Volume refractometry of liquids using stable optofluidic Fabry-Perot resonator with curved surfaces

This work reports a simple, miniaturized optical sensing module for liquid refractometry. It is based on a stable Fabry-Perot resonator consisting of two silicon cylindrical mirrors with a cylindrical lens in the core. The lens is formed by a capillary tube through which the analyte to be analyzed passes. This setup enables volume refractometry, where light propagates through the sample realizing high interaction depth. The cylindrical surfaces achieve light confinement, reducing the light escaping loss encountered in classical cavities with straight mirrors; and hence high quality factor (Q) over 1,000 is attained. Exploiting this high Q, we adopt uncommon refraction index (RI) measurement criterion: we operate at a fixed wavelength and detect the power drop caused as a consequence to the spectral shift with RI change. Performing experimental testing using a tunable laser and a power-meter, the normalized spectra for different mixture ratios of acetone and deionized water are obtained. The wavelength corresponding to the maximal power transmission from pure acetone is taken as a reference. A vertical line at this wavelength cuts the successive transmission curves and enables measuring the power drop in the linear region, and from it the refractive index change Delta n above the refractive index of the reference solution can be determined for 0.0023<Delta n<0.0045. Sensitivity up to 4,094 dBm/RIU is achieved. A wider range is still accessible by the conventional method of tracing the shift in peak wavelengths: a range of Delta n=0.0163 RIU can be scanned, with a sensitivity of 221 nm/RIU. Error analysis has been accomplished, and the design parameters of the device are discussed to evaluate the performance.