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arXiv:2006.07400 · Optica

Frequency-domain optical coherence tomography with undetected mid-infrared photons

Aron Vanselow, Paul Kaufmann, Ivan Zorin, Bettina Heise, Helen M. Chrzanowski, Sven Ramelow

Published Oct 28, 2020• 109 citations
arXivHTMLPDF

Abstract

Mid-infrared (mid-IR) light scatters much less than shorter wavelengths, allowing greatly enhanced penetration depths for optical imaging techniques such as optical coherence tomography (OCT). However, both detection and broadband sources in the mid-IR are technologically challenging. Interfering entangled photons in a nonlinear interferometer enables sensing with undetected photons, making mid-IR sources and detectors obsolete. Here we implement mid-IR frequency-domain OCT based on ultra-broadband entangled photon pairs spanning from 3.3 to 4.3 µm. We demonstrate 10 µm axial and 20 µm lateral resolution 2D and 3D imaging of strongly scattering ceramic and paint samples. By intrinsically being limited only by shot noise, we observe times more sensitivity per integration time and power of the probe light. Together with the vastly reduced footprint and technical complexity, our technique can outperform conventional approaches with classical mid-IR light sources.

DOI
10.1364/optica.400128

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Mid-infrared (mid-IR) light scatters much less than shorter wavelengths, allowing greatly enhanced penetration depths for optical imaging techniques such as optical coherence tomography (OCT). However, both detection and broadband sources in the mid-IR are technologically challenging. Interfering entangled photons in a nonlinear interferometer enables sensing with undetected photons, making mid-IR sources and detectors obsolete. Here we implement mid-IR frequency-domain OCT based on ultra-broadband entangled photon pairs spanning from 3.3 to 4.3 µm. We demonstrate 10 µm axial and 20 µm lateral resolution 2D and 3D imaging of strongly scattering ceramic and paint samples. By intrinsically being limited only by shot noise, we observe times more sensitivity per integration time and power of the probe light. Together with the vastly reduced footprint and technical complexity, our technique can outperform conventional approaches with classical mid-IR light sources.
Optical coherence tomography (OCT) is an appealing technique for bio-imaging, medicine, and material analysis. For many applications, OCT in mid- and far-infrared (IR) leads to significantly more accurate results. Reported mid-IR OCT systems require light sources and photodetectors which operate in mid-IR range. These devices are expensive and need cryogenic cooling. Here, we report a proof-of-concept demonstration of a wavelength tunable IR OCT technique with detection of only visible range photons. Our method is based on the nonlinear interference of frequency correlated photon pairs. The nonlinear crystal, introduced in the Michelson-type interferometer, generates photon pairs with one photon in the visible and another in the IR range. The intensity of detected visible photons depends on the phase and loss of IR photons, which interact with the sample under study. This enables us to characterize sample properties and perform imaging in the IR range by detecting visible photons. The technique possesses broad wavelength tunability and yields a fair axial and lateral resolution, which can be tailored to the specific application. The work contributes to the development of versatile 3D imaging and material characterization systems working in a broad range of IR wavelengths, which do not require the use of IR-range light sources and photodetectors.
Owing to its capacity for unique (bio)-chemical specificity, microscopy with mid-infrared (IR) illumination holds tremendous promise for a wide range of biomedical and industrial applications. The primary limitation, however, remains detection, with current mid-IR detection technology often marrying inferior technical capabilities with prohibitive costs. Here, we experimentally show how nonlinear interferometry with entangled light can provide a powerful tool for mid-IR microscopy while only requiring near-IR detection with a silicon-based camera. In this proof-of-principle implementation, we demonstrate widefield imaging over a broad wavelength range covering 3.4 to 4.3 μm and demonstrate a spatial resolution of 35 μm for images containing 650 resolved elements. Moreover, we demonstrate that our technique is suitable for acquiring microscopic images of biological tissue samples in the mid-IR. These results form a fresh perspective for potential relevance of quantum imaging techniques in the life sciences.
Metrology with undetected photons is an emerging technique that leverages quantum effects and photon correlations (entanglement) to retrieve valuable information in a target spectral range (e.g., mid-infrared, mid-IR) using measurements in an easily accessible domain (e.g., visible, near-IR). The underlying quantum process of spontaneous parametric down-conversion (SPDC) is utilized to generate non-degenerate correlated signal and idler photons to serve as detection and probing photons, respectively. Sensing with undetected photons enables important advantages, such as ultra-low probe powers, room-temperature operation, and shot-noise-limited detection. In this contribution, we apply a quantum nonlinear interferometer based on an SPDC source to perform applied mid-IR spectroscopy, mid-IR microscopy, and mid-IR optical coherence tomography (OCT) as among the most promising techniques for quantum-based routine non-destructive testing. Moreover, we characterize the system, benchmark it against classical systems, and provide a prospective outlook for this new technology.
Sensing with undetected photons has become a vibrant, application-driven research domain with a special focus on the mid-infrared (mid-IR) wavelength region. Since the mid-IR contains spectral bands with highly specific and strong molecular absorbance signatures, often referred to as fingerprints, a multitude of different samples and their compositions can be detected and quantified spectroscopically. Enhancing this spectroscopic method with imaging capabilities leads to a powerful technique for environmental monitoring and biomedical applications that enables automated diagnostics while omitting time-consuming and non-reversible labeling steps. To evade the shortcomings of state-of-the-art instruments for mid-IR hyperspectral microscopy—namely cost, complexity, power consumption, and performance, which stem from technological challenges in mid-IR detection and light sources—we construct a proof-of-concept nonlinear interferometer in a wide-field imaging arrangement. This nominally narrowband imaging technique is then expanded to acquire broadband spectral information through capturing images for varying interferometer displacement and applying a pixelwise Fourier-transform of the resulting interferograms, yielding high-resolution infrared spectra for each camera pixel. For the broadband range of 2300−3100cm −1 , covering the important CH-stretch band, we perform hyperspectral imaging that simultaneously resolves 3500 spatial modes, each with a spectral resolution of 10cm −1 , leveraging in total around 10 5 spatio-spectrally entangled photon modes. Our image acquisition uses a commercial sCMOS camera, while a medium-power and compact continuous-wave pump laser is the only necessary light source. For a moderate speed of 360 voxel/s, we obtain a predominantly shot-noise-influenced signal-to-noise ratio (SNR) of 50. We further demonstrate the practicality of our novel hyperspectral imaging technique for microplastics detection and bio-imaging tasks and outline engineering solutions to increase its speed by several orders of magnitude. This shows that our quantum imaging technique is highly promising for applications requiring compact, cost-effective label-free analyses.
Nonlinear SU(1,1) quantum interferometers based on non-degenerate optical parametric down-conversion exhibit strong unbalanced group velocity dispersion (GVD). This feature is intrinsic to this type of interferometer as correlated photons of vastly different frequencies propagate through a dispersive nonlinear crystal; consequently, the dispersion arises from the source itself. The resulting GVD degrades the axial point-spread function (PSF) in optical coherence tomography (OCT) with undetected photons; and physical compensation is less straightforward, in particular for non-degenerate broadband regimes due to the limited number of suitable materials. In this contribution, we analyze dispersion in bulk nonlinear interferometry and describe its implications for OCT imaging. Aspects of hardware compensation are addressed, and a novel empirical numerical method of compensation is proposed. The approach is based on the extraction of the phase component directly from the time-domain modality (high precision linearized quantum Fourier transform infrared spectrometer) and its injection into the mid-IR spectral-domain OCT signals (central wavelength of around 3770 nm) before the Fourier transform. The proposed method is compared with an alternative numerical technique. The results demonstrate a 2.2-fold improvement in axial resolution and outperform the alternative correction method in overall imaging performance.