24 - Diffraction gratings

Summary

Diffraction gratings have been used in spectroscopy and other studies of electromagnetic phenomena for nearly two centuries. Josef Fraunhofer (1787–1826), the discoverer of the dark lines in the solar spectrum, built the first gratings in 1819 by winding fine wires around two parallel screws. Henry Rowland made significant contributions to the fabrication of precise, large-area, high-frequency ruled gratings in the 1880s. Robert Wood, who succeeded Rowland in the chair of experimental physics at Johns Hopkins University in 1901, used these ruled gratings extensively in his researches and discovered, among other things, the “anomalous” behavior of metallic gratings, which he first published in 1902. John William Strutt (Lord Rayleigh) developed a theoretical model of these gratings around 1907 and was successful in explaining certain features of Wood's anomalies. However, it is only during the past thirty years or so that a thorough understanding of nearly all aspects of the behavior of diffraction gratings has been achieved through the consistent application of Maxwell's equations with the help of advanced analytical and numerical techniques.

Modern gratings having a few thousand lines per millimeter with near-perfect periodicity are fabricated over fairly large areas (grating diameters of around one meter or so are possible). The groove shapes can be controlled to be sinusoidal, rectangular, triangular, or trapezoidal, and one can obtain shallow or deep grooves (relative to the groove width) by current manufacturing techniques.