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arXiv:1709.09456 · Journal of Optics

Parametric down-conversion in ppLN ridge waveguide: a quantum analysis for efficient twin photons generation at 1550 nm

Ramesh Kumar, Joyee Ghosh

Published May 25, 2018• 19 citations
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Abstract

Abstract We study modal characteristics of a customized ridge waveguide in lithium niobate designed to generate twin photons at telecom wavelength. A quantum analysis of SPDC predicts the possible down conversion processes and optimizes the input beam parameters for fundamental mode emission. Further, a joint spectral amplitude analysis ensures that the parameters are ambient for generating frequency-correlated photons throughout the phase-matching spectrum. Moreover, negatively correlated degenerate signal/idler photon pairs in the fundamental mode emission occurs at 1550 nm. A key result of our paper is an explicit expression and plot for comparing the generated signal power in case of waveguides as compared to bulk crystals. Our calculations clearly show a dependence of ℒ 3/2 for a waveguide of length ℒ , compared to a linear dependence in case of a bulk crystal.

DOI
10.1088/2040-8986/aac7da

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Abstract We study modal characteristics of a customized ridge waveguide in lithium niobate designed to generate twin photons at telecom wavelength. A quantum analysis of SPDC predicts the possible down conversion processes and optimizes the input beam parameters for fundamental mode emission. Further, a joint spectral amplitude analysis ensures that the parameters are ambient for generating frequency-correlated photons throughout the phase-matching spectrum. Moreover, negatively correlated degenerate signal/idler photon pairs in the fundamental mode emission occurs at 1550 nm. A key result of our paper is an explicit expression and plot for comparing the generated signal power in case of waveguides as compared to bulk crystals. Our calculations clearly show a dependence of ℒ 3/2 for a waveguide of length ℒ , compared to a linear dependence in case of a bulk crystal.
We study modal characteristics of a customized ridge waveguide in Lithium Niobate designed to generate twin photons at telecom wavelength. A quantum analysis of SPDC predicts the possible down conversion processes and optimizes the input beam parameters for fundamental mode emission. Further, a Joint Spectral Amplitude (JSA) analysis ensures the generation of signal/idler at 1550 nm in the customized LN waveguide. A calculation of the parametric down conversion (PDC) signal power shows a dependence of L^(3/2) for a waveguide compared to a linear dependence in case of bulk crystal.
In this paper, we study the possible parametric down conversion processes in a periodically poled customized Lithium Niobate (LiNbO3) ridge waveguide. Our analysis of spontaneous parametric down-conversion (SPDC), first, with a Gaussian pump beam mode and second, with an anti-symmetric Hermite-Gaussian HG (1,0) pump beam mode predict the possible down conversion processes in each case. From our JSA analysis, it is evident that the generated photons pairs in all these cases are negatively correlated and have orthogonal polarizations. In case of the former, degenerate photon pairs are emitted at 1550 nm with the highest efficiency in the fundamental waveguide mode. While, in case of the latter, non-degenerate photon pairs in different higher order spatial modes are generated. Such photons, thus, have multiple degrees of freedom, like polarization and spatial modes, which can be further harnessed towards hyper-entangled photons for quantum information applications.
We demonstrate a spectrally correlated photon-pair source at telecom wavelengths (spanning across the S-, C-, and L-bands), based on type-0 spontaneous parametric downconversion (SPDC) in a fiber-coupled Zn-indiffused MgO doped periodically poled lithium niobate (PPLN) ridge waveguide. Modal analysis of the waveguide performed through numerical finite element method (FEM) simulation indicates that device temperature can be used to dramatically vary and control the emission spectrum. Efficient photon-pair generation is measured over a broad wavelength range from ∼1520 − 1580 nm [full width at half maximum (FWHM) > 45 nm] with a coincidence-to-accidental ratio (CAR) as high as ∼668 and spectral brightness ∼2.5 × 10 7 pairs/s/mW/nm. Such sources can be employed in wavelength division multiplexed (WDM) quantum key distribution (QKD) over existing fiber-optic networks.
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.