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Spectrometers – devices that distinguish different wavelengths of light and are used to determine the chemical composition of everything from laboratory materials to distant stars – are large devices with six-digit price tags, and tend to be found in large laboratories or observatories of universities and industries.
A new breakthrough by MIT researchers could produce tiny, accurate and powerful spectrometers that can be mbad-produced using conventional chip-making processes. This approach could pave the way for new uses of spectrometry that would previously have been impossible physically and financially.
The invention is described today in the journal Nature Communicationsin an article by Juejun Hu, Associate Professor of Materials Science and Materials Engineering, Derek Kita, Ph.D. student, Brando Miranda, Research Assistant, and five others.
The researchers explain that this new approach to manufacturing on-chip spectrometers could offer major advantages in terms of performance, size, weight and energy consumption, compared to current instruments.
Other groups have tried to make chip-based spectrometers, but the challenge is inherent: the ability of a device to spread light according to its wavelength, using any optical system conventional, depends heavily on its size. "If you reduce performance, performance deteriorates," says Hu.
Another type of spectrometer uses a mathematical approach called Fourier transform. But these devices are always subject to the same size constraint: long optical paths are essential to achieve high performance. Since high performance devices require tunable and long optical path lengths, miniaturized spectrometers are generally inferior to their table counterparts.
Instead, "we used a different technique," says Kita. Their system is based on optical switches, which can instantly reverse a beam of light between different optical paths, which can be of different lengths. These fully electronic optical switches eliminate the need for moving mirrors, which are required in current versions, and can easily be manufactured using standard chip making technology.
According to Kita, eliminating moving parts, there is a huge advantage in terms of robustness. You can put it on the table without causing any damage. "
By using path lengths in power increments of two, these lengths can be combined in different ways to replicate an exponential number of discrete lengths, thereby leading to a potential spectral resolution increasing exponentially with the number of on-chip optical switches. It is the same principle that allows a scale to accurately measure a wide range of weights by combining only a small number of standard weights.
To prove the concept, the researchers used an industry-standard semiconductor manufacturing service to build a device with six sequential switches, producing 64 spectral channels, with built-in processing capability to control the device and treat its output. By expanding to 10 switches, the resolution would increase to 1024 channels. They designed the device as a plug-and-play unit that can be easily integrated into existing optical networks.
The team also used new machine learning techniques to reconstruct detailed spectra from a limited number of channels. The method they developed works well for detecting wide and narrow spectral peaks, says Kita. They were able to demonstrate that its performance matched the calculations, opening up a wide range of development possibilities for various applications.
The researchers explain that such spectrometers could find applications in sensing devices, material badysis systems, coherent optical tomography in medical imaging and optical network performance monitoring, on which most current digital networks are based. . Already, some companies interested in the team have contacted the team, promising huge benefits in terms of size, weight and energy consumption, says Kita. There is also an interest in real-time monitoring applications of industrial processes, adds Hu, as well as for environmental detection for industries such as oil and gas.
This work "is a very interesting approach because it allows for a high-resolution spectrometer on a small footprint," says Gunther Roelkens, professor at the University of Ghent in Belgium, who was not connected to this research . "This device enables applications such as on-chip spectroscopic sensors, which are a hot topic of research."
"The challenge for future research will be to extend the wavelength coverage while maintaining the same resolution," adds Roelkens. "In addition, addressing different bands of wavelength will allow many new applications."
The team also included David Favela, MIT undergraduate, Jerome Michon, postgraduate student, former postdoctoral fellow Hongtao Lin, researcher Tian Gu and staff member David Bono. The research was funded by the National Science Foundation, the MIT SENSE.nano, the US Department of Energy and the Saks Kavanaugh Foundation.
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