A continuously pumped iodine laser amplifier

Volume 84, number 3,4

OPTICS COMMUNICATIONS

15 July 1991

A continuously pumped iodine laser amplifier In H e o n H w a n g a n d K w a n g S. H a n Department of Physics and Spaceborne Photonics Institute, Hampton University, Hampton, VA 23668, USA

Received 14 February 1991

The amplification characteristicswere studied for a continuouslypumped iodine laser amplifier using n-C3FTIas the amplifying medium. A small-signalamplification of 5 was obtained from a 15 cm long amplifier pumped with 1000 AM0 solar radiation by passing the oscillator output through the amplifier three times.

1. Introduction

The development of solar-pumped lasers is actively investigated around the world [ 1-3 ]. I f an efficient laser material could be found, a solar-pumped laser would be a very inexpensive source of coherent radiation. A ground-based solar-pumped laser could be employed in various industries as a substitute of electrically pumped lasers, which are in growing demand in various manufacturing processes. A space-based solar-pumped laser may play an important role in future space applications. Among various applications, power transmission to distant space vehicles or planets is easily conceivable as a prime power supply. When a laser is employed for power transmission, the size of the transmitter and receiver can be made many orders of magnitude smaller than those for microwaves due to the shorter wavelength of the laser light. For this reason, research on a direct solar-pumped iodine laser has been continuing in the NASA Langley Research Center since 1980 [4-6 ]. The ultimate goal of the research is to develop a direct solar-pumped laser system which can be employed for efficient power distribution to distant space vehicles. A cw laser may have the best applicability in space power transmission. But long-distance power transmission requires a high-quality laser beam with minimum divergence. The beam divergence of a laser depends on the wavelength of the laser, the geometry of the laser cavity and the transverse electric and magnetic (TEM) mode structure of the oscillation.

The lowest beam divergence is usually accomplished with the lowest TEM mode of the laser output. However, it is difficult to operate a large laser system in the TEMoo mode with high power. An alternative laser system for space power transmission is a high repetition rate pulsed laser. A master-oscillator power-amplifier (MOPA) structure of a pulsed laser system can provide good beam quality with low divergence and high peak power at the same time [ 7 ]. Such a high-power laser is required for laser propulsion of space vehicles [ 8 ]. A cw laser amplifier may be considered to achieve high power, but the saturation intensity is so high that the extraction efficiency is usually very low [ 7 ]. In contrast to the cw amplifier, the saturation energy fluence of a pulsed amplifier is low enough that a moderately sized oscillator can easily saturate a considerably larger sized amplifier. In this investigation, the amplification characteristics of a continuously solar-simulator-pumped iodine laser amplifier were studied and the experimental measurement of the small-signal amplification was compared with the calculation from a simplified iodine laser kinetic model.

2. Iodine laser kinetics and rate equations

The laser media used in the iodine laser experiments are mostly perfluoroalkyl iodides such as nC3F7I, i-CaF7I.or t-C4FgI. Among the perfluoroalkyl iodides, t-C4F9I is known to have the highest recom-

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169

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bination rate after the lasing process [9]. Moreover, this chemical has a wider absorption band ( ~ 50 nm) with peak absorption at a longer wavelength ( ~ 288 nm) compared with other iodides. When these perfluoroalkyl iodides, RI, are exposed to uv light in the range of 270 nm, the molecule dissociates into an excited iodine atom I* and a radical R. The quantum yield of excited atomic iodine by photodissociation is near unity in the whole spectral range of the absorption band [ 10]; therefore the energy level diagram can be approximated as shown in fig. 1. Excited atomic iodine has a long upper state lifetime of about 130 ms. Such a long lifetime cannot be maintained, however, due to collisional quenching by the parent molecule and other impurities contained in the laser gas. Excited atomic iodine returns to the ground state by the stimulated emission of laser radiation. Most of the ground-state iodine atoms combine with the radical to form the original iodide molecule. But, some of the ground-state iodine atoms form the iodine molecule, I2, by a three-body collisional process, I + I + M - + I 2 + M , where M denotes the collision partner. The iodine molecule is the strongest quencher of excited atomic iodine and the most effective assisting partner for iodine molecule formation through the three-body reaction. Therefore, once the formation of iodine molecules has started, it accelerates the formation. This is why we cannot operate the iodine laser in cw mode without gas flow through the laser tube. A continuously pumped iodine laser amplifier also cannot be oper-

RI*

Q.

RI

12"

D.

ated without gas flow in the amplifier tube for the same reason. When a broad-band light source such as solar radiation is used as the pumping source, iodine molecules are photodissociated by photons in the visible range of the spectrum ( ~ 500 nm). This photodissociation contributes to the generation of excited atomic iodine; however the amount is insignificant to iodine laser performance. A more significant contribution of this photodissociation to the iodine laser is the reduction of the concentration of iodine molecules, which is the strongest quencher of excited atomic iodine. In spite of the photodissociation of the iodine molecules by photons in the visible spectrum of the pumping radiation, gas flow is necessary in a cw iodine laser oscillator or a continuously pumped iodine laser amplifier to remove the excess amount of iodine molecules produced in the tube such that the density of iodine molecules could be stabilized at a low equilibrium density. With these kinetic characteristics of the iodine laser material, we can establish the approximate rate equations for the excited state and ground state atomic iodine in the amplifier tube assuming uniform flow of the gas in the tube [6] as follows: d[I*] Idt= WplRI ] - ( l / z ) [I*] - Q ~ [RI] [ I * ] - Q 2 [ I 2 ] [ I * ] ,

(l)

d [ I ] / d t = ( l / r ) [I*] + Q , [RI] [I*] +Qz[I2][I*]-k2[R][I].

(2)

The quantities in square brackets represent the particle density of each chemical species. Q~ and Q2 are the quenching rate coefficients of excited atomic iodine by the parent molecule and molecular iodine, respectively. Wp is the pumping rate, r is the spontaneous emission lifetime of the excited-state iodine atom, and kz is the recombination rate of the groundstate iodine atom and the radical. The pumping rate Wp was calculated assuming that every absorbed photon from the pumping source in the laser medium generates one excited iodine atom. Therefore the pumping rate is

1

4

W p - [RI] D

12

Fig. 1. A simplified energylevel diagram of the iodine laser. 170

15 July 1991

× {-F(2)[l-exp(-a~(2)[RI]D)] d

ci2

(3)

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OPTICS COMMUNICATIONS

for the present experimental setup, in which the pumping light is introduced through the side wall of the amplifier tube. D in eq. (3) represents the amplifier tube diameter, F(~.) is the pumping photon fluence through the amplifier wall and t&(;t) is the absorption cross section of the amplifying medium. The photon fluence is obtained from a detailed measurement of the irradiance on the amplifier tube surface by the solar simulator. The irradiance by the solar simulator is compared with that by the sun in outer space in fig. 2. The absorption curves of the three iodine laser materials are also presented in fig. 2. When, from previous kinetic considerations of continuous pumping [ 6 ], the spatially averaged density of iodine molecules at equilibrium is assumed to depend on the laser gas flow velocity as [I2] = [I2]o

if flow velocity > 6 m / s , (4)

the two rate equations (1) and (2) can be solved easily. [I2] o in eq. (4) represents the initial density of iodine molecules in the iodide gas. The functional form of the density of iodine molecules was chosen phenomenologically based on our experimental results [6]. The distribution of excited-state and ground-state atomic iodine along the tube axis is obtained by transforming the solutions of eqs. ( 1 ) and (2) to a space dependent form through the relation x = vt of the gas flow, i.e., '

'

'

t

.

.

.

.

i

.

.

.

.

solar irrndlance

s

---~. / /

°

!

.*_=

.

/ \\

' L,~,

\ f~

/J

i

'

x 500

I

/;

\ .lar-almulalor

g

. f

/

[I*] = [ I * ] o [ 1 - - e x p ( - - x / ~ e ) ] ,

(5)

[I] = ( l/k2 re) { 1 - e x p ( -k2ze[I* ]o X [x/vre + e x p ( - x / v r e ) - 1 ] )} ,

(6)

where [I*]o = Wp[RI]r~

(7)

and the effective lifetime, z~, is given as (1/re) = ( l / z ) + Q , [RI] +Q2[I2]

•

(8)

From eqs. (5) and (6), the population inversion is almost equivalent to the density of excited iodine [I*], because [I] is about three orders of magnitude smaller than [I*] along the tube axis in the present experimental setup. The small-signal amplification is now simply calculated as I

= [ I 2 ] o ( 6 / v ) 2 if flow velocity < 6 m / s ,

,

15 July 1991

.

Ass =exp(tro f [I*] d x ) , o

where l represents the length of the amplifier. However, due to the complex level structure of the iodine atom, not all excited iodine atoms can contribute to the amplification. The population in the F = 3 hy, perfine level of the excited iodine atom contributes to the amplification due to the fact that the iodine laser oscillates usually at the wavelength of the 3--,4 transition. When the duration of the extracting pulse is long compared with the relaxation time of the hyperfine sublevels of the excited state of the iodine atom, and ground-state atomic iodine is depleted by fast recombination with the radical, then both states behave as if each of them is single level and the degeneracy of the ground state is infinity. Therefore, the amplification is contributed to by the total number of excited iodine atoms. When, on the contrary, the pulse duration is much shorter than the relaxation time of the sublevels, only about 44% of the excited iodine atoms can be involved in the amplification [ 11 ]. Therefore, the small-signal amplification of the iodine laser amplifier should be expressed as l

~= n ' - , ~ ~

-200

. . . . . . .

250 300 wavelength(nm)

'~'-~

""~-

350

0

Ass =exp(tro f ,[I* ] d x ) ,

(9,

0

Fig. 2. The absorption cross sections of iodides and the AM0 solar irradiance compared with the measured irradiance from the solar simulator.

where tro is the peak stimulated emission cross section of the 3--.4 transition and 7 is a number between 171

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OPTICS COMMUNICATIONS

0.44 and 1.0 depending on the duration of the extracting pulse.

3. Experiment A solar-simulator-pumped iodine laser oscillator was employed as an amplifier after removal of the cavity mirrors. The experimental setup is shown in fig. 3. This amplifier system was previously used for solar-simulator-pumped cw iodine laser research and produced more than 10 W cw output power when it was used as an oscillator [ 5 ]. To increase the signal to noise ratio of the amplification measurement, we employed a triple-pass geometry by using two plane mirrors. A XeCl-laser-pumped iodine laser oscillator was used for the measurement of the small-signal amplification. This oscillator can generate 3 mJ output energy with a pulse length of about 20 ns and can be run at a pulse repetition rate of 5 Hz. The details and the output characteristics of the XeCl-laser-pumped iodine laser oscillator are described in ref. [ 12 ]. The amplifier tube diameter is 20 m m and the pumped length is 15 cm. The laser material used in this experiment was n-C3F7 I. The small-signal amplification was measured for different gas flow velocities at various n-C3FTI pressures. The iodide gas was vacuum distilled at - 10°C before use to reduce the impurities and the concentration of iodine molecules was further reduced by passing the iodide gas

through a cooled copper mesh filter. The laser gas speed inside the amplifier tube was controlled by adjusting the valves of the evaporator and the condenser. The highest achievable gas speed was about 8 m/s.

4. Results and discussion

For the calculation of the small-signal amplification from the model, the pumping rate was calculated from the measured spectral irradiance on the surface of the amplifier tube by using eq. (3). The pumping rate by the solar simulator was almost equivalent to that by 1000 AM0 solar radiation when the solar simulator was driven with a 300 A electrical current. The initial iodine molecule density [ 12] 0 was assumed to be 2 p p m of iodide molecules; even the iodide gas was distilled very carefully. This amount of iodine molecules is due to its finite vapor pressure even at low temperature. The kinetic coefficients and the other parameters for the calculation of the smallsignal amplification are given in table 1. The peak stimulated emission cross section fro of the 3--.4 transition in eq. (9) was calculated from the relation [ 13 ]

n" C3F71 r'-'~" gas in I I cooling water

~

n'C3F71 out

gas

iodine laser beam

mirror

!

from oscillator mirror

15 July 1991

I

to d e t e c t o r amplifier tube

I Ii

: ,

i CW Ar arc lamp

I I

Fig. 3. The experimentalarrangement for the measurement of the small-signalamplification with the continuously pumped iodine laser amplifier. 172

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OPTICS COMMUNICATIONS

Table 1 Kinetic coefficients and spontaneous lifetime 3 x 10- ]7cma/s 6× 10-'t cm3/s 7× 10-12 cma/s 7.9s -l

Qj

Q2 k2 1/r

'~

5 .....

10 torr ) 20 torr I cal

"4'9

--.--

30 torr

// //

/

~-Jt " " . \'x ~ ' " ~ -

/tic

/,

'11,

"

15 July 1991

the flow velocity range from 3 m / s to 6 m / s for different gas pressures. The discrepancy between the experimental result and the calculation at flow speeds below 3 m / s may be due to iodine molecule attachment to the window and the amplifier tube wall. At such a low flow velocity, the iodine molecules generated can attach to the tube wall and the windows. The discrepancy at higher flow velocity is not yet understood clearly. The incompleteness of the model may be the main reason o f the discrepancy.

"''-: 5. Conclusion

=~

"1',,

•

Ig

J- 10

E 1 .r

torr

-?-

'7.'. I ox.

' i

~

-.-l-- 30 tort ' i

' i

' i

÷

~

9

flow speed (m/sec)

Fig. 4. The small-signal amplification dependence on the flow speed in the amplifier tube at different n-C3FTIpressures for the triple-pass amplification experiment. cro = 7 (A34/47~2) ().z/A VD) for < 20 T o r r ,

=7(A34/4rt2){)t2/tAUD+( p - 2 0 ) X for > 20 T o r r ,

14× 106]} (10)

where p is the iodide gas pressure. The second expression o f eq. (10) reflects the pressure broadening o f the linewidth by the iodide molecule. The Doppler linewidth A~,D is about 250 M H z at a gas temperature o f 300 K and the Einstein coefficient A34 o f the 3-~4 transition is 5.1 s - I . The measured small-signal amplification is compared with the calculation from eq. (9) in fig. 4. The pulse repetition rate of the iodine laser oscillator was 2 Hz. The experimental result was the same when the oscillator was operated at 5 Hz. In the calculation, allowance has been made for the triple-pass geometry o f the experiment. Also the value o f Xchosen in this calculation is 0.5 from the fact that the laser oscillator output pulse has only a duration o f 20 ns, which is comparable to the relaxation time of the excited sublevels. The agreement between the experimental results and the calculation is fairly good in

In this experiment, we studied the possibility o f iodine laser amplification by a continuously p u m p e d amplifier. A simplified kinetic model o f the continuously pumped iodine laser amplifier gives a good explanation o f the experimental results. The highest amplification obtained in this experiment is about 5 at a gas flow speed o f about 6 m/s, which agrees well with the calculation from the model. This high amplification at such a low pumping rate ( Wp ~ 4 s - ~) is due to the long excited-state lifetime o f atomic iodine. Although the present analytical model gives a good explanation o f salient features of the amplifier, a more complete model for the continuously pumped iodine laser amplifier is necessary to account for the effects of neglected chemical reactions on the amplifier performance.

Acknowledgement

This work was supported in part by NASA Grant NAG-I-1091. The authors appreciate the help o f Dr. Bagher M. Tabibi and Mr. Donald H. Humes in the experimental measurement.

References

[ 1] J.H. Lee and W.R. Weaver, Appl. Phys. Len. 39 ( 1981) 137. [2] M. Weksler and J. Shwartz, IEEE J. Quantum Electron. 24 (1988) 1222. [3] H. Opower, F. Lindner and W. Zittel, SPIE 972 (1988) 336. [4 ] W.R. Weaver and J.H. Lee, J. Energy 7 ( 1983) 498. [ 5 ] J.H. Lee, M.H. Lee and W.R. Weaver, in: Proc. Intern. Conf. on Lasers '86, ed. R.W. McMillan (1986) p. 150. 173

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OPTICS COMMUNICATIONS

[ 6 ] I.H. Hwang and B.M. Tabibi, J. Appl. Phys. 68 (1980) 4983. [ 7 ] A.E. Siegman, Lasers (Univ. Science Books, Mill Valley, CA, 1986) Ch. 7. [8]J. Kare, UCRL-101139 preprint, Lawrence Livermore National Laboratory ( 1989 ). [9] T.L. Andreeva, G.N. Birich, I.I. Soberman, V.N. Sorokin and I.I. Struk, Sov. J. Quantum Electron. 7 (1978) 1230.

174

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[ 10 ] T.P. Zhurilo, V.Yu. Zalesskii and A.M. Kokushkin, Sov. J. Quantum Electron. 15 ( 1985 ) 669. [ 11 ] W. Fuss and K. Hohla, Z. Naturforseh. 31a (1976) 569. [12] I.H. Hwang, K.S. Hart and J.H. Lee, Optics Comm. 70 (1989) 341. [ 13 ] G. Brederlow, E. Fill and K.J. Witte, The high-power iodine laser (Springer, New York, NY 1983) Ch. 2.