Accurate thin-film measurement method based on a distribution of laser intensity emitted from optical fiber: Proposal of step light emitted model for ray-tracing simulation

The accurate measurement of thin liquid films in various environments is essential for several industrial applications. In this regard, measurement procedures involving the use of optical fibers are preferred because such fibers are resistant to heat and pressure. Herein, we propose a high-accuracy method to measure liquid films with thicknesses of <100 μm (about fiber diameter) on the basis of the variation in the intensity distribution of the laser light emitted from the optical fiber; the thickness is measured by using the light reflected from the air– liquid interface (called the glare light in this study). First, instead of using light with a Gaussian distribution (characteristic of conventional graded-index fibers), we consider a stepped-index-type optical fiber with a step distribution. We model the distribution of the light emitted from the optical fiber and analyze the reflected light, i.e., glare light, via the ray-tracing method. We model three distributions: the Gaussian, point-like, and step distributions, and then we found that the step-distribution-based approach facilitates high-resolution measurements of liquid films with thicknesses less than optical fiber diameter. Moreover, the reflected light intensities for different film thicknesses closely agreed with the experimental results obtained using a steppedindex fiber. Remarkably, the intensity of the reflected light linearly decreases with the increase in the film thickness when using the step distribution. The numerical results quantitatively agreed with experiments; therefore, these results indicate the possibility of numerical calibration for liquid-film measurements with the use of the proposed step distribution model.