Thickness Measurement of Single-Mode Fiber Coating Layer Based on Forward Brillouin Scattering

The coating layer is essential for protecting and applying optical fibers. Nowadays, the thickness measurement of the coating layer relies on point measurement schemes, such as the side-looking light distribution method and mechanical method. However, there are some problems with the above methods, such as the tradeoff between resolution and field of view (FOV), the cutting of the measurement position, and the long measurement time. To overcome these problems, we propose a new method based on forward Brillouin scattering (FBS), which measures the coating layer thickness of single mode fibers (SMFs). The physical properties of the transverse acoustic field involved in FBS make it possible to measure the thickness of the fiber coating. The corresponding relationship between the transverse acoustic mode frequency and the diameter of the coating layer can be calculated. The coating layer thickness can be measured with high precision by measuring the frequency spectrum of FBS corresponding to multiple transverse acoustic modes at a given temperature. In the verification experiment, three types of SMF coated with polyimide are used as the sensing fiber to measure the thickness of the coating layer at multiple temperatures. The experimental results show that the error in the coating layer thickness obtained is less than $0.05~\mu $ m. To the best of our knowledge, this is the first demonstration of nondestructive, high-precision monitoring of fiber coating thickness.