Real-Time Double-Layer Thin Film Thickness Measurements Using Modified Sagnac Interferometer with Polarization Phase Shifting Approach

Abdullahi Usman,Rapeepan Kaewon,Chutchai Pawong,Apichai Bhatranand,Yuttapong Jiraraksopakun

doi:10.3390/photonics8120529

Abstract

This paper describes a modified Sagnac interferometer with a self-referenced polarization and phase-shifting technique for real-time thickness measurement of single- and double-layer transparent thin films. The proposed interferometric setup generated outstanding rotating linearly polarized light with a degree of polarization (DOP) of 99.40%. A beam splitter placed at the interferometer output separated the beam into two identical linearly polarized beams. One of the beams served as a reference, while the other served as a sensing arm. The output linear polarizer set at 45° relative to a reference plane was positioned anterior to the photodetectors to get rotating light intensities for phase shift measurement; hence, the intensities at various polarizations of 0°, 45°, and 90° were automatically acquired without any polarizing device adjustments. These intensities were then transformed into a phase retardation introduced by a sample, and the resulting phase shift was eventually converted into film thickness. The samples were properly prepared, with pure BK7 substrate being deposited by WO3-, Ta2O5-, and WO3/Ta2O5 films of known thicknesses. The thickness measurement obtained from the proposed system yielded reading errors of 1.3%, 0.2%, and 1.3/2.5% for WO3-, Ta2O5-, and WO3/Ta2O5 films, respectively. The mathematical theory was effectively demonstrated and empirically confirmed. The experimental results show that the proposed setup has a lot of potential for real-time, non-destructive thickness assessment of transparent thin films without the need to modify polarizing device orientations.

Highlights

One of the most significant properties of light for optical measurements of a material is the phase shift
To overcome difficulties described above, this paper proposes real-time simultaneous measurements of sample and reference arms using an automated phase shifting approach with balanced photodetectors located at the output
The proposed proposedapproach approachoffers offersa few a few advantages over previous works, and advantages over ourour previous works,. Both reports, an wave extra plate wave was plateinserted was inserted in the interferometric to. In both an extra in the interferometric system system to induce induce phase retardation viaand its fast and slow axes,itwhich it is not required in thearrangecurrent phase retardation via its fast slow axes, which is not required in the current arrangement

Summary

Introduction

One of the most significant properties of light for optical measurements of a material is the phase shift. This is achieved by the interference of two lights propagating through the same medium and this scheme is famously called interferometry. Michelson interferometer is the simplest arrangement using a beam splitter to generate fringes from coherent beams. Mach–Zehnder interferometer, developed after Michelson configuration, consists of a reference arm, a sample arm, and two beam splitters. It minimizes environmental interactions, but a small change in path length affects intensity and refractive index of the

Results

Discussion

Conclusion