An iodine photodissociation laser using t-C4F9I as the active material was pumped by an XeCl laser. An iodine laser output energy of 3 mJ with pulse duration of 25 ns was obtained when the pumping pulse energy was 80 mJ, the iodide pressure was 70 torr, and the reflectance of the output mirror was 85 percent. The high pumping efficiency and low threshold pump power achieved in this experiment are attributable to the high absorption cross section at the pump laser wavelength (308 nm) of the iodide used

Han, Kwang S.

Hwang, In Heon

English

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/N91 : . 117 '
A
XeC! LASER PUMPED IODINE LASER USING t-C4F9I
In H. Hwang and Kwang S. Han
Department of Physics
Hampton University
Hampton, VA
ABSTRACT
An iodine photodissociation laser using t-C4FgI as the active material was pumped by an XeC1
laser. An iodine laser output energy of 3 mJ with pulse duration of 25 ns was obtained when the pumping
pulse energy was 80 mJ, the iodide pressure was 70 torr, and the reflectance of the output mirror was
85%. The high pumping efficiency and low threshold pump power achieved in this experiment are
attributable to the high absorption cross section at the pump laser wavelength (308 nm) of the iodide
used.
398
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XeC! LASER PUMPED IODINE LASER USING t-C4F9I
I. Introduction
Since the atomic iodine photodissociation laser was discovered by Kasper and Pimentel in 1964
[1], numerous research efforts have been concentrated on this laser to increase the power level needed
for laser fusion [2,3]. The fusion-experiment-oriented iodine laser is constructed in a master-oscillator-
power-amplifier (MOPA) architecture [4].
The iodine laser is also being considered as a direct solar pumped laser for space applications such
as the laser propulsion for an orbital transfer vehicle. The first solar simulator pumped iodine laser was
reported in 1981 by Lee, et. al. [5]. This laser was operated in quasi-continuous-wave mode. After this
report, various iodine compounds were tested and evaluated as candidates for direct solar pumped laser
materials [6,7,8].
Although many space applications are best served by a continuous wave (CW) laser, a continuously
pulsed laser also has good applicability in space, especially for laser propulsion. If weak pumping (such
as solar radiation pumping) of laser materials exists, use of a MOPA system is essential for obtaining
high peak power when the laser material has a long upper-state lifetime like an iodine atom. In previous
research, the feasibility of the oscillator-amplifier scheme was proposed and tested for the solar pumped
iodine laser [7.9].
In order to be incorporated in the MOPA system, the iodine laser oscillator must generate
temporally smooth and short pulses. In the fusion-oriented-iodine-laser-experiment the short pulse is
provided by mode-locking of the flashlamp-pumped oscillator. However, the repetition rate of the
flashlamp-pumped oscillator is usually very low (<5Hz), and the mode-locking devices are generally
complicated. An excimer laser pumped iodine laser oscillator was demonstrated by Fill et. al. [10]. In
their experiment, a 500-mJ excimer laser was used for the pumping of the iodine laser, and a temporally
smooth laser pulse with duration from 2.6 ns to 12 ns was obtained by using i-C3F7I as the laser material.
In this report, a XeCI laser pumped iodine laser using t-C4F9I is described. This laser oscillator
has much a higher efficiency and a lower threshold pump power than the system using i-C3FTI. The laser
output energy dependence on the gas-filled pressure and on the reflectance of the output mirrors are
measured for both longitudinal and transverse pumping.
II. XeCI Excimer Laser
A laboratory built XeCI laser was used for the pumping of the iodine laser in this experiment. The
electrical circuit of the XeCI laser is shown in Fig. 1. A pyrex tube with an I.D. of 0.1 m and a length of
0.66 m was used as the laser chamber. The two brass electrodes were rounded in order not to develop
an arc discharge in the discharge volume. The width of the electrodes was 20 mm, and the length was
about 0.45 m. The separation between the two electrodes was 20 ram. Seventeen pairs of arc arrays
were located beside the electrodes so that the discharge volume could be preionized uniformly. The
optimized gas composition was HCI: Xe: Ar: He = 0.2% : 5.9% : 19.5% : 74.4% at a total pressure of
2 atm. The addition of argon gas improved the discharge uniformity at the higher operating voltage.
399
When the XeCI laser was operated at the charging voltage of 25 kV, the output energy was 80 mJ per
pulse. If the charging voltage was increased, the output energy also increased, but an arc discharge
developed between the electrodes. Thus, operation above 25 kV was not pursued. A typical laser pulse
is shown in Fig. 2. The half width (FWHM) of the XeCI laser pulse was about 25 nsec, and no significant
variation of the laser pulse was found for prolonged operation.
high voltage 500K 300K
,r.,on,z.,
pu,,e_50pF/ -_ I _ P
IOM e- ?
-- 300K 300K -_
w
Figure 1
Electrical circuit diagram of the XeCI excimer laser.
"O
E
O
Q
1""
V
50ns/div.
Figure 2
A typical pulse shape of the XeCI laser output.
400
IIL Iodine Laser Experiment
The iodide used in this experiment was t-C4FgI (perfluoro-tertiary-butyl iodide). This material was
chosen mainly due to the absorption cross section at the wavelength of XeCI laser light (308 nm). The
published data show that the absorption cross section of t-C4FgI at 308 nm is about 3.6 x 10 -10 cm 2 [7].
Also, t-C4F9I has shown good chemical reversibility in a flashlamp-pumped system [6,7], and thus the
iodine molecule buildup in the laser cell is minimized. The main disadvantage of this iodide is the low
vapor pressure (--85 tort) at room temperature. The other chemical kinetic properties are nearly the
same or better than other commonly used perfluoralkyl iodides in the flashlamp-pumped experiment
[7]. The quantum yield of excited atomic iodine in the UV photodissociation is nearly unity [11].
A quartz cuvette with a square cross section of 1 cm 2 and length of 50 mm was used as the iodine
laser cell. The windows of the laser cell were nearly perpendicular to the optic axis of the laser cell.
Both longitudinal and transverse pumping of the iodine laser were employed.
When the iodine laser was pumped longitudinally, a dichroic mirror was used to introduce the
XeCI laser light into the iodine laser cell. The dichroic mirror transmitted about 80% of the XeCI laser
light and fully reflected the iodine laser light. The XeCI laser light was directed through the dichroic
mirror and was focused into the center of the laser cell by a quartz spherical lens of focal length 0.2 m.
When the iodine laser was pumped transversely, a quartz cylindrical lens of focal length 0.3 m was used
to focus the XeCI laser light into the iodine laser cell.
The output energy dependence on the gas filled pressure at different output mirror reflectances
is shown in Figs. 3 and 4. As shown in Fig. 3, there is an optimum iodide pressure for each output mirror
reflectance for the longitudinal pumping. However, for the transverse pumping, the laser output energy
increases monotonically with the t-C4FqI pressure as shown in Fig. 4 The results in Figs. 3 and 4 were
taken when the laser was operated at 2-Hz repetition rate. When the pulse repetition rate was increased
to 5 Hz, the laser output energy per pulse was reduced slightly (=10%) due to the reduction of the
XeCI laser pumped energy.
A typical laser output is shown in Fig. 5. The half width (FWHM) of the pulse is about 25 nsec.
The iodine laser onset is delayed from the XeCI pump laser and occurs at the end of the pump pulse.
The pulse shape is nearly the same for both pumping geometries.
IV. Discussion
In this experiment, a XeCI laser pumped iodine laser was developed and tested by using t-C4F9I
as the laser material. This lasant t-C4F9I has a larger absorption cross section at the XeCI laser line
compared with other iodides used in the laser experiment, such as i-C3F7I. The larger absorption cross
section allows the high utilization of pumping energy in short gain length and low pressure operation
of the iodine laser. The low-pressure operation of the iodine laser is suitable for single longitudinal
mode output because of the reduced pressure broadening of the gain profile. The single-longitudinal
mode operation of the laser oscillator is necessary to obtain a temporally smooth pulse.
The iodine laser described in this report was operated up to 5 Hz, which was limited by the power
supply of the XeCI laser. Further increase of the repetition rate can be made by scaling up the pump
laser system. The iodine laser was operated in a sealed-off mode. There was no significant reduction of
the laser output energy after a few hundred pumpings with a single fill t-C4FgI in the laser cell. There
401
wasalso no noticeable change in the t-C4F9I gas after a few hundred pumpings and no deposits on the
wall of the laser cell (contrary to the report of Ref. 10 where t-C3FTI was used). This may be attributable
to the superior chemical reversibility of the iodide used.
R
E
e.
_g
"i
O
2 -
1 B
-.-11---/1=85%
R=30%
_a,-- R=4%
_'o _o' :io 4'0 s'o 6o 7'o ,_'o
pressure(totr)
Figure 3 - Iodine output energy dependence on the gas
fill pressure in the laser cell at each output mirror
reflectance. The iodine laser was pumped longitudinally
with the XeCI laser of pulse energy 80 mJ.
"O
E
Q
1,-
W_ry fV%_
I/
[
v w
L.I
1F
l
50ns/div.
....
XeCI laser
iodine laser
Figure 4 - Iodine laser output energy dependence on the
gas fill pressure when the iodine laser was pumped
transversely. The pumping energy from the XeCI laser
was fixed to 80 mJ.
402
3L
E
v
O
"_ 2
O.
e_
e-
u 1
u
-n
CI.
-i
O
--i----. R=85%
_b 2'0 _'o 4'o s'o _o ro so
pre$sure(torr)
Figure 5 - A typical iodine laser pulse shape which is
compared with the pumping pulse. (a) XeCI pump laser
pulse. (b) Iodine laser output pulse.
V. Conclusion
An iodine laser oscillator pumped by an XeCI laser was developed using t-C4F9I as the laser
material, and a 3 mJ laser output energy was obtained with only 80 mJ pumping energy. Compared with
previous results, the pumping energy was dramatically reduced in this experiment, The pumping
efficiency (i.e., the ratio of the iodine laser energy and the XeCI laser energy) was 3.75%. This
experiment also demonstrated a repetitive operation of the iodine laser oscillator with a stable output.
Since the threshold pumping energy (20 m J) is low for t-C4F9I, a moderate size XeCI laser may suffice
for the pumping of the high-repetition-rate laser oscillator required for solar pumped master-oscillator
power-amplifier systems.
Acknowledgments
This work is supported by NASA grant NAG1-441.
References
1. J.V.V. Kasper and G.C. Pimentel, Appl. Phys. Lett. 5, 231 (1964).
2. G. Brederlow, E. Fill and ILL Witte, "High-Power Iodine Laser," Springer-Verlag, New
York, N.Y. (1983).
4O3
3. S.B. Kormer, Izvesteriya Akademii Nauk SSSR. Seriya Fizicheskaya 44, 2002 (1980).
4. K.J. Witte, Czech. J. Phys. B34, 790 (1984).
5. J.H. Lee and W.R. Weaver, Appl. Phys. Lett. 3.,9_,137 (1981).
6. J.H. Lee, J.W. Wilson, T. Anderson, D.H. Hume, W.R. Weaver and B.M. Tabibi, Optics
Comm. _ 367 (9185).
7. B.M. Tabibi, M.H. Lee, J.H. Lee and W.R. Weaver, Proceedings of International
Conference on Lasers' 86, pp 144-149 (1986).
8. R.J. Deyoung, IEEE. J. Quant. Electron. _ 1019 (1986).
9. I.H. Hwang, J.H. Lee and M.H. Lee, Optics Comm. 58, 47 (1986).
10. E. Fill, W. Skrlac and K.J.K. Witte, Optics Comm. 37, 123 (1981).
11. A.B. AJekseev, A.M. Pravilov, I.I. Didorov and V.A. Skorohodov, Soy. J. Quantum
Electron. 17, 1539 (1987).
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XeCl laser pumped iodine laser using t-C4F9I

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