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SUBMITTED TO:

FINAL

/,u/j _ Z a_-.;-"

REPORT -" /_-" .-.d

National Aeronautics and

Space Administration

Langley Research Center

Hampton, Virginia 23665

INSTITUTION: Department of Physics

Hampton University

Hampton, Virginia 23668

TITLE OF RESEARCH: Direct Solar-Pumped

Amplifier

Iodine Laser

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NASA GRANT NUMBER:

PERIOD COVERED BY THIS REPORT:

NAG-1-441

March 1, 1984-

1990

February 28,

PRINCIPAL INVESTIGATOR: Kwang S. Han

CO-PRINCIPAL INVESTIGATOR: In Heon Hwang

RESEARCH ASSOCIATE: Larry V. Stock

(NA_A-CR-I86_42) uIRECT SOLAR-PUMPED IODINE

LASER A_PLIFIER Final Report, I Mar. 1984 -

2B Feb. Iq90 (Hampton Inst-) 17 p CSCL 20E

N90-220-_0

Unc1_

_3/36 0Z7ZZ97
Abstract

This report gives the final summary of the research work performed

under the NASA grant NAG1-441 entitled "Direct Solar-Pumped IodineLaser Amplifier" which ha.s been performed from March 1, t984 to

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February 28,1990_The optimum conditions of a solar pumped iodine laser

are found in this research for the case of a continuous wave operation and

a pulsed operation. The optimum product of the pressure(p) inside the

laser tube and the tube diameter(d) was pd=40~50 torr-cm on the contrary

to the case of a high intensity flashlamp pumped iodine laser where the

optimum value of the product is known to be pd=150 torr-cm. The pressurediameter product is less than 1/3 of that of the high power iodine laser.

During the research period, various laser materials were also

studied for solar pumping. Among the laser materials, Nd:YAG is found to

have the lowest laser threshold pumping intensity of about 200 solar

constant. The Rhodamine 6G was also tested as the solar pumped laser

material. The threshold pumping power was measured to be about 20,000

solar constant.

The amplification experiment for a continuously pumped iodine laser

amplifier was performed using Vortek solar simulator and the

amplification factors were measured for single pass amplification and

triple pass amplification of the 15 cm long amplifier tube. The

amplification of 5 was obtained for the triple pass amplification.
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I. Introduction

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A solar pumped laser may serve as a prime power source for the

future space investigation. Although there could be various alternatives

for power generation in space, the lasers can easily be employed in the

power distribution to various distant space systems such as the orbital

transfer vehicles, lunar transfer vehicles and various satellites, due to

the highly directional characteristics of the beam. The laser beam can be

concentrated in a small area with high power density due to its shorter

wavelength compared with that of the micro-wave and its low divergence

angle from generation, which is very advantageous property for certain

application such as the laser propulsion of space vehicles.

The research on the development of "Direct Solar Pumped Iodine

Laser Amplifier" was initiated from March 1, 1984, and continued for 6

years at NASA Langley Research Center. During the research period, most

of our efforts were concentrated on the development of the basic concept

of the Solar Pumped Iodine Laser MOPA (master-oscillator poweramplifier). On the other hand, various laser materials were also tested its

possibility as the solar pumped laser candidate.

The MOPA concept is employed in the high power fusion laser

development using energy storable laser medium such as Nd÷3:YAG(or

Glass), CO2 and atomic iodine that have long excited level lifetime. The

iodine laser MOPA system is considered as a fusion driver and is

extensively studied in Max Planck Institute at Garching, West Germany[I].

However, the solar pumped iodine laser should somewhat be different in

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operational conditions from the short pulse flashlamp pumped high power

laser used in laser fusion research.

The MOPA system is very advantageous to obtain high peak power in

space, however, the material should have sufficient long upper state

lifetime to store high energy from the weak solar pumping. Among the

various laser materials, the atomic iodine has long upper state lifetime of

130 msec. The population inversion in the iodine laser is mostly obtained

by photodissociation with UV radiation of the perfluoro-alkyliodide such

as i-C3FTI or n-C3F71 which absorbs radiation in the range from 230 nm to

320 nm. When the iodides are dissociated with UV radiation, it produces

almost 100% of the excited atomic iodine. Subsequently, the excited

atomic iodine transits to the ground state and generates the laser

radiation at the wavelength 1.315 I_m.

In this research, we could identify the solar pumped iodine laser

amplifier characteristics under the solar simulating radiation pumping.

We found that the operational condition of the solar pumped iodine laser is

quite different from that of the high power flashlamp pumped short pulse

iodine laser.

In addition to the experimental works, the theoretical modeling of

the iodine laser was performed to understand the physics involved in the

laser performance and to provide the optimum experimental conditions.

However, due to the many unknown physical and chemical processes in the

iodine laser action, the theoretical calculation does agree with the

experimental results in a very limited experimental situation.

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II. Iodine Laser Amplifier Experiment

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As a first step of the experiment, we constructed a long flashlamp

pumped iodine laser amplifier. To simulate the solar radiation in space, a

parabolic cylindrical reflector was employed to make the light from the

flashlamp parallel. Another same parabolic cylindrical collector was used

for the collection of the light into the laser amplifier tube. The flashlamp

was 115 cm long and the inner diameter was 5 mm. The flashlamp was

driven with a 12.5 I_F capacitor at various charging voltage and the pulse

width was about 100 i_sec at 30% of the peak.

The amplifier tube was made of a UV transmission tailored quartz

tube (Germisil) of length 117 cm. The inner diameter of the amplifier tube

was 11.5 mm. To compare the pumping intensity by the flashlamp with the

AM0 (air mass zero) solar radiation, the spectral irradiance was measured

from inside of the tube. The radiation energy in the absorption band of iC3F71 concentrated into the amplifier was measured to be 0.09% of the

electrical energy applied to the flashlamp. From this measurements, the

pumping intensity was estimated to be equivalent to 3.0x104 solar

constants and 2.4x104 solar constants when the flashlamp was driven

with electrical energy 1.41 kJ and 0.97 kJ respectively.

To investigate the amplification characteristics of the amplifier,

we set up a small iodine laser oscillator. The oscillator was pumped with

a 10 cm long flashlamp and was enforced to lase in TEMoo mode by

inserting 2 mm diameter aperture in the laser cavity. The laser output

from the oscillator was about 6 mJ with pulse duration of 4 I_sec. The

amplification was measured at different !njection times of the laser

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pulse from the oscillator to the amplifier. The highest amplification by

this experimental arrangement was about 3.2 ( output laser energy/input

laser energy) at the injection time of about 100 I_sec from the beginning

of the amplifier pumping. This results shows that the energy storage of

the amplifier is the highest at the end of the amplifier pumping and proves

the long excited state lifetime.

We also studied the temperature effect of the laser gas on the

amplification by varying the temperature of the amplifier tube. Keeping

the iodide molecule density the same in the amplifier tube at each

temperature, the amplification was measured at different injection time

of the oscillator output into the amplifier. The experimental results

shows that the amplification reaches its maximum at the injection time

of around 100 i_sec for the temperature range from room temperature (23

oC) to 141 oc. However. the amplification after 100 i_sec reduces

drastically for hotter gas. This indicates that the higher the temperature

of the gas, the shorter the energy storage time in the amplifier tube after

pumping. The detail of the experimental set up and results are described in

Appendices 1 and 2.

We also tested other perfluoro-alkyliodides as the amplifying

medium of the solar pumped iodine laser amplifier such as t-C4Fgl and nC4F91 with the same experimental set up. The n-C4Fgl has very similar UV

absorption characteristics with the n-C3F71 due to the similar chemical

structure. However, the t-C4Fgl has the widest absorption band in the UV

range of spectrum among other iodine laser materials and also the

absorption peak is at 290 nm which is much longer wavelength compared

with the absorption peak wavelength of other iodides[2]. The fact that the

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t-C4Fgl has the absorption peak at the longer wavelength indicates the

high rate of the solar spectrum utilization in the pumping and high

efficiency of the laser performance due to the low spectral irradiance in

the solar spectrum below 250 nm. The amplification factor obtained with

the t-C4Fgl is about twice of that obtained with i-C3F71. Moreover, the

energy storage capability after the amplifier pumping is superior than

other iodides (see Appendix 3). This measurement is quite contradictory

with the high measured value of the quenching rate of the excited state

atomic iodine by the parent molecule[3]. This requires more careful and

precise measurement of the quenching rate of the excited iodine by the

parent molecule.

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III. Continuous Wave Iodine Laser

The best material for the solar radiation pumped laser is the one

which can be pumped continuously and can produce CW laser radiation. A

couple of research group did experiment on the CW solar pumped laser

using some solid state laser materials such as Nd÷:YAG[4],[5]. However, the

solid state laser has definitely some drawbacks for the development of

scaled-up solar pumped laser system. The most serious one is the heat

management produced in the course of pumping. The produced heat is very

difficult to remove due to the very low heat conductivity of the solid

material. Moreover, the heat removal from the laser material should rely

on the radiation when the laser is operated in space, which requires

unacceptably large radiator.

Another difficulty of the solid state laser material is the available

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size. Due to the limited durable power density of the solid material, a

large size of laser material is necessary to obtain high power laser

output. Not only the weight of the laser material is a disadvantage but

also the crystal growing is another problem.

In contrast to the solid laser material, the heat management in gas

or liquid laser material is comparatively easy. The heat produced in the

laser could be removed by circulating the medium. Moreover, the

refractive index change in gas is usually negligibly small compared with

the solid material, which causes the thermal lensing in solid laser

material.

In this research period, we tested the CW laser possibility of the

perfluoro-alkyl iodide by pumping with a solar simulator. The solar

simulator used in this research was an argon arc lamp which could

generate optical output upto 45 kW. As a first attempt we installed a 12

mm I.D. laser tube in an elliptic cylindrical pumping cavity with the solar

simulator. The iodide used was n-C3FTI in this experiment and the iodide

gas was flowed through the laser tube with a evaporator-condenser unit.

With this system, we could obtained 2 W CW laser output for over 2 hours

(see Appendix 4). This experimental result is the first CW solar simulator

pumped iodine laser.

In the subsequent experiment, we used a 16 mm I.D. laser tube and

obtained 10 W CW laser output with pumping power of about 1300 solar

constant in the absorption band of the n-C3F71. This output power is the

highest ever obtained in the solar pumped gas laser experiment. The

detailed experimental configurations and conditions are described in

Appendix 5.

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IV. Other Laser Materials

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In this period of research, Nd:YAG, Cr:Nd:GSGG and Nd:YLF were tested

as the solar pumped solid laser materials by using the Tamarack solarsimulator. Among the three laser materials, Nd:YLF shows the lowest laser

threshold pump power of about 210 solar constants with a 1/8" diameter

by 3" long laser rod[6]. Nd:YAG shows slightly higher laser threshold pump

intensity of 236 solar constants. Cr:Nd:GSGG shows the highest laser

threshold pump power of 1,500 solar constants and even the solar

utilization efficiency is the highest among the three laser materials.

Moreover, it has been shown that Cr:Nd:YAG crystal cannot be pumped with

higher than 2,000 solar constants due to the serious thermal lens effect.

This crystal shows also very weak resistance to the thermal shock. This

crystal was very hard to pump with higher than 2,000 solar constant due

to the crystal breakage originated from the thermal stress.

The heat conductivity of Nd:YAG is the highest among the three

crystals and thus the thermal lens effect is the lowest, which indicates

the possibility of high solar constant pumping. However, this crystal has

the lowest solar utilization efficiency among the three crystals due to the

narrow absorption band and the spectral mismatch between the absorption

band and the solar irradiance. The three solid laser materials shows the

possibility of solar pumped laser. However, due to the high thresold pump

intensity, heat removal from the laser rod looks like the major issue when

they are pumped with highly concentrated solar radiation.

Another laser material tested in this period of research was

rhodamine 6G solution in methanol. The absorption band of this solution

matches very well with the peak of solar irradiance so that the solar

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utilization efficiency reaches to 15%. However, the lifetime of excited

state is so short that the laser threshold pump power exceeds 20,000

solar constant, which is obtainable with a very elaborate solar collector.

the experimental details and results are shown in Appendices 6 and 7.

V. Solar-Simulator Pumped MOPA

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Although many space applications are best served by CW lasers, a

continuosly pulsed laser also has good applicability in space, especially

for laser propulsion. Among various schemes of continuously pulsed laser,

a solar-pumped MOPA(master-oscillator power-amplifier) is a suitable

system for generation of high peak power. However, due to weak solar

irradiance on earth, a laser material with very long excited state lifetime

is necessary to store large amount of energy in amplifier. The excited

state lifetime of the atomic iodine is known to be the longest among

various laser materials(~130 msec).

In this research period, we attempted to test the solar-simulator

pumped laser as a continuously pumped amplifier. To perform this

experiment, we had to develop an iodine laser oscillator which could

generate highly repetitive pulses. Most of flashlamp pumped iodine laser

oscillators are operated below 5 Hz. To increase the repetition rate, a

XeCI laser pumped iodine laser was developed in this research period. The

laser material used in the oscillator was t-C4F91. As mentioned in the

previous section, this laser material is much superior in XeCI laser

pumping than n-C3F71 or i-C3F71 because this chemical has very high

photochemical stability. This chemical allowed us to operate the

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oscillator almost sealed-off manner. This oscillator could be operated

upto 5 Hz which is limited by the power supply for the XeCI laser.

Moreover, this chemical showed much higher laser efficiency than n-C3F71

or i-C3F71. The detailed experimental set up and results are shown in

Appendices 8 and 9.

With this oscillator, we performed an amplification measurement

with the continuously solar-simulator pumped amplifier for the first

time. The measured small signal amplification was 1.5 with 15 cm long

continuously pumped amplifier. When we fold the oscillator beam path

twice such that the oscillator beam could pass the amplifier three times,

the measured small signal amplification was 5 for both 2 Hz operation and

5 Hz operation. This fact indicates that we could increase the repetition

rate much higher. A brief report of the experimental results are shown in

Appendix 10.

VI. Theoretical Modeling

In parallel with the experimental works, a theoretical modeling of

the iodine laser was conducted. The modeling efforts for the iodine laser

were concentrated on the pulsed iodine laser by other research groups

aiming on the development of high power fusion driver. In our case, the

modeling effort should be concentrated on a CW laser or a long pulse

pumped laser.

In the modeling of a CW iodine laser, due to the molecular iodine

accumulation, the inclusion of gas flow is very important. To reduce the

molecular iodine the iodide gas should be circulated and reprocessed for

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long term operation. However in this research period, a kinetic model for a

static fill iodine laser was developed and compared to the experiments. A

complete kinetic model of the flowing gas iodine laser is under

development. The previous modeling results are shown in Appendices 11

VII. Conclusions

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The purpose of this research was to realize a solar-pumped iodine

laser and to establish a scaling law for the future development of a large

scale solar-pumped laser system in space. We tested the iodine laser as a

future solar-pumped laser and tested various aspects of the laser system

using a solar-simulator which simulates the AMO solar radiation. In the

course of this research, we obtained a 10 W laser output power for the

first time. Also we have first run a continuously pumped iodine laser

amplifier and measured the small signal amplification. Among the various

findings of our experiment, the following features are very important for

the future research.

1. The operational conditions of the solar-pumped iodine laser are

quite different from that of the high intensity flashlamp pumped iodine

laser. Therefore, the characteristics of the solar-pumped iodine laser

should be more clearly manifested.

2. The new iodine laser material t-C4Fgl, which was found in the

Langley Research Center, needs more precise study to measure the kinetic

coefficients and requires concentrated experimental efforts to compare

with other iodine laser chemicals such as i-C3F71 or n-C3F71 that are

commonly used in iodine laser experiment.

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3. The modeling effort should be more stressed to establish a

realistic model for a real solar-pumped iodine laser system and to

establish a scaling law for the development of a high power space-based

solar-pumped iodine laser.

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References

[1]. G. Brederlow, E. Fill and K. J. Witte, "The High-Power Iodine

LaSer", (Springer-Verlag, New York, N. Y., 1983).

[2].

[3].

J. H. Lee, J. W. Wilson, T. Enderson, D. H. Humes, W. R. Weaver and B. M.

f

Tabibi, Optics Comm. 53, 367(1985).

V. Yu. Zalesskii, L. S. Ershov, A. M. Kokushkin and S. S. Polikarpov, Sov.

J. Quant. Electron. 11, 498 (1981).

[4].

[5].

H. Arashi,Y. Oka, N. Sasahara, A. Kaimai and M. Ishigame, Jap. J. Appl.

Phys. 23, 1051 (1984).

M. Weksler and J. Schwartz, IEEE J. Quant. Electron. 24, 1222 (1988).

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Appendices

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A Solar Pumped Iodine Laser Amplifier

I. H. Hwang and K. S. Han,

LIA Symposia Proceedins vol. 47, ICALEO'84 (1984) 87-91•

A Long-Pulse Amplifier For Solar-Pumped Iodine

I. H. Hwang, J. H. Lee and M. H. Lee,

Optics Communications vol. 58, (1986) 47-52.

Laser

Perfluorobutyl Iodides As Gain Media For A Solar-Pumped

Laser Amplifier

B. M. Tabibi, M. H. Lee, J. H. Lee and W. R. Weaver,

Proceedings of the International Conference on Lasers'86,

144-149.

Iodine

(1986)

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Solar-Pumped Photodissociation Iodine Laser

J. H. Lee, W. R. Weaver, D. H. Humes, M. D. Williams and M. H. Lee,

AlP Conference Proceedings No. 146, (1985) 179-180•

High Power CW Iodine Laser Pumped By Solar Simulator

J. H. Lee, M. H. Lee and W. R. Weaver,

Proceedings of the International Conference on Lasers'86,

150-156.

(1986)

Feasibility Of Solar-Pumped Dye Lasers

J. H. Lee, K. C. Kim and K. H. Kim,

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.

Proceedings of the International Conference on Lasers'86, (1986)

174-178.

Threshold Pump Power Of A Solar-Pumped Dye Laser

J. H. Lee, K. C. Kim and K. H. Kim

Applied Physics Letters vol. 53, (1989) 2021-2022.

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10.

11.

XeCI Laser Pumped Iodine Laser Using t-C4Fgl

I. H. Hwang, K. S. Han and J. H. Lee

Optics Communications vol. 70, (1989) 341-344

XeCI-Laser-Pumped Iodine Laser Oscillator With t-C4Fgl

I. H. Hwang and J. H. Lee,

Proceedings of the International Conference on Lasers'88, (1988)

217-221.

A Continuously Pumped Iodine Laser Amplifier

I. H. Hwang, B. M. Tabibi, J. H. Lee and D. H. Humes,

LEOS '89 Proceedings, (1989) 240-241

A Model For The Kinetics Of A Solar-Pumped Long Path Laser

EXperiment

L. V. Stock, J. W. Wilson and R. J. DeYoung,

NASA Technical Memorandum 87668 (1986).

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12. A Kinetic Model For A Solar-Pumped Iodine Laser

L. V. Stock and J. W. Wilson,

AlP Conference Proceedings voi. 172 (1987) 99-102.

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