Highly precise thickness measurement of multilayer films based on the cross-correlation algorithm using a widely tunable MG-Y laser

Abstract

Inspired by the demodulation algorithm of Fabry–Perot composite sensors in the field of fiber-optic sensing, this paper proposes a method based on a widely tunable modulated grating Y-branch (MG-Y) laser combined with the cross-correlation algorithm to achieve a highly precise measurement of the optical thickness of each layer of a multilayer optical sample. A sample consisting of a double glass stack was selected, and the interference spectrum of the stacked sample was acquired using a widely tunable MG-Y laser. A fast Fourier transform (FFT) algorithm combined with a finite impulse response (FIR) bandpass filter was utilized to separate the different frequency components of the multilayer optical sample. The normalized spectra of each layer were reconstructed using the Hilbert transform. Subsequently, a cross-correlation algorithm was employed to process the normalized spectrum and determine the optical thickness of each layer with high precision. The samples were measured at predetermined locations, with 150 consecutive measurements performed to assess the repetition of the thickness. The standard deviation of these measurements was found to be lower than 1.5 nm. The results show that the cross-correlation algorithm is advantageous in the optical thickness measurement of multilayer films.