Coherently Enhanced Wireless Power Transfer

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Coherently Enhanced Wireless Power Transfer

Alex Krasnok, Denis G. Baranov, Andrey Generalov, Sergey Li, and Andrea Alù

Phys. Rev. Lett. 120, 143901 – Published 2 April 2018

See Focus story: Wave Trick May Lead to Wireless Charging at a Distance

Abstract

Extraction of electromagnetic energy by an antenna from impinging external radiation is at the basis of wireless communications and wireless power transfer (WPT). The maximum of transferred energy is ensured when the antenna is conjugately matched, i.e., when it is resonant and it has an equal coupling with free space and its load. This condition, however, can be easily affected by changes in the environment, preventing optimal operation of a WPT system. Here, we introduce the concept of coherently enhanced WPT that allows us to bypass this difficulty and achieve dynamic control of power transfer. The approach relies on coherent excitation of the waveguide connected to the antenna load with a backward propagating signal of specific amplitude and phase. This signal creates a suitable interference pattern at the load resulting in a modification of the local wave impedance, which in turn enables conjugate matching and a largely increased amount of extracted energy. We develop a simple theoretical model describing this concept, demonstrate it with full-wave numerical simulations for the canonical example of a dipole antenna, and verify experimentally in both near-field and far-field regimes.

Received 3 December 2017

DOI:https://doi.org/10.1103/PhysRevLett.120.143901

Physics Subject Headings (PhySH)

Electromagnetism Engineering Light propagation, transmission & absorption Photonics

Condensed Matter, Materials & Applied PhysicsAtomic, Molecular & OpticalInterdisciplinary PhysicsGeneral Physics

Focus

Wave Trick May Lead to Wireless Charging at a Distance

Published 2 April 2018

A technique involving wave interference may improve the practicality of charging a phone wirelessly at some distance from the power source.

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Authors & Affiliations

Alex Krasnok¹, Denis G. Baranov^2,3, Andrey Generalov⁴, Sergey Li⁵, and Andrea Alù^1,6,7,8,*

¹Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, Texas 78712, USA
²Department of Physics, Chalmers University of Technology, 412 96 Gothenburg, Sweden
³Moscow Institute of Physics and Technology, Dolgoprudny 141700, Russia
⁴Department of Electronics and Nanoengineering, Aalto University, 02150 Espoo, Finland
⁵ITMO University, St. Petersburg 197101, Russia
⁶Photonics Initiative, Advanced Science Research Center, City University of New York, New York 10031, USA
⁷Physics Program, Graduate Center, City University of New York, New York 10016, USA
⁸Department of Electrical Engineering, City College of The City University of New York, New York 10031, USA

^*alu@mail.utexas.edu

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Issue

Vol. 120, Iss. 14 — 6 April 2018

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Images

Figure 1
Sketch of a coherently enhanced WPT system. The incident radiation ( $s_{F}$ ) excites the antenna, which reradiates back into free space at a rate $1 / τ_{F}$ and couples to the waveguide at a rate $1 / τ_{w}$ creating a forward-propagating waveguide mode ( $s_{w}^{-}$ ). Coherent excitation of the waveguide with a backward propagating mode ( $s_{w}^{+}$ ) with a specific amplitude and phase can retune the matching condition and largely improve the WPT performance.
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Figure 2
Analytical modeling of coherently enhanced WPT. (a) Spectrum of the transferred energy ( $| s_{w}^{-} |^{2}$ ) in the passive case (no back-propagating signal) for antennas with different waveguide couplings. The decay times are measured in the units of $1 / ω_{0}$ . (b) Energy balance $Σ$ for a resonant matched (gray dashed curve) and resonant mismatched antenna with $τ_{F} = 10$ and $τ_{w} = 100$ (color solid curves). Solid area indicates the region of available values of the energy balance. (c) Energy balance and total power scattered by the mismatched antenna $P_{sca}$ for the optimal phase of the auxiliary mode. The two values are equal at the point of restored conjugate matching. (d) Enhancement factor $F$ defined as the ratio of maximal achievable energy balance assisted by the auxiliary signal to the transferred energy $| s_{w}^{-} |^{2}$ without coherent illumination as a function of $τ_{w} / τ_{F}$ .
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Figure 3
Numerical simulations of the proposed approach of coherently enhanced WPT for the canonical example of a dipole antenna. (a) Energy balance ( $| s_{w}^{-} |^{2} - | s_{w}^{+} |^{2}$ ) as a function of the auxiliary signal intensity ( $| s_{w}^{+} |^{2}$ ) for different relative phase values. The black dashed line shows the energy level received by the dipole antenna without auxiliary signal. Solid area indicates the region of energy benefit, where the coherently assisted energy balance exceeds that for $s_{w}^{-} = 0$ . (b),(c) Poynting vector distribution around the microwave antenna with the auxiliary signal of amplitude $| s_{w}^{+} | = 12$ and relative phases of 0° (b) and 180° (c).
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Figure 4
Experimental verification of coherently enhanced wireless energy transfer. (a) Measured spectrum of $s_{11}$ and $s_{22}$ absolute values for transmitting and receiving antennas placed next to each other. Inset: Photo of the experimental setup. Two coaxial cables are attached to the vector network analyzer ports and terminated with waveguide to coaxial adapters at the free ends. One port serves as the transmitter, while the other serves as the power receiver. (b),(c) The energy balance $| s_{w}^{-} |^{2} - | s_{w}^{+} |^{2}$ vs power of the additional signal $| s_{w}^{+} |^{2}$ for two different frequencies. The shaded area corresponds to all values of the relative phase between the external signal and the auxiliary mode. The power of 1 mW is fed to port 1 in both cases. (d) Dependence of the energy transfer enhancement factor $F$ on the distance between the WCAs at fixed frequency of 7 GHz.
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