Multiwall Carbon Nanotubes-Integrated Fiber Microsphere Probe Light Intensity Sensor With Harmonic Vernier Effect

Reliable and highly sensitive fiber-based light intensity sensors are increasingly demanded for integrated and miniaturized optical sensing platforms. In this work, we demonstrate a highly sensitive all-fiber light intensity sensor based on a hollow microsphere Fabry–Pérot (FP) cavity probe coated with multiwalled carbon nanotubes (MWCNTs). Light intensity detection is realized through the photothermal conversion of MWCNTs and is further enhanced by a harmonic Vernier mechanism. The harmonic order of the FP cavity can be flexibly controlled by adjusting the air cavity length and the diameter of the microsphere. Microsphere FP cavities operating at the traditional, first-order harmonics, and second-order harmonics Vernier effect are fabricated experimentally. Temperature response measurements reveal that sensitivity increases significantly with the harmonic order. For the same harmonic order, a larger envelope-free spectral range of the Vernier spectrum leads to higher temperature sensitivity. A maximum temperature sensitivity of 0.392 nm/°C is achieved for the probe operating at the second order harmonic. By further functionalizing the probe with MWCNTs to realize photodetection, a light intensity sensitivity as high as 2.5 nm/mW is achieved. These results demonstrate that the hollow microsphere structure provides an excellent platform for realizing high-order harmonic Vernier effects, while the MWCNTs coating enhances the light intensity response, enabling a highly sensitive and integrated all-fiber light intensity sensor.