Beam Shaping for Wireless Optical Charging with Improved Efficiency

crystals

Article Beam Shaping for Wireless Optical Charging with Improved Efﬁciency

Lei Tian, Jiewen Nie and Haining Yang *

Department of Electronic Science and Engineering, Southeast University, 2 Sipailou, Xuanwu District, Nanjing 210018, China; [email protected] (L.T.); [email protected] (J.N.) * Correspondence: [email protected]

Abstract: Optical wireless charging is a nonradiative long-distance power transfer method. It may potentially play an important role in certain scenarios where access is challenging, and the radio frequency power transfer is less efficient. The divergence of the optical beam over distances is a key limiting factor for the efficiency of any wireless optical charging system. In this work, we propose and experimentally demonstrate a holographic optical beam shaping system that can restrict the divergence of the optical beam. Our experimental results showed up to 354.88% improvement in the charging efficiency over a 10 m distance.

Keywords: optical wireless power transfer; beam shaping; phase modulation

1. Introduction Wireless charging [1–4] has attracted considerable attention in recent years due to its Citation: Tian, L.; Nie, J.; Yang, H. operational flexibility; this is especially true for underwater environments as the design of Beam Shaping for Wireless Optical the charging cable and sockets can be complicated and often impractical. Wireless charging Charging with Improved Efficiency. can be implemented in various ways. Magnetic coupling charging [5,6] can transmit energy Crystals 2021, 11, 970. https:// at the kilowatt level with high efficiency. However, it only supports wireless charging doi.org/10.3390/cryst11080970 within 1 m as the power declines rapidly with the increasing charging distance [7,8]. The transmission distance of microwave charging is much longer [9]. However, its charging Academic Editors: Akihiko efficiency is limited due to the quick divergence of the microwave beams. The propagation Mochizuki, Kohki Takatoh loss of microwaves is also extremely high in the water. Therefore, it is not suitable for and Jun Xu underwater applications [3,10]. Wireless optical charging has gained considerable interest in recent years due to its flexibility, strong directivity, and support for long transmission Received: 15 July 2021 ranges [11]. It is also free of electromagnetic interference. In addition, blue and green Accepted: 15 August 2021 Published: 17 August 2021 wavelengths correspond to the low attenuation window of the seawater, which make them suitable for underwater applications [12]. The sources for wireless optical charging are

Publisher’s Note: MDPI stays neutral laser diode [13,14], light-emitting diodes (LED) [15,16], vertical-cavity surface-emitting with regard to jurisdictional claims in laser (VCSEL) [17], etc. The performance of the wireless optical charging system has been published maps and institutional affil- optimised in previous works. Most of these works mainly focused on fully exploiting the iations. power processing capacity and conversion efficiency of laser and PV cells. The operating point of the laser can be tuned to improve the electrical-to-optical efficiency [18,19]. The material and temperature of the photovoltaic (PV) cell can also be optimised for higher efficiency [20]. However, the divergence of the optical beam over distances [21] may be a key factor for the system’s overall efficiency. To our knowledge, this issue has not been Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. properly addressed so far. This article is an open access article In this work, we designed and demonstrated a holographic optical beam shaping distributed under the terms and system that could improve the overall charging efficiency. In our demonstration, the conditions of the Creative Commons divergence of the optical beam was restricted for different charging distances. The power Attribution (CC BY) license (https:// on the PV cell significantly increased in both free space and water compared with the creativecommons.org/licenses/by/ conventional system. Our results showed that this technique has great potential for the 4.0/). application of underwater wireless charging.

Crystals 2021, 11, 970. https://doi.org/10.3390/cryst11080970 https://www.mdpi.com/journal/crystals Crystals 2021, 11, x FOR PEER REVIEW 2 of 8

Crystals 2021, 11, 970 conventional system. Our results showed that this technique has great potential for2 the of 8 application of underwater wireless charging.

2. System Setup 2. System Setup Figure 1 shows the design of our proposed optical launching unit with the beam shapingFigure capability.1 shows A the pigtailed design single-mode of our proposed laser opticaldiode (Thorlabs launching LP450-SF15, unit with the Newton, beam NJ,shaping USA) capability. operating Aat pigtailed 450 nm was single-mode used as th lasere light diode source. (Thorlabs Then LP450-SF15,a fibre-coupled Newton, collima- NJ, torUSA) (Thorlabs operating F671FC-405, at 450 nm Newton, was used NJ, as United the light States) source. was Then used a to fibre-coupled slow the divergence collimator of the(Thorlabs laser beam. F671FC-405, The wavefront Newton, of the NJ, collimat USA) wased beam used was to slow spatially the divergencemodulated ofby thea phase- laser beam. The wavefront of the collimated beam was spatially modulated by a phase-only only liquid crystal on silicon (LCOS) device (CamOptics COVIS-2K, Cambridge, UK). This liquid crystal on silicon (LCOS) device (CamOptics COVIS-2K, Cambridge, UK). This LCOS device has a bit depth of 8 bit, i.e., 256 unique phase levels between 0 and 2.5π. The LCOS device has a bit depth of 8 bit, i.e., 256 unique phase levels between 0 and 2.5π. The phase flicker of the LCOS was optimised [22,23]. The reflectance of the LCOS device was phase flicker of the LCOS was optimised [22,23]. The reflectance of the LCOS device was ~80%; however, it can be improved by applying a dielectric mirror coating on the LCOS ~80%; however, it can be improved by applying a dielectric mirror coating on the LCOS backplane [24]. The desired beam can be generated with the Fourier transform lens (L1, f1 backplane [24]. The desired beam can be generated with the Fourier transform lens (L1, = 500 mm) in the system. A 4f magnification system based on L2 (f2 = 250 mm) and L3 (f3 f1 = 500 mm) in the system. A 4f magnification system based on L2 (f2 = 250 mm) and = 500 mm) were used to further enlarge the shaped beam. For the proof-of-concept demon- L3 (f3 = 500 mm) were used to further enlarge the shaped beam. For the proof-of-concept stration, a beam splitter (BS) was used in the current system; this can be replaced with a demonstration, a beam splitter (BS) was used in the current system; this can be replaced prism to further increase the energy efficiency of this optical launching unit [25]. with a prism to further increase the energy efficiency of this optical launching unit [25].

FigureFigure 1. 1. SetupSetup of of the the optical optical launching launching unit. unit.

TwoTwo types of beam shaping techniques were investigated in this work. The The first first one usesuses holograms that exhibitexhibit radiallyradially symmetricsymmetric cubic cubic phase phase profiles, profiles, which which is describedis described by bythe the following following equation equation [26 [26,27]:,27]: 3 P𝑃= angle 𝑎𝑛𝑔𝑙𝑒𝑒e−ib|r||| (1)(1) where r is the radial distance and b is a coefficient optimised for different charging dis- where r is the radial distance and b is a coefficient optimised for different charging distances. tances. The second type utilises holograms that follow the Fresnel lens shape [28,29], which is The second type utilises holograms that follow the Fresnel lens shape [28,29], which described by: is described by: 2 P = angle e−ibr (2) 𝑃 𝑎𝑛𝑔𝑙𝑒𝑒 (2) where r is the radial distance and b is related to focal length of the Fresnel lens. Again, the wherevalue ofr isb thecan radial be optimised distance for and different b is related charging to focal distances. length of the Fresnel lens. Again, the valueFigure of b can2 shows be optimised the general for different architecture charging of our distances. testbed. A pigtailed single-mode laser diodeFigure (Thorlabs 2 shows LP450-SF15, the general Newton, architecture NJ, USA) of operatingour testbed. at 450A pigtailed nm was drivensingle-mode by Thorlabs laser diodeLDM9LP (Thorlabs to provide LP450-SF15, a constant Newton, DC of NJ, 35 Unit mAed with States) an outputoperating power at 450 of nm approximately was driven by4.37 Thorlabs mW. This LDM9LP driver unitto provide also integrated a constant a thermoelectric DC of 35 mA coolerwith an module output to power maintain of ap- the proximatelylaser performance 4.37 mW. during This the driver experiment. unit also Thenintegrated the laser a thermoelectric was fed from cooler the single-mode module to maintainfibre into the an opticallaser performance launching unit during for collimationthe experiment. and beam Then shapingthe laser before was fed entering from the single-modechannel. At fibre the receiver into an side,optical a low-costlaunching PV unit cell for was collimation used to collect and beam the incidentshaping opticalbefore enteringpower. In the this channel. way, the At received the receiver optical side, power a low-cost was converted PV cell was into electricused to powercollect tothe charge inci- denta target optical device. power. A source In this meter way, (KEITHLEY-2400, the received optical Portland, power OR, was United converted States) into was electric used to powercollect to the charge voltage a target and current device. passing A source through meter the(KEITHLEY-2400, cell. In this work, Portland, we tested OR, wireless United optical charging in both free spaces and water environments.

Crystals 2021, 11, x FOR PEER REVIEW 3 of 8

States) was used to collect the voltage and current passing through the cell. In this work, Crystals 2021, 11, 970 3 of 8 States)we tested was usedwireless to collect optical the charging voltage in and both curr freeent spaces passing and through water environments. the cell. In this work, we tested wireless optical charging in both free spaces and water environments.

Figure 2. General architecture of the testbed. FigureFigure 2. 2.GeneralGeneral architecture architecture of ofthe the testbed. testbed. 3. Results 3. 3.Results ResultsFirst, simulations based on the Fourier optics theory [30] were conducted to investi- gateFirst, theFirst, performancesimulations simulations based of based the on optical on the the Fourier Fourier launching optics un theoryit. It should [[30]30] werewere be noted conducted conducted that a to 4fto investigate systeminvesti- of gateL2the theand performance performance L3 had a magnification of theof the optical optical launchingfactor launching of 2. unit.The un transmissionit. It shouldIt should be be notedmedium noted that that was a 4fa assumed 4f system system ofto of L2be L2homogeneousand and L3 L3 had had a a magnificationandmagnification had a refractive factorfactor index ofof 2.2. Theof The 1. transmission transmissionThe results were medium medium shown was was in assumedFigure assumed 3. toFigure to be be homogeneous and had a refractive index of 1. The results were shown in Figure3. Figure3a homogeneous3a corresponded and tohad the a refractivebeam propagation index of 1.behaviour The results when were the shown optical in beam Figure shaping 3. Figure was corresponded to the beam propagation behaviour when the optical beam shaping was not 3anot corresponded applied. It diverged to the beam with propagation a fixed angle behaviour while propagating when the throughoptical beam the medium. shaping Figure was applied. It diverged with a fixed angle while propagating through the medium. Figure3b not3b applied. corresponded It diverged to the with case a whenfixed angle a radially while symmetric propagating cubic through phase the modulation medium. Figureapplied corresponded to the case when a radially symmetric cubic phase modulation applied with 3bwith corresponded b = 4.95 to 10 the. Thecase beamwhen showeda radially a symmetricstrong self-focusing cubic phase characteristic modulation within applied the b = 4.95 × 1011. The beam showed a strong self-focusing characteristic within the first withfirst b ~20= 4.95 m before 10 .diverging. The beam The showed highest a strong intensity self-focusing of the modulated characteristic beam within was 7.8 the dB ~20 m before diverging. The highest intensity of the modulated beam was 7.8 dB higher firsthigher ~20 thanm before the Gaussiandiverging. beam The athighest the same intensity distance. of the This modulated simulation beam results was when 7.8 dB the than the Gaussian beam at the same distance. This simulation results when the Fresnel lens higherFresnel than lens the phase Gaussian hologram beam with at thea 𝑏 samevalue distance. of 1.25 This 10 simulation was illustrated results in Figure when 3c.the In phase hologram with a b value of 1.25 × 108 was illustrated in Figure3c. In this case, the Fresnelthis case, lens the phase position hologram of the with maximum a 𝑏 value light of intensity 1.25 10 was was similar illustrated to Figure in Figure 3b. A further3c. In position of the maximum light intensity was similar to Figure3b. A further improvement thisimprovement case, the position of 4.01 of dB the was maximum achieved. light After intensity passing was through similar theto Figure focal point,3b. A furtherthe light of 4.01 dB was achieved. After passing through the focal point, the light field diverged improvementfield diverged of as 4.01 a Gaussian dB was achieved.beam. The After simulation passing results through showed the focal that thepoint, Fresnel the lightphase hologramas a Gaussian was beam.more effective The simulation than the results radially showed symmetric that the cubic Fresnel phase phase hologram. hologram was fieldmore diverged effective as than a Gaussian the radially beam. symmetric The simulation cubic phase results hologram. showed that the Fresnel phase hologram was more effective than the radially symmetric cubic phase hologram.

Figure 3.3. TheThe phasephase holograms holograms and and simulated simulated characteristics characteristics of the of launchedthe launched beams beams (a) without (a) without beam shaping; (b) with the cubic modulation beam shaping when b = 4.95b × 10 4.9511;( c) 10 with the Fresnel Figurebeam 3.shaping; The phase (b) withholograms the cubic and modulatisimulatedon char beamacteristics shaping of when the launched beams (; a()c )without with the b 81.25 10 beamFresnelmodulation shaping; modulation beam (b) with shaping beam the shapingcubic when modulati b when= 1.25 ×on10 beam. shaping. when b 4.95 10; (c) with the Fresnel modulation beam shaping when b 1.25 10. Subsequently, thethe impactimpact of ofb bvalues values on on the the system system of of beam beam propagation propagation characteristics characteris- wasticsSubsequently, was analysed. analysed. As the shownAs impactshown in Figure ofin bFigure values4, the 4, on focusingthe the focusing system position position of beam of the of propagation beamthe beam increased increased characteris- with with the value of b when the cubic modulation was implemented. Although the peak light intensity ticsthe was value analysed. of b when As shown the cubic in Figure modulation 4, the focusing was impl positionemented. of the Although beam increased the peak with light decreased for the longer transmission distances, it was always signiﬁcantly higher than the the value of b when the cubic modulation was implemented. Although the peak light standard Gaussian beam without beam shaping.

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intensity decreased for the longer transmission distances, it was always significantly higher than the standard Gaussian beam without beam shaping. Crystals 2021, 11, 970 intensity decreased for the longer transmission distances, it was always significantly4 of 8 higher than the standard Gaussian beam without beam shaping. 60 6 Focusing distance Peak intensity with cubic modulation 4 60 Peak intensity of the Gaussian beam 6 50 Focusing distance 2 Peak intensity with cubic modulation 4 Peak intensity of the Gaussian beam 0 50 2 40 -2 0 -4 40 -2 30 -6 -4 -8 30 -6 20 -10 -8 -12 20 -10 10 -14 -12 -16 10 -14 0 -18 0 5 10 15 20 25 30-16 The b value/*e11 0 -18 0 5 10 15 20 25 30 Figure 4. The trend of focusingThe distance b value/*e11 and intensity with b value when loading the cubic modu-

lation hologram. FigureFigure 4. The 4. The trend trend of focusing of focusing distance distance and and intensit intensityy with with b value b value when when loading loading the the cubic cubic modu- modula- lationtionFigure hologram. hologram. 5 showed corresponding results when the Fresnel modulation was used. The trend wasFigure similar5 showed to that correspondingobserved in Figure results 4. whenThe notable the Fresnel difference modulation was that was the used. inten- The sity attenuationFigure 5 showed was very corresponding small at different results focusingwhen the distances Fresnel modulation with Fresnel was modulation. used. The trendtrend was was similar similar to that to that observed observed in Figure in Figure 4. The4. The notable notable difference difference was was that that the the inten- inten- Comparedsity attenuation with cubic was modulati very smallon, atFresnel different modulation focusing distancescan increase with the Fresnel light modulation.intensity moresity attenuation significantly. was very small at different focusing distances with Fresnel modulation. ComparedCompared with with cubic cubic modulati modulation,on, Fresnel Fresnel modulation modulation can can increase increase the thelight light intensity intensity moremore significantly. signiﬁcantly. 60 6 Focusing distance Peak intensity with Fresnel modulation 4 60 Peak intensity of the Gaussian beam 6 50 Focusing distance 2 Peak intensity with Fresnel modulation 4 Peak intensity of the Gaussian beam 0 50 2 40 -2 0 -4 40 -2 30 -6 -4 -8 30 -6 20 -10 -8 -12 20 -10 10 -14 -12 -16 10 -14 0 -18 00.511.522.533.54-16 The b value/*e8 0 -18 00.511.522.533.54 FigureFigure 5. The 5. trendThe trend of focusing of focusingThe distance b value/*e8 distance and intensit and intensityy with b value with bwhen value loading when the loading Fresnel the mod- Fresnel

ulationmodulation hologram. hologram. Figure 5. The trend of focusing distance and intensity with b value when loading the Fresnel mod- ulationIn hologram.ourIn our experiment, experiment, the thevoltage-current voltage-current curve curve of the of thePV PVcell cell was was measured measured by bya source a source metre.metre. The The measurement measurement was was conducted conducted when when the the PV PV cell cell was was placed 10 mm awayaway fromfrom our ourlaunching launchingIn our experiment, unit unit and and the thethe beam voltage-current beam shaping shaping was curvewas not not applied. of theapplied. PV The cell The results was results measured were were plotted by plotted a in source Figure in 6. metre.The The maximum measurement power generated was conducted in this when specific the setting PV cell was was 1.91 placedµW with 10 m a loadaway resistance from ourof launching 5250 Ω. unit and the beam shaping was not applied. The results were plotted in Subsequently, we measured the maximum electrical power generated by the PV cell under different beam shaping configurations. The experiment was first conducted in the free space between 4 m and 10 m. The value of b was optimised for each transmission distance. Figure7 illustrated the maximum power generated at each plane. It can be Crystals 2021, 11, x FOR PEER REVIEW 5 of 8

Figure 6. The maximum power generated in this specific setting was 1.91 µW with a load resistance of 5250 Ω.

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seen that the generated power decreased over the distance. However, the drop was most Figuresigniﬁcant 6. The when maximum no beam power shaping generated was applied. in this When specific the Fresnelsetting phasewas 1.91 modulation µW with was a load resistanceused, the decreaseof 5250 Ω in. power was the slowest. The value of b used in each measurement was listed in Table1.

Figure 6. Voltage vs. current curve of the PV cell.

Subsequently, we measured the maximum electrical power generated by the PV cell under different beam shaping configurations. The experiment was first conducted in the free space between 4 m and 10 m. The value of b was optimised for each transmission distance. Figure 7 illustrated the maximum power generated at each plane. It can be seen that the generated power decreased over the distance. However, the drop was most sig- nificant when no beam shaping was applied. When the Fresnel phase modulation was used, the decrease in power was the slowest. The value of b used in each measurement

was listed in Table 1. FigureFigure 6. 6. VoltageVoltage vs. current curvecurve ofof the the PV PV cell. cell.

FigureFigure 7. 7.TheThe maximum maximum electrical electrical power power obtained obtained by by so sourceurce metre metre conversion conversion at at different different planes. planes.

Table 1. The values of b used in the experiments.

Focusing Distance/m Cubic Modulation/(×1011) Fresnel Modulation/(×108) 4 0.7324 0.4189 5 1.1719 0.6981 6 1.7578 0.9774 7 2.4902 1.2566 8 3.8086 1.5359 9 5.2734 1.8151 10 8.2031 2.0944 Figure 7. The maximum electrical power obtained by source metre conversion at different planes.

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Table 1. The values of b used in the experiments.

Focusing Distance/m Cubic Modulation/(𝟏𝟎𝟏𝟏) Fresnel Modulation/(𝟏𝟎𝟖) 4 0.7324 0.4189 5 1.1719 0.6981 6 1.7578 0.9774 7 2.4902 1.2566 8 3.8086 1.5359 9 5.2734 1.8151 Crystals 2021, 11, 970 10 8.2031 2.0944 6 of 8

Figure 8 showed the relative improvement in the charging efficiency at each plane when Figurecompared8 showed with the the result relatives measured improvement without in thebeam charging shaping. efﬁciency At 10 m, atthe each charging plane efficiencywhen compared increased with by the246% results when measured the Fresnel without modulation beam shaping.was used. At 10 m, the charging efﬁciency increased by 246% when the Fresnel modulation was used.

FigureFigure 8. 8. TheThe efficiency efﬁciency of of power power improvement. improvement.

WeWe then then conducted conducted the the same same experiment experiment in in a a water water tank. tank. The The attenuation attenuation coefficient coefficient ofof water water in in the the tank tank was was measured measured as as ~0.25/ ~0.25/m,m, which which was was similar similar to to the the Jerlov Jerlov II II water water conditions.conditions. The The experimental experimental results results underwate underwaterr were were nearly nearly the the same same as as those those in in free free space.space. Since Since the the tank tank was was 5 5m m long, long, only only the the situations situations at at5 m 5 mand and 10 10m mwere were tested. tested. At At5 m,5 m,the the radially radially symmetric symmetric cu cubicbic phase phase modulation modulation increas increaseded power power by by 21.33% 21.33% over over the the GaussianGaussian and and the the Fresnel Fresnel phase phase modulation modulation by by 26.22%. 26.22%. At At 10 10 m, m, the the former former increased increased by by 176.24%176.24% and and the the latter latter by by 354.88%. 354.88%. At At a a longer longer distance, distance, the the beam beam size size without without shaping shaping becamebecame significantly significantly larger. larger. The The beam beam shaping shaping techniques techniques were were able able to to concentrate concentrate the the opticaloptical power power at at an an arbitrary arbitrary receiver receiver plane. plane. Therefore, Therefore, its its effect effect became became more more apparent apparent at at aa longer longer distance, distance, improving improving th thee overall overall system system efficiency. efficiency. 4. Conclusions 4. Conclusions In this work, we demonstrated two holographic optical beam shaping techniques In this work, we demonstrated two holographic optical beam shaping techniques that that could improve the optical energy transfer efficiency in free spaces and underwater could improve the optical energy transfer efficiency in free spaces and underwater envi- environments. In a proof-of-concept experiment, both techniques demonstrated focusing ronments. In a proof-of-concept experiment, both techniques demonstrated focusing ca- capability at different distances. The beam shaping based on the Fresnel lens modulation pability at different distances. The beam shaping based on the Fresnel lens modulation performed better than the radially symmetric cubic phase modulation. We believe the performedholographic better optical than beam the radially shaping symmetric technique hascubic the phase potential modulation. for application We believe in underwater the hol- ographicwireless optical chargingbeam shaping systems, technique where thehas accessibilitythe potential is for poor, application and the RF in transmissionunderwater wirelessis inefficient. optical Althoughcharging systems, the power where currently the accessibility generated is in poor, this and work the is RF low, transmission it could be significantly improved by using more powerful laser sources and more efficient PV cells. Using the beam splitter in our current configuration also limited the overall efficiency of the system. However, it can be easily solved by replacing the beam splitter with a prism, as

previously mentioned. Alternatively, transmissive phase-only spatial light modulators can also be used.

Author Contributions: Conceptualization, H.Y.; methodology, H.Y.; validation, L.T. and J.N.; formal analysis, L.T., J.N. and H.Y.; writing—original draft preparation, L.T.; writing—review and editing, L.T., J.N. and H.Y.; supervision, H.Y.; funding acquisition, H.Y. All authors have read and agreed to the published version of the manuscript. Funding: This research and APC were funded by the Natural Science Foundation of Jiangsu Province (BK20200351); Jiangsu Special Professorship. Crystals 2021, 11, 970 7 of 8

Acknowledgments: The authors would like to thank Chaogang Lou for providing the source metre. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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