Depth-resolved local reflectance spectra measurements in full-field optical coherence tomography

Abstract

Full-field optical coherence tomography (FF-OCT) is a widely used technique for applications such as biological imaging, optical metrology, and materials characterization, providing structural and spectral information. By spectral analysis of the backscattered light, the technique of spectroscopic-OCT enables the differentiation of structures having different spectral properties, but not the determination of their reflectance spectrum. For surface measurements, this can be achieved by applying a Fourier transform to the interferometric signals and using an accurate calibration of the optical system. An extension of this method is reported for local spectroscopic characterization of transparent samples and in particular for the determination of depth-resolved reflectance spectra of buried interfaces. The correct functioning of the method is demonstrated by comparing the results with those obtained using a program based on electromagnetic matrix methods for stratified media. Experimental measurements of spatial resolutions are provided to demonstrate the smallest structures that can be characterized.

Local inspection of refractive index and thickness of thick transparent layers using spectral reflectance measurements in low coherence scanning interferometry

“…To determine both parameters accurately, great care must therefore be taken during the data acquisition process. We previously demonstrated that the numerical aperture (NA) of the objectives could significantly modify the amplitude of the reflectance spectrum reconstructed from a FT of the fringes [16]. By keeping the NA fully open during the image stack acquisition, errors are induced in the amplitude of the spectrum and we then show that the determination of the refractive index is wrong, subsequently distorting the measurement of the thickness (Fig.…”

“…For this first result, the refractive index was assumed to be constant in the model to speed up the algorithm. Usually spread over an area of 0.79 µm², the diffraction spot becomes more extended (2.27 µm²) because of the necessity of reducing the system NA, resulting in a lateral resolution that decreases from 0.5 µm to 0.85 µm [16].…”

Spatially‐Resolved Spectroscopic Characterization of Reflective and Transparent Materials at a Micro‐Meter Scale Using Coherence Scanning Interferometry

“…[10][11][12] We have previously demonstrated that for depth-resolved spectral measurements, it is necessary to decrease the effective NA of the optical system, in this case by closing the aperture diaphragm, in order to reduce the errors that distort the results. [13] Consequently, the lateral resolution slightly decreases to 0.85 μm. For these kinds of measurements the notion of axial resolution is added.…”

“…Usually, the signal is selected using a window with a length which is approximately the same as that of the interferogram. [13] This gives a spectral resolution of 30 nm at a wavelength of 650 nm for both cases. The measurable wavelength range is set by both the illumination spectrum of the source and the spectral response of the camera.…”

“…However, to keep high quality results, the measurements are only performed in the range where the signal to noise ratio is greater than a certain threshold value. [8,13] This threshold limits the measurement at 550 nm towards the small wavelengths and at 1000 nm toward higher wavelengths.…”