Light’s Speed Mismatch Weakens Advanced Medical Scans, Researchers Find

Researchers have identified a significant source of degradation in optical coherence tomography (OCT) utilising undetected photons, stemming from unbalanced group velocity dispersion within nonlinear interferometers. Ivan Zorin and Paul Gattinger, both from the Research Center for Non-Destructive Testing in Linz, Austria, detail how this dispersion arises intrinsically from the process of non-degenerate optical parametric down-conversion, impacting axial resolution. Their work addresses this challenge by analysing dispersion in bulk nonlinear interferometry and proposing a novel empirical numerical compensation method, extracting phase information from time-domain spectrometry and injecting it into mid-IR spectral-domain OCT signals. This innovative approach demonstrably improves axial resolution by a factor of 2.2 and surpasses the performance of existing numerical correction techniques, representing a substantial advancement in OCT imaging capabilities.

This breakthrough addresses a fundamental limitation of quantum-based imaging techniques that utilise correlated photons generated through non-degenerate optical parametric down-conversion.

The inherent unbalanced group velocity dispersion within these interferometers, originating from the source itself, typically degrades image quality in OCT applications. Researchers tackled this challenge by extracting phase information directly from time-domain measurements using a high-precision linearized Fourier transform infrared spectrometer.
This extracted phase component is then injected into mid-infrared spectral-domain OCT signals, operating around a central wavelength of 3770nm, prior to Fourier transformation. The innovative approach leverages the Wiener, Khinchin theorem and its cross-correlation generalization to relate the bi-photon coherence function in the time domain to the complex spectrum.

By precisely compensating for dispersion, the study demonstrates a significant enhancement in imaging performance compared to alternative numerical correction techniques. The work establishes a new benchmark for nonlinear interferometric OCT, paving the way for more sensitive and cost-effective mid-infrared imaging systems.

Nonlinear SU(1,1) interferometry, based on quantum phenomena like entanglement and bi-photon interference, is gaining prominence as an alternative to conventional optical metrology. These techniques employ non-degenerate spontaneous parametric down-conversion to generate correlated photon pairs, enabling sensing with undetected photons.

This approach eliminates the need for direct optics and detectors in challenging spectral ranges, utilising mature shot-noise limited detection technologies instead. Adapting this interferometric principle to coherent imaging and spectroscopy, including Fourier-transform infrared spectroscopy and optical coherence tomography, offers advantages in the mid-infrared spectral range where classical instrumentation faces technological and economic constraints.

The research focuses on addressing dispersion arising from the propagation of correlated signal and idler photons within the nonlinear crystal of the interferometer. This dispersion is an intrinsic property of the source, making physical compensation complex, particularly in broadband, non-degenerate regimes.

The proposed numerical method directly measures the phase using quantum Fourier transform infrared spectroscopy and applies this correction to the OCT signals. Experimental results confirm that this technique not only improves axial resolution by a factor of 2.2 but also surpasses the performance of existing numerical correction methods, demonstrating a substantial advancement in nonlinear interferometric imaging.

Empirical phase retrieval for dispersion correction in mid-infrared spectral-domain optical coherence tomography

A high precision linearized Fourier transform infrared spectrometer served as the foundation for a novel dispersion compensation method in optical coherence tomography. Researchers addressed unbalanced group velocity dispersion inherent in nonlinear SU(1,1) interferometers, a consequence of correlated photons with differing frequencies propagating through a dispersive nonlinear crystal.

This dispersion degrades the axial point-spread function in OCT, and conventional physical compensation proves challenging, especially in non-degenerate broadband regimes. The study involved extracting the phase component directly from time-domain signals obtained via the spectrometer. This extracted phase information was then injected into the mid-IR spectral-domain OCT signals, with a central wavelength around 3770nm, prior to Fourier transformation.

This innovative approach constitutes an empirical numerical method designed to counteract the effects of dispersion on image resolution. The technique was specifically developed for use with mid-IR spectral-domain OCT, leveraging the advantages of nonlinear interferometry for sensing with undetected photons.

Performance of the new method was rigorously compared against an alternative numerical correction technique. Results demonstrated a 2.2-fold improvement in axial resolution using the proposed phase injection method. Furthermore, overall imaging performance surpassed that achieved with the alternative correction, highlighting the efficacy of this approach in mitigating dispersion-related artifacts. The work details a practical solution for enhancing the clarity and precision of mid-IR OCT imaging, particularly in applications where conventional dispersion compensation is limited.

Numerical dispersion compensation enhances axial resolution in mid-infrared spectral-domain optical coherence tomography

Axial resolution in optical coherence tomography (OCT) improved by a factor of 2.2 following the implementation of a novel numerical compensation method. This advancement directly addresses strong unbalanced group velocity dispersion intrinsic to nonlinear SU(1,1) interferometers utilizing non-degenerate optical parametric down-conversion.

The research focused on mitigating dispersion arising from correlated photons of differing frequencies propagating through a nonlinear crystal, a source of degradation in axial point-spread function quality. The newly developed compensation technique extracts phase information from time-domain measurements obtained via a high-precision linearized Fourier transform infrared spectrometer.

This extracted phase component is then injected into mid-infrared spectral-domain OCT signals, with a central wavelength around 3770nm, prior to Fourier transformation. By directly addressing the phase component, the method effectively counteracts dispersion and enhances image clarity. Comparative analysis revealed that this approach outperforms an alternative numerical correction technique in overall imaging performance.

The method leverages the relationship between the mutual bi-photon coherence function in the time domain and the complex spectrum in the frequency domain, as described by the Wiener, Khinchin theorem and its cross-correlation generalization. This allows for precise phase measurement and subsequent compensation within the OCT imaging modality.

Implementation of this technique facilitates higher-order dispersion compensation, particularly crucial for strong non-degeneracy and broadband emission scenarios. The ability to store the derived phase for sequential spectral interferograms further enhances the efficiency and applicability of the method. This work demonstrates a significant step towards robust and high-resolution mid-infrared OCT imaging using nonlinear interferometry.

Mid-infrared axial resolution enhancement via phase compensation of down-converted signals

Nonlinear interferometers utilising non-degenerate optical parametric down-conversion are subject to substantial unbalanced group velocity dispersion originating intrinsically from the source itself. This dispersion arises because correlated photons with differing frequencies propagate through a dispersive nonlinear crystal, ultimately degrading axial resolution in optical coherence tomography (OCT) when undetected photons are used.

Researchers developed and analysed a novel empirical numerical method for compensating this dispersion, extracting the phase component from time-domain data and injecting it into mid-infrared spectral-domain OCT signals prior to Fourier transformation. The proposed compensation technique demonstrated a 2.2-fold improvement in axial resolution and surpassed the performance of an alternative numerical method.

This approach is particularly advantageous for non-destructive testing applications, enabling mid-infrared OCT with cost-effective near-infrared components and ultra-low probing power of 60 pW, ensuring minimal sample alteration. Investigations revealed that the energy entanglement of generated photons results in a cumulative group-delay dispersion across signal and idler spectral bands, necessitating overall net dispersion compensation rather than matching dispersion within interferometer arms as in classical systems.

The study highlighted that materials such as potassium titanyl phosphate and lithium niobate, commonly used in mid-infrared metrology, exhibit negative net dispersion, potentially allowing for counterintuitive improvements in axial resolution by adding positively dispersive materials. Acknowledging the source of substantial uncompensated dispersion lies within the nonlinear crystal itself, the findings underscore the importance of careful optical component selection and system engineering to minimise effective dispersion. Future work could focus on deliberately choosing materials, such as replacing dichroic mirror substrates with silicon, to further reduce dispersion and optimise axial resolution in these non-classical OCT systems.