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1.0. INTRODUCTION [top]
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 telecom-munications, gratings are being used to increase the capacity of fiber-optic networks using wavelength division multiplexing (WDM). Gratings have also 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 [top]
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 surface-relief pattern is generated by casting or molding the relief pattern of another grating (see Chapter 5).
1.2. A BRIEF HISTORY OF GRATING DEVELOPMENT [top]
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 value to modern spectroscopists. He continued to rule gratings until his death in 1901.
After Rowland's great success, many people set out to rule diffraction gratings. The few who were successful sharpened the scientific demand for gratings. As the advantages of gratings over prisms and interferometers for spectroscopic work became more apparent, the demand for diffraction gratings far exceeded the supply.
1.3. HISTORY OF THE NEWPORT GRATINGS OPERATION [top]
In 1947, Bausch & Lomb decided to make precision gratings available commercially. In 1950, through the encouragement of Prof. George R. Harrison of MIT, David Richardson and Robert Wiley of Bausch & Lomb succeeded in producing their first high quality grating. This was ruled on a rebuilt engine that had its origins in the University of Chicago laboratory of Prof. Albert A. Michelson. A high fidelity replication process was subsequently developed, which was crucial to making replicas, duplicates of the painstakingly-ruled master gratings. A most useful feature of modern gratings is the availability of an enormous range of sizes and groove spacings (up to 10,800 grooves per millimeter), and their enhanced quality is now almost taken for granted. In particular, the control of groove shape (or blazing) has increased spectral efficiency dramatically. In addition, interferometric and servo control systems have made it possible to break through the accuracy barrier previously set by the mechanical constraints inherent in the ruling engines.5
During the subsequent decades, we have produced thousands of master gratings and many times that number of high quality replicas. In 1985, Milton Roy Company acquired Bausch & Lomb's gratings and spectrometer operations; in 1995 it sold these operations to Life Sciences International plc as part of Spectronic Instruments, Inc. – at this time, the gratings operations took the name Richardson Grating Laboratory. In 1997, Spectronic Instruments was acquired by Thermo Electron Corporation, and the gratings operation was called Thermo RGL for a time before being transferred to Thermo Electron's subsidiary, Spectra-Physics.
In 2004, Spectra-Physics was acquired by Newport Corporation, a leading global supplier of advanced-technology products and systems to the semiconductor, communications, electronics, research and life and health sciences markets. Newport provides components and integrated subsystems to manufacturers of semiconductor processing equipment, biomedical instrumentation and medical devices, advanced automated assembly and test systems to manufacturers of communications and electronics devices, and a broad array of high-precision systems, components and instruments to commercial, academic and government customers worldwide. Newport's innovative solutions leverage its expertise in photonics instrumentation, lasers and light sources, precision robotics and automation, sub-micron positioning systems, vibration isolation, optical components and optical subsystems to enhance the capabilities and productivity of its customers' manufacturing, engineering and research applications.
During these changes in corporate ownership, we have continued to uphold the traditions of precision and quality established by Bausch & Lomb over fifty years ago.
1.4. DIFFRACTION GRATINGS FROM NEWPORT [top]
The gratings operation of Newport Corporation, which is known throughout the world as "the Grating Lab", is a unique facility in Rochester, New York, containing not only the Newport ruling engines and holographic recording chambers (both of which are used for making master gratings) but the replication and associated testing and inspection facilities for supplying replicated gratings in commercial quantities.
To achieve the high practical resolution characteristic of high-quality gratings, a precision of better than 1 nm (= 0.001 µm) in the spacing of the grooves must be maintained. Such high precision requires extraordinary control over temperature fluctuation and vibration in the ruling engine environment. This control has been established by the construction of specially-designed ruling cells that provide environments in which temperature stability is maintained at ± 0.01 °C for weeks at a time, as well as vibration isolation that suppresses ruling engine displacement to less than 0.025 µm. The installation can maintain reliable control over the important environmental factors for periods in excess of six weeks, the time required to rule large, finely-spaced gratings.
Newport has facilities for coating and testing master and replica substrates, as well as special areas for the controlled replication process itself. In order to produce the finest gratings with maximum control and efficiency, even storage, packing and shipping of finished gratings are part of the same facility.
In addition to burnishing gratings with a diamond tool, an optical interference pattern can be used to produce holographic gratings. Master holographic gratings require strict maintenance of the recording optical system to obtain the best contrast and fringe structure. Newport produces holographic gratings in its dedicated recording facility, in whose controlled environment thermal gradients and air currents are minimized and fine particulates are filtered from the air. These master gratings are replicated in a process identical to that for ruled master gratings.
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