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

摘要

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 between. The lens is formed by a capillary tube through which the analyte 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; hence, a high-quality factor (Q) over 1000 is attained. Exploiting this high Q, we adopt the refractive index (Rl) measurement criterion: operating at a fixed wavelength and detecting the power drop as a consequence to the spectral shift with Rl change. This technique showed that measuring Rl change Δn above the Rl of the reference solution can be determined for 0.0023 < Δn < 0.0045. Sensitivity up to 4094 dBm/RIU is achieved. A wider range is still achievable by the conventional method of tracing the shift in peak wavelengths: a range of Δn = 0.0163 RIU can be scanned, with a sensitivity of 221 nm/RIU. Error analysis has been accomplished, and the device's design parameters are discussed to evaluate the performance.