Demonstration of Fourier-domain Quantum Optical Coherence Tomography for a fast tomographic quantum imaging

Abstract

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.

Figures (24)

• Figure 1: Quantum OCT comprises (a) a photon pair source, an interferometer where the produced photons propagate, and a detection part measuring coincidence $C$ of the photons' arrival at two detectors. (b) Time-domain Q-OCT is based on single-photon-sensitive, single-pixel detectors, and outputs (c) a dip when the object arm length equals the reference arm length. (d) The Fourier-domain Q-OCT incorporates additional dispersive elements enabling wavelength discrimination, producing (e) a 2D joint spectrum which is Fourier transformed, with half the diagonal being the A-scan. The joint spectrum contains fringes along the diagonal, here, a single-frequency modulation appearing when a mirror for an object is moved away from the zero delay location and which - when Fourier transformed - produces a peak at a location proportional to the distance from the zero delay location. Such generation of different single-frequency modulations in the spectrum by different reflectors inside the object is what underpins the Fourier-domain OCT approaches. BS - beamsplitter, $\lambda_1$, $\lambda_2$ - wavelength of the photons in the pair, SPD - single-photon-sensitive detector.
• Figure 2: Fourier-domain Quantum OCT setup uses broadband laser light at 775 nm, Coherent Chameleon Ultra+, 80 MHz repetition rate, 10 nm bandwidth, to pump the ppKTP, 24.7 $\mu$m poling period, crystal and generate photon pairs at 1550 nm. One photon from the pair propagates in the object arm where it is reflected from the object. The other photon from the pair propagates in the reference arm, whose length and produced polarisation match those of the object arm. The photons overlap at the 50:50 fibre splitter where they quantum interfere. The wavelength-dependent coincidence rate is measured using two fibre spools, superconducting single-photon detectors SSPDs, Scontel, and a photodiode-triggered time-tagging device, Qutag by Qutools. A Time-domain Quantum OCT signal is acquired by translating the motorised stage in the reference arm. Conventional OCT is performed by connecting a second fibre splitter to a Menlo T-light laser (central wavelength of 1550 nm and the total bandwidth of 160 nm) at one end and to FB1 and FB2 at the other one. The classical fringes are then acquired using an optical spectrum analyser, Yokogawa AQ6374E, connected to one of the output ports of the 50:50 fibre splitter. M1-2 -- mirrors, L1-3 -- lenses (f=125 mm, 75 mm, 50 mm), $\lambda$/2 -- half-wave plate, $\lambda$/4 -- quarter-wave plate, PBS - polarisation beamsplitter, fibre connectors: FB1 and FB2 -- 8-mm focal length lenses in front of HP1550 single-mode fibres, FB3 and FB4 -- collimation package, Thorlabs FC1550-f6.37 mm, FB5 and FB6 -- lens with f=11 mm in front of the fibres.
• Figure 3: (a) The input joint spectrum is distributed diagonally. (b) After its counter-clockwise rotation by 45 degrees, the joint spectrum, whose signal distribution is now along the X axis, can undergo either (c) slower 2D Fourier transformation in order to produce 2D Fourier transform, or (d) its faster row-wise mean and 1D Fourier transformation calculation, both alternative ways to obtain (e) an A-scan.
• Figure 4: With a mirror as an object, (b) the whole joint spectrum covering a 398 nm wavelength range is acquired by varying the time delay in the two detection channels of the Quantum OCT setup and stitching the acquired frames. (a) A single frame, corresponding to a 102 nm wavelength range, is used for characterising the setup and comparing its performance with the conventional OCT. The anti-diagonal FWHM in both cases is 3.2 nm (see section S5). 2D Fourier transformation reveals the degrading effects of chromatic dispersion, the more so, the broader spectrally the photon pairs: (c) while the elements of the 2D Fourier transform of a single frame deviate from the expected Gaussian-like peaks (compare with Fig. \ref{['fig:ascan-calculation']}c), (d) the elements of the 2D Fourier transform of the whole joint spectrum are considerably distorted. (e) As expected, the A-scan resolution for the single frame (red curve), 4.3 $\mu$m, is worse than for the whole joint spectrum (purple curve), 10.6 $\mu$m. Also, uncompensated third-order dispersion is visible in the latter in the form of one-sided modulations (green circle in the inset).
• Figure 5: Applying a fibre-dispersion-compensating vector - conceptually represented by white arrows - (a) to a target joint spectrum whose (c) 2D Fourier transform is of a poor quality results in (b) a straightened joint spectrum whose (d) 2D Fourier transform shows high-quality sharp elements. Because the fibre dispersion compensation does not change the average statistics of the joint spectrum, (e) the original A-scan, plotted in pink, and the fibre-dispersion-compensated A-scan, plotted in blue, are the same.