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Frontiers in Optics

Temperature-Tunable Telecom Photon-Pairs via Cascaded χ(2) Process in PPLN Waveguide

Ashish Samantaray, Vikash Kumar Yadav, Joyee Ghosh, Vivek Venkataraman

Published Jan 1, 2023• 0 citations
Publisher

Abstract

We explore the cascaded χ(2) nonlinear process of second-harmonic generation (SHG) + spontaneous parametric down-conversion (SPDC) for telecom photon-pair generation. We experimentally verify our analytical prediction of discrete temperature-tunable signal/idler sidebands around a ~ 1550-nm pump.
DOI
10.1364/fio.2023.jm7a.127

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We report detailed experimental studies in a single wavelength-band system of correlated photon-pair generation in a 1.5 μm telecommunication wavelength-band using cascaded χ(2):χ(2) processes, second-harmonic generation, and the following spontaneous parametric down conversion (c-SHG/SPDC), in a periodically poled LiNbO3 (PPLN) ridge-waveguide device. By using a PPLN module with 600%/W of the SHG efficiency, we achieved a coincidence-to-accidental ratio (CAR) of 2380 at 1.8×10−4 of the mean number of the signal photon per pulse. Detailed investigation on the origin of uncorrelated noise photons was also discussed in this paper. We revealed that the noise photons mainly originated from spontaneous Raman scattering induced in pigtail optical fibers and also that the PPLN device itself had poor contribution to the noise photons. This feature of the c-SHG/SPDC process is promising for the realization of a noise-photon-free, high-purity quantum entangled photon-pair source.
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We generate quantum-correlated photon pairs using cascaded χ ( 2 ) : χ ( 2 ) traveling-wave interactions for second-harmonic generation (SHG) and spontaneous parametric down-conversion (SPDC) in a single periodically-poled thin-film lithium-niobate (TFLN) waveguide. When pulse-pumped at 50 MHz, a 4-mm-long poled region with nearly 300%/Wcm2 SHG peak efficiency yields a generated photon-pair probability of 7±0.2 × 10−4 with corresponding coincidence-to-accidental ratio (CAR) of 13.6±0.7. The CAR is found to be limited by Stokes/anti-Stokes Raman-scattering noise generated primarily in the waveguide. A Raman peak of photon counts at 250 cm−1 Stokes shift from the fundamental-pump wavenumber suggests most of the noise that limits the CAR originates within the lithium niobate material of the waveguide.
Spontaneous parametric down-conversion (SPDC) presents a convenient method of producing highly correlated heralded entangled photon pairs for quantum applications and networking. Of particular interest is Type-II SPDC, where signal and idler photons possess orthogonal polarisations, permitting straightforward entanglement in the useful polarisation basis. To the best of our knowledge, this has hitherto not been demonstrated in Zn-indiffused MgO:PPLN ridge waveguides, which offer excellent mode matching to standard telecoms fibers [1]. A particular difficulty in achieving SPDC in waveguides is to establish optimal phase-matching conditions. A common approach [2] is to perform standard SHG from the target SPDC wavelength first, leveraging the greater conversion efficiencies available in this regime. For example, in this work we target generation of Type-II SPDC photon pairs at 1310nm telecoms wavelengths suited to long-distance quantum networking protocols such as quantum key distribution [3], prompting initial testing of phase matching conditions via Type-II SHG.
Abstract Integrated photon-pair sources are crucial for scalable photonic quantum systems. Thin-film lithium niobate is a promising platform for on-chip photon-pair generation through spontaneous parametric down-conversion (SPDC). However, the device implementation faces practical challenges. Periodically poled lithium niobate (PPLN), despite enabling flexible quasi-phase matching, suffers from poor fabrication reliability and device repeatability, while conventional modal phase matching (MPM) methods yield limited efficiencies due to inadequate mode overlaps. Here, we introduce a layer-poled lithium niobate (LPLN) nanophotonic waveguide for efficient photon-pair generation. It leverages layer-wise polarity inversion through electrical poling to break spatial symmetry and significantly enhance nonlinear interactions for MPM, achieving a notable normalized second-harmonic generation (SHG) conversion efficiency of 4615% W-1 cm-2. Through a cascaded SHG and SPDC process, we demonstrate photon-pair generation with a normalized brightness of 3.1×106 Hz nm-1 mW-2 in a 3.3 mm long LPLN waveguide, surpassing existing on-chip sources under similar operating configurations. Crucially, our LPLN waveguides offer enhanced fabrication reliability and reduced sensitivity to geometric variations and temperature fluctuations compared to PPLN devices. We expect LPLN to become a promising solution for on-chip nonlinear wavelength conversion and non-classical light generation, with immediate applications in quantum communication, networking, and on-chip photonic quantum information processing.