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

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.