Diffraction is the bending of a wave as it passes around a corner or through an opening. The phenomena can be best observed using the prism experiment or Young’s Double-Slit Experiment. In the prism experiment, white light is passed through a prism and viewed on a white screen as it exits the prism.
On the white screen, you will observe an array of colors as each wavelength in the visible spectrum is bent to a different degree, effectively separating the white light into its constituent colors. Young’s Double Slit experiment demonstrates the same principle by passing light through a small slit and observing the light on a screen when it exits on the other side.
The discovery of light diffraction was monumental in optical physics because it proved light’s wave-particle duality. That is, it proved that light exhibits properties of both waves and particles. In this blog, we cover the applications of diffraction gratings for spectrometry tools in modern technology.
What is a Diffraction Grating?
Evolved from Young’s Double Slit experiment, diffraction gratings are the preferred method of light scattering in many spectrometers. A diffraction grating is a device that splits electromagnetic radiation into its constituent wavelengths. In a nutshell, a diffraction grating comprises slits of varying widths to match the wavelengths of the different colors of the visible spectrum. When white light is incident on the grating, its constituent colors are separate as they bend through the slit that matches their respective wavelengths.
While they are fairly simple devices, diffraction gratings for spectrometry tools have established a foothold in modern spectrometry and shaped the technology of our lives.
The discovery of diffraction launched the scientific field of spectroscopy, the study of the interaction of matter and electromagnetic radiation. Since then, diffraction gratings have contributed significantly to modern science and are incorporated in many common spectrometry tools, including spectrophotometers and monochromators. They are generally prefered over prisms because they do not absorb UV or infrared radiation.
Types of Diffraction Gratings and Their Associated Spectrometry Tools
In general, there are four types of diffraction gratings: ruled gratings, holographic gratings, transmission gratings, and reflection gratings.
Ruled gratings are created by physically etching several parallel grooves onto a reflective surface. Applications that require a narrow wavelength, such as spectrometers and monochromators, often benefit from having a ruled grating blazed at that specific wavelength. Commons applications for ruled gratings are:
- Fluorescence Excitation
- Analytical Chemistry
- Life Sciences
- Space Sciences
Note: The wavelength of electromagnetic radiation that yields the greatest absolute efficiency of the ruled diffraction grating is referred to as the blaze wavelength.
Holographic gratings are developed by using a photolithographic process to generate an interference pattern between two UV beams, creating a sinusoidal index of refraction variation in a piece of optical glass. Generally, ruled diffraction gratings are lighter and cheaper than holographic gratings but they exhibit more stray light. On the other hand, holographic diffraction gratings are better for stray light performance but tend to have lower efficiency.
One popular style of grating is the transmission grating. This type of grating is created by scratching or etching a transparent substrate with a repetitive, parallel structure. In a transmission diffraction grating, light passes through the material on which the grating is etched.
Transmission gratings are particularly useful in fixed grating applications such as spectrographs.
Transmission gratings have relatively low polarization sensitivity when compared to reflection gratings because incident light is not reflected by a mirror coating. Transmission gratings are particularly effective in compact, in-line configurations because light is transmitted through the grating. Transmission gratings are great in monochromators and spectrometers.
A reflective grating is traditionally made by depositing a metallic coating on an optic and ruling parallel grooves in the surface. Reflective gratings are also commonly made by replicating a master diffraction grating version with epoxy and/or plastic. In all cases, light is reflected off of the ruled surface at different angles corresponding to different orders and wavelengths.
As evident in their descriptions, the four types of diffraction gratings listed are not necessarily mutually exclusive, and diffraction gratings are capable of incorporating components of multiple different types.
Diffraction Gratings for Spectrometry
Diffraction gratings are commonly used in monochromators, spectrometers, lasers, wavelength division multiplexing devices, optical pulse compression devices, and many other optical instruments. CDs and DVDs are good, easily observable examples of diffraction gratings. Reflecting sunlight off a CD or DVD onto a white wall will yield light of different colors i.e, different wavelengths of the visible spectrum.
Perhaps the most elementary application of diffraction gratings for spectrometry tools, spectrometers are used to separate white light into its constituent wavelengths.
In some ways, monochromators are kind of the reverse of spectrometers. While spectrometers separate white light into all its constituent colors, monochromators are devices used to filter out all but a narrow band of electromagnetic energy. This particular application of diffraction gratings for spectrometry tools is very useful when tunable monochromatic light is needed.
Diffraction gratings are often used in lasers for wavelength tuning. That is, calibrating the laser to emit a specific wavelength of electromagnetic radiation.
Holographic diffraction gratings have widespread use in optical communications and industrial measurement across near infrared spectral regions in which high performance and environment resistance are necessary.
Diffraction gratings have also found a foothold in the pulse compression technology. The gratings used for these applications tend to be made of high-purity monolithic fused silica, which is ideal for certain laser wavelengths. This application of diffraction gratings for spectrometry tools is generally found in laser material processing, the semiconductor industry and in the medical industry for refractive cornea correction.
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