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Chapter 29:
Echelle and Arrayed Waveguide Gratings for WDM and Spectral Analysis
The diffraction grating story is one of a remarkable success. When in 1789 American astronomer David Rittenhouse made the first diffraction grating by wrapping a wire around the threads of two fine-pitch screws and used it to measure the wavelength of light, no one could foresee the impact this device would have on our lives. Diffraction gratings have been, for more than a century, an essential tool for uncovering the world at both microscopic and cosmological scales by means of spectroscopy. Also, the profound recent changes in the way our society communicates, including the Internet, have been made possible thanks to various diffraction grating-based devices that are key elements in modern telecommunication optical networks. The foundations of diffraction grating technology were laid in the first half of the nineteenth century by pioneering work of Joseph von Fraunhofer. To make his gratings, he built the first ruling engine. With such ruled gratings he discovered dark lines in light emitted by several substances and observed similar spectral lines in light from the sun and other stars. Later, he used these absorption lines, now known as Fraunhofer's lines, as markers for precise measurements of the refractive index of glass used in achromatic lenses. He also derived and experimentally verified the grating equation. In the 1870s, Rayleigh predicted that gratings could outperform even the most powerful prisms in terms of spectral resolution, and several such gratings were soon made. Toward the end of the nineteenth century, Henry A. Rowland at John Hopkins University was producing gratings of unprecedented high spectral resolution and accuracy. The availability of new and accurate spectroscopic data obtained with such gratings was the key for fundamental discoveries on the nature of spectral lines by Rydberg, Balmer, Runge, Zeeman, and others. These spectral studies were of singular importance paving the road toward the era of atomic physics and quantum mechanics. However, the grating alone does not suffice for making accurate spectroscopic measurements. It needs to be specifically positioned in an optomechanical setup capable of collecting the input light and focusing the spectrally dispersed light by the grating on an output port. The latter can be either a simple slit aperture in the case of a monochromator, or a photographic film (in modern devices often a photodetector array) in the case of a spectrometer. Essential for the development of spectroscopic instruments was a discovery, made by Rowland, that diffraction gratings can be formed on a concave (rather than plane) substrate. This way, the light is simultaneously dispersed and focused, obviating the need for additional focusing optics (lenses). Rowland also proposed a grating configuration that improves focusing, eliminates the primary coma, and minimizes spherical aberration.
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