphire laser and an optical parametric amplifier. Symbols used P, Brewster dispersing prism X, titanium-sapphire crystal OC, output coupler B, acousto-optic pulse selector (Bragg cell) FR, Faraday rotator and polarizer assembly DG, diffraction grating BBO, p-barium borate nonlinear crystal.

Treacy E B 1969 Optical pulse compression with diffraction gratings IEEE J. Quantum. Electron. 5 454-8... 

A wavelength selector that uses a diffraction grating or prism, and that allows for a continuous variation of the nominal wavelength. 

The construction of a typical monochromator is shown in Figure 10.12. Radiation from the source enters the monochromator through an entrance slit. The radiation is collected by a collimating mirror, which reflects a parallel beam of radiation to a diffraction grating. The diffraction grating is an optically reflecting surface with... 

Typical grating monochromator with inset showing the dispersion of the radiation by the diffraction grating. 

Another approach to multielemental analysis is to use a multichannel instrument that allows for the simultaneous monitoring of many analytes. A simple design for a multichannel spectrometer consists of a standard diffraction grating and 48-60 separate exit slits and detectors positioned in a semicircular array around the diffraction grating at positions corresponding to the desired wavelengths (Figure 10.50). 

Grossman, W. E. L. The Optical Characteristics and Production of Diffraction Gratings, /. Chem. Educ. 1993, 70, 741-748. Palmer, C. Diffraction Gratings, Spectroscopy 1995, 10(2),... 

The dispersing element to be described in Section 3.3 splits up the radiation into its component wavelengths and is likely to be a prism, diffraction grating or interferometer, but microwave and millimetre wave spectroscopy do not require such an element. 

Although prisms, as dispersing elements, have been largely superseded by diffraction gratings and interferometers they still have uses in spectroscopy and they also illustrate some important general points regarding dispersion and resolution. 

As in all Fourier transform methods in spectroscopy, the FTIR spectrometer benefits greatly from the multiplex, or Fellgett, advantage of detecting a broad band of radiation (a wide wavenumber range) all the time. By comparison, a spectrometer that disperses the radiation with a prism or diffraction grating detects, at any instant, only that narrow band of radiation that the orientation of the prism or grating allows to fall on the detector, as in the type of infrared spectrometer described in Section 3.6. 

The dispersing elemenf is usually a diffraction grating or an inferferomefer wifh a beamsplitter made from silicon-coafed or germanium-coafed quartz or calcium fluoride. 

Dispersing elements may be either prisms (glass for the visible, quartz for the nearultraviolet) or, more often, diffraction gratings for which a Czemy-Tumer mounting, shown in Figure 3.17, may be used. 

The dispersing element is a diffraction grating preferably used under conditions of grazing incidence (6 in Equation 3.9 about 89°) to improve the reflectance. The grating may also be concave to avoid the use of a focusing mirror. 

Figure 8.28 shows how the X-rays fall on the solid or liquid sample which then emits X-ray fluorescence in the region 0.2-20 A. The fluorescence is dispersed by a flat crystal, often of lithium fluoride, which acts as a diffraction grating (rather like the quartz crystal in the X-ray monochromator in Figure 8.3). The fluorescence may be detected by a scintillation counter, a semiconductor detector or a gas flow proportional detector in which the X-rays ionize a gas such as argon and the resulting ions are counted.

The cavity may be tuned to a particular transition by a prism or, preferably, by replacing one of the mirrors (not the output mirror) at one end of the cavity by a diffraction grating. 

Fig. 12. Cut-out drawing of a distributed feedback (DFB) laser showing the active region and a diffraction grating, under the active layer, which produces...

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