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.
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.
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 square root 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: Q-OCT with joint spectrum detection (JS-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.
Quantum Optical Coherence Tomography (Q-OCT) is a non-classical equivalent of Optical Coherence Tomography and is able to provide a twofold axial resolution increase and immunity to resolution-degrading dispersion. The main drawback of Q-OCT are artefacts which are additional elements that clutter an A-scan and lead to a complete loss of structural information for multilayered objects. Whereas there are very practical and successful methods for artefact removal in Time-domain Q-OCT, no such scheme has been devised for Fourier-domain Q-OCT (Fd-Q-OCT), although the latter modality-through joint spectrum detection-outputs a lot of useful information on both the system and the imaged object. Here, we propose two algorithms which process a Fd-Q-OCT joint spectrum into an artefact-free A-scan. We present the theoretical background of these algorithms and show their performance on computer-generated data. The limitations of both algorithms with regards to the experimental system and the imaged object are discussed.
Using spectrally correlated photon pairs instead of classical laser light and coincidence detection instead of light intensity detection, Quantum Optical Coherence Tomography (Q-OCT) outperforms classical OCT in several experimental terms. It provides twice better axial resolution with the same spectral bandwidth and it is immune to even-order chromatic dispersion, including Group Velocity Dispersion responsible for the bulk of axial resolution degradation in the OCT images. Q-OCT has been performed in the time domain configuration, where one line of the two-dimensional image is acquired by axially translating the mirror in the interferometer's reference arm and measuring the coincidence rate of photons arriving at two single-photon-sensitive detectors. Although successful at producing resolution-doubled and dispersion-cancelled images, it is still relatively slow and cannot compete with its classical counterpart. Here, we experimentally demonstrate Q-OCT in a much faster Fourier-domain configuration, theoretically proposed in 2020, where the reference mirror is fixed and the joint spectrum is acquired by inserting long fibre spools in front of the detectors. We propose two joint spectrum pre-processing algorithms, aimed at compensating resolution-degrading effects within the setup. While the first one targets fibre spool dispersion, an effect specific to this configuration, the other one removes the effects leading to the weakening of even-order dispersion cancellation, the latter impossible to be mitigated in the time-domain alternative. Being additionally contrasted with both the time-domain approach and the conventional OCT in terms of axial resolution, imaging range and multilayer-object imaging, Fourier-domain Q-OCT is shown to be a significant step forward towards a practical and competitive solution in the OCT arena.
By using entangled photon pairs instead of a laser light, Quantum Optical Coherence Tomography (Q-OCT) outperforms conventional OCT, providing twice better axial resolution and immunity to image-degrading even-order chromatic dispersion. Despite these very sought-after features, its applicability is hindered by long acquisition times, image-scrambling artefacts and chromatic dispersion which reappears in configurations enabling the artefact suppression. Here, we experimentally demonstrate Q-OCT with a Fourier-domain acquisition scheme which allows both fast imaging and immunity to the re-appearance of chromatic dispersion. By delivering two-dimensional joint spectra, our Fourier-domain Q-OCT enables comfortable manipulation of spectral relationships between the photon pairs, and consequently the removal of the dispersion effects. We report attractive acquisition times, accompanied by a demonstration of two joint spectrum pre-processing algorithms, aimed at controlling dispersion effects within the setup. While the first algorithm reverses the effects specific to the joint spectral acquisition, the other one removes the effects leading to the weakening of even-order dispersion cancellation, impossible to be mitigated in other Q-OCT implementations. Contrasted with both the original, time-domain implementation and the conventional OCT, Fourier-domain Q-OCT is shown to be an important step forward towards becoming a practical and competitive solution.
Quantum Optical Coherence Tomography (Q-OCT) presents many advantages over its classical counterpart, Optical Coherence Tomography (OCT): it provides an increased axial resolution and is immune to even orders of dispersion. The core of Q-OCT is quantum interference of negatively correlated entangled photon pairs obtained in a Hong-Ou-Mandel configuration. This two-photon interference can be observed in the time domain in the form of dips or in the Fourier domain by means of a joint spectrum. The latter approach proved to be practical in the sense that it alleviated the requirement posed on light in Q-OCT to exhibit strict negative correlations, since the negative correlations can be easily extracted in the Fourier domain as the main diagonal of the joint spectrum. In this work, we investigate the use of this spectral approach in which quantum interference is obtained with classical low-intensity light pulses. We report theoretical calculations and their experimental validation and show that although such classical light is much easier to launch into an experimental system, it offers limited benefits as compared to Q-OCT based on entangled light. We analyse the differences in the characteristics of the joint spectrum obtained with entangled photons and with classical light and explain the origins of these differences.
We report Fourier-domain Quantum Optical Coherence Tomography employing a photon pair source covering a very broad wavelength range spanning from around 1350 to 1750 nm. With nearly 180 nm full width at half maximum, it enables high axial resolution of 2.7 μm, most commonly achieved only in classical OCT working at a much shorter wavelength of 800 nm. The light of such considerable bandwidth is especially vulnerable to axial-resolution-degrading effects of chromatic dispersion, and although the even-order dispersion - responsible for the bulk of resolution degradation - is inherently cancelled, the odd-order dispersion becomes non-negligible, with the third-order-dispersion effects already observed in various Quantum OCT setups. We introduce a dimensionless parameter χ₃, dependent on the photon pair source and medium parameters, facilitating prediction of the appearance of these image-distorting dispersion effects. Also, we show that this issue - impossible to be removed in time-domain Quantum OCT - is easily mitigated in its Fourier-domain configuration by mere post-processing of a joint spectrum, resulting in the high axial resolution being maintained throughout most of the imaging depth, here approx. 0.35 mm. Finally, we show that the imaging depth can be extended to 0.7 mm if the fibre spools - central to the joint spectral detection - are extended from 5 km to 10 km, however at the expense of a longer acquisition time.