Resolution-enhanced optical coherence tomography based on classical intensity interferometry

Extracting Group Velocity Dispersion values using quantum-mimic Optical Coherence Tomography and Machine Learning

Quantum-mimic Optical Coherence Tomography (Qm-OCT) images are cluttered with artefacts - parasitic peaks which emerge as a by-product of the algorithm used in this method. However, the shape and behaviour of an artefact are uniquely related to Group Velocity Dispersion (GVD) of the layer this artefact corresponds to and consequently, the GVD values can be inferred by carefully analysing them. Since for multi-layered objects the number of artefacts is too high to enable layer-specific analysis, we employ a solution based on Machine Learning. We train a neural network with Qm-OCT data as an input and dispersion profiles, i.e. depth distribution of GVD within an A-scan, as an output. By accounting for noise during training, we process experimental data and estimate the GVD values of BK7 and sapphire as well as provide a qualitative GVD value distribution in a grape and cucumber. Compared to other GVD-retrieving methods, our solution does not require user input, automatically provides dispersion values for all the visualised layers and is scalable. We analyse the factors affecting the accuracy of determining GVD: noise in the experimental data as well as general physical limitations of the detection of GVD-induced changes, and suggest possible solutions.

Intensity correlation OCT is a classical mimic of quantum OCT providing up to twofold resolution improvement

Quantum Optical Coherence Tomography (Q-OCT) uses quantum properties of light to provide several advantages over its classical counterpart, OCT: it achieves a twice better axial resolution with the same spectral bandwidth and it is immune to even orders of dispersion. Since these features are very sought-after in OCT imaging, many hardware and software techniques have been created to mimic the quantum behaviour of light and achieve these features using traditional OCT systems. The most recent, purely algorithmic scheme—an improved version of Intensity Correlation Spectral Domain OCT named ICA-SD-OCT—showed even-order dispersion cancellation and reduction of artefacts. The true capabilities of this method were unfortunately severely undermined, both in terms of its relation to Q-OCT and its main performance parameters. In this work, we provide experimental demonstrations as well as numerical and analytical arguments to show that ICA-SD-OCT is a true classical equivalent of Q-OCT, more specifically its Fourier domain version, and therefore it enables a true two-fold axial resolution improvement. We believe that clarification of all the misconceptions about this very promising algorithm will highlight the great value of this method for OCT and consequently lead to its practical applications for resolution- and quality-enhanced OCT imaging.

Fourier domain quantum optical coherence tomography

Quantum optical coherence tomography (Q-OCT) is the non-classical counterpart of optical coherence tomography (OCT), a high-resolution 3D imaging technique based on white-light interferometry. Because Q-OCT uses a source of frequency-entangled photon pairs, not only is the axial resolution not affected by dispersion mismatch in the interferometer but is also inherently improved by a factor of two. Unfortunately, practical applications of Q-OCT are hindered by image-scrambling artefacts and slow acquisition times. Here, we present a theoretical analysis of a novel approach that is free of these problems: Fourier domain Q-OCT (Fd-Q-OCT). Based on a photon pair coincidence detection as in the standard Q-OCT configuration, it also discerns each photon pair by their wavelength. We show that all the information about the internal structures of the object is encoded in the joint spectrum and can be easily retrieved through Fourier transformation. No depth scanning is required, making our technique potentially faster than standard Q-OCT. Finally, we show that the data available in the joint spectrum enables artefact removal and discuss prospective algorithms for doing so.

All-depth dispersion cancellation in spectral domain optical coherence tomography using numerical intensity correlations

In ultra-high resolution (UHR-) optical coherence tomography (OCT) group velocity dispersion (GVD) must be corrected for in order to approach the theoretical resolution limit. One approach promises not only compensation, but complete annihilation of even order dispersion effects, and that at all sample depths. This approach has hitherto been demonstrated with an experimentally demanding ‘balanced detection’ configuration based on using two detectors. We demonstrate intensity correlation (IC) OCT using a conventional spectral domain (SD) UHR-OCT system with a single detector. IC-SD-OCT configurations exhibit cross term ghost images and a reduced axial range, half of that of conventional SD-OCT. We demonstrate that both shortcomings can be removed by applying a generic artefact reduction algorithm and using analytic interferograms. We show the superiority of IC-SD-OCT compared to conventional SD-OCT by showing how IC-SD-OCT is able to image spatial structures behind a strongly dispersive silicon wafer. Finally, we question the resolution enhancement of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sqrt{2}$$\end{document}2 that IC-SD-OCT is often believed to have compared to SD-OCT. We show that this is simply the effect of squaring the reflectivity profile as a natural result of processing the product of two intensity spectra instead of a single spectrum.

Experimental Demonstration of Spectral Intensity Optical Coherence Tomography

We demonstrate experimentally spectral-domain intensity optical coherence tomography using a Mach-Zehnder interferometer with balanced detection. We show that the technique allows for a point spread function with reduced full-width at half maximum compared to conventional optical coherence tomography. The method further provides benefits similar to those of chirped-pulse interferometry in terms of dispersion cancellation but only requires a broadband incoherent source and standard detectors. The measurements are in excellent agreement with the theoretical predictions. Finally, we propose an approach that enables the elimination of potential artefacts arising from multiple interfaces.

Intensity-interferometric spectral-domain optical coherence tomography with dispersion cancellation

We describe a technique for cancelling group-velocity dispersion in spectral-domain (SD) optical coherence tomography (OCT) based on classical intensity correlations. As a classical analogue of quantum OCT, a Hong-Ou-Mandel interferometer is combined with a conventional SD-OCT setup, and correlations between different spectral intensities are calculated. It is shown theoretically that a simple computational procedure used in SD-OCT enables scanless cross-sectional imaging with both dispersion cancellation and a factor-of-√2 resolution enhancement. The method involves no ultrafast detectors and works with common light sources.

Resolution improvement in spectral-domain optical coherence tomography based on classical intensity correlations

We describe a resolution-improved spectral-domain (SD) optical coherence tomography (OCT) based on classical intensity correlations as a counterpart of the recently proposed intensity-based time-domain OCT. Similarly to quantum OCT, the setup employs the Hong-Ou-Mandel interferometer at the output of the conventional SD-OCT. It is shown theoretically that a factor-of-√2 improvement in axial resolution is obtained with this arrangement. Some unique and useful features of our proposed system are addressed.