Miniature Fiber-Optic Microflowmeter Based on Fabry-Perot Interferometer and Carbon Nanotubes

A novel fiber-optic micro flowmeter based on a Fabry-Perot interferometer (FPI) and carbon nanotubes (CNTs) has been proposed and investigated. The proposed sensor integrates a Fabry-Perot cavity on the tip of a single-mode fiber (SMF), which is used as the sensing probe. The Fabry-Perot cavity comprises an SMF and a short hollow-core fiber (HCF) section. The HCF is partially filled with CNTs-doped polydimethylsiloxane (CNTs-PDMS), which emits heat obviously due to the excellent photothermal effect of CNTs-PDMS when illuminated by a laser. Under a certain flow velocity, the heat would be taken away by a microfluid as it flows over the surface of the Fabry-Perot cavity until the thermal equilibrium is established. Besides, due to the thermal expansion of CNTs-PDMS, the variation of heat will result in a change in the cavity length of FPI. By monitoring the dip wavelength variation of the sensor, the microflow rate will be determined. The experimentally measured maximum flow rate sensitivity is 68.52 nm/( $\mu $ L/s) in the 0– $2~\mu $ L/s range. The proposed sensing probe is ultracompact, and the footprint is extremely small (the length of the sensing part is only $126~\mu $ m), which has potential in flow rate detection in narrow spaces and in vivo applications in biomedicine.