Fiber Evanescent Wave Spectroscopy (FEWS) is an efficient way to collect optical spectra in situ, in real time and even, hopefully, in vivo. Thanks to selenide glass fibers, it is possible to get such spectra over the whole mid-infrared range from 2 to 12 μm. This working window gives access to the fundamental vibration band of most of biological molecules. Moreover selenide glasses are stable and easy to handle, and it is possible to shape the fiber and create a tapered sensing head to drastically increase the sensitivity. Within the past decades, numerous multi-disciplinary studies have been conducted in collaboration with the City Hospital of Rennes. Clinical trials have provided very promising results in biology and medicine which have led to the creation in 2011 of the DIAFIR Company dedicated to the commercialization of fiber-based infrared biosensors. In addition, new glasses based on tellurium only have been recently developed, initially in the framework of the Darwin mission led by the European Space Agency (ESA). These glasses transmit light further into the far-infrared and could also be very useful for medical applications in the near future. Indeed, they permit to reach the vibrational bands of biomolecules laying from 12 to 16 μm where selenide glasses do not transmit light anymore. However, while Se is a very good glass former, telluride glasses tend to crystallize easily due to the metallic nature of Te bonds. Hence, further work is under way to stabilize the glass composition for fibers drawing and to lower the optical losses for improving their sensitivity as bio-sensors.
Chalcogenide glasses are a matchless material as far as mid-infrared (IR) applications are concerned. They transmit light typically from 2 to 12 μm and even as far as 20 μm depending on their composition, and numerous glass compositions can be designed for optical fibers. One of the most promising applications of these fibers consists in implementing fiber evanescent wave spectroscopy, which enables detection of the mid-IR signature of most biomolecules. The principles of fiber evanescent wave spectroscopy are recalled together with the benefit of using selenide glass to carry out this spectroscopy. Then, two large-scale studies in recent years in medicine and food safety are exposed. To conclude, the future strategy is presented. It focuses on the development of rare earth-doped fibers used as mid-IR sources on one hand and tellurium-based glasses to shift the limit of detection toward longer wavelength on the other hand.
Chalcogenide glasses are well known materials due to their transparency in the infrared optical range and their ability to
be drawn into optical fibers. Such fibers can transmit light from 1 to 20 μm depending on the composition of the glass
constituting the fiber. Besides, microstructured silica fibers have been successfully used as fiber sensors as the holes can
be filled with liquid or gas to achieve overlap of the mode field (doing the sensing) and the sample. Since gas generally
shows a characteristic optical absorption spectrum in the mid-infrared, it can be detected selectively and quantitatively in
a given environment by analysing mid-IR spectra, in a region where silica fibers can't be used due to their low
transmission. Microstructured optical fibers made of chalcogenide glass will permit to implement this measurement and
detect species such as CO2 through its absorption band near 4.2 μm.
Among the measures to reduce CO<sub>2</sub> emissions, capture and geological storage holds out promise for the future in the
fight against climate change. The aim of this project is to develop a remote optical sensor working in the mid-infrared
range which will be able to detect and monitor carbon dioxide gas. Thus, chalcogenide glasses, transmitting light in the
1-6 μm range, are matchless materials. The first of our optical device is based on the use of two GeSe<sub>4</sub> chalcogenide
optical fibers, connected to an FTIR spectrometer and where CO<sub>2</sub> gas can flow freely through a 4 mm-spacing between
fibers. Such sensor system is fully reversible and the sensitivity threshold is about 0.5 vol.%. Fiber Evanescent Wave
Spectroscopy technology was also studied using a microstructured chalcogenide fiber and first tests led at 4.2 μm have
provided very promising results. Finally, in order to explore the potentiality of integrated optical structures for microsensor,
sulphide or selenide Ge25Sb10S(Se)65 rib waveguide were deposited on Si/SiO<sub>2</sub> wafer substrates, using pulsed
laser deposition and RF magnetron sputtering deposition methods. The final aim of this study is to develop a rib
waveguide adapted for middle-IR including an Y-splitter with a reference beam and sensor beam targeting an accurate