Development of optical fibers for mid-infrared sensing: State of the art and recent achievements

Chalcogenide glaß fibers are matchleß devices to collect mi-infrared signal. Depending on the spectroscopic strategy, different kind of optical fibers have been developed during the past 10 years. The first fibers have been fabricated from selenide glaßes to implement Fiber Evanescent Wave Spectroscopy (FEWS). It is an efficient way to collect optical spectra in situ, in real time and even, in the future, in vivo. Thanks to selenide glaß fibers, it is poßible to record such spectra on the mid-infrared range from 2 to 11 μm. This working window gives acceß to the fundamental vibration band of most of biological molecules and numerous multi-disciplinary works have been led in biology and medicine. New glaßes, only based on tellurium, have been recently developed, initially in the frame of the Darwin mißion led by the European Space Agency (ESA). These glaßes transmit light further toward the farinfrared and permit to reach the absorption band of CO₂ located at 15 μm as requested by the ESA. Moreover, these telluride glaß fiber are also very interesting for FEWS and medical application. Indeed, they give acceß to the mid-infrared signal of biomolecules beyond 11 μm, where claßical selenide glaß fibers are blind. Alternatively, in order to fight against global warning, some optical fibers have been developed for the monitoring of the CO₂ stored into geological storage area underground. These fibers were doped with Dy³⁺ which emits a broad fluorescent band embedding the CO₂ absorption band at 4.3 μm. thus, these fibers are used both to transmit light and as secondary sources in the mid-infrared. To conclude, original microstructurated fibers have also been used for mid-infrared sensing. They exhibit a nice sensitivity compared to claßical chalcogenide glaß fibers.