Dual-Comb Spectroscopy With Video-Rate Imaging for Food, Agricultural and Pharmaceutical Applications

Posted Mar 27, 2020 by Universidad Carlos III de Madrid

Researchers from Universidad Carlos III de Madrid (Spain) have used an advanced analytical technique known as dual-comb spectroscopy to rapidly acquire extremely detailed hyperspectral images. By acquiring a full spectrum of information for each pixel in a scene with high sensitivity and speed, the new approach could greatly advance a wide range of scientific and industrial applications such as chemical analysis and biomedical sensing.

Dual-comb spectroscopy has revolutionized optical spectroscopy by providing unmatched spectral resolution and accuracy as well as short acquisition times without moving parts, The new direct hyperspectral dual-comb imaging approach will make it possible to expand most of the point-detection capabilities of current dual-comb systems to create a spectral image of an entire scene.

Dual-comb spectroscopy uses two optical sources, known as optical frequency combs, that emit a spectrum of colors – or frequencies – that are perfectly spaced like the teeth on a comb. As reported in Optica, The Optical Society's journal for high impact research, this is the first time that a dual-comb spectrum has been directly detected using a video camera.

Researchers demonstrate spectral interrogation of a 2D object in just one second, more than three orders of magnitude faster than previous demonstrations. This fast acquisition time enables dual-comb hyperspectral imaging of fast or dynamic processes, which wasn’t possible before. Although the work was performed using near-infrared wavelengths, the researchers say that the concept can be easily transferred to a variety of spectral regions, widening the number of possible applications.

In particular, expanding the approach to the terahertz and millimeter-wave spectral regions would open many new opportunities for nondestructive testing and product inspection in the food, agricultural and pharmaceutical industries. In the mid-infrared and the near-infrared regions, it could also enhance the performance of chemical imaging, 3D mapping, and surface topography technologies.

Video-Rate Detection

Dual-comb spectrometers work by interfering light from two closely matched optical frequency combs. This mixing process generates a signal known as an interferogram at rates that are typically in the tens of megahertz (million times per second), too fast to capture with even the fastest high-speed video cameras. The research team stretched the interferograms generated by their system up to a second to make it possible to detect the dual-comb interference signal using a video camera. This allows the spectral analysis of an entire scene, instead of just a point.

To do this the researchers built a system based on a very simple electro-optic dual-comb source made mostly of optical fiber components. The use of two acousto-optic modulators let them offset the optical combs by an arbitrarily low frequency, to create ultra-slow interferograms. The researchers used the new method to acquire hyperspectral images of ammonia gas escaping from a bottle. They achieved an optical resolution of 1 GHz (0.0033 cm-1) at video rates of 25 frames per second, with each frame containing 327,680 individual spectral measurements. According to the researchers, the resolution they achieved allows easy distinction between different gases and is 100 times better than current commercial equipment.

This enables the researchers to easily identify and distinguish between different gases. The resolution demonstrated in this first experimental demonstration is two orders of magnitude better than that of current commercial equipment. Simplicity is one of the main strengths of the system. It worked flawlessly and could be implemented in any optics laboratory.

The work is part of a larger project funded by the ATTRACT initiative (Horizon 2020), which aims to develop a fast hyperspectral imaging system that uses the terahertz region of the electromagnetic spectrum for inspection, quality control, and classification of agricultural and food products. The researchers are now working to develop a terahertz dual-comb source to demonstrate the method in this spectral region.

Click here for more details on ‘Direct Hyperspectral Dual-Comb Imaging’.