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Photonics Research

Classical imaging with undetected photons using four-wave mixing in silicon core fibers

Meng Huang, Dong Ho Wu, Haonan Ren, Li Shen, Thomas Wade Hawkins, John M. Ballato, Ursula J. Gibson, Martynas Beresna, Radan Slavı́k, John E. Sipe, Marco Liscidini, Anna C. Peacock

Published Jan 10, 2023• 10 citations
Publisher

Abstract

Undetected-photon imaging allows for objects to be imaged in wavelength regions where traditional components are unavailable. Although first demonstrated using quantum sources, recent work has shown that the technique also holds with classical beams. To date, however, all the research in this area has exploited parametric down-conversion processes using bulk nonlinear crystals within free-space systems. Here, we demonstrate undetected-photon-based imaging using light generated via stimulated four-wave mixing within highly nonlinear silicon fiber waveguides. The silicon fibers have been tapered to have a core diameter of ∼ 915 nm to engineer the dispersion and reduce the insertion losses, allowing for tight mode confinement over extended lengths to achieve practical nonlinear conversion efficiencies ( ∼ − 30 dB ) with modest pump powers ( ∼ 48 mW ). Both amplitude and phase images are obtained using classically generated light, confirming the high degree of spatial and phase correlation of our system. The high powers ( > 10 nW ) and long coherence lengths ( > 4 km ) associated with our large fiber-based system result in high contrast and stable images.

DOI
10.1364/prj.473239

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Feb 20, 2017•0 citations
Undetected-photon imaging allows for objects to be imaged in wavelength regions where traditional components are unavailable. Although first demonstrated using quantum sources, recent work has shown that the technique also holds with classical beams. To date, however, all the research in this area has exploited parametric down-conversion processes using bulk nonlinear crystals within free-space systems. Here, we demonstrate undetected-photon-based imaging using light generated via stimulated four-wave mixing within highly nonlinear silicon fiber waveguides. The silicon fibers have been tapered to have a core diameter of ∼ 915 nm to engineer the dispersion and reduce the insertion losses, allowing for tight mode confinement over extended lengths to achieve practical nonlinear conversion efficiencies ( ∼ − 30 dB ) with modest pump powers ( ∼ 48 mW ). Both amplitude and phase images are obtained using classically generated light, confirming the high degree of spatial and phase correlation of our system. The high powers ( > 10 nW ) and long coherence lengths ( > 4 km ) associated with our large fiber-based system result in high contrast and stable images.
We demonstrate theoretically the direct (filter-free) generation of spectrally uncorrelated photon pairs, and consequently high-purity heralded single photons at telecommunication wavelengths, in SOI (silicon-on-insulator) nanowaveguides via spontaneous four-wave mixing (SFWM). The inherent strong waveguide dispersion in such high-index contrast structures is employed for discrete-band phase-matching in order to suppress the pair spectral correlations, and to produce photons in the fundamental spatial mode for easy in-out coupling to single-mode optical fibers. Additionally, by optimizing the source parameters like waveguide length, dispersion and pump bandwidth, the group-velocity of generated photons is also engineered to further eliminate spectral correlations. Extensive numerical mode simulations for various waveguide widths are used to design intrinsically pure photon sources via joint spectral amplitude (JSA) engineering. Purity$>9595% is shown to be achievable in a silica-clad SOI waveguide with a cross-section 220 × 600 nm95^{2}fortypicalvaluesoflengthandpumpbandwidth.Thewavelengthoftheseheraldedsinglephotonscanbetunedintherangeof1266–1363nm(for typical values of length and pump bandwidth. The wavelength of these heralded single photons can be tuned in the range of 1266–1363 nm (fortypicalvaluesoflengthandpumpbandwidth.Thewavelengthoftheseheraldedsinglephotonscanbetunedintherangeof1266–1363nm(\sim$O-band) by varying the pump wavelength. Considering the high accuracy provided by silicon nanofabrication techniques, the feasibility of generating single photons across the wavelength range 1333–1425 nm (E-band) and 1437–1510 nm (S-band) for slightly different waveguide widths (625 nm & 650 nm) is also demonstrated. These filter-free integrated photon sources could be important for large-scale linear optical quantum computation (LOQC) and long-distance fiber-optic quantum key distribution (QKD) networks.
Interference measurements with undetected photons employ entangled photon pairs in order to overcome classical limitations in measurement sensitivity. With the undetected photon technique, one photon (signal) interacts with an object, and the signature of the interaction is stored in its entangled counterpart (idler): the interacting photon remains undetected, while the detection is performed on the photon that did not interact with the object. While the signal photon can be chosen in any spectral region suitable for the interaction, the idler photon is generated in a spectral region where detectors are efficient, scalable, and cheap. To date, the configurations proposed are in bulk setups. In this work, we propose a novel configuration on an integrated device, with the advantages of the reduced dimension, the lower cost and the robustness to alignment. In our experiment, we pumped a silicon-on-silica integrated circuit with a classical beam at a wavelength of 1.568 μm. Via intermodal spontaneous four-wave mixing, we generated highly non-degenerate time-energy entangled signal and idler photons at 1.99 μm and 1.29 μm, respectively. As we have integrated two identical sources of entangled photons in series, the photon pairs generated in the two sources are indistinguishable, and controlling the phase of the pump beam and the phase of the signal photons after the first nonlinear source, we observed interference patterns with 24% maximum visibility in the idler photon counts. We successfully measured the dephasing induced on the signal photons by measuring only the idler ones.
We obtained the phase and intensity images of an object by detecting classical light which never interacted with it. With a double passage of a pump and a signal laser beams through a nonlinear crystal, we observe interference between the two idler beams produced by stimulated parametric down conversion. The object is placed in the amplified signal beam after its first passage through the crystal and the image is observed in the interference of the generated idler beams. High contrast images can be obtained even for objects with small transmittance coefficient due to the geometry of the interferometer and to the stimulated parametric emission. Like its quantum counterpart, this three-color imaging concept can be useful when the object must be probed with light at a wavelength for which detectors are not available.
We experimentally demonstrate undetected photon phase sensing on a silicon photonic chip. We use a novel design, implementing an innovative integrated SU(1,1) interferometer based on transverse waveguide modes, that leverages intermodal spontaneous four-wave mixing to generate highly non-degenerate idler–signal photons. In our experiment, we retrieve the phase applied to the signal photons at wavelength 1972 nm by detecting only the correlated idler photons at 1291 nm, demonstrating the first on-chip phase sensing based on undetected photons. This result paves the way for novel generations of quantum refractometric sensors.
Single photon sources are a key element for quantum computing, quantum key distribution (QKD) and quantum communications. In particular, producing single photons at telecommunications wavelengths is valuable for QKD protocols and would enable realizing the quantum internet. The preferred method for their generation has long been spontaneous down conversion in bulk crystals, which suffers from connection loss to fiber networks. In-fiber spontaneous four-wave mixing provides a viable alternative as a photon pair source due to being compatible with existing fiber networks. We present an all-fiber photon pair source based on degenerate four-wave mixing in a 400 m Highly-Nonlinear fiber, with signal and idler wavelengths generated at 1552.5 nm and 1557 nm respectively. The source consists of CW pump laser operating at 1554.75 nm, which is slightly detuned from the zero group velocity dispersion wavelength into the normal dispersion regime. After pair generation in the highly-nonlinear fiber, three arrayed waveguide gratings are employed to spatially separate signal and idler, and provides a 120 dB pump power reduction. Firstly the source is modelled and experimentally characterized in the well known classical regime of stimulated four-wave mixing. The effect of fiber cooling on spontaneous Raman scattering is modelled and characterized, and a 30% reduction in spontaneous emission is found when cooling the fiber to -77°C. In the low power regime the coincidence to accidental count ratio is simulated and measured. An increase in the coincidence to accidental count ratio is observed when cooling the fiber.