LEC+8+GEOMETRIC OPTICS

1. SPECTROSCOPY AND GRATINGS
“It is difficult to point to another single device that has brought more
important experimental information to every field of science than the
diffraction grating. The physicist, the astronomer, the chemist, the
biologist, the metallurgist, all use it as a routine tool of unsurpassed
accuracy and precision, as a detector of atomic species to determine the
characteristics of heavenly bodies and the presence of atmospheres in
the planets, to study the structures of molecules and atoms, and to
obtain a thousand and one items of information without which modern
science would be greatly handicapped.”
¾ J. Strong, “The Johns Hopkins University and diffraction gratings,”
J. Opt. Soc. Am. 50, 1148-1152 (1960), quoting G. R. Harrison.
1.0. INTRODUCTION
Spectroscopy is the study of electromagnetic spectra – the wavelength
composition of light – due to atomic and molecular interactions. For many years,
spectroscopy has been important in the study of physics, and it is now equally
important in astronomical, biological, chemical, metallurgical and other analytical
investigations. The first experimental tests of quantum mechanics involved
verifying predictions regarding the spectrum of hydrogen with grating
spectrometers. In astrophysics, diffraction gratings provide clues to the
composition of and processes in stars and planetary atmospheres, as well as
offer clues to the large-scale motions of objects in the universe. In chemistry,
toxicology and forensic science, grating-based instruments are used to determine
the presence and concentration of chemical species in samples. In telecommunications,
gratings are being used to increase the capacity of fiber-optic
networks using wavelength division multiplexing (WDM). Gratings have also
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found many uses in tuning and spectrally shaping laser light, as well as in
chirped pulse amplification applications.
The diffraction grating is of considerable importance in spectroscopy, due to
its ability to separate (disperse) polychromatic light into its constituent
monochromatic components. In recent years, the spectroscopic quality of
diffraction gratings has greatly improved, and Newport has been a leader in this
development.
The extremely high accuracy required of a modern diffraction grating
dictates that the mechanical dimensions of diamond tools, ruling engines, and
optical recording hardware, as well as their environmental conditions, be controlled
to the very limit of that which is physically possible. A lower degree of
accuracy results in gratings that are ornamental but have little technical or scientific
value. The challenge to produce precision diffraction gratings has attracted
the attention of some of the world s most capable scientists and technicians.
Only a few have met with any appreciable degree of success, each limited by the
technology available.
1.1. THE DIFFRACTION GRATING
A diffraction grating is a collection of reflecting (or transmitting) elements
separated by a distance comparable to the wavelength of light under study. It
may be thought of as a collection of diffracting elements, such as a pattern of
transparent slits (or apertures) in an opaque screen, or a collection of reflecting
grooves on a substrate (also called a blank). In either case, the fundamental
physical characteristic of a diffraction grating is the spatial modulation of the
refractive index. Upon diffraction, an electromagnetic wave incident on a grating
will have its electric field amplitude, or phase, or both, modified in a predictable
manner, due to the periodic variation in refractive index in the region near the
surface of the grating.
A reflection grating consists of a grating superimposed on a reflective
surface, whereas a transmission grating consists of a grating superimposed on a
transparent surface.
A master grating (also called an original) is a grating whose surface-relief
pattern is created “from scratch”, either by mechanical ruling (see Chapter 3) or
holographic recording (see Chapter 4). A replica grating is one whose surfacerelief
pattern is generated by casting or molding the relief pattern of another
grating (see Chapter 5).
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1.2. A BRIEF HISTORY OF GRATING DEVELOPMENT
The first diffraction grating was made by an American astronomer, David
Rittenhouse, in 1785, who reported constructing a half-inch wide grating with
fifty-three apertures.2 Apparently he developed this prototype no further, and
there is no evidence that he tried to use it for serious scientific experiments.
In 1821, most likely unaware of the earlier American report, Joseph von
Fraunhofer began his work on diffraction gratings.3 His research was given
impetus by his insight into the value that grating dispersion could have for the
new science of spectroscopy. Fraunhofer s persistence resulted in gratings of
sufficient quality to enable him to measure the absorption lines of the solar
spectrum, now generally referred to as the Fraunhofer lines. He also derived the
equations that govern the dispersive behavior of gratings. Fraunhofer was interested
only in making gratings for his own experiments, and upon his death, his
equipment disappeared.
By 1850, F.A. Nobert, a Prussian instrument maker, began to supply
scientists with gratings superior to Fraunhofer s. About 1870, the scene of
grating development returned to America, where L.M. Rutherfurd, a New York
lawyer with an avid interest in astronomy, became interested in gratings. In just
a few years, Rutherfurd learned to rule reflection gratings in speculum metal that
were far superior to any that Nobert had made. Rutherfurd developed gratings
that surpassed even the most powerful prisms. He made very few gratings,
though, and their uses were limited.
Rutherfurd s part-time dedication, impressive as it was, could not match the
tremendous strides made by H.A. Rowland, professor of physics at the Johns
Hopkins University. Rowland s work established the grating as the primary
optical element of spectroscopic technology.4 Rowland constructed sophis -
ticated ruling engines and invented the concave grating, a device of spectacular
2 D. Rittenhouse, “Explanation of an optical deception,” Trans. Amer. Phil. Soc. 2, 37-42
(1786).
3 J. Fruanhofer, “Kurtzer Bericht von den Resultaten neuerer Versuche über die Sesetze des
lichtes, und die Theorie derselbem,” Ann. D. Phys. 74, 337-378 (1823).
4 H. Rowland, “Preliminary notice of results accomplished on the manufacture and theory of
gratings for optical purposes,” Phil. Mag. Suppl. 13, 469-474 (1882); G. R. Harrison and E.
G. Loewen, “Ruled gratings and wavelength tables,” Appl. Opt. 15, 1744-1747 (1976).