Optics & Laser Technology 47 (2013) 194–198

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Optics & Laser Technology

0030-39

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journal homepage: www.elsevier.com/locate/optlastec

Efficiency of continuous-wave solar pumped semiconductor lasers

Stanley Johnson a,n, Franko Kuppers b, Stanley Pau a

a College of Optical Sciences, University of Arizona, 1630 East University Boulevard, Tucson, AZ 85721, USAb Institute for Microwave Engineering and Photonics, TU Darmstadt, Merckstr. 25, 64283 Darmstadt, Germany

a r t i c l e i n f o

Article history:

Received 23 May 2012

Accepted 29 August 2012Available online 12 October 2012

Keywords:

Semiconductor laser

Solar pumped laser

92/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.optlastec.2012.08.037

esponding author. Tel.: þ1 520 247 5706.

ail address: stanj@optics.arizona.edu (S. Johns

a b s t r a c t

We report the results of an efficient solar pumped semiconductor laser system that uses high efficiency

multi-junction photovoltaic cells and laser diodes in order to achieve the sunlight to laser light

conversion efficiency of over 10% without any active cooling and concentration optics. Semiconductor

lasers with wavelength from 445 nm to 1550 nm are powered directly by an array of photovoltaic (PV)

cells under one sun illumination (100 mW/cm2). The maximum energy efficiency reaches 10.34% at

976 nm with an output power of 4.31 W. This system is inherently more efficient than direct solar

pumped lasers that have been studied in the past and could play a key role in future renewable energy

production and power beaming applications.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Efficient conversion of sunlight to laser light has been pursuedfor applications in materials processing [1], fuel generation [2],power beaming [3], propulsion [4], and free-space telecommuni-cation [5]. Solar energy is an excellent renewable energy sourceand efficient conversion of solar energy to fuel using solar energy-pumped lasers can provide an important renewable energysource [1]. Existing direct solar pumped solid state lasers [6,7]with energy efficiency between 1% [8–10] and 4% [11] and opticalamplifier with an optical–optical conversion efficiency of 33%using an arc-metal-halide lamp pump [12] require bulky concen-trating optics to achieve population inversion. The idea of directsolar pumped semiconductor lasers has been explored theoreti-cally [13,14]. However, little experimental work has been done inthis area because the lasing threshold of the proposed lasersrequires high solar concentration of the order of 250 (25 W/cm2)and a sophisticated cooling system.

Here we demonstrate an efficient, compact system for sunlightto laser light conversion without the cost and complexity of asolar concentrator and consequent heat management. We reportthe results for semiconductor lasers with wavelength from445 nm to 1550 nm powered directly by an array of photovoltaic(PV) cells under one sun illumination (100 mW/cm2). The max-imum energy efficiency reaches 10.34% (12.8% transient peak) at976 nm with an output power of 4.31 W (4.51 W) without theuse of an impedance matching system. Semiconductor lasers havethe advantages of small device form factor and high efficiency[15,16]. Different wavelengths of laser light can be generated by

ll rights reserved.

on).

selection of suitable fixed or tunable [17] wavelength laser diodesand the system can be scaled to higher powers using beamcombining optics [18], without the thermal lensing effects thatlimit other high power solar pumped lasers [19]. Using separatedevices for light collection and generation allows for maximumflexibility as new higher efficiency devices can be used whenavailable and one can circumvent the trade-off between opticalconfinement for lasing action and the need for larger devicestructures for solar absorption [14] that is inherent in a directsolar pumped laser. Direct solar pumped semiconductor lasers areestimated to have an efficiency of 35% [13]. The theoreticalefficiency of diode lasers can approach 100% [20]. The theoreticalmaximum for a PV cell is 85.0% for AM1.5 direct normalirradiance [21]. Practically, semiconductor lasers have reached73% efficiency [16] and triple junction solar cells have reached anefficiency of 35.871.5% (AM1.5) [22], hence it should be possibleto achieve a conversion efficiency of over 25% with currenttechnology.

2. Experiment setup

GaInP2/GaAs/Ge triple junction solar cells with a rated effi-ciency of 24% at AM0 solar irradiance (135.3 mW/cm2) are used inthe experiment. The PV cells are connected to form an arrayclosely matched to the current and voltage requirements of agiven semiconductor laser (see Table 1 for various configura-tions). The PV array is cooled passively by ambient air currents.The array is mounted on a single axis sun tracker and is setup for normal incidence of sunlight. Operation of the system atone sun concentration relaxes the requirement for precise suntracking. A pyranometer sensor (SP-110, Apogee Instruments)

Table 1Laser diode parameters.

Laser wavelength (nm) Material, device package ITHRESHOLD (A) VNOMINAL (V) PMAX (W) PV array configuration,nominal array voltage

445 InGaN/GaN, TO-56 0.18 4.2 1 11 cells in series-parallel, 4 V

635 AlGaInP/GaAs, F-mount 0.77 2.1 0.5 17 cells in parallel, 2 V

808 AlGaAs/GaAs, F-mount 0.81 1.7 5 17 cells in parallel, 2 V

915 InGaAs/GaAs, F-mount 0.45 1.6 5 17 cells in parallel, 2 V

976 InGaAs/GaAs, F-mount 0.42 1.5 5 17 cells in parallel, 2 V

1550 InGaAsP/InP, F-mount 0.50 1.2 1 14 cells in parallel, 2 V

Fig. 1. Experimental setup. (a) The schematic of the solar pumped laser,

PS – pyranometer sensor, DL – data logger, IS – integrating sphere, PM – power meter.

(b) Photograph of the setup. (c) Power spectra of the sun [23] and the laser diodes used

in the experiment.

S. Johnson et al. / Optics & Laser Technology 47 (2013) 194–198 195

mounted on the PV array measures incident solar irradiance(Fig. 1).

Six laser diodes from 445 nm to 1550 nm were tested inde-pendently (see Table 1). The range of wavelength has applicationsin display, telecommunication and solar fuel conversion. Eachlaser diode is mounted on a heat sink with a built in heat pipe andconnected to the PV array using a total of six American WireGauge (AWG) 8 wire segments (three segments in parallel foreach path) in order to reduce circuit resistance. Thermal paste isused to transfer heat efficiently from the laser diode to the heatsink. The laser diode-heat sink assembly dissipates heat bynatural convection alone and no forced air or active coolingscheme is used. A ThorLabs S302C thermal power head is usedto measure the laser output power. The high power lasers�808 nm, 915 nm and 976 nm, with an output power of up to5 W, are first coupled to an integrating sphere followed by thesensor head. The laser diode, heat sink, integrating sphere,thermal power head and the data logging electronics are housedin a light-sealed box. The experiment was conducted on the roofof the Meinel building at the University of Arizona in Tucson,Arizona (321N latitude) and data were logged from 9 am to 3 pmon days with good sunshine. The light-sealed box was coveredwith a cardboard box for shade.

In addition to using high efficiency laser diodes and PV cells, inorder to achieve a high sunlight to laser conversion efficiency, the

laser diodes should operate at or close to the maximum powerpoint of the PV array [24]. As different semiconductor materialsare used to fabricate the laser diodes in order to meet thebandgap requirement for a particular wavelength, each laserdiode operates at a slightly different point on the current–voltage (I–V) curve of the PV array. The operating point for eachlaser system is the point of intersection of the laser diode andPV array I–V curves as shown in Fig. 2(c) and (e). The experi-mental maximum power point of the PV array is denoted by adiamond in Fig. 2. In Fig. 2(c), we see that the 808 nm (red curve),915 nm (green curve) and 976 nm (yellow curve) lasers haveoperating points close to the maximum power point of the PVarray, making them excellent candidates for solar laser systemswith high efficiency.

3. Experiment results and discussion

The solar pumped semiconductor laser can be modeled by asimplified equivalent circuit as shown in Fig. 3(a). The voltagesacross the junctions are related by Kirchhoff’s law:

1

e

XN

i ¼ 1

ðEifc�Ei

fvÞ ¼1

eðELD

fc �ELDfv Þþ IUðRLD

s þRPVs Þ, ð1Þ

where e is the charge of the electron, Eifc and Ei

fv are the quasi-Fermi levels of the n and p regions of a single junction in anN junction PV cell, ELD

fc and ELDfv are the quasi-Fermi levels of the

n and p regions of the laser diode, RLDs and RPV

s are the equivalentseries resistances of the laser diode and PV array respectively andI is the circuit current. In the ideal coupling condition between thelaser diode and the PV array, the source impedance matches theload impedance, i.e. RPV

s ¼ RLDs , resulting in maximum power

transfer from source (PV array) to load (semiconductor laser).This is equivalent to requiring that the operating point of the laserdiode match the maximum power point of the PV array. Thus,high efficiency solar laser systems for a particular wavelengthcan be built by careful selection of devices that meet the impe-dance matching condition, eliminating the need for impedancematching electronics [25].

Operation of the solar pumped semiconductor laser requirespopulation inversion. The semiconductor laser threshold is givenby the Bernard–Duraffourg condition [26]:

ELDfc �ELD

fv 4_olaser, ð2Þ

where olaser is the laser frequency. Eqs. (1) and (2) together givethe condition for operation of the semiconductor solar laser:

XN

i ¼ 1

ðEifc�Ei

fvÞ4ELDfc �ELD

fv 4_olaser: ð3Þ

Thus, a solar laser for a particular wavelength can be designedby connecting PV cells in series to satisfy the threshold conditionof Eq. (3). Connecting cells in series is equivalent to a multiplephoton pumping process (Fig. 3(b)) that enables the energy of

Fig. 2. Photovoltaic cell and laser diode characteristics. (a) I–V curves for a single PV cell under varying illumination. (b) Efficiency for the single PV cell under varying

illumination. (c) I–V curves for laser diodes and the 2 V PV cell array, the operating points of the 635 nm and 1550 nm laser diodes do not match the maximum power point

of the PV array (impedance mismatch). (d) Efficiency (dashed curves) and output power (solid curves) of the 2 V laser diodes. (e) I–V curves for 445 nm laser diode and the

4 V PV cell array. (f) Efficiency (dashed curve) and output power (solid curve) of the 445 nm laser diode. (For interpretation of the references to color in this figure caption,

the reader is referred to the web version of this article.)

Fig. 3. (a) Equivalent circuit of the solar pumped semiconductor laser. (b) Lower

energy photons can pump a higher energy transition by series connection of PV

cells. G denotes the ground state and E denotes the excited state of a two-level

system. (c) Solar spectral utilization (blue) for a 445 nm direct solar pumped laser.

(d) Solar spectral utilization for a triple junction solar cell with bandgaps of

1.85 eV (blue), 1.42 eV (green) and 0.67 eV (brown). (For interpretation of the

references to color in this figure caption, the reader is referred to the web version

of this article.)

S. Johnson et al. / Optics & Laser Technology 47 (2013) 194–198196

multiple lower energy photons to be summed up for pumping ahigher energy transition, giving higher conversion efficiency ascompared to a direct solar pumped laser. This is especiallyimportant for solar lasers with wavelengths shorter than the solarspectrum peak of about 500 nm with most of the spectrumlacking the high energy photons needed for the pumping process.For example, a 445 nm direct solar pumped laser has a solarspectral utilization as shown by the blue shaded area inFig. 3(c) and would only be able to utilize about 14% of solarenergy (see Appendix). Triple junction PV cells have a higher solarspectral utilization as shown by the sum of the blue, green andbrown shaded areas in Fig. 3(d), and the solar energy utilization isabout 67% (see Appendix). The intermediate energy levelsrequired for multiple photon pumping are not easily available insolid state materials for direct solar pumped lasers but itsequivalent can be realized in the electrical domain by seriesconnection of PV p–n junctions.

Fig. 4 shows the results for the 976 nm laser system. The solarirradiance, laser output power and PV array temperature aremonitored during the experiment, and the system efficiency,defined as the ratio of the output power of the semiconductorlaser to the solar power incident on the PV cells forming the array,is calculated. The 976 nm laser system gives a steady stateefficiency of 10%, which does not change much during the daywhen the PV array temperature changes from 55 1C to 65 1C. Theother semiconductor lasers were also tested for a day each, theresults of which are displayed in Fig. 5. The 976 nm laser systemhas the highest efficiency followed closely by the 915 nm lasersystem. In addition to their I–V characteristics that enable them tooperate close to the maximum power point of the PV array(Fig. 2(c)), maximizing power transfer, the two laser diodes havehigh efficiencies and contribute to the high system efficiencies.Although the 808 nm laser is closely matched to the PV array,

Fig. 4. The 7 day data for the 976 nm solar pumped semiconductor laser. Occasional dips in the solar irradiance are due to intermittent cloud cover.

Fig. 5. The 1 day data for different solar pumped semiconductor lasers.

S. Johnson et al. / Optics & Laser Technology 47 (2013) 194–198 197

the laser diode itself has a lower efficiency (Fig. 2(d)), resultingin lower system efficiency. The 445 nm laser system has anefficiency of about 3%.

As the PV array is cooled passively, the ambient temperaturecan greatly influence the PV array temperature. As the bandgap ofthe p–n junctions of the PV cell decreases with increasingtemperature, it results in lower PV array voltage and systemefficiency as PV array temperature rises. This effect is most visible

when a laser diode operates at close to the open circuit voltage ofthe PV array, as is the case with the 635 nm laser. The other laserstested do not have such a pronounced dependence of efficiencyon array temperature as they operate away from the open circuitvoltage of the PV array.

The temperature of the active region of the laser diode iscritically important to the output power of the device [27]. In thisexperiment, as no attempt was made to maintain the laser diode

S. Johnson et al. / Optics & Laser Technology 47 (2013) 194–198198

at a fixed temperature due to the use of passive cooling, thedevice temperature rises with increasing drive current, resultingin lowering of the efficiency of the laser diodes (Fig. 2(d) and (f)).Hence, for a passively cooled system, it is important to operatethe laser diode at a drive current that maximizes efficiency. Thiscould be in conflict with the need for higher laser output power orthe impedance matching condition for a given PV array config-uration. This trade-off can be clearly seen in the data for the1550 nm laser diode (Fig. 2(c) and (d)), with the maximumefficiency point occurring at a diode current of 2 A while themaximum output power is achieved for a diode current of 4 A.The use of high capacity heat sinks for efficient heat removal fromthe laser diode will enable the maximum efficiency point of thelaser diode to be reached at close to the maximum output power.

4. Conclusion

We have implemented solar pumped semiconductor lasersthat can operate at multiple wavelengths with steady stateenergy efficiency from 2% to 10% at one sun concentration.Passive cooling was used for both the PV array and the laserdiode. For systems with matched I–V characteristics of thePV array and the laser diode, we demonstrate good power transferand high conversion efficiency. The high efficiency of this systemas compared to a direct solar pumped laser is due to the inherentability of multi-junction PV cells to utilize the solar spectrummore efficiently and due to recent advances in high efficiencysemiconductor lasers. The lack of concentrator optics merits thissystem to be considered as a simple and efficient alternative topresent direct solar pumped laser schemes. A system combiningPV cells and laser diodes as demonstrated here could play animportant role in solar fuel conversion, free-space telecommuni-cation and power beaming applications.

Acknowledgments

The authors wish to thank Dr. R. Kostuk and D. Zhang for helpwith solar cell characterization, G. Myhre, A. Lennartz andK. Erwin for help with the experimental setup, N. Anderson,J. Czapla-Myers and S. Biggar for equipment loan and W. Hsufor comments on the manuscript. This work is partially funded bythe Arizona Technology Research Infrastructure Fund (TRIF).

Appendix

With the sun modeled as a blackbody at 5780 K, a 445 nmdirect solar pumped laser would only be able to utilize about 14%of solar energy, as shown by the integral:Z 445 nm

0

l445 nm

USðlÞsT4

dl¼ 0:14 with SðlÞ ¼2pc2h

l5U

1

ehc=lkT�1:

The use of triple junction PV cells increases the energyutilization to about 67%, as shown by the sum of integrals:

Z 669 nm

0

l669 nm

USðlÞsT4

dlþZ 872 nm

669 nm

l872 nm

USðlÞsT4

dl

þ

Z 1840 nm

872 nm

l1840 nm

USðlÞsT4

dl¼ 0:67:

Here, c is the speed of light, h is the Planck’s constant, s is theStefan–Boltzmann constant, k is the Boltzmann constant, l is thewavelength and T is the temperature of the blackbody. The triplebandgap PV cell is modeled with bandgaps of 1.85 eV, 1.42 eV and0.67 eV.

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