Fiber-Loop Ring-Down Spectroscopy for Enhanced Detection of Absorption with Limited Path Length

Abstract

Pulsed, near-infrared laser light is coupled into an optical fiber, which is wound into a loop using a fiber splice connector. The light pulses traveling through the fiber-loop are detected using a photomultiplier detector. It is found that once the light is coupled into the fiber it experiences very little loss and the light pulses do a large number of round trips before their intensity is below the detection threshold. This method resembles “cavity ring-down absorption spectroscopy” and is well suited to characterize low-loss processes in fiber optic transmission independent from power fluctuations of the light source. It is demonstrated that the technique is useful as an absorption spectroscopic technique of very small sample volumes. This has been shown by recording an absorption spectrum of 7 x 10”¹⁵ mol of the dye 1,1′-diethyl-4,4′-dicarbocyanine iodide (DDCI) in 7 × 10¹² L of dimethylsulfoxide.