Mid-IR optical sensor for CO2 detection based on fluorescence absorbance of Dy3+:Ga5Ge20Sb10S65 fibers

The Dy3+ doped Ga5Ge20Sb10S65 bulk glass provides good emission efficiency in the middle infrared with a better brightness than the blackbody sources. Sulfide single index fibers doped with Dy3+ (500-3000 ppm) were drawn with optical attenuation of about 1-3 dB/m, suitable to develop fluorescence sources for chemical analysis by optical absorption in middle infrared. They particularly present a broad emission around 4-5 μm, making them attractive for CO2 detection. Considering the low cost and efficient Dy3+ pumping scheme by means of a commercial laser diode, the Dy3+ sulfide fiber reveals potential for developing a CO2 optical sensor. Using the 4.35 μm broad emission of a Dy3+ doped Ga5Ge20Sb10S65 fiber combined with a differential measurement technique, the CO2 gas concentration measurement was carried out fruitfully. For this specific application, the CO2 detection threshold was fixed at about 400 ppm corresponding to atmospheric concentration and was successfully reached for a cell length of 1.5 cm with a resolution of about 5%. The sensitivity of the setup is mainly related to the size of the cell, deliberately reduced to develop a robust and compact system for natural geological sites.