A scientific team made up of researchers from France, Canada and Spain have just published a paper in which they show a new on-chip broadband Fourier transform spectrometer that operates at near-infrared wavelengths and has a large surface area of light gathering.
This new spectrometer manages to expand the operating bandwidth with respect to the state of the art, thus allowing the same device to detect different types of substances with the same high resolution and sensitivity that characterizes this type of spectrometer.

Why are spectrometers important?

Optical spectroscopy has been used in many applications for the identification of specific molecules and compounds using the interaction between light and the matter to be studied, through the physical phenomena of light emission, scattering or absorption.

In particular, the spectral range of near infrared light, which covers the wavelengths between 800 nm and 2500 nm, has once again aroused great interest with applications ranging from the food industry, chemical and biological analysis, research medical and even environmental supervision. These applications often demand high spectral resolution, wide operating bandwidth, and high optical performance.

Benchtop spectrometers achieve high spectral resolution (they differentiate compounds very well from others) and broadband operation for the detection of multiple substances; however, these spectroscopic systems are based on bulky optical instruments.

In contrast, miniaturized spectrometers, such as the one in this study that occupies 9.8 × 0.9 mm ² , have emerged as a promising solution for performing analysis outside the laboratory with portable devices or remote sensing. mounted on microsatellites and unmanned vehicles.

Fourier transform spectrometers

Fourier transform (FT) spectrometers consist of an interferometer that splits light into two light beams. A small variation in its optical path is applied to one of the beams, so that when they are combined again, the two beams are added to form an interferogram as a function of the difference in the optical path of the interferometer arms.
The spectrum of the input light is obtained by the Fourier transform of the interferogram detected at the output.
This method achieves a higher resolution and also a higher sensitivity.

The scientific team has exploited the wide bandwidth of their recent high-performance beam splitter , and has also managed to implement a large 320 × 410 μm2 light collection surface to better exploit the Jacquinot advantage (or optical performance).

Optical performance is a measure of the amount of light that the system uses, that is, of the amount of light from the source that reaches the detector at the output. Due to the fact that Fourier transform spectrometers do not introduce dispersive elements with energy losses, the intensity of the signal to be detected is much greater than in other spectrometers based on diffraction grating, slits or raw materials.

Manufacturing and results

The device, which only occupies 9.8 × 0.9 mm ² , was manufactured using 248 nm deep ultraviolet lithography, exhibiting more than 13 dB improvement in optical performance compared to a device of single opening. The measured spectral resolution varies between 29 pm and 49 pm within the operating wavelength range of 1260 - 1600 nm.

These results pave the way for the implementation of cost-effective on-chip spectrometers for near-infrared multi-target applications such as environmental studies or chemical analysis.

Link to the article

The work is a collaboration of the Institute of Optics with the Center de Nanosciences et de Nanotechnologies (France), STMicroelectronics (France), the National Research Council Canada and the Center for Research in Photonics of the University of Ottawa (Canada)