Micro Fabry–Perot interferometer based on a microsphere lens for motor rotation speed measurement

In this study, a micro Fabry–Perot interferometer (FPI) based on a microsphere lens for motor rotation speed measurement is proposed and experimentally demonstrated. A 5 μm diameter microsphere is attached to the tip of a tapered single-mode fiber. The FPI can produce photonic nanojets (PNJs) because of the presence of the microsphere. The PNJs can produce small laser spots and enhance the backscattering light from the surface of the object. In this phenomenon, the speckle signal required in measuring motor rotation speed can be easily detected. Experimental results show that the relative errors are less than 0.1%.