This work reports a simple, miniaturized optical sensing module for liquid refractometry. It is based on a stable Fabry–Pérot 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 () over 1000 is attained. Exploiting this high , we adopt the refractive index (RI) measurement criterion: operating at a fixed wavelength and detecting the power drop as a consequence to the spectral shift with RI change. This technique showed that measuring RI change above the RI of the reference solution can be determined for . Sensitivity up to is achieved. A wider range is still achievable by the conventional method of tracing the shift in peak wavelengths: a range of can be scanned, with a sensitivity of . Error analysis has been accomplished, and the device’s design parameters are discussed to evaluate the performance.