Spectroscopy on the go: Innovations in the design of portable broadband spectrometers

Spectrometers are advanced tools that let us analyze the makeup of light by separating it into its constituent wavelengths, much like how a prism breaks white light into a rainbow pattern. The way light interacts with matter at different wavelengths (or frequencies) provides valuable information about all sorts of physical, chemical, and biological processes. This is why spectrometers are used in many fields of science and engineering, including materials science, chemical analysis, and climate science. Thanks to recent advances in manufacturing technology and electronics, portable spectrometers have become a reality. These small devices extend the use cases of spectral analysis to a vast variety of applications, proving useful in field astronomy, environmental monitoring, industrial control systems, food safety analysis, and medicine.

Unfortunately, the design and manufacturing of portable broadband spectrometers remains challenging. These spectrometers must operate over a broad range of frequencies, spanning from the near-infrared region (NIR) to the high end of the visible region (VIS). To separate broadband light into its constituent wavelengths, grooved optical components called "gratings" can be used. However, gratings suffer from an inherent trade-off between size, operating frequency range, spectral resolution and efficiency; their performance on these fronts is ultimately constrained based on their shape and the disposition of their grooves.

Against this backdrop, the Brussels Photonics research team from Vrije Universiteit Brussel, Belgium, has been working on an innovative design for a high-performing yet portable broadband spectrometer. In their study, published in the Journal of Optical Microsystems, the team reports an innovative strategy based on the use of concave gratings.

Thanks to their curved shape, concave gratings can better focus different wavelengths of light to different angles compared to planar gratings, making them ideal for broadband applications. In their design, however, the researchers adopted a concave grating with variable spacing. Simply put, the grooves in the grating were not periodically arranged as in a conventional grating, but rather strategically positioned to compensate optical aberrations and achieve the highest possible spectral resolution.

To minimize the size of the spectrometer while maintaining a high performance, the team used a two-channel grating instead of a single-channel grating. This allowed them to optimize one portion of the grating for VIS light and the other portion for NIR light, giving them more freedom by relaxing design constraints. To fabricate the two-channel grating simultaneously on the same substrate, the proposed spectrometer employs a beam splitter that reflects light in the VIS range and lets light in the NIR range pass through with a folded mirror to align both channels, effectively dividing the broadband problem into two more manageable halves. Moreover, the researchers used a single 2D sensor to capture the light spread by both gratings instead of employing two independent line sensors, keeping the design and calibration simple.

Thanks to these innovative design strategies, the proposed spectrometer has no moving mechanical parts and a relatively small number of optical and electronics components. This not only simplifies manufacturing and assembly but also ensures small device size. Indeed, the spectrometer can fit in a volume of only 26 × 12 × 10 mm³, and a fully functional prototype with 2D sensor and housing only occupies 37 × 29 × 27 mm³. Despite its small size, the optical resolution of the spectrometer in the VIS and NIR channels was less than 1.6 nm and 3.1 nm, respectively.

Notably, the researchers also demonstrated that their concave gratings can be easily mass produced via a molding approach. To this end, they made brass molds bearing the inverse shape of the concave gratings using precision diamond tooling. After that, by simply pressing and heating up a piece of polymethyl methacrylate against the mold, they could quickly impart the desired grating shape onto the polymer. The gratings produced using this technique exhibited high diffraction efficiency, reaching up to 70% and 60% in the VIS and NIR channels, respectively.

Overall, this study shows an innovative design strategy to manufacture portable broadband spectrometers with exceptional performance. With any luck, these ideas will open the field of spectroscopy to many more applications!

Read the Gold Open Access paper by Shcheglov et al., “Miniaturized two-channel broadband spectrometer based on variable-spacing concave blazed gratings,” J. Opt. Microsys. 3(2) 024501 (2023) doi: 10.1117/1.JOM.3.2.024501.