Solar Pumped Lasers for Free Space Laser Communication

Changming Zhao, Haiyang Zhang, Zhe Guan, Zitao Cai, Dongbing He and Yongheng Wang

Beijing Institute of Technology, Beijing, China

Keywords: Solar Pumped Lasers, Multi-Frequency, Free Space Laser Communication.

Abstract: Solar Pumped Lasers (SPL) is a kind of lasers that can transform solar light into laser directly, with the

advantages of least energy transform procedure, higher energy transform efficiency, higher reliability, and

longer lifetime, which is suitable for use in unattended space system, for solar light is the only form of energy

source in space. In order to exploring the possibility of using SPL for free space laser communication, multi-

frequency SPL is investigated and solar pumped laser amplification is initiated. The first demonstration of

SPL used in free space laser communication is also conducted in our group.

1 INTRODUCTION

Solar pumped laser (SPL) is a special kind of lasers,

that can transform broad-band, incoherent solar light

into narrow-band, coherent laser directly, with the

advantages of least energy transform procedure,

higher energy transform efficiency, higher reliability

and longer lifetime. It is suitable for application in

unattended space system, such as space laser

communication, space to earth wireless power

transmission and space laser propulsion(Mori et

al.,2006; Guan et al.,2017; Oliveira et al.2016; Yabe

et al., 2008).

SPL has a comparable long research history as

lasers itself. Shortly after lasers was invented in 1960,

solar light was considered as the pumping source of

solid state lasers, for all solid state lasers were light-

pumped and solar light is the mostly common light

source we encountered in daily life. SPL was first

demonstrated by Z. J. Kiss from RCA Laboratories in

1963(Kiss et al., 1963), a Dy

: CaF

crystal was

cooled in liquid nitrogen and pumped by solar light.

Shortly thereafter, systems using solar light to pump

different kinds of laser mediums were considered.

Among various laser mediums (solid, liquid and

gaseous), solid state lasers appear to be most

competitive because of stable performance, lower

pumping threshold, and potential efficient of solar-to-

laser power conversion. The first Solar-pumped solid

laser was reported by Young from the American

Optical Company in 1966(Young, 1966), 1W of

continuous wave laser output was obtained at room

temperature via a Nd:YAG crystal. In 1988, Weksler

and Shwartz from Weizmann Institute of Science,

Israel, through a compound parabolic concentrator

(CPC) obtained 60 W CW output power of laser from

a Nd:YAG rod with a slope efficiency about

2%(Weksler and Shwartz, 1998) . In 2007, T. Yabe

from Tokyo Institute of Technology demonstrated an

18.7 W/m

laser output from a Cr

, Nd

co-doped

YAG ceramic with Fresnel lens as the primary solar

light concentrator, corresponding to a total slope

efficiency of 2.9%(Yabe et al., 2007). In 2011, Liang

from Universidade NOVA de Lisboa, Portugal, 19.3

W/m

laser collection efficiency was achieved from a

Φ425mm Nd:YAG rod, which is pumped by a 0.64

Fresnel lens(Liang and Almeida, 2011). T. Yabe’s

group reported a SPL with 120 watts CW output in

2012(Dinh et al., 2012). They used a 4 m

Fresnel lens

as first solar energy collector and a Φ6mm Nd:YAG

rod, the collection efficiency was 30W/m

. For

further thermal management, grooved Nd:YAG laser

rod was firstly used in SPL in 2014(Xu et al., 2014).

Zhao´s group from Beijing Institute of Technology

achieved 27 W laser power by utilizing a Φ695mm

Nd:YAG grooved rod pumped by a 1.03 m

Fresnel

lens, corresponding to a slope efficiency of 9.0%. The

grooved Nd:YAG rod offered better heat dissipation

and reduced the thermal lens effect, compared with

that of unpolished rod, leading to a superior efficiency

and beam quality. In 2017, D. Liang and J. Almeida

used the heliostat-parabolic mirror system to pump a

Nd:YAG rod and obtained a collection efficiency of

31.5 W/m

(Liang et al., 2017). The highest collection

efficiency of 32.1W/m

, for the time being, was

achieved by Beijing Institute of Technology, using a

268

Zhao, C., Zhang, H., Guan, Z., Cai, Z., He, D. and Wang, Y.

Solar Pumped Lasers for Free Space Laser Communication.

DOI: 10.5220/0007569702680275

In Proceedings of the 7th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2019), pages 268-275

ISBN: 978-989-758-364-3

grooved and bonding Nd:YAG/YAG crystal rod and

a Fresnel lens as the primary concentrator(Guan and

Zhao, 2018).

The technology maturity and efficiency of SPL

has arrived at a stage for application in a suitable

situation. And free space laser communication is

probably the best chance for SPL to be used, for solar

light is the only form of energy source in space.

1064nm wavelength laser has become one of a major

wavelength for laser communication. In 2007,

Germany launched TerraSA R-X satellite for

scientific and commercial purposes, where laser

communication terminal were used for inter-satellite

laser communication with a laser band of

1064nm(Liu, 2007.). Switzerland Contraves Space

Centre designed OPTEL high-performance laser

communication terminal series with 1064nm

wavelength, which use 808nm laser diode as the

pump source and Nd:YAG as the gain

medium(Baister et al.). Therefore we consider using

Nd:YAG crystal and its 1064nm wavelength as our

selection.

In order to increase the data rate of laser

communication system, multi-wavelength or multi-

frequency operation of the lasers is demanded,

completed by the technology of wavelength division

multiplexing(WDM). The possibility of multi-

wavelength and multi-frequency operation for solar

pumped both Nd:YAG lasers and amplifier are

investigated in the paper. One to three-frequency

operation of solar pumped Nd:YAG laser is realised

in experiment. A new bonding Nd:YAG slab

amplifier is designed and initial experiment is

performed. Based on the solar pumped Nd:YAG

lasers, the first solar pumped free space laser

communication system is demonstrated.

2 SOLAR PUMPED

MULTI-LONGITUDINAL

MODE LASERS

In free space laser communication systems, the total

data rate is equal to the data rate of one frequency

channel multiply the number of frequency channels.

Data rate is improved either by increase the date rate

of single frequency channel or by increase the number

of frequency channels. To increase the number of

frequency channel have two methods, the first one is

to oscillate simultaneously more wavelengths in a

certain laser medium, and the second one is to

oscillate simultaneously more longitudinal modes

within a single emitting wavelength, while keeping

the frequency separation suitable for frequency

discrimination.

For the mostly used laser medium Nd:YAG, there are

several emitting wavelengths around 1064nm, and

their emitting cross section are comparable with its

of 1064nm, means the possibility of lasing separately

under certain conditions, such as the wavelength of

1052nm, 1062nm, 1065nm, and 1074nm.

For a particular wavelength, such as 1064nm, its

linewidth of florescence (0.45nm) will allow multi-

longitudinal mode oscillating simultaneously, with

each longitudinal mode represents a frequency

channel. Indeed, Nd:YAG laser is usually oscillating

in multi-longitudinal mode without mode selecting

design. But, the multi-longitudinal mode output here

means to get the demanded number of longitudinal

mode output from a specially designed laser.

2.1 Design and Experimental Setup

Nd:YAG crystal is a four-level system, the most

important wavelength of 1064nm at room

temperature is generated by the two-level transition,

3/2



11/2

, and the average fluorescence linewidth

of the transition is 120GHz. The broadening of the

spectrum of solid laser gain medium is mainly due to

uniform widening caused by lattice thermal motion

and no-uniform widening caused by lattice defects.

Nd:YAG crystal is of good quality and isotropic, and

dominated by uniform widening in the whole

temperature range. The uniform broaden line has a

Lorentz line shape, the closer to the gain curve centre

frequency, the higher gain the longitudinal mode

obtained. According to the principle of resonator

longitudinal mode selection, the number of the lasing

longitudinal mode m is determined by the width of

gain curve 𝛥𝜈

𝐷

of the laser medium and the

longitudinal mode spacing 𝛥𝜈

𝑞

of the resonator,

written as:

𝑚 = (𝛥𝜈

𝐷

/𝛥𝜈

𝑞

) + 1

(1)

And

𝛥𝜈

𝑞

= 𝑐/2𝑛𝐿

(2)

Where c is the vacuum speed of light, n is the

refractive index of laser medium at certain

wavelength, and L is the geometry length of the

resonator. From equations (1) and (2), we can

calculate the resonator length for a given number of

m. In fact, for a small number of m, L is small too, and

Solar Pumped Lasers for Free Space Laser Communication

269

the laser rod is in fact a disk. For m=1,2,3, L will be

0.8mm, 1.1mm, and 2.4mm, respectively.

The experimental setup of multi-longitudinal

mode laser is shown in fig.1. It is composed of solar

light collecting system, the laser medium and the

temperature control system. The solar light collecting

system consists of focusing lens, coupling lenses, and

large aperture fibre. The solar light is focused and

coupled into the large aperture fibre firstly, and after

transmitted in the fibre, it is coupled into the laser

medium by the second coupling optics. The purpose

of using fibre as light transmitter is because of its

flexibility. The size of the laser medium is Ф6×0.8～

2.4mm, with its two surface form the resonator. One

surface is coated with AR coating @808nm and HR

coating @1064nm, as the pump light entrance,

another surface is coated with PR coating @1064nm,

as output coupler. The laser medium is temperature

controlled by a TEC cooler.

Figure 1: The experiment setup of solar-pumped multi-

frequency lasers.

2.2 Experimental Results

Before pumped by solar light, the multi-frequency

laser is firstly pumped by an 808nm LD indoors. The

output transverse mode is monitored by a laser beam

analyser, and the longitudinal mode is monitored by

a F-P spectrum analyser. From the measurement

result of the laser beam analyser, TEM

output from

the multi-frequency lasers is ensured.

The longitudinal mode of output laser for different

length of resonation is show in Fig.2, which

corresponding to one longitudinal mode, two

longitudinal modes and three longitudinal modes,

respectively.

For the single longitudinal mode output, the

linewidth is measured based on the fibre delayed

homodyne beat note method, and result is show as

Fig.3. From where, one can see, 16KHz of linewidth

is obtained. For the two longitudinal mode output,

the spacing of the longitudinal mode is 74.4GHz. For

the three longitudinal mode output, the spacing of the

longitudinal mode is 34.3GHz. In the experiments of

two frequencies and three frequencies, the measured

frequency spacing is a little bit smaller than

theoretical prediction raising from thermal expansion

of the laser medium.

Figure 2: The output longitudinal mode from LD pumped

different length of resonator lasers.

Figure 3: The linewidth of single frequency output based on

the measurement method of fibre delayed homodyne beat

note.

The output power of LD and thereafter the solid

state lasers is adjusted by tuning the pumping electric

current of LD. The output power of different

resonator length versus the pumping current is show

in Fig.4.

PHOTOPTICS 2019 - 7th International Conference on Photonics, Optics and Laser Technology

270

Figure 4: The output power of different resonator length

lasers versus pumping current.

After complication of indoors experiment, solar

pumped experiment outdoors is performed. The

picture of experimental facility is show in Fig. 5.

Figure 5: The picture of solar pumped multi-frequency

lasers experimental facility.

The experiment is performed in Beijing under

clear (with light cloud) weather, with measured solar

radiation density of 770W/m

. The output power of

different resonator length lasers versus pumping solar

power is show in Fig.6. Under the complexity outdoor

environment, SHR wavelength meter (SOLAR

LASER SYSTEM Co., ltd) is used to replace the F-P

interferometer as the frequency (wavelength)

measurement instrument, with the measurement

precision of 0.012nm around 1064nm. Fig.6 showed

the three wavelength output and the measured data for

each wavelength. The output wavelength is

1064.754nm, 1064.877nm and 1065.011nm,

respectively, corresponding to frequency of

281414.406GHz, 281449.906GHz and

281482.406GHz. Also, the measured frequency

(wavelength) spacing is a little bit different from

theoretical prediction, raising from thermal expansion

of the laser medium. The output power of each

frequency is different, with higher power in the

central frequency, for which obtaining higher gain

compared with the another two frequencies.

Figure 6: The three wavelength output and measured data

for each wavelength.

3 SOLAR PUMPED ND:YAG

SLAB AMPLIFIER

Another way to use the solar pumped lasers for free

space laser communication is to amplify

simultaneously multi-frequency seed lasers by the

same solar pumped amplifier. A new bonding slab

solar pumped amplifier is designed and initial

experiment is performed.

3.1 Design of the Bonded

YAG/Nd:YAG/YAG Slab

Amplifier

The rod and disk geometry is usually adopted in laser

amplifier. For the purpose of solar pump, the

matching between focused solar light spot and the

laser medium is a new problem. The primary focusing

optics used is a larger aperture Fresnel lens, with the

size of 1.40m1.05m and focus length of 1.20m. The

measured diameter of focusing spot is 11.2mm,

corresponding to the 9.2mrad divergence angel of

solar light. The effect area of the Fresnel lens,

eliminating shade area of support mechanic is

1.03m

.Based on above consideration, a new slab

crystal amplifier is designed, with the size of

18mm12mm5mm. The slab is bonded with

YAG/Nd:YAG/YAG to form the sandwich structure,

show in Fig.7.

The upper layer is YAG crystal, with thickness of

1mm, the middle layer is Nd:YAG crystal, with

thickness of 3mm, and the bottom layer is YAG

crystal, with thickness of 1mm. The entrance and exit

windows for seed laser are coated with AR

coatings@1064nm （ T99.8% ） . The seed laser

travels in zigzag rout inside the gain medium. The

front (except the part of seed laser entrance and exit)

and rear surface of the crystal are coated with HR

Solar Pumped Lasers for Free Space Laser Communication

271

Figure 7: Bonded YAG/Nd:YAG/YAG crystal structure

and its coatings at each surfaces.

coating@1064nm. The upper surface of the crystal is

coated with AR coating@300-900nm (T95%), for

solar light to irradiate. The bottom surface of the

crystal is coated with HR coating@300-900nm

(T95%), for remaining solar light re-absorption. The

bonding crystal is favourable for protecting the

coatings under high temperature and increase the

utilization of pump solar light.

3.2 Initial Experimental Result of the

Solar Pumped Laser Amplifier

The experimental setup of the solar pumped laser

amplifier is show in Fig.8, which is composed of

Fresnel lens, seed laser, bonded slab laser crystal, and

temperature control system.

Figure 8: The experimental setup of the solar pumped laser

amplifier.

A 1064nm single mode seed fibre laser is firstly

aligned through a fibre port and then enter into the

laser crystal with a certain angle. After four times of

reflection inside the crystal, the amplified laser

emitted through the exit.

The beam profile of the seed laser, of the laser

after four times of reflection inside the crystal, and of

the laser after amplification are show in Fig.9(a), (b),

and (c), respectively, showing the beam quality

remain the same after amplification.

Figure 9: Comparison of the beam profile of the seed

laser(a), of the laser after four times of reflection inside the

crystal(b), and of the laser after amplification(c).

The gain of solar pumped laser amplifier versus

pumping solar power is show in Fig.10. The max gain

is 1.25, much lower the theoretical simulation.

Figure 10: The gain of solar pumped laser amplifier versus

pumping solar power.

4 DEMONSTRACTION OF FREE

SPACE LASER

COMMUNICATION USING

SOLAR PUMPED LASER

A free space laser communication system with a solar

pumped laser as the signal transmitter was

demonstrated. A 0.6m × 0.6 m Fresnel lens was used

as the primary concentrator to collect the solar light.

6.8 W continuous wave laser power was obtained

from a 4 mm diameter grooved Nd:YAG rod. The

output intensity was modulated with a video signal

via a LiNbO

Mach–Zehnder optoelectronic

modulator. The video signal with a resolution of

19201080/frame and the frame rate of 25 Hz was

transmitted over five-meter free space in real time

with high fidelity. The transmission rate was 125

Mbps and bit error rate was lower than 10

−6

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272

4.1 Experimental Setup of the Free

Space Laser Communication

System

The free space laser(FSL) communication system

was composed of a SPL with fibre coupler, a Mach–

Zehnder modulator (MZM), a set of optical

transmitting/receiving antennas, an avalanche photo

diode (APD), a demodulator and a monitor, as shown

in figure 11.

Figure 11: Scheme of the free-space laser communication

system. SPL, solar-pumped laser; MZM, Mach–Zehnder

modulator; APD, avalanche photo diode.

A more compact SPL was designed to meet the

requirements for FSL communication. A 0.6m×0.6 m

Fresnel lens which supplied by Shandong Yuying

Optical Instrument Co., LTD was used as the primary

concentrator. The Fresnel lens was made of

Polymethyl Methacrylate (PMMA) material. The

focal length of the Fresnel lens was 0.89 m. A conical

cavity was used to further concentrate the solar light

into the laser rod, a quartz tube filled with cooling

water confines the solar light in the crystal. The

conical cavity also was cooled by cooling water. The

fibre coupling system with a single lens and a

multimode fibre was designed. Schematic of the SPL

is shown in figure 12.

Figure 12: Schematic of the solar pumped laser with Fresnel

lens and conical cavity.

The solar light was converged by a Fresnel lens and

was focused into the laser head. The input window of

the conical cavity was 30mm in diameter and 50mm

in length. The inner wall of the cavity was gold plated.

The outer diameter of quartz tube was 9mm. These

parameters were numerically optimized by

TracePro@ software. The grooved rod was cooled by

deionized water at 4.5lmin

−1

flow rate. The focal

length of the coupling lens was 100 mm and the

diameter of the fibre was 62.5 µm. We use a grooved

laser rod for a better heat dissipation effect. The

1.0at% grooved Nd:YAG rod with 4mm diameter,

70mm length, 0.6mm grooved pitch and 0.1mm

grooved depth was used.

4.2 Experimental Result of the Free

Space Laser Communication

System

The solar irradiance in Beijing during the experiment

was 930 W/m

. The Fresnel lens had an effective solar

energy collection area of 0.35 m

. For 326W solar

power at the surface of Fresnel lens, output couplers

of different reflectivity with same radius of curvature

(RoC) of 900 mm were tested individually to

maximize the output laser power. Figure 13 shows the

results of laser output with respect to various input

solar power levels. The maximum output power was

6.8 W corresponding to a slope efficiency of 3.9%

when R=97% output coupler was used. The optical-

optical efficiency was 2.1%. The maximum output

power from the R= 99%, R=95%, and R=97% output

coupler were 5.2W, 6.0W, and 6.8W, respectively.

Figure 13: Laser output power versus solar power at the

surface of Fresnel lens for three output couplers with

different reflectivity.

The video signal from a media player with the

resolution of 19201080/frame and the frame rate of

25 Hz is transmitted. A LiNbO

MZM modulates

Solar Pumped Lasers for Free Space Laser Communication

273

laser intensity with the encoded video signal. The

bandwidth of the MZM is 10GHz, and the frequency

limit of the video signal carrier is 4 GHz in the

experiments. Optical transmitting antenna collimates

the output light from the fibre and sends into free

space within 40 µrad angle of divergence.

Meanwhile, the optical receiving antenna focuses the

laser signal onto a high speed APD gain controller,

then a data decision circuit, successively. The decoder

restores the digital video signal and transmits to a

monitor.

During the testing of the free space laser

communication system with a distance of 5 meters, a

Lecory Labmaster 10-36Zi oscilloscope was used for

monitoring the communication system. The bit error

rate (BER) was measured lower than 10

−6

. The video

transmission rate was measured higher than 125

Mbps from an eye diagram shown in figure 14.

Figure 14: The 125 Mbps eye diagram of the demodulated

signals in 1.3 ns/div was measured by oscilloscope.

A real-time, high-fidelity transmission of video

signals had been realized in this system. The video

signal was divided into two paths: one was

transmitted by the free space laser communication

system, and the other one directly connected to the

display as a comparison. Figure15 shows a snapshot

of the video signal before and after the transmission.

Figure 15: The original and transmitted video snapshot.

5 CONCLUSION

A research works on the free space laser

communication system using solar pumped Nd:YAG

laser and related research is explored. Based on the

advantages of solar pumped laser, a simple, high

efficiency, and long lifetime free space laser

communication system is feasible in the near future.

Multi-frequency output solar pumped laser is

realised. The initial progress in the amplification of

1064nm seed laser by solar pump is achieved. A free

space laser communication system based on a SPL is

built. A high resolution video signal was transmitted

by the laser beam in a 5 meters free-space. 125 Mbps

bit rate was demonstrated, BER was measured lower

than 10

−6

. The feasibility of using SPL for high rate

communication application in free space was

demonstrated for the first time.

We hope our research can promote the

development of SPLs as well as their applications.

Future efforts will be made to increase the number of

channel for communication, and increase the data

transmission rate.

ACKNOWLEDGMENTS

This work is supported by the National Natural

Science Foundation of China (61775018).

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