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Research in Optical Sciences

Spatial-Mode-Selective Quantum Frequency Conversion in a χ^(2) Slab Waveguide

Michael Vasilyev, Young B. Kwon, Yu-Ping Huang

Published Jan 1, 2014• 2 citations
Publisher

Abstract

We numerically investigate single-spatial-mode-selective up-conversion in a diffractionless χ(2) slab waveguide by optimizing its one-dimensional pump profile. Singular-value decomposition of the Green’s function predicts ~5-dB discrimination between the first two modes’ conversion efficiencies.
DOI
10.1364/hilas.2014.jw2a.52

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We analyze sum-frequency generation (SFG) in a χ(2) slab waveguide with the goal of achieving a single spatial-mode operation. We first develop Green’s function formalism for the SFG equations and then perform singular-value decomposition (SVD) of the Green’s function. By adjusting the spatial profile of the pump, we manipulate the SVD spectrum to maximize the up-conversion of one signal mode while minimizing the up-conversion of all others, which opens a possibility of realizing a spatial-mode-selective quantum frequency converter for future optical communications.
Spatial-mode-selective frequency conversion is potentially useful for both classical and quantum communication applications. By a judicious choice of the quasi-phase-matching period in a Kai(2) multimode waveguide, such conversion can be achieved with high efficiency (close to 100%) and with low crosstalk (< -20 dB). For space-division multiplexing application with classical signals, where each spatial mode represents a separate signal channel, the selective conversion of a spatial mode without disrupting other signal modes can be used for reconfigurable spatial-mode de-multiplexing. This classical de-multiplexing capability can be also extended to the quantum regime, where the quantum state of the signal is preserved during frequency conversion, owing to the unitary nature of the sum-frequency generation (SFG) process. Building upon our previous experimental demonstration of the classical spatial-mode-selective frequency up-conversion in a two-mode PPLN waveguide, here we report the extension of this work into the single-photon-level regime. The signal (1540 nm) in either a single mode (TM00 or TM01) or a superimposition mode (TM00+TM01, TM00+iTM01) of the waveguide is selectively up-converted into TM01 SFG mode, by interacting with an appropriate pump mode (1560 nm). An accurate measurement of the single-photon-level SFG signals requires thorough filtering of the unwanted photons contributed by the second harmonic of the pump, residual pump noise extending to the signal band, and the Raman noise generated in the waveguide. We have investigated these unwanted photon sources and suppressed them by a combination of thin-film-interference and volume-Bragg-grating filters. Resulting single-photon-counting measurements show >70% internal conversion efficiency, better than -12dB crosstalk, and >100 ratio of the signal to background photon counts for all selected modes and mode superpositions.
We study sum-frequency generation (SFG) in a multimode PPKTP waveguide. We show that under proper quasi-phasematching, it can support one of the two scenarios. In the first, a single pump mode up-converts several different signal modes to different SFG modes. In the second, several different pairs of signal and pump modes are converted to the same SFG mode. By adjusting the relative phases and magnitudes of the pump modes, any superposition of the corresponding signal modes can be selected for up-conversion without affecting other modes, which can be used for spatial-mode de-multiplexing in both classical and quantum communications.
We investigate spatial-mode-selective frequency up-converters of quantum states from infrared to visible region, which could be useful not only for interfacing the optical fiber links with quantum memories and for increasing the photon detection efficiency, but also for classical demultiplexing of spatial modes that are otherwise difficult to discriminate in both spatial and spatial-frequency domains. We consider two approaches: first, based on sum-frequency generation (SFG) in 2D free space, and second, based on SFG in a multimode waveguide with 2D confinement. For the latter approach, we find that under proper quasi-phase-matching arrangement, several different pairs of signal and pump modes are converted to the same SFG mode. By adjusting the relative phases and magnitudes of the pump modes, any superposition of the corresponding signal modes can be selected for up-conversion without affecting other modes.
Quantum frequency conversion (FC) in nonlinear optical media is a powerful tool for temporal-mode selective manipulation of light. Recent attempts at achieving high mode selectivities and/or fidelities have had to resort to multi-dimensional optimization schemes to determine the system's natural Schmidt modes. Certain combinations of relative-group velocities between the relevant frequency bands, medium length, and temporal pulse widths have been known to achieve good selectivities (exceeding 80%) for temporal modes that are nearly identical to pump pulse shapes, even for high conversion efficiencies. Working in this parameter regime using an off-the-shelf, second-harmonic generation, MgO:PPLN waveguide, and with pulses on the order of 500 fs at wavelengths around 800 nm, we verify experimentally that model-predicted Schmidt modes provide the high temporal-mode selectivity expected. The good agreement between experiment and theory paves the way to the implementation of a proposed two-stage FC scheme that is predicted by the present theory to reach near-perfect (100%) selectivity.