Silver halide fiber-based evanescent-wave liquid droplet sensing with room temperature mid-infrared quantum cascade lasers

Quantum cascade lasers and unclad silver halide fibers were used to assemble mid-infrared fiber-optics evanescent-wave sensors suitable to measure the chemical composition of liquid droplets. The laser wavelengths were chosen to be in the regions which offer the largest absorption contrast between constituents inside the mixture droplets. A pseudo-Beer-Lambert law fits well with the experimental data. Using a 300μm diameter fiber with a 25 mm immersion length, the signal to noise ratios correspond to 1 vol.% for a-tocophenol in squalane and 2 vol.% for acetone in aqueous solution for laser wavenumbers of 1208 cm^-1 and 1363 cm^-1, respectively.