Thin liquid film thickness measurement using stepped-index-type optical fiber sensors

Abstract

Optical fiber liquid film sensors utilizing the reflection intensity at the gas-liquid interface exhibit excellent environmental robustness. However, this technique needs to improve its sensitivity when measuring liquid film thicknesses approaching the fiber core diameter. In this study, we propose a method for accurately measuring liquid film thickness around the fiber core diameter by changing the spatial intensity distribution of laser light emitted from the fiber. We employ stepped-index-type optical fibers (SI-type) instead of the commonly used graded-index-type fibers (GI-type) to alter the spatial distribution and investigate the relationship between reflection intensity and liquid film thickness. The reflection intensity of SI-type optical fibers exhibits a linear response over a range approximately eight times the core radius, significantly enhancing sensitivity for thin films. In addition, we use a theoretical model for the spatial intensity distribution of light emitted from SI-type optical fibers, demonstrating a close match between the predictions and experimental results, and discussing the linear response. Finally, we demonstrate the liquid film thickness measurement of the liquid jet impinging surface.

Keywords

• gas-liquid two-phase flow
• measurement
• film thickness
• optical fiber
• laser intensity
• impinging jet