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

• Innovative thermal blood flowmeter combining cascaded Fabry-Pérot Interferometer (CFPI) and multimode Fibre (MMF). • Enhanced harmonic Vernier effect improves temperature sensitivity, achieving 26.67±1.07 nm/°C, which is 117× higher than a single polymer cavity. • Cascaded CFPI structure enables to generate Vernier effect in a compact design, eliminating the need for a separate reference cavity. • Wide flow measurement range (20–300 ml/min) with temperature variation below 1 °C. Localised accurate blood flow measurement is crucial in various clinical scenarios for diagnosing and treating vascular conditions. An innovative thermal blood flowmeter that comprises cascaded Fabry-Pérot Interferometer (CFPI) and a multimode fibre (MMF) is presented. The MMF is connected to a green LED for modulating fluid temperature, and the resulting temperature change is inversely proportional to the flow rate. The CFPI consists of a polymer and air cavities of lengths 285.1 μ m and 408.4 μ m , respectively. The CFPI temperature sensitivity is enhanced due to the high thermo-expansion of the polymer compared to silica fibre. The typical discrete FPI system based on the Vernier effect comprises one sensing cavity and a reference cavity of constant length. In contrast, the CFPI is more compact and further enhances the sensor's sensitivity because the lengths of the two cavities change in opposite directions. The CFPI's temperature sensitivity is improved 117.4 times compared to a single polymer cavity and can reach 26.67 ± 1.07 nm/ °C. Flow in a phantom system can be measured over a range from 20 ml/min to 300 ml/min.