Download COIL—Chemical Oxygen Iodine Laser

WDS'06 Proceedings of Contributed Papers, Part II, 81–85, 2006.
ISBN 80-86732-85-1 © MATFYZPRESS
COIL—Chemical Oxygen Iodine Laser
I. Picková, M. Tichý
Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic.
V. Jirásek, O. Špalek, J. Kodymová, M. Čenský, J. Schmiedberger
Institute of Physics, Academy of Sciences of the Czech republic, Prague, Czech Republic.
Abstract. This article provides a short review of our research on the chemical
oxygen iodine laser (COIL). Experimental setup and methods used to produce
required chemical components (oxygen in excited singlet delta state and atomic
iodine) and also methods used to measure the parameters of COIL are described
here. COIL is the laser in which iodine atoms in excited state (lasing medium) are
obtained by the chemical reactions. The wavelength of COIL is 1315 nm, which
is in the near infrared region, and is suitable for propagation of light through the
atmosphere or through optical fibres. In our recent experiments both singlet oxygen
and atomic iodine are produced chemically, but also atomic iodine generation in
electric discharge will be investigated.
Introduction
The Chemical Oxygen Iodine Laser was first demonstrated at the Air Force Weapons
Laboratory in 1977. Since that time new methods of COIL operation were investigated to
improve the efficiency of the laser.
This laser belongs to the family of high power chemical gas lasers. Principle of chemical
lasers is that they efficiently convert energy derived from chemical bonds and reactions into
excited states of atoms or molecules and then into the laser beam. These chemical reactions
usually take place in gaseous media. Very huge advantage of the chemical lasers is then the
high quality of the beam. This is due to the homogeneity of the lasing media in a resonator
(usually low pressure gas). COIL has also the advantage that it can work either in continuous
or in pulsed regime.
Apart from COIL there exists another type of the chemical gas laser, which is HF or DF
laser, which operates at wavelength 2,7 µm. More about chemical lasers can be found for
example in Perram.
In the COIL atomic iodine is pumped by near resonant energy transfer from excited oxygen
molecule O2 (1 ∆g ) (so called singlet delta oxygen) to atomic iodine in a ground state. Singlet
oxygen itself is unsuitable for lasing as it has very long lifetime (∼ 45 min). This is however
the advantage for the transfer of singlet oxygen in the device.
Both the singlet oxygen and atomic iodine can be produced by different methods with
different efficiency. Our task is to maximize the efficiency of both these processes and therefore
improve the COIL operation.
Experimental setup
COIL device consists of three main parts: a) singlet oxygen generator, b) supersonic nozzle,
where chemical components are introduced and atomic iodine is produced and c) laser cavity
with resonator, where the beam of light is produced by stimulated emission of radiation on the
I(2 P1/2 ) → I(2 P3/2 ) transition. Scheme of the experimental setup can be seen in Fig. 1.
All parts of the COIL and processes which take place there will be further described in the
next sections.
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PICKOVA ET AL.: COIL—CHEMICAL OXYGEN IODINE LASER
Figure 1. Simple scheme of the experimental setup of the COIL laser. Singlet oxygen is
produced in the singlet oxygen generator (SOG) in the reaction (2). Atomic iodine is then
produced in the nozzle in the reaction (3) or (12). Then the energy is transferred through the
reaction (23) from singlet oxygen to atomic iodine. In the laser cavity either laser beam is
produced or diagnostics is applied to measure the concentration of I ∗ .
Singlet oxygen generation
In singlet oxygen generator oxygen in the O2 (1 ∆g ) state is produced. The yield of singlet oxygen is defined as a ratio of concentrations of singlet oxygen and all oxygen present in
generator:
[O2 (1 ∆g )]
Y =
(1)
[O2 ]
We need to have high enough yield (over 15%) for the good laser operation, but the required
quantity depends also on the method of I atoms generation.
There are many types of singlet oxygen generator (SOG). Classical method of generation
is the chemical reaction between BHP (basic hydrogen peroxide - mixture of H2 O2 and KOH)
and chlorine Cl2 , which is usually diluted by N2 or He:
Cl2 + 2KOH + H2 O2 → O2 (1 ∆g ) + 2KCl + 2H2 O
(2)
It is the liquid-gas surface reaction (BHP being liquid), so the possibly largest surface of liquid
is needed for the contact of liquid and gas components.
Singlet oxygen can also be generated in electric discharge, see for example Carroll et al.,
2004, Napartovich et al., 2001. Different types of discharge (RF, DC, microwave) can be used
in generator. One of the greatest problems with discharge SOG is the difficulty to sustain high
enough yield of singlet oxygen at higher pressures due to the quenching of this state with atoms
and molecules created in discharge (most harmful are N , O and O3 ).
Atomic iodine generation
Originally, from the beginning of the COIL development, atomic iodine has been produced
from iodine molecule I2 which is dissociated by the singlet oxygen molecule. For one iodine
molecule dissociation about 4–6 oxygen molecules are consumed. That diminishes the fraction
of molecules able to pump the iodine and so diminishes the COIL efficiency. Another problem
is that I2 is really strong quencher of I ∗ . I2 is also solid at the room temperature and must be
heated to sublimate to gas phase.
Another method which has been developed is the discharge generation of atomic iodine
from different molecules such as I2 , HI or CH3 I. Different types of discharge have been and
still are examined, such as DC, RF or microwave discharge.
New method of atomic iodine generation was investigated. It is the chemical reaction of
hydrogen iodide HI with atomic chlorine or fluorine. For further information see Špalek et al.,
2002, Špalek et al., 2004, Jirásek et al., 2001.
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PICKOVA ET AL.: COIL—CHEMICAL OXYGEN IODINE LASER
Atomic iodine generation via chlorine
Atomic iodine generation in the reaction of hydrogen iodide with atomic chlorine is described in the following equation:
k = 1.64 × 10−10 cm3 molec−1 s−1
HI + Cl → HCl(HCl∗ ) + I,
(3)
Here k is the rate constant of this reaction and abbreviation molec stands for the molecule.
First there must be atomic chlorine produced in the reaction with N O:
ClO2 + 2N O → Cl + 2N O2
(4)
This actually is not simple chemical reaction, but the chain of more reactions ([5] – [7])
and there are more reactions describing mainly Cl atoms losses (reactions [8] – [11]):
k = 3.4 × 10−13 cm3 molec−1 s−1
ClO2 + N O → N O2 + ClO,
ClO + N O → N O2 + Cl,
k = 1.7 × 10
−11
−11
Cl + ClO2 → 2ClO,
k = 5.9 × 10
−31
Cl + N O2 + He → ClN O2 + He,
k = 7.2 × 10
−31
ClO + N O2 + He → N O3 Cl + He,
k = 1.0 × 10
Cl + Cl + He → Cl2 + He,
k = 6.4 × 10
ClN O2 + Cl → Cl2 + N O2 ,
k = 3.0 × 10
−33
−11
(5)
3
−1 −1
s
(6)
3
−1 −1
s
(7)
6
−2 −1
s
(8)
6
−2 −1
(9)
6
−2 −1
s
(10)
3
−1 −1
(11)
cm molec
cm molec
cm molec
cm molec
s
cm molec
cm molec
s
Losses of Cl atoms occur mostly in the termolecular reactions (reactions of three molecules).
Importance of the termolecular reactions increases with pressure and decreases with temperature.
Atomic iodine generation via fluorine
Like atomic chlorine also atomic fluorine can be used for I atoms generation. Here is the
reaction of atomic fluorine with HI molecule:
HI + F → HF (HF ∗ ) + I,
k = 1.51 × 10−10 e−608.3/T cm3 molec−1 s−1
(12)
Atomic fluorine is created from the molecular fluorine F2 in reaction:
F2 + N O → F + N OF (N OF ∗),
k = 7 × 10−13 e−1150/T cm3 molec−1 s−1
(13)
This is not the only one process taking place during the fluorine atoms generation, there are
some more reactions involving atomic fluorine, iodine, N O and diluting gas (helium in this case,
fluorine must be diluted for safety reasons). Following reactions describe fluorine atoms losses:
k = 1.1 × 10−31 cm6 molec−2 s−1
F + N O + He → N OF + He,
F + F + He → F2 + He,
k = 5.3 × 10
−34
−31
F + N O + N O → N OF + N O,
k = 1.7 × 10
−10
F + I2 → IF + I,
k = 4.3 × 10
(14)
6
−2 −1
s
(15)
6
−2 −1
(16)
cm molec
cm molec
3
s
−1 −1
cm molec
s
.
(17)
And in following reaction iodine atom losses are described:
I + I + He → I2 + He,
I + I + I2 → I2 + I2 ,
I + N O + He → IN O + He,
I + IN O → I2 + N O,
I + F2 → IF + F,
k = 3.8 × 10−33 cm6 molec−2 s−1
k = 3.7 × 10
−30
−33
k = 5.5 × 10
−10
k = 2.6 × 10
−14
k = 1.9 × 10
83
(18)
6
−2 −1
s
(19)
6
−2 −1
s
(20)
3
−1 −1
s
(21)
3
−1 −1
(22)
cm molec
cm molec
cm molec
cm molec
s
PICKOVA ET AL.: COIL—CHEMICAL OXYGEN IODINE LASER
There can be also included the reaction of atomic iodine with the wall, which causes another
losses of iodine atoms.
Equations described here are then used for modeling of the chemical reactions (chemical
kinetics). For the concentration estimation we use the fourth order Runge Kutta algorithm. As
we now prepare mainly the experiments with atomic iodine generation via fluorine, this system
is of great importance for us.
Laser operation
Energy pumping of the laser can be described by the following reaction:
O2
1
∆g + I ↔ O2
3
Σg + I ∗
(23)
with kf (= 7.8 × 10−11 cm3 molec−1 s−1 ) and kb (= 1.04 × 10−10 e−403/T cm3 molec−1 s−1 ) being
the forward and backward direction rate constants. The equilibrium rate constant is defined as
a ratio of these rate constants:
kf
Keq =
= 0.75e403/T
(24)
kb
We need to set the equilibrium conditions in such state so excited iodine concentration would
be high enough compared with the ground state iodine concentration. This is the condition for
laser population inversion, which is necessary for laser operation.
Experiments
Two main types of measurement have been done in the laser cavity: a) small signal gain
measurement and b) laser output power measurement.
Small signal gain is the quantity which shows us, whether the population inversion was
attained in the cavity. In this case, the following condition is fulfilled:
[I] ≤ 2 · [I ∗ ].
(25)
Small signal gain is measured in the laser cavity without mirrors (so lasing is disabled)
and diode laser with the same wavelength (1315 nm) is used. Beam from this laser can cause
either stimulated emission, if there is population inversion present and then we measure the
gain or absorption can occur otherwise. This diode laser is tunable, so we can measure also
close wavelengths and get also the broadening of this spectral line, which is mainly caused by
the Doppler broadening, from which we can estimate the temperature in the cavity. Doppler
broadening occurs due to the different directions and velocities of molecules in the cavity. Small
signal gain depends on the flows of gases, on the flow rates of gases and on the experiment
configuration.
When we enable lasing, we can measure the laser output power. The beam is split and
one part is sent to power meter. We measure only integral power (we reached over 400 W so
far), not the profile of the beam. We also make the burn pattern on the wooden desk to see the
homogeneity of the light generation in the resonator.
There are now two sets of experiments prepared. One of them is the further investigation
of the atomic iodine generation via atomic fluorine with new device which is now built. So
far we built new singlet oxygen generator (spray type) and some experiments have been made
(measurement of the BHP flow or distribution of BHP droplets diameters). The second set of
experiments includes the generation of atomic iodine in RF-discharge from molecules such as
I2 , HI and CH3 I.
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PICKOVA ET AL.: COIL—CHEMICAL OXYGEN IODINE LASER
Conclusion
COIL is the chemical gas laser in which the substantial part of energy is produced by the
chemical reactions. In classical COIL there are two main chemical reaction: singlet oxygen
generation and atomic iodine generation. Besides these two, many more reactions can be found
there, some of them causing the losses of desired atoms or molecules. This laser is also high
power laser, which wavelength 1315 nm is suitable for propagating trough the air or optical
fibres.
On the COIL device we measured the small signal gain and laser output power and besides also the flows of gases and pressure in the main parts of COIL. Our work should be the
contribution to the further COIL improvement, so it can be used in different applications.
Both singlet oxygen and atomic iodine can be produced in electric discharge. This seems
to be better for laser safety (fluorine, chlorine, BHP etc. can be removed). However, discharge
methods still have some difficulties to produce required components with hight yield or in high
pressure conditions.
References
Jirásek, V., O. Špalek, J. Kodymová, M. Čenský, Chemical generation of atomic iodine for chemical
oxygen-iodine laser. I. Modeling of reaction systems, Chemical physics, 269 , 167-178, 2001.
Špalek, O., V. Jirásek, M. Čenský, J. Kodymová, I. Jakubec, G. D. Hager, Chemical generation of atomic
iodine for chemical oxygen-iodine laser. II. Experimental results, Chemical physics, 282, 147-157, 2002.
Špalek, O., M. Čenský, V. Jirásek, J. Kodymová, I. Jakubec, G. D. Hager, Chemical Oxygen-Iodine
Laser Using a New Method of Atomic Iodine Generation, IEEE Journal of Quantum Electronics, 49,
564-570, 2004.
Carroll, D. L., J. T. Verdeyen, D. M. King, J. W. Zimmerman, J. K. Laystrom, B. S. Woodard, N.
Richardson, K. Kittell, M. J. Kushner, W. C. Solomon, Measurement of positive gain on the 1315 nm
transition of atomic iodine pumped by O2 (a1 ∆) produced in and electric discharge, Applied physics
letters, 85, 1320-1322, 2004.
Napartovich, A. P., A. A. Deryugin, I. V. Kochetov, Discharge production of the singlet delta oxygen
for an iodine laser Journal of physics D: Applied physics, 34, 1827-1833, 2001.
Perram, G. P., Chemical lasers, web source, Air Force Institute of Technology, date not found.
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