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Optics Letters

Proposal for in-fiber generation of telecom-band polarization-entangled photon pairs using a periodically poled fiber

L. G. Helt, Eric Y. Zhu, Marco Liscidini, Li Qian, John E. Sipe

Published Jul 8, 2009• 24 citations
Publisher

Abstract

We treat spontaneous parametric downconversion in a periodically poled fiber, quasi-phase matched to allow for the generation of photon pairs at wavelengths within the low-loss telecommunications window. For an appropriate pump polarization, the unusual properties of such a fiber's effective chi(2) result in a biphoton wave function that is symmetric upon simultaneous exchange of downconverted photon frequencies and polarizations and that is nonzero over a wide range of downconverted frequencies. This could lead to a significant technical simplification of sources of in-fiber telecom-band polarization-entangled photons.

DOI
10.1364/ol.34.002138

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We treat spontaneous parametric downconversion in a periodically poled fiber, quasi-phase matched to allow for the generation of photon pairs at wavelengths within the low-loss telecommunications window. For an appropriate pump polarization, the unusual properties of such a fiber's effective chi(2) result in a biphoton wave function that is symmetric upon simultaneous exchange of downconverted photon frequencies and polarizations and that is nonzero over a wide range of downconverted frequencies. This could lead to a significant technical simplification of sources of in-fiber telecom-band polarization-entangled photons.
In this Letter, we report a polarization-entangled photon-pair source based on type-II spontaneous parametric downconversion at telecom O-band in periodically poled silica fiber (PPSF). The photon-pair source exhibits more than 130 nm ( ∼ 24 T H z ) emission bandwidth centered at 1306.6 nm. The broad emission spectrum results in a short biphoton correlation time, and we experimentally demonstrate a Hong–Ou–Mandel interference dip with a full width of 26.6 fs at half-maximum. Owing to the low birefringence of the PPSF, the biphotons generated from type-II SPDC are polarization-entangled over the entire emission bandwidth, with a measured fidelity to a maximally entangled state greater than 95.4%. The biphoton source provides the broadest bandwidth entangled biphotons at O-band to our knowledge.
We experimentally demonstrate the direct generation of polarization-entangled photon pairs in an optical fiber at room temperature by exploiting type-II phase-matched spontaneous parametric down-conversion. A second-order nonlinearity is artificially induced in the 17-cm-long weakly birefringent step-index fiber through the process of thermal poling, and quasi-phase-matching allows for the generation of entangled photons in the 1.5-micron telecom band when the fiber is pumped at 775 nm. A greater-than 80:1 coincidence-to-accidental ratio is achieved, limited mainly by multiphoton pair generation. Without the need to subtract accidentals or to compensate for walk-off, the raw two-photon interference visibility is found to be better than 95%, and violation of Bell's inequality is observed by more than 18 standard deviations. This makes for a truly alignment-free, plug-and-play source of polarization-entangled photon pairs.
In this paper, we address the issue of the generation of nondegenerate cross-polarization-entangled photon pairs using type-II periodically poled lithium niobate. We show that, by an appropriate engineering of the quasi-phase-matching grating, it is possible to simultaneously satisfy the conditions for two spontaneous parametric down-conversion processes, namely, ordinary pump photon down conversion to either extraordinary signal and ordinary idler paired photons or to ordinary signal and extraordinary idler paired photons. In contrast to single type-II phase matching, these two processes, when enabled together, can lead to the direct production of cross-polarization-entangled states for nondegenerate signal and idler wavelengths. Such a scheme should be of great interest in applications requiring polarization-entangled nondegenerate paired photons with, for instance, one of the entangled photons at an appropriate wavelength being used for local operation or for quantum storage in an atomic ensemble and the other one at the typical wavelength of 1550 nm for propagation through an optical fiber.