Daniel E. Raible

Glenn Research Center, Cleveland, Ohio

Brian R. Fast

Rockwell Automation, Cleveland, Ohio

Dragos Dinca

Glenn Research Center, Cleveland, Ohio

Taysir H. Nayfeh

Cleveland State University, Cleveland, Ohio

Andrew K. Jalics

QinetiQ North America, Cleveland, Ohio

Comparison of Square and Radial Geometries for 

High Intensity Laser Power Beaming Receivers

NASA/TM—2012-217255

March 2012

ISBN: 978-1-4244-9686-0
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Daniel E. Raible

Glenn Research Center, Cleveland, Ohio

Brian R. Fast

Rockwell Automation, Cleveland, Ohio

Dragos Dinca

Glenn Research Center, Cleveland, Ohio

Taysir H. Nayfeh

Cleveland State University, Cleveland, Ohio

Andrew K. Jalics

QinetiQ North America, Cleveland, Ohio

Comparison of Square and Radial Geometries for 

High Intensity Laser Power Beaming Receivers

NASA/TM—2012-217255

March 2012

ISBN: 978-1-4244-9686-0

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

International Conference on Space Optical Systems and Applications (ICSOS)

sponsored by the Institute of Electrical and Electronics Engineers

Santa Monica, California, May 11–13, 2011
Acknowledgments

This work was conducted by the Industrial Space Systems Laboratory (ISSL) at Cleveland State University under grant from the 

AFRL-Revolutionary Munitions Directorate at Eglin AFB. The ISSL team would like to express their thanks to: Bernie Sater and 

Greenfi eld Solar for the advice given to the ISSL from years of experience developing the VMJ cells. Jim Soeder, Ray Beach, 

Fred Wolff, and the NASA John H. Glenn Research Center for supporting the efforts of the ISSL. Ken Edwards and the Eglin 

Air Force Research Laboratory for the research partnership with the ISSL in further developing this technology. Hobson Lane,

Bob Rice, and Northrop Grumman’s Space Technology division for the time and support given to the ISSL during the demonstrations. 

NASA’s Space Communications and Navigation (SCaN) program for supporting the publication and presentation of this research.

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Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2012-217255 1 

Comparison of Square and Radial Geometries for High Intensity 

Laser Power Beaming Receivers 

Daniel E. Raible 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Brian R. Fast 

Rockwell Automation 

Cleveland, Ohio 44124 

Dragos Dinca 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Taysir H. Nayfeh 

Cleveland State University 

Cleveland, Ohio 44115 

Andrew K. Jalics 

QinetiQ North America 

Cleveland, Ohio 44135 

Abstract 

In an effort to further advance a realizable form of wireless 

power transmission (WPT), high intensity laser power beaming 

(HILPB) has been developed for both space and terrestrial 

applications. Unique optical-to-electrical receivers are 

employed with near infrared (IR-A) continuous-wave (CW) 

semiconductor lasers to experimentally investigate the HILPB 

system. In this paper, parasitic feedback, uneven illumination 

and the implications of receiver array geometries are considered 

and experimental hardware results for HILPB are presented. 

The TEM00 Gaussian energy profile of the laser beam presents a 

challenge to the effectiveness of the receiver to perform 

efficient photoelectric conversion, due to the resulting nonuniform illumination of the photovoltaic cell arrays. In this 

investigation, the geometry of the receiver is considered as a 

technique to tailor the receiver design to accommodate the 

Gaussian beam profile, and in doing so it is demonstrated that 

such a methodology is successful in generating bulk receiver 

output power levels reaching 25 W from 7.2 cm2

 of 

photovoltaic cells. These results are scalable, and may be 

realized by implementing receiver arraying and utilizing higher 

power source lasers to achieve a 1.0 m2

 receiver capable of 

generating over 30 kW of electrical power. This type of system 

would enable long range optical “refueling” of electric 

platforms, such as MUAV’s, airships, robotic exploration 

missions and provide power to spacecraft platforms which may 

utilize it to drive electric means of propulsion. In addition, a 

smaller HILPB receiver aperture size could be utilized to 

establish a robust optical communications link within 

environments containing high levels of background radiance, to 

achieve high signal to noise ratios. 

1.0 Introduction 

In the 20th and 21st century, we as a society have become 

increasingly dependent on the immediate availability of 

electrical power. Electrical power is a commodity that is 

especially valuable for military operations and emergency first 

responders. However, many times the nature of these missions 

do not offer the convenient luxury of flipping a switch to 

instantaneously receive electrical power. These operations 

often require assets to be placed into remote locations where 

conventional power delivery infrastructures do not exist or 

may have been disrupted. Therefore other means of 

distributing electrical power are being investigated to meet 

these particularly challenging needs. These methods would not 

require large physical infrastructures and are known as 

wireless power transmission (WPT). This technology is 

similar to the wireless telecommunications industry. Cellular 

phones have grown in popularity over the past decades due to 

the availability, reliability, and the low associated cost. In 

areas where conventional land-based communications lines do 

not exist such as remote villages, wireless technology has 

become the standard method of electronic communication. In 

a similar manner, WPT may be used to enable power delivery 

to areas where conventional transmission lines would be 

prohibitively expensive, or technically infeasible. 
NASA/TM—2012-217255 2 

The WPT system would consist of a source, to transmit 

energy wirelessly, and a receiver, to collect and convert the 

energy either for immediate use or storage. Removing the 

transmission lines significantly increases the flexibility and 

capability during military operations and emergency missions. 

However, removing the distribution lines adds significant 

engineering challenges for providing sufficient energy in a 

reliable manner. One specific WPT technology includes high 

intensity laser power beaming (HILPB). HILPB consists of a 

high energy laser that would transmit optical energy at a 

specific target, for the purpose of providing remote power and 

potential high bandwidth communications capability. The 

target would consist of an optical receiver optimized to 

maximize the power conversion process from optical to 

electrical energy. This paper will specifically investigate the 

performance and scalability of the power receiver. 

At the core of the power receiver is a single Vertical MultiJunction (VMJ) photovoltaic cell. The small 0.8 cm VMJ cell 

is essentially 40 vertically oriented current sources connected 

in series (Refs. 1 and 2). The cell is radically different from 

traditional photovoltaic devices due to its ability generate 

large current densities on the order of watts-per-square 

centimeter, while delivering a nominal 24 V under load 

(Ref. 3). The primary limitation of this type of cell is apparent 

when the illumination of the cell is non-uniform, which is the 

typical unconditioned TEM00 Gaussian case from most laser 

sources (Ref. 4). In this condition, one junction suffers from a 

lower level of illumination than its neighboring junctions, and 

the bulk output current of the cell will be limited by the single 

lowest capacity junction. Therefore, when designing a larger 

array which includes multiple VMJ cells the type of 

illumination across the cell becomes a major design 

consideration. This paper will examine multiple types of 

geometric arrangements and analyze the performance of the 

entire power receiver when illuminated by a laser that exhibits 

a typical TEM00 Gaussian profile. The arrangements will 

include an optimization of the VMJ cell packing density and a 

strategy which attempts to balance the illumination level on 

the VMJ cells across each junction. 

The remainder of the paper is organized by presenting the 

developed HILPB system in Section 2.0, basic theory when 

connecting multiple cells in parallel in Section 3.0, Section 4.0 

presents the results of a multi cell array optimizing the 

packing density, Section 5.0 presents the results of a multi cell 

array optimizing the illumination across each junction of a 

cell, and Section 6.0 presents the concluding results. 

2.0 The HILPB System 

The HILPB system is constructed around one subsystem to 

transmit the energy optically, and another to receive that 

energy and convert it back into electrical energy. The design 

and construction of the power receiver addresses four main 

issues. First, the receiver needs to provide sufficient thermal 

dissipation in order to handle the excess, electrically 

unconvertible energy. Second, the materials in the power 

receiver must have similar coefficients of thermal expansion 

to avoid stress fracturing during thermal cycling. Third, the 

electrical paths and connections in the receiver must feature 

low resistivity in order to maintain good end-to-end power 

efficiency. Lastly, the receiver must provide electrical 

isolation for the VMJ cells as well as the electrical routing and 

the interconnections. 

The HILPB receiver has been designed around a highly 

efficient photovoltaic technology developed by scientists at 

NASA GRC for use in photovoltaic concentrator systems, 

known as VMJ cells. Utilizing a vertical configuration of 

p+nn+ unit junctions (see Fig. 1), the VMJ photovoltaic cells 

offer unique advantages over the conventional planar solar 

cells; a typical cell has an area of approximately 0.8 cm2

 and 

can withstand solar intensities that exceed 2500 suns. 

The current HILPB system has been designed and 

constructed to enable in-air refueling capability for perpetual 

flight micro unmanned aerial vehicles (MUAVs) (Ref. 5). For 

this application, great considerations have been made in both 

the form factor and the weight of the developed hardware, to 

accelerate the integration of the prototype test articles and 

electronics into a working airframe. Materials such as 

aluminum nitride substrates, boron nitride epoxies, Kovar 

alloy conductors and silver paste adhesives were selected to 

meet the thermal, electrical and mechanical rigors of the 

receiver (Refs. 6 to 9). These materials were used to construct 

the HILPB receiver on the face of a thermal heat pipe unit, and 

a cross-sectional illustration of the material stack-up is shown 

in Figure 2. 
NASA/TM—2012-217255 3 

The power receiver is coupled with a system composed of 

high energy lasers and optics, custom data collection 

electronics and software, a power management and 

distribution system, and a variable load (Ref. 10). Various 

receivers have been constructed and evaluated with the system 

to investigate the effects of different cell arraying geometries, 

which are described in the next sections. These tests were 

conducted by the Industrial Space Systems Laboratory at 

Northrop Grumman’s Laser Facility at Space Technology 

Park. 

3.0 Photovoltaic Array Cell 

Backfeeding 

The purpose of the initial series of tests was to investigate 

the potential for back-feeding of VMJ cells when they are 

wired in a parallel arrangement. This effect is encountered in a 

conventional solar array under conditions where the 

photovoltaic cells are subjected to non-uniform illumination 

(such as from partial shading or debris on the panel), and this 

is commonly solved by inserting blocking diodes between 

each cell and the array bus to prevent the current flow from 

back-feeding into a weaker cell. The downside of doing this is 

that a voltage drop will occur across the diodes during normal 

operation of the array, resulting in slightly lower array output 

power. 

Since the beam profile of the laser is non-uniform, this 

phenomenon could be encountered when using a parallel array 

of VMJ cells for HILPB. To investigate this, an experiment 

was conducted on a multi-cell receiver, using two of the 

available VMJ cells (referred to as the top and left cell) 

connected in parallel. These particular cells were chosen 

because they represent two different grades of VMJ cells, with 

one offering consistently better performance than the other in 

the NASA GRC flash lamp range. This would establish the 

conditions for a natural imbalance to occur in the array, even 

if both cells received exactly equal illumination. This 

imbalance was exacerbated by obscuring the weaker grade cell 

so as to insure an imbalanced condition. Low-loss diodes were 

selected for the blocking application, and these could be 

introduced into the circuit to observe their effects. Ammeters 

were used to measure the amount and polarity of the 

individual branch current contributions from both cells. 

The tests were conducted using a 200 W continuous wave 

980 nm laser, with the beam illuminating the two cells in a 

manner as to allow for each to receive a similar profile. Both 

of the cells were oriented with the direction of their junctions 

aligned towards the center of the beam, so that they both 

received equivalent amounts and profile of illumination. The 

test was conducted with three different amounts of beam 

overfill to vary the overall profile illuminating the cells, and 

was repeated both with and without the blocking diodes in the 

circuit. The receiver temperature was maintained within 50 to 

60 °C for the duration of the experiment using a variable speed 

cooling fan flowing air across the heat pipe unit. The results of 

these tests are shown in Figures 3 to 5, and the relative 

diameter and placement of the beam overfill is illustrated. 

The negative currents may be observed in the above tables 

when the blocking diodes are not present, and represent the 

back-feeding of the current from the strong cell (top) into the 

weak cell (left). The addition of the blocking diodes does 

correct the backflow, but at the expense of the inherent power 

loss through the diodes, to the extent that the diodes did not 

improve the overall output power of the receiver for any of the 

experimental results, and it was observed in these instances 

that their presence was more detrimental than beneficial. This 

is due to the diode losses exceeding the branch current gains, 

leading to a decrease in bulk output power. 

It is therefore concluded that for HILPB applications, the 

use of blocking diodes as typically employed in photovoltaic 

arrays is not necessary. Furthermore, the detrimental receiver 

output power losses due to the inclusion of the blocking 

diodes exceeds the parasitic losses induced from cell backfeeding. The omission of the diodes yields a receiver design 

with fewer components, and permits a further investigation 

into receiver geometries as presented in the next two sections. 

4.0 9-Cell Square Array 

To explore the hypothesis of an alternate cell configuration 

and quantify the degree of its improvement, two receiver 

geometries are considered, a 9-cell square array and a 9-cell 

radial orientation array. The 9-cell square array simply orients 

all of the p+nn+ junctions for each cell in the same direction, 

which optimizes packing density but generates the maximum 

possible junction-to-junction output variation when 

illuminated with a Gaussian laser beam. The receiver is 
NASA/TM—2012-217255 4 

constructed in a similar manner to the initial HILPB prototype 

(Ref. 10), and is hard-wired to provide three separate stacks of 

three paralleled cells (Fig. 6). For the purposes of these tests 

each stack is electrically connected together to produce a total 

of nine parallel cells. 

The first objective of the experiment was to establish a 

baseline maximum power measurement for the subsequent tests. 

The 9-cell square receiver was illuminated with a 980 nm fibercoupled laser. The receiver position was adjusted in three axes 

to obtain the maximum output power with the laser’s radiant 

power set to 200 W, and the crest of this procedure resulted in a 

30 percent illumination as depicted in the Figure 7. The peak 

output of the receiver in this configuration was measured to be 

23 W, and the temperature at the heat pipe face was regulated 

within 50 to 60 °C during the test. 

The 23 W result represents a receiver with the maximum 

photovoltaic cell packing density, and with no regard for 

accommodating the non-uniform profile of the impinging 

beam. In the next section an alternate receiver geometry will 

be compared with this baseline figure. 

5.0 9-Cell Radial Array 

The 9-cell radial orientation array places all of the junctions 

in eight of the cells on a radial to the center-point of the 

receiver (Fig. 8). When centered with the laser beam, the 

Gaussian profile will extend down the length of each p+nn+ 

junction, rather than appear across the junctions. In this way, 

the junction-to-junction delta is minimized, resulting in an 

equal average illumination per cell. A compromise to this rule 

is made by placing a single cell at the center of the array, for 

the purpose of maximizing packing density. 

In this design each cell has its own dedicated pair of 

electrical leads, and as a design tradeoff to implement the 

radial concept it can be seen that the packing density is much 

lower than 9-cell square array. This receiver also features two 

additional miniature VMJ cells mounted in the vacant 

triangular corners, which were not used for this test. 

For the next part of the experiment, the radial orientation 

receiver was used with all 9 of the 40-junction VMJ cells 

connected in a parallel arrangement. The same conditions 

were used as with the previous test to align the receiver at full 

(200 W) laser radiant power, starting with the inner grouping 

of five cells. The z-axis was adjusted to achieve a range that 

would maximize the overall power converted by the receiver 

array, resulting in a peak overfill condition. Each of the 

individual cells in the array were then disconnected and 

characterized in order to determine their contribution to the 

overall output of the receiver. The resulting output I-V curves 

are shown below in Figures 9 to 13, and the relative diameter 

and placement of the beam overfill is illustrated. 
NASA/TM—2012-217255 5 

It is interesting to note that the center cell, where the beam 

profile contains the most energy, did not have a higher output 

power than the four immediately surrounding cells. This may 

be attributed to the fact that unlike the surrounding cells, the 

center cell does not comply with the radial configuration, and 

even though it is receiving the highest intensity portion of the 

laser beam, it is unable to convert the energy efficiently since 

it is seeing an uneven power distribution across its p+nn+ 

junctions. This is an indication into the magnitude of the 

detrimental effect of uneven power distribution across the 

p+nn+ junctions, and is also a first glimpse into the validity of 

the radial arrangement of the surrounding cells. 

For the next part of the test, data sets were collected from 

both the inner four and five cell arrangements wired in 

parallel. These tests were performed with an optical alignment 

illuminating the five inner cells and at the 200 W laser power 

level. The resulting output I-V curves are shown below in 

Figures 14 and 15, and the relative diameter and placement of 

the beam overfill is illustrated. 

A comparison of the results obtained in the grouped cell 

tests with the single cell tests shows that the four inner cells 

are again contributing the majority of the radial receiver’s total 

output. Although the center cell is illuminated by the most 

intense part of the laser beam, it is converting less energy than 

its four individual neighboring cells. As stated earlier, this 

may be attributed to the center cell not complying with the 

radial configuration. 

The five-cell output power of 23.935 W was calculated to 

have an optical-to-electrical conversion efficiency of 

22.75 percent based on the overfill level of the beam. 

Comparing these results with those of the square array 

demonstrates that five cells in the radial orientation can 
NASA/TM—2012-217255 6 

outperform nine cells in the square orientation, even with the 

packing density losses, providing further validation of the 

radial array concept. 

For the final portion of the test, the overfill level of the 

receiver was incrementally increased to involve more of the 

corner cells in the overall output. As with all of the previous 

parts of this experiment the laser was operated at 200 W, and 

the receiver temperature was regulated within 50 to 60 °C. The 

results may be observed in Figures 16 to 18. 

The smallest beam overfill level (Fig. 16) yielded the 

highest output power generated by a receiver during the 

experiment, at 25.206 W. At this level of overfill, the opticalto-electrical conversion efficiency was calculated to be 

26.2 percent. The highest optical-to-electrical conversion 

efficiency was found to be with the largest beam overfill level, 

at 44.39 percent, but this occurred at the expense of total 

receiver output power dropping to 22.488 W. The receiver 

setup used to achieve these experiments is shown in Figure 19. 

6.0 Conclusions and Future Work 

In summary, the radial orientation of the p+nn+ junctions 

was able to generate over 25 W of continuous output power 

from only 9 small VMJ cells, and under different overfill 

levels the same receiver was able to achieve 44.39 percent 

optical-to-electrical conversion efficiency. This demonstrates 

one potential solution to the Gaussian distribution issue, by 

outperforming the peak performance obtained from the 

conventional 9-cell square receiver array. 

Future work in manufacturing chevron-shaped VMJ cells 

will have further potential improvements on array 

performance, by increasing the packing density in a radial 

array. A different approach may be to investigate the 

possibility of manufacturing curved junctions within the cell, 

to inversely match the profile of the impinging beam. 

Further studies into investigating other laser frequencies or 

more complex optical illumination strategies will also have an 

impact on the overall receiver performance, and will be 

carried out to determine system impact. It is the opinion of the 

research team that the results presented in this paper 

demonstrates that HILPB reliably achieves significant 

performance levels for a wide range of missions or operations, 

and is scalable to achieve larger energy demands. 
NASA/TM—2012-217255 7 

References 

1. B.L. Sater, N.D. Sater, “High voltage silicon VMJ solar 

cells for up to 1000 suns intensities,” Photovoltaic 

Specialists Conference, 2002. Conference Record of the 

Twenty-Ninth IEEE, Publication Date: 19-24 May 2002, 

on page(s): 1019- 1022, ISSN: 1060-8371, ISBN: 0-

7803-7471-1. 

2. C. Goradia, B.L. Sater, “A first order theory of the p/+/-

n-n/+/ edge-illuminated silicon solar cell at very high 

injection levels,” IEEE Transactions on Electron 

Devices. Vol. ED-24, pp. 342–351. Apr. 1977. 

3. D. Raible, “High intensity laser power beaming for 

wireless power transmission,” Master’s Thesis, 

Cleveland State University, May, 2008. 

4. J. Alda, “Laser and Gaussian beam propagation and 

transformation,” University Complutense of Madrid, 

Madrid, Spain, Encyclopedia of Optical Engineering, 2003. 

5. T.H. Nayfeh, D. Raible, D. Dinca, D. Avanesian, “High 

intensity laser power beaming (HILPB) receiver to 

enable in-flight remote refueling of electric miniature 

unmanned aerial vehicles (MUAVs),” Final Program 

Report to the Eglin Air Force Research Laboratory, 

Industrial Space Systems Laboratory, Cleveland State 

University, February 15, 2010. 

6. K. Miwa, A. Fukumoto, “First principles calculation of 

the structural, electronic and vibrational properties of 

gallium nitride and aluminum nitride,” Physical Review 

B, Volume 48 Number 11, Spetember 15, 1993. 

7. P. Bujard, “Thermal conductivity of boron nitride filled 

epoxy resins: temperature dependence and influence of 

sample preparation,” Ciba-Geigy Ltd., Fribourg; 

Thermal Phenomena in the Fabrication and Operation of 

Electronic Components: I-THERM '88, InterSociety 

Conference on; 11-13 May 1988. 

8. G. Liu, B. Cui, R.J. Chen, “Thermal expansion behavior 

of copper/Kovar alloy composite,” Northeast Univ. 

Technol. (China). Vol. 9, no. 4, pp. 498-501. Dec. 1988. 

9. SPI Silver Paste Plus datasheet, 

http://www.2spi.com/catalog/spec_prep/plus_prop.html 

10. T.H. Nayfeh, B. Fast, D. Raible, D. Dinca, N. Tollis, A. 

Jalics, “High intensity laser power beaming architecture 

for space and terrestrial missions,” Proceedings of the 

18th AFRL/NASA Advanced Space Propulsion 

Workshop; November 15–17, 2010; University of 

Colorado at Colorado Springs, Colorado Springs, 

Colorado; (2010).
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13. SUPPLEMENTARY NOTES

14. ABSTRACT

In an effort to further advance a realizable form of wireless power transmission (WPT), high intensity laser power beaming (HILPB) has 

been developed for both space and terrestrial applications. Unique optical-to-electrical receivers are employed with near infrared (IR-A) 

continuous-wave (CW) semiconductor lasers to experimentally investigate the HILPB system. In this paper, parasitic feedback, uneven 

illumination and the implications of receiver array geometries are considered and experimental hardware results for HILPB are presented. 

The TEM00 Gaussian energy profile of the laser beam presents a challenge to the effectiveness of the receiver to perform efficient 

photoelectric conversion, due to the resulting non-uniform illumination of the photovoltaic cell arrays. In this investigation, the geometry of 

the receiver is considered as a technique to tailor the receiver design to accommodate the Gaussian beam profile, and in doing so it is 

demonstrated that such a methodology is successful in generating bulk receiver output power levels reaching 25 W from 7.2 cm2 of 

photovoltaic cells. These results are scalable, and may be realized by implementing receiver arraying and utilizing higher power source 

lasers to achieve a 1.0 m2 receiver capable of generating over 30 kW of electrical power. This type of system would enable long range 

optical “refueling” of electric platforms, such as MUAV’s, airships, robotic exploration missions and provide power to spacecraft platforms 

which may utilize it to drive electric means of propulsion. In addition, a smaller HILPB receiver aperture size could be utilized to establish a 

robust optical communications link within environments containing high levels of background radiance, to achieve high signal to noise 

ratios. 

15. SUBJECT TERMS

Optics; Photonics; Laser power beaming; Wireless power transmission 

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