Diffraction grating

A diffraction grating is an optical component made of numerous closely spaced parallel lines or grooves. Diffraction gratings are used for dispersing light by spatially separating light of different wavelengths. The shiny, reflective reverse sides of CDs and DVDs are good examples of an everyday diffraction grating. In most fields of spectral analysis, which involves studying a signal by breaking it down into its basic components, diffraction gratings are used as an alternative to prisms. Diffraction gratings first began to enjoy widespread use after physics professor Henry Augustus Rowland developed a device called a ruling engine in the late nineteenth century that led to significant improvements in their manufacture. Since that time, the availability of high quality diffraction gratings has virtually revolutionized the field of analytical spectroscopy. Such gratings are often used in a wide array of optical instruments, some of which include spectrometers, monochromators, wavelength division multiplexing devices, and optical pulse compressing devices.

Background

Scottish mathematician and astronomer James Gregory first observed diffraction gratings in the seventeenth century. He successfully outlined many of the basic principles of the concept while studying the iridescent qualities of certain bird feathers. Renowned Philadelphia inventor David Rittenhouse subsequently created the first manmade diffraction gratings in the late eighteenth century. German physicist Joseph von Fraunhofer built another early diffraction grating device in 1821. Although functional, these early diffraction gratings were imprecise. As a result, they were not widely used in the scientific community.

The development of high quality diffraction gratings began in the late nineteenth century thanks to Henry Augustus Rowland. Born in 1848, Rowland was a Pennsylvania native who took an avid interest in science from an early age and eventually became a professor of physics at Johns Hopkins University. In addition to fulfilling his teaching duties, Rowland spent a great deal of time attending his scientific pursuits. Among other things, he successfully recalibrated the value of the ohm and created an improved version of James Joule's paddle-wheel experiment for measuring the mechanical equivalent of heat.

Rowland first turned his attention to diffraction gratings in the 1870s. Seeking to improve their precision, he built a machine called a ruling engine that had a diamond tip for etching the grating. The ruling engine was equipped with a screw that was used to shift the diamond tip a very short distance between each line etched during the process of creating a diffraction grating. This allowed for the manufacture of the most precise and useful diffraction gratings that had yet been created. Rowland used his own diffraction gratings to study the solar spectrum with spectrometers. These studies eventually led to his creation of photographic map of that spectrum in 1888. In the years that followed his invention of the ruling engine, Rowland's improved diffraction gratings, which were far superior to any previously available, came to be used by scientists all over the world. Rowland himself became so closely associated with diffraction gratings that he was even depicted with one in a portrait painted of him by artist Thomas Eakins in 1897.

Overview

A diffraction grating is similar to a prism in that it separates the colors in white light to create a spectrum. The main difference is that while prisms produce spectrums through refraction, diffraction gratings produce spectrums through the diffraction of light that is reflected by the narrow lines in the grating's surface. Diffraction gratings typically consist of an underlying layer of optical material onto which numerous parallel lines or grooves are etched. The surface of the optical material is then coated with some sort of reflective material. As Rowland realized, the etching quality and spacing of the grooves on a diffraction grating has a significant impact on the grating's overall performance.

When light strikes a diffraction grating, it is split and diffracted into separate beams traveling in different directions. The spacing of the grooves in the grating and the wavelength of the light determine the precise direction of these beams. The grooves on a diffraction grating can be either transmissive or reflective. Diffraction gratings as a whole can take one of two main forms: ruled or holographic. Ruled gratings are made by physically imprinting grooves on a reflective surface with the use of a ruling engine. Holographic gratings are made using a special photolithographic process in which an interference pattern is generated to preferentially expose parts of a coating made from a photosensitive resin known as photoresist.

Diffraction gratings are used in many types of optical equipment, including monochromators and spectrometers. Monochromators are optical devices that transmit a narrow band of wavelengths of light chosen from a range of selectable wavelengths. A spectrometer, meanwhile, is a device used to record the spectrum of an object. It works by dispersing the light from the object in question so that it can be processed by a photodetector.

The most common examples of diffraction gratings are CDs and DVDs. The diffraction grating qualities of CD and DVD media are a direct result of their manufacture. The readable side of CDs and DVDs has an underlying plastic layer featuring numerous small pits that are arranged in a spiral shape. A thin layer of metal applied in the manufacturing process serves to make the pits even more visible. This all leads CDs and DVDs to have a grating that produces a spectrum when held at a sharp angle to a light source.

Diffraction gratings can occur in nature. Peacocks and some other bird species have feathers with natural diffraction grating capable of producing constructive interference, a phenomenon that leads to visible iridescence. Natural diffraction grating can also be seen in the antennae of seed shrimp and butterfly wings. Materials like abalone shell, mother-of-pearl, and opal have naturally occurring diffraction grating properties as well. The most common example of natural diffraction grating is seen in striated muscle. This effect is particularly visible in cooked beef, where it often results in the appearance of rainbow-like colorations on the surface of meat that is sliced against the grain of the muscle fiber.

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