Comparison of envelope detection techniques in coherence scanning interferometry

Abstract

The aim of this work is to make a comparison of the most current signal processing techniques used to analyze the fringe signal in coherence scanning interferometry (CSI), a major technique for optical surface roughness measurements. We focus here on classical AM-FM signal-processing algorithms such as the Hilbert transform (HT), the five-sample adaptive (FSA), and the continuous wavelet transform (CWT). We have recently also introduced a new family of compact and robust algorithms using the Teager-Kaiser energy operator (TKEO). We propose an improved version of TKEO using a combination of different techniques of pre-filtering and demodulation processing to remove the noise and offset component and to retrieve the fringe envelope to either determine the surface height information or to separate adjacent transparent layers. In particular, as a pre-filtering approach, we have focused on empirical mode decomposition in combination with the Savitzky-Golay filter. An added Gaussian post-filtering is helpful for a precise peak extraction. The experimental results show that TKEO performs better than CWT in terms of computation time and provides a better surface extraction than HT and FSA. Results have been obtained on synthetic and real data taken from a layer of resin on a silicon substrate.

Development of surface reconstruction algorithms for optical interferometric measurement

“…A scanning actuator such as a piezoelectric transducer (PZT) drives the interference objective to make continuous motion along the vertical axis, which synchronizes the scan of focus and optical path length [32]. Figure 2 [57] shows a typical layout of CSI using a Linnik objective. The sample is placed on a motorized axial translation stage to perform a single vertical scan.…”

“…The sample is placed on a motorized axial translation stage to perform a single vertical scan. The peak positions of the fringe envelope along the scanning direction are measured, which corresponds to the height of the surface [57,58]. To achieve the accurate zero OPD position, Zhou et al [59] employed a laser Michelson interferometry system to calibrate the displacement of the PZT stage in an SWLI system.…”

Simultaneous dual-band line-field confocal optical coherence tomography: application to skin imaging

“…The axial resolution of our imaging system is defined as the FWHM of the envelop of the interference pattern from a glass/air interface. For this purpose, the interference pattern is flattened to remove the confocal reflection from the interface, and the envelope is extracted using the method proposed in [21] (see Fig. 3).…”

Line-field confocal optical coherence tomography operating simultaneously at 800 nm and 1300 nm center wavelengths

“…The axial resolution of the imaging system is defined as the full width at half maximum (FWHM) of the envelop of the interference pattern. The method presented in [21] is used to flatten the interference pattern to remove the confocal reflection and to extract the envelop of the fringes. Axial resolutions of 1.2 µm and 2.7 µm are measured in the VIS/nIR band and the nIR band respectively.…”