Evanescent-wave sensing and spectroscopy
Optical cavity-based detection methods have been widely used for sensing and spectroscopy with lasers or incoherent light sources. Using optical fibers, it is easy to build cheap, small-size, high-finesse resonators that do not require special care in terms of alignment, cleaning and insulation. Optical fiber resonators are particularly convenient for liquid detection and analysis, as demonstrated in recent works. Radiation-matter interaction may occur either by custom-built coupling interfaces or evanescent-field access elements such as core-exposed fibers and tapered fibers. In this activity, we perform high sensitivity liquid sensing & spectroscopy by evanescent-wave absorption cavity ring-down spectroscopy in a fiber-ring resonator. The frequency of the cavity modes is kept resonant with a near-infrared laser or an optical frequency comb using a Pound-Drever-Hall (PDH) locking scheme. The detection limit is characterized through repeated cavity lifetime measurements and a comparison with various noise contributions. Exploiting the intrinsic sensitivity and noise immunity of cavity ring-down spectroscopy technique, we show that liquid absorption can be detected down to a level which is nearly a factor 10 above the shot noise limit. The optical comb allows to cover entire absorption bands of liquid species and distinguish among different chemical components in a liquid sample. A thorough comparison between the experimental results and various noise contributions is provided also pointing out that different expressions can be used to calculate shot noise equivalent absorbance. As a proof of principle, polyamines detection in aqueous solutions has been already carried out demonstrating sensitivity to the sample evaporation with a minimum detectable absorbance of 1.8 × 10<sup>-7</sup> Hz<sup>-1/2</sup>, which is to our knowledge the best sensitivity limit reported to date for evanescent-wave sensors.