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Physical Review A

Generation of squeezed vacuum pulses using cascaded second-order optical nonlinearity of periodically poled lithium niobate in a Sagnac interferometer

Kenichi Hirosawa, Yasuyuki Ito, Hidetake Ushio, Hisayuki Nakagome, Fumihiko Kannari

Published Oct 26, 2009• 7 citations
Publisher

Abstract

Squeezed vacuum pulses up to $\ensuremath{-}1.7\text{ }\text{dB}$ at telecom-band $(\ensuremath{\sim}1550\text{ }\text{nm})$ is generated with femtosecond laser pulses using cascaded ${\ensuremath{\chi}}^{(2)}$ optical nonlinearity in a periodically poled lithium niobate crystal placed in a Sagnac interferometer. In spite of group velocity mismatch at short laser pulse pumping, sufficient cascaded nonlinear wave mixing is obtainable at the wavelength shifted from the phase matching wavelength for second harmonic generation. The theoretical model prediction agrees well with the experimental results.

DOI
10.1103/physreva.80.043832

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Lithium niobate is a critical material extensively used in nonlinear optics. The recent development of nanophotonic lithium niobate devices further extends their importance with enhanced nonlinearity and scalability. However, the performance of nonlinear optical processes in lithium niobate is influenced by the photorefractive effect. In this work, we study the impact of the photorefractive effect on second-order nonlinear optical processes in nanophotonic lithium niobate waveguides. We simulate the parametric down-conversion for squeezed light generation and second-harmonic generation in nanophotonic lithium niobate waveguides, considering the photorefractive effect. The squeezing generation with parametric down-conversion only experiences a constant wavelength shift as the refractive index change remains constant. On the other hand, the second-harmonic generation spectrum is distorted, and a decrease in nonlinear efficiency is observed. This is because the strength of the photorefractive effect varies along the waveguide. We further show that the ideal efficiency of second-harmonic generation can be recovered by the adapted poling technique, where the phase matching condition change caused by the photorefractive effect is compensated by adjusting the poling period of lithium niobate waveguides. Eventually, we analyze the cascaded process of second-harmonic generation and parametric down-conversion for squeezed light generation, and show that squeezing level up to 17.6 dB can be realized using 10 mW power with current performance of nanophotonic lithium niobate devices.

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Squeezed vacuum pulses up to \ensuremath−1.7 dB\ensuremath{-}1.7\text{ }\text{dB}\ensuremath−1.7 dB at telecom-band (\ensuremath∼1550 nm)(\ensuremath{\sim}1550\text{ }\text{nm})(\ensuremath∼1550 nm) is generated with femtosecond laser pulses using cascaded \ensuremathχ(2){\ensuremath{\chi}}^{(2)}\ensuremathχ(2) optical nonlinearity in a periodically poled lithium niobate crystal placed in a Sagnac interferometer. In spite of group velocity mismatch at short laser pulse pumping, sufficient cascaded nonlinear wave mixing is obtainable at the wavelength shifted from the phase matching wavelength for second harmonic generation. The theoretical model prediction agrees well with the experimental results.
We report on wavelength-shifting based on cascaded second-order nonlinearities1 in periodically-poled lithium niobate. Wavelength shifting from a signal s at λs=λp+Δ to a frequency-shifted signal f at λf=λp−Δ is obtained under the action of a pump p at λp: the effect is realized through the generation of the second harmonic (sh) of p and subsequent parametric amplification of s driven by sh. In the latter process f is also generated. Exploiting the advantages of quasi-phase-matching, efficient shifting is achieved at pump intensities about two orders of magnitude lower than in traditional second-order materials such as BBO.2
We determine the theoretical singly resonant second-harmonic generation squeezing performance of several cavity designs and nonlinear materials. We show that, for the doubling of the 1064-nm output of Nd:YAG lasers, monolithic cavities made from periodically poled LiNbO3 (PPLN) will produce the largest amount of squeezing. We also present experimental results for a free-space cavity with a PPLN sample that yielded slightly less than 0.6 dB of measured squeezing. The cavity performance as a function of input power agrees with our theoretical predictions and shows the superior performance of periodically poled materials for the production of bright squeezed light.
Discrete frequency-bin entanglement is an essential resource for applications in quantum information processing. In this Letter, we propose and demonstrate a scheme to generate discrete frequency-bin entanglement with a single piece of periodically poled lithium niobate waveguide in a modified Sagnac interferometer. Correlated two-photon states in both directions of the Sagnac interferometer are generated through cascaded second-order optical nonlinear processes. A relative phase difference between the two states is introduced by changing the polarization state of pump light, thus manipulating the two-photon state at the output of the Sagnac interferometer. The generated two-photon state is sent into a fiber polarization splitter, and then a pure discrete frequency-bin entangled two-photon state is obtained by setting the pump light. The frequency entanglement property is measured by a spatial quantum beating with a visibility of 96.0±6.1%. The density matrix is further obtained with a fidelity of 98.0±3.0% to the ideal state. Our demonstration provides a promising method for the generation of pure discrete frequency-bin entanglement at the telecom band, which is desired in quantum photonics.
Lithium niobate is a critical material extensively used in nonlinear optics. The recent development of nanophotonic lithium niobate devices further extends their importance with enhanced nonlinearity and scalability. However, the performance of nonlinear optical processes in lithium niobate is influenced by the photorefractive effect. In this work, we study the impact of the photorefractive effect on second-order nonlinear optical processes in nanophotonic lithium niobate waveguides. We simulate the parametric down-conversion for squeezed light generation and second-harmonic generation in nanophotonic lithium niobate waveguides, considering the photorefractive effect. The squeezing generation with parametric down-conversion only experiences a constant wavelength shift as the refractive index change remains constant. On the other hand, the second-harmonic generation spectrum is distorted, and a decrease in nonlinear efficiency is observed. This is because the strength of the photorefractive effect varies along the waveguide. We further show that the ideal efficiency of second-harmonic generation can be recovered by the adapted poling technique, where the phase matching condition change caused by the photorefractive effect is compensated by adjusting the poling period of lithium niobate waveguides. Eventually, we analyze the cascaded process of second-harmonic generation and parametric down-conversion for squeezed light generation, and show that squeezing level up to 17.6 dB can be realized using 10 mW power with current performance of nanophotonic lithium niobate devices.