Daniel E. Raible and Dragos Dinca

Glenn Research Center, Cleveland, Ohio

Taysir H. Nayfeh

Cleveland State University, Cleveland, Ohio

Optical Frequency Optimization of a High Intensity 

Laser Power Beaming System Utilizing VMJ

Photovoltaic Cells

NASA/TM—2012-217256

March 2012

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

Glenn Research Center, Cleveland, Ohio

Taysir H. Nayfeh

Cleveland State University, Cleveland, Ohio

Optical Frequency Optimization of a High Intensity 

Laser Power Beaming System Utilizing VMJ

Photovoltaic Cells

NASA/TM—2012-217256

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 Air Force Base. In addition, 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 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 demonstration. LIMO Inc. for accommodating the ISSL team during the experimental tests and visit of the facilities in 

Dortmund, Germany.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

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—2012-217256 1 

Optical Frequency Optimization of a High Intensity Laser Power 

Beaming System Utilizing VMJ Photovoltaic Cells 

Daniel E. Raible and Dragos Dinca 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Taysir H. Nayfeh 

Cleveland State University 

Cleveland, Ohio 44115 

Abstract

An effective form of wireless power transmission (WPT) 

has been developed to enable extended mission durations, 

increased coverage and added capabilities for both space and 

terrestrial applications that may benefit from optically 

delivered electrical energy. The high intensity laser power 

beaming (HILPB) system enables long range optical 

“refueling” of electric platforms such as micro unmanned 

aerial vehicles (MUAV), airships, robotic exploration 

missions and spacecraft platforms. To further advance the 

HILPB technology, the focus of this investigation is to 

determine the optimal laser wavelength to be used with the 

HILPB receiver, which utilizes vertical multi-junction (VMJ) 

photovoltaic cells. Frequency optimization of the laser system 

is necessary in order to maximize the conversion efficiency at 

continuous high intensities, and thus increase the delivered 

power density of the HILPB system. Initial spectral 

characterizations of the device performed at the NASA Glenn 

Research Center (GRC) indicate the approximate range of 

peak optical-to-electrical conversion efficiencies, but these 

data sets represent transient conditions under lower levels of 

illumination. Extending these results to high levels of steady 

state illumination, with attention given to the compatibility of 

available commercial off-the-shelf semiconductor laser 

sources and atmospheric transmission constraints is the 

primary focus of this paper. Experimental hardware results 

utilizing high power continuous wave (CW) semiconductor 

lasers at four different operational frequencies near the 

indicated band gap of the photovoltaic VMJ cells are 

presented and discussed. In addition, the highest receiver 

power density achieved to date is demonstrated using a single 

photovoltaic VMJ cell, which provided an exceptionally high 

electrical output of 13.6 W/cm2

 at an optical-to-electrical 

conversion efficiency of 24 percent. These results are very 

promising and scalable, as a potential 1.0 m2

 HILPB receiver 

of similar construction would be able to generate 136 kW of 

electrical power under similar conditions. 

1.0 Introduction 

The concept of wireless power transmission has been 

around for almost two centuries, beginning with Callan and 

Marconi, but it was the pioneering work of Nikola Tesla that 

furthered the reality of this concept around the turn of the 

twentieth century (Ref. 1). Following Tesla’s pioneering work, 

microwave technology refined during and after the second 

world war became the primary means of exploring wireless 

power transmission. Although microwave technology is 

capable of delivering massive amounts of energy, its 

efficiency and practical use become limited when utilized over 

longer distances due to diffractive losses. Inductive transfer is 

another form of wireless power transmission, and may be 

found in applications such as charging small hand held 

devices. This form of power transmission, however, is 

generally only effective over small distances (cm or less). 

With the development of high power semiconductor lasers 

along with advances in photovoltaic cell technology, a new 

avenue in the realm of wireless power transmission has been 

opened for research and development. 

HILPB is a relatively new technology which has been 

developed to enable long range wireless power transmission 

for both space and terrestrial applications. The foundation and 

potential practical use of HILPB is based on high wall-plug 

efficiency lasers, specialized photovoltaic cells and the proven 

technological ability to accurately direct optical energy over 

long distances (kilometers or greater) with demonstrated 

systems such as the Airborne Laser (ABL), Tactical High 

Energy Laser (THEL), and various free-space optical 

communication systems. Applications that may utilize 

optically delivered electric energy include robotic exploration, 

airships, MUAVs as well as spacecraft and other resource 

constrained platforms that may utilize electric energy as 

means of propulsion and capability. 
NASA/TM—2012-217256 2 

The HILPB system is made up of two major components: a 

high intensity laser source that is used to transmit the energy, 

and a power receiver to convert the optical energy into usable 

electrical energy. Technological advances driven by the 

manufacturing industry for cutting and welding applications 

and the defense industry for imaging and directed energy 

weapons in the past decade have led to a new generation of 

compact high energy semiconductor lasers that have greatly 

improved electrical to photonic energy conversion 

efficiencies. These advances offer a multitude of options to 

select for the HILPB source. The receiving portion consists of 

the photovoltaic based system that has been developed around 

the VMJ cells. The highly efficient silicon based VMJ cells 

have been developed at the NASA GRC and are capable of 

sustaining and converting solar energy intensities greater than 

2500 suns. 

Based on the recent performance results, HILPB is being 

investigated as the enabling technology to realize a perpetual 

flight MUAV application for the Air Force Research 

Laboratory (AFRL) Munitions Directorate at Eglin Air Force 

Base in Florida. Among other applications, HILPB may also 

become the enabling technology for exploring the far side of 

the Moon, by establishing a wireless electrical grid 

infrastructure for NASA (Ref. 2). To further advance the 

HILPB technology, different investigations are currently being 

conducted in an effort to maximize the overall end-to-end 

system efficiency and increase the delivered power density of 

the HILPB system. This paper investigates the optimal 

operating frequency of the semiconductor source laser to be 

used with the current HILPB power receiver, in order to 

improve the conversion efficiencies at continuous high energy 

illumination. To that end, initial spectral characterization of 

the VMJ cells performed at NASA GRC are presented and 

used as the basis for formulating the theoretical hypothesis 

required to optimize the overall efficiency of the HILPB 

system. The experimental hardware presented in Section 3.0 

and the results in Section 4.0 offer specific valuable insight 

and validate the theoretical hypothesis. 

2.0 The HILPB System 

The HILPB system is constructed around two separate 

subsystems: one to transmit the energy optically, and another 

to receive that energy and convert it into electrical energy. 

While the former half of the HILPB system is accomplished 

through the use of commercial-off-the-shelf (COTS) high 

intensity lasers and optics, the latter half is a critical-path 

component of the HILPB system. The HILPB power receiver 

has been designed and constructed to withstand high energy 

illumination, efficiently convert optical energy into electrical 

energy, dissipate the thermal energy (undesirable byproduct), 

all while maintaining a practical form factor for system 

applicability. 

The HILPB receiver has been developed around the VMJ 

cells, a highly efficient photovoltaic technology that has been 

developed by scientists at NASA GRC for use in photovoltaic 

concentrator systems. Utilizing a vertical configuration of 

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

offer unique advantages over the conventional planer solar 

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

 and 

can withstand solar intensities that exceed 2500 suns. Various 

tests have been conducted at NASA GRC’s Large Area Pulsed 

Solar Simulator (LAPSS) to characterize a single 40-junction 

VMJ cell, with the results of these tests depicted in Figure 2. 

Utilizing flash tests of broadband optical energy, a single VMJ 

cell is capable of producing 40.4 W at 24 V from transient 

flash illuminations of 211 W/cm2 with an optical to electrical 

conversion efficiency of 20 percent. 

The current HILPB receiver has been designed and 

constructed for the in-air refueling capability of perpetual 

flight MUAVs, a terrestrial military application as described in 

Section 1.0. The design requirements for integrating the VMJ 

cells with a high performance thermal management system 
NASA/TM—2012-217256 3 

had to meet stringent size and weight constraints, while 

addressing the following design challenges: sufficient thermal 

dissipation for the excess heat energy, low resistivity electrical 

connections, electrical isolation for the VMJ cells and the 

routing of the wire interconnections (Refs. 3 to 7). To address 

the thermal challenge, the prototype receiver implemented a 

COTS Zalman microprocessor heat pipe unit, which was 

modified to utilize the airflow from the propeller in order to 

maximize the surface cooling effect as illustrated in Figure 3. 

To avoid stress and fracturing during thermal cycles, the VMJ 

cells are mounted on the thermal management system using a 

series of epoxies and substrate materials with similar 

coefficients of thermal expansion. A ferrite-nickel-cobalt alloy 

wire was used to route the electrical paths between the 

photovoltaic cells. A cross-sectional illustration of the receiver 

stack-up is shown Figure 4. 

A typical VMJ cell is constructed using an array of 

miniature silicon vertical junction unit cells, which are 

integrally bonded and series connected, as illustrated in 

Figure 1. With vertical oriented junctions, the thickness of the 

cell is not dependent on the semiconductor material of the 

wafer, but can be adjusted during manufacturing to optimize 

the cell’s overall performance by tuning the penetration depth. 

These cells are also known as edge-illuminated multi-junction 

cells due to their orientation to illumination, since the 

illuminated face of the cell is oriented at the side of the p+nn+ 

junctions. Depending on the end application, the total number 

of junctions can be varied at the manufacturing stage in order 

to meet the desired bus voltage requirements. A 40-junction 

VMJ cell has an area of 0.8 cm2 and a nominal load voltage of 

24 VDC. This structural design allows for high packing 

densities when constructing an array, and makes the VMJ 

technology ideal for use in high power density conversion 

systems. 

The rugged high voltage VMJ cell technology offers a linear 

low series parasitic resistance at high intensities, which 

contributes to the high overall conversion efficiency. The 

edge-illuminated design and construction of the VMJ cell 

offers a major and unique advantage, as there is no need for 

electrical contacts obscuring the illuminated face. With no 

sheet resistivity components, there is more convertible surface 

area for the photonic energy to enter the cell and less potential 

for carrier losses. Furthermore, the spectral response at low 

and high frequencies is improved due to the equal collection 

probability of an excess carrier to be generated at any depth 

from the impinging energy (Refs. 8 and 9). 

Although a variety of semiconductor materials may be used 

to manufacture the VMJ cell, a silicon based cell was selected 

for this particular application due to the high technology 

readiness level (TRL) to construct a system demonstration. 

The selection of the wafer composition used to manufacture 

the VMJ cell affects the complete performance and 

architecture of the HILPB system as it is directly responsible 

for the spectral and thermal response of the power beaming 

receiver. For a silicon based VMJ cell, the thermal derating 

response appears to be linear in nature since its output power 

density decreases by less than 1 percent for every 10 of 

temperature change, as illustrated in Figure 5. 

Maximizing the conversion efficiency and power density of 

the HILPB system is directly depended on matching the 

wavelength of the source laser with the frequency response of 

the VMJ cell under high levels of illumination. The band-gap 

of the VMJ cell can be determined directly from the 

semiconductor material used in the manufacturing process. 

These particular VMJ cells were constructed using silicon 

wafers, and the characteristic frequency response of the cell is 

depicted in Figure 6 as obtained from LAPSS. The silicon 

spectral response of the VMJ cell clearly illustrates that a near 

infra-red (NIR) laser with a wavelength in the vicinity of 

1000 nm would maximize the output performance of the 

power receiver. 

One key aspect that is illustrated by the quantum efficiency 

plot shown in Figure 6 is the specific optimal conversion 

frequency of a VMJ cell. The quantum efficiency and thus the 

conversion efficiency of a VMJ cell increases as the 

wavelength of the incident photons approaches the band gap 

for silicon. The band gap for silicon ranges from 1.125 to 

1.2 eV, depending on its crystalline structure, in this particular 

case the peak response is approximately 1000 nm. Based on 

the quantum efficiency peak range of Figure 6, if incident 

laser energy in the vicinity of 800 to 1000 nm is used with a 

VMJ cell, its theoretical conversion efficiency may approach 

50 to 60 percent. 
NASA/TM—2012-217256 4 

The linear transient and steady state behavior of a typical 

silicon based VMJ cell theoretically should be maintained 

even at high laser power concentrations. The peak conversion 

efficiency at high intensities may be affected since high 

injections levels may cause the band gap to narrow due to the 

change in the carrier generation and recombination process, 

where the Shockley equations describing the current-voltage 

relationship of p-n junctions become less applicable. Hence, in 

order to maintain high conversion efficiency at high intensity 

levels the energy wavelength must be within this nonlinear 

band gap (Refs. 10 and 11). 

It must be noted here that for semiconductor materials, the 

coefficient of absorption decreases significantly beyond the 

cutoff wavelength of the impinging energy, as illustrated by 

the knee curves of Figure 7. At high injection levels the 

narrow band-gap of silicon may be affected as there is a 

possibility that the knee of the silicon curve may shift, further 

affecting the conversion efficiency of the VMJ cells. In 

addition, there are two scenarios that may limit the overall 

conversion efficiency and creating additional thermal stress 

due to the excess heat byproduct. First, the thickness of the 

semiconductor material may lead to saturation by limiting the 

total depth of penetration for the impinging energy, thus 

limiting the efficient photoelectric conversion. Another 

scenario that may limit the overall conversion efficiency is the 

increase in wavelength of the impinging light due to the 

oblique collisions in the capture cross section as the light 

penetrates through the silicon material. Empirical investigation 

of the optimal frequency under high levels of steady-state 

illumination is necessary to validate the transient response 

curves. 

3.0 Experiment Design 

The best range of narrow band illumination for the VMJ cell 

was identified based on the quantum efficiency plot shown in 

Figure 6. The particular peak frequency must be verified 

experimentally to ensure that saturation does not occur at 
NASA/TM—2012-217256 5 

continuous high intensities. The experimental set-up consists 

of a single VMJ cell receiver and three similar continuouswave, fiber-coupled semiconductor lasers with distinct 

operating frequencies. Since the current technology does not 

offer a tunable laser at the wavelength range of interest, three 

standard high intensity lasers were selected from 

Lissotschenko Mikrooptik GmbH (LIMO) Laser Applications 

Laboratory in Dortmund, Germany. The wavelengths of the 

three lasers were selected to be in close range of the VMJ 

narrow band gap: 808 nm (LIMO70-F200-DL808), 940 nm 

(LIMO70-F200-DL940), and 976 nm (LIMO70-F200-

DL976). All three LIMO lasers are complete systems that 

include power, control, water chiller, and a diode module; a 

typical LIMO laser system is shown in Figure 8. 

The experimental rig consists of the following components: 

a single VMJ cell receiver mounted to a 3-axis translational 

positioned system, laser source, adjustable mechanical stage 

for the optical fiber, cooling fan, custom data acquisition 

electronics (DATAQ), and a logging personal computer. A 

pictorial representation of the experimental set-up is illustrated 

in Figure 9. 

In order to characterize the performance of the receiver, an 

automated, high performance data acquisition (DATQ) system 

was designed and constructed to record voltage, current, and 

temperature information simultaneously. The DATAQ system 

is made up of three individual subsystems: an active variable 

load that can be adjusted from open-circuit to short-circuit in 

order to trace the characteristic I-V curve, sensing circuitry 

and data processing. The focus during the design of the 

DATAQ system was to achieve high data integrity and 

reliability. To that end, many design decisions were made to 

improve the accuracy and quality of the measured states, such 

as using open-loop/Hall-Effect current sensors to minimize 

noise interference. For accuracy purposes, the information is 

represented by 16-bit words (19 bits effective). In addition, to 

improve the overall system reliability, the DATAQ has been 

divided into two separate printed circuit boards (PCB) as 

depicted in Figure 10, with special attention given to the board 

layout phase of the design. Data logging and processing is 

achieved by utilizing a Xilinx Spartan-3 development board, 

which includes a field programmable gate array (FPGA) that 

contains a total of 200 k gates. Figure 11 illustrates the 

functional block diagram of the DATAQ system. The data 

transmits via the RS-232 interface to a personal computer at a 

rate of 6 Hz. Lastly, a custom Graphical User Interface (GUI) 

was designed to plot and display the I-V characteristic curve 

of the receiver in real-time. 
NASA/TM—2012-217256 6 

For experimental consistency purposes, all three lasers have 

a 70 W capacity and the fiber is 200 μm in diameter. All 

optical equipment was fastened down to the work surface, and 

only one laser fiber was used for all experiments; this way the 

laser diode module became the only hardware variable during 

each test trial. An initial alignment was performed prior to 

conducting all tests, by using the laser pilot beam to visually 

align the laser beam at the center of the VMJ cell. At low laser 

intensities the receiver was adjusted using the 3-axis 

translational positioner for peak output power generation. For 

all tests, the laser power was increased to cover the full power 

range of each LIMO laser system. The point of maximum 

output power was achieved when 26 W of laser energy 

directly illuminated the VMJ cell, overfilling its surface area. 

4.0 Experimental Results 

The experimental results from the conducted tests are 

illustrated in Tables I, II, and III, and the maximum output 

power attained is highlighted in each corresponding table. 

TABLE I.—940 nm WAVELENGTH RESULTS 

LIMO70-F200-DL940 

Current 

setpoint, 

A 

Rated optical 

power, 

W 

Operating 

optical power, 

W 

Adj. optical 

window 

transmittance,

W 

10 9.9 9.207 8.608545 

20 25.7 23.901 22.347435 

30 40.9 38.037 35.564595 

40 55.1 51.243 47.912205 

50.8 70 65.1 60.8685 

LIMO70-F200-DL940 

Impinging 

optical power, 

W 

VMJ peak 

power, 

W 

Conversion 

efficiency, 

% 

Receiver 

temperature,

°C 

3.6586 1.5198 41.54 24.4 

9.4977 3.5398 37.27 25 

15.1150 5.0675 33.23 27.6 

20.3627 6.1611 30.26 30.1 

25.8691 6.8263 26.39 32 

TABLE II.—976 nm WAVELENGTH RESULTS 

LIMO70-F200-DL976 

Current 

setpoint, 

A 

Rated optical 

power, 

W 

Operating 

optical power, 

W 

Adj. optical 

window 

transmittance,

W 

10 9.8 9.114 8.52159 

20 25.4 23.622 22.08657 

30 40.9 38.037 35.564595 

40 56 52.08 48.6948 

50 70 65.1 60.8685 

LIMO70-F200-DL976 

Impinging 

optical power, 

W 

VMJ peak 

power, 

W 

Conversion 

efficiency, 

% 

Receiver 

temperature,

°C 

3.6217 1.5481 42.74 24.1 

9.3868 3.5402 37.71 24.7 

15.1150 5.1722 34.22 27.5 

20.6953 6.4459 31.15 30.1 

25.8691 7.2430 28.00 32.6 

TABLE III.—808 nm WAVELENGTH RESULTS 

LIMO70-F200-DL808 

Current 

setpoint, 

A 

Rated optical 

power, 

W 

Operating 

optical power, 

W 

Adj. optical 

window 

transmittance,

W 

10 1.7 1.581 1.478235 

20 20.3 18.879 17.651865 

30 38.9 36.177 33.825495 

40 56.4 52.452 49.04262 

49.4 70 65.1 60.8685 

LIMO70-F200-DL808 

Impinging 

optical power, 

W 

VMJ peak 

power, 

W 

Conversion 

efficiency, 

% 

Receiver 

temperature,

°C 

0.6282 0.3131 49.83 22.5 

7.5020 1.8050 24.06 28.7 

14.3758 3.0695 21.35 35.4 

22.8431 3.9837 19.11 41.8 

25.8691 4.6798 18.09 47.2 
NASA/TM—2012-217256 7 

Furthermore, the output power characteristic I-V curves 

corresponding to each LIMO laser system are illustrated in 

Figure 12. Corresponding images of the experiments are also 

shown in Figure 12, and if viewing this document in color 

please note that the images are shown in three different colors 

even though all three laser beams are in the NIR region, which 

is invisible to the human eye. The coloring effect is due to the 

digital camera used during the experiment aliasing the NIR 

frequencies in the experimental hardware rig. 

Figures 13 and 14 illustrate the response at each frequency 

as a function of input intensity, and conversion efficiency. It is 

shown here that the response across varying irradiance is fairly 

linear, while the efficiency curve for the 808 nm test exhibits a 

strong decrease above nominally low optical inputs. 

The experimental results illustrated in Tables I, II, III, and 

Figures 12, 13, and 14 illustrates that the 976 nm wavelength 

laser offers the optimal performance when used with the 

silicon based VMJ cells. Furthermore, the 976 nm wavelength 

laser offers better efficiency and higher electrical output as 

depicted in Figure 15 and 16 comparisons, respectively. 

An interesting phenomenon is illustrated in Figure 15. The 

808 nm wavelength laser is shown to achieve 50 percent 

efficiency at lower laser intensities, followed by a drop in 

efficiency with increasing levels. At first, this is may indicate 

that the VMJ cell reached saturation. However, that may not 

be the case since the sharp drop is only associated with the 

808 nm wavelength. The associated hardware pictures also 

depict a high level of scattering for the 808 nm tests, which 

may indicate that some of the laser energy is reflected back 

and away from the surface of the VMJ cell, and is thus not 

contributing to the electrical output of the cell. 

These data sets may be compared with an earlier experiment 

conducted with a very similar test setup, but utilizing a LIMO 

1064 nm source laser. In that experiment, the laser source was 

similarly operated at a peak optical output power of 70 W 

(with 25.86 W illuminating the receiver), and after fine tuning 

the alignment of the receiver the single illuminated VMJ cell 

was able to deliver 4.08 W of electrical power. This 

performance indicates a 15.8 percent optical-to-electrical 

efficiency, which is below the performance achieved with 

either the 808, 940 or 976 nm sources. This is because the 

1064 nm source is beyond the band gap of the silicon VMJ 

cell, as illustrated in Figure 6. 

Lastly, a separate experiment was conducted while at the 

LIMO laser laboratory in Dortmund, Germany, to determine 

the highest power density achievable with a single VMJ 

photovoltaic cell. Using the selected 976 nm laser system, one 

VMJ cell produced 10.74 W from 44.72 W of impinging laser 

energy, which is equivalent to an output power density of 

13.6 W/cm2

 from 56.6 W/cm2 of total laser energy. Hence, the 

total optical-to-electrical conversion efficiency was 

approximately 24 percent. This represents the highest output 

power density achieved to date in HILPB, with a single VMJ 

silicon-based photovoltaic cell. 
NASA/TM—2012-217256 8 
NASA/TM—2012-217256 9 

5.0 Conclusion 

In conclusion the optimal frequency of the laser source to be 

used with the current HILPB receiver should be in the vicinity 

of 976 nm, which corresponds favorably to commercially 

available laser systems. Appreciative responses may be realized 

at adjacent frequencies including 808, 940, and 1064 nm, but 

they are significantly reduced when compared with the 

performance results obtained near 976 nm. This capability may 

be utilized in a multi-beam/multi-wavelength system, in which 

the VMJ cells may act as sensors for an acquisition beacon laser 

operating at an adjacent wavelength to the main beam. 

Depending on the specific application, the cell design and 

semiconductor material may also be selected to maximize 

different design parameters such as high output voltages (high 

energy band-gaps) or high output currents (low energy bandgaps). These tests also demonstrate the highest output power 

density achieved thus far at 13.6 W/cm2

. 

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4. TITLE AND SUBTITLE

Optical Frequency Optimization of a High Intensity Laser Power Beaming System Utilizing 

VMJ Photovoltaic Cells 

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6. AUTHOR(S)

Raible, Daniel, E; Dinca, Dragos; Nayfeh, Taysir, H. 

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WBS 439432.07.01.17.02 

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National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

An effective form of wireless power transmission (WPT) has been developed to enable extended mission durations, increased coverage and 

added capabilities for both space and terrestrial applications that may benefit from optically delivered electrical energy. The high intensity 

laser power beaming (HILPB) system enables long range optical “refueling” of electric platforms such as micro unmanned aerial vehicles 

(MUAV), airships, robotic exploration missions and spacecraft platforms. To further advance the HILPB technology, the focus of this 

investigation is to determine the optimal laser wavelength to be used with the HILPB receiver, which utilizes vertical multi-junction (VMJ) 

photovoltaic cells. Frequency optimization of the laser system is necessary in order to maximize the conversion efficiency at continuous 

high intensities, and thus increase the delivered power density of the HILPB system. Initial spectral characterizations of the device 

performed at the NASA Glenn Research Center (GRC) indicate the approximate range of peak optical-to-electrical conversion efficiencies, 

but these data sets represent transient conditions under lower levels of illumination. Extending these results to high levels of steady state 

illumination, with attention given to the compatibility of available commercial off-the-shelf semiconductor laser sources and atmospheric 

transmission constraints is the primary focus of this paper. Experimental hardware results utilizing high power continuous wave (CW) 

semiconductor lasers at four different operational frequencies near the indicated band gap of the photovoltaic VMJ cells are presented and 

discussed. In addition, the highest receiver power density achieved to date is demonstrated using a single photovoltaic VMJ cell, which 

provided an exceptionally high electrical output of 13.6 W/cm2 at an optical-to-electrical conversion efficiency of 24 percent. These results 

are very promising and scalable, as a potential 1.0 m2 HILPB receiver of similar construction would be able to generate 136 kW of electrical 

power under similar conditions. 

15. SUBJECT TERMS

Optics; Photonics; Laser power beaming; Wireless power transmission 

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