Thermal blood flowmeter based on cascaded Fabry-Pérot Interferometers utilising the enhanced harmonic Vernier effect

An enhanced Vernier effect-based CFPI blood flowmeter has been developed and tested. By generating the enhanced Vernier effect and using a polymer (NOA65) within the cavity, the temperature sensitivity of the CFPI was improved 117.4 times compared to a single polymer cavity, reaching 26.67±1.07 nm/℃ with R-square 0.9957. The CFPI was combined with multimode fibre, and a green LED was utilized to heat model blood. As the blood temperature decreases with increasing blood flow, and flow rate can be obtained by temperature monitoring. The maximum temperature change recorded in this test was 0.3°C, minimizing the issue of excessive heating temperature changes in traditional thermal anemometry, making the CFPI sensor suitable for blood flow measurement. Ultimately, the CFPI sensor obtained blood flow rate information ranging from 20 to 300 ml/min. Clinically, this value can cover the arm total blood flow rate at rest [5] but can be further improved by increasing the output power of the heating light source. The sensor exhibits excellent accuracy, repeatability, and insensitivity to pressure, demonstrating its potential for blood flow rate monitoring.