Fiber-tip optic sensor based on a three-beam interferometer for simultaneous measurement of flow rate and temperature

Precise monitoring of the microfluidic flow rate is a key factor in microfluidic chips. However, the temperature of the microfluidics often affects the measuring results of the optical flow rate sensor. Here, we present a fiber-tip optical sensor based on a three-beam interferometer that can measure microfluidic flow rate and temperature simultaneously. The optical fiber sensor is composed of UV adhesive at the bottom of the microcavity sealed by liquid paraffin, and a section of air is reserved in the hollow core fiber (HCF). The flow of microfluidics will affect the pressure of the air cavity formed by UV adhesive and the paraffin. Microfluidics with different flow rates in the channel of the microfluidic chip have different effects on the position of the liquid paraffin in the cavity. Because of the small viscous force between the paraffin and the inner wall of the HCF, the length of the air cavity changed greatly as the flow rate changed resulting in a high flow rate sensitivity. The UV adhesive as a temperature measuring part has a high thermo-optical coefficient (TOC), as a temperature measuring part, utilized to measure the temperature of the microfluidics. The length variation of air cavity and refractive index (RI) of UV adhesive will cause the shifting of the dips in the interference spectrum. The rate sensitivity can reach -22.51 nm/(mm/s) in the range of 0.17 mm/s to 1.17 mm/s. In addition, the sensor has a stable temperature sensitivity of 13.513 nm/℃. This work provides a low-cost sensing platform for measuring various physical parameters in microfluidic chips, which is of great significance for on-chip biochemical reactions.