Investigations on Exponential Lifetime Measurements for Fluorescence Thermometry

Lifetime-based methods have been, on the whole, one of the most successful schemes for fiber optic temperature sensing, using fluorescent materials whose response is intensity independent. Several approaches for determining the fluorescence lifetime, and with that the measurand, have been investigated. An experimental comparison of direct and indirect measurement methods, i.e., involving actual signals from representative optical media instead of simply using Monte Carlo simulations, has been carried out. Direct fitting methods, including Marquardt, log-fit and Prony, were used to estimate the fluorescence lifetime of a Cr3+:YAG-based sensor system and the results were compared. An agreement to better than 0.5% between Marquardt and log-fit algorithms and an agreement of about 1.5% between Marquardt and Prony approaches was found. Thus, a temperature reproducibility, of 0.5 and 1.2 °C, respectively, can be obtained with the Cr3+:YAG sensor system. An indirect measurement approach based on a phase-locked (analog-to-digital signal processor) (A-DSP) was also tested. It was found that when the A-DSP output is used to estimate the lifetime, it performs only slightly better than using direct fitting methods. On the contrary, when the whole A-DSP sensor system was directly calibrated against temperature, the measurement accuracy improves by at least a factor of 10.