Microwave excited excimer laser and method

A microwave pumped excimer laser and method.

This invention relates generally to excimer lasers and more particularly to 
microwave excited excimer lasers and method. 
Excimer or exciplex lasers are laser systems using gas-phase molecular 
species in which the upper laser level is a bound, stable state and the 
lower level is an unbound, unstable dissociative state. This 
quasi-molecular system has been termed an "excimer" or exciplex", and is 
derived from a contraction of "excited state molecule" or "excited state 
complex", since the "molecule", or atomic complex exists only in the 
excited state. Appropriate sets of states are found in many combinations 
of atoms including all the noble or rare gases. For example, molecules 
formed of two Xe atoms, Xe.sub.2, do not exist, but excited state 
molecules do exist, Xe.sub.2 *, where the "*" indicates an excited, 
energetic state. If such a state is formed it normally lasts until it 
emits a photon and makes a transition to the lower, unbound state, and 
flies apart: Xe.sub.2 *.fwdarw.Xe+Xe+photon. 
The general requirement for net laser gain is that there be more population 
(atoms or molecules) in the upper, energetic level than in the lower 
level. Since excimers exist only in the upper level a population inversion 
has been obtained if any are created. In addition, all of the population 
can be extracted and contribute to the laser energy. 
The first excimer lasers were made using Xe.sub.2 * and Ar.sub.2 *. The 
rare-gas halogen excimers are based on the same principle, but are formed 
by a rare gas atom (Column VIII of the periodic chart: Ne, Ar, Kr, Xe) and 
a halogen atom (Column VII of the periodic chart: F, Cl, Br, I). An even 
newer class are the metal-halide systems such as HgBr.* Following is a 
table showing typical, or some excimer laser systems and their 
wavelengths: 
______________________________________ 
Species Wavelength 
______________________________________ 
Ar.sub.2 125 nm 
Kr.sub.2 146 
Xe.sub.2 173 
XeCl 308 
XeF 351 
ArF 193 
KrF 249 
KrCl 222 
XeBr 282 
HgBr 500 
______________________________________ 
Thus the primary problem of making an excimer laser reduces to one of 
creating the excited state atomic complex, such as Xe.sub.2 * or XeCl*. 
The various kinetic processes which result in the production of such 
species have been studied extensively. The two dominant formation channels 
are ionic combination, e.g., Xe.sup.+ +Cl.sup.- +M.fwdarw.XeCl*+M, and 
neutral collisions with excited atoms, e.g., Xe*+HCl.fwdarw.XeCl*+H. In 
the first case M represents the required third body necessary to conserve 
both momentum and energy in the collision In the second case, the H from 
the "donor" molecule HCl can perform this function, if it is available. 
Other collisions requiring a third body are also important, such as, 
Xe*+Cl+M.fwdarw.XeCl* +M. The important point is that the rate of 
formation of the excimers is dependent on the rate of such 3-body 
collisions and thus is very sensitive to the total pressure High pressures 
are necessary (&gt;1 atm) so that the formation rates exceed the natural 
radiative lifetime of the excimer levels, typically 10 nS. For a number of 
reasons, including corrosive properties, excitation efficiency, and 
optical losses, excimer lasers are usually made using a high pressure 
"inert" buffer gas (usually He, Ne, or Ar) to provide a high density of 
"third body" collision partners, "doped" with a small percentage of the 
"active" ingredients, e.g. Xe, and HCl or Cl.sub.2. For example, our best 
mixtures were 0.3% Xe, 0.05% HCl, and 99.5% Ne at a total pressure of 2 
atm or greater. For conventional discharge pumped excimer lasers the 
mixture might be 5% Xe, 0.2% HCl and 94.8% He, Ne, or Ar. 
Although high pressures are necessary and desirable for excimer lasers such 
pressures make it difficult to couple energy into the system to create the 
necessary excited atoms or ions. The first lasers used electron beam 
excitation, which is effective, but very complex, expensive, inefficient, 
and has a low pulse repetition rate. Present commercial systems use 
pulsed, avalanche electric discharge between two parallel plates on each 
side of the gas. This works but has a number of stability problems. The 
discharge is sensitive to gas composition, electrode irregularities, 
uniformity of preionization and the characteristics of the driving source. 
These result in very short gain times, and therefore laser pulses, before 
the discharge turns into an arc, or spark. The severity of this problem is 
directly proportional to the pressure and limits operation to near 1 or 2 
atmospheres. It also influences the choice of gas mixtures. For such short 
times the laser light cannot make many (usually no more than 2) passes 
through the laser and thus there is little or no opportunity to use 
standard laser techniques to refine and modify the characteristics of the 
output light. Such techniques include spatial mode control, frequency 
tuning, linewidth narrowing, and mode locking to form very short, high 
peak power pulses. All of these techniques generally require a number of 
transits of the light through the modifying element (filter, modulator, 
etc.) and thus long periods of effective excitation and gain. 
Because of its instability, the discharge can effectively excite the gas 
only for periods of about 20 nS. Special experimental techniques using 
electron beams, or x-rays have extended this, but are not presently 
commercially viable. 
Excimer lasers are important because they provide high power in the 
difficult to access UV spectral region. In addition they are efficient and 
scalable to large volume and energy. However, the present methods for 
excitation of the molecule or complex have not proved completely 
satisfactory. 
We have discovered that the molecules or complexes can be excited at high 
pressures with high power microwave energy. The microwave excitation of 
high pressure gases is much less sensitive to details of the plasma. 
The power deposition is independent of gas composition. Uniform 
preionization, or even ionization, is not critical. Local discharges or 
arcs, which are deliterious in discharge excitation, do not prevent 
microwave excitation in other regions of the plasma. 
It is an object of the present invention to provide an improved excimer 
laser and method. 
It is another object of the present invention to provide an excimer laser 
in which the lasing species are excited or pumped into their upper or 
energetic level by microwave energy. 
It is another object of the present invention to provide an improved method 
for exciting the lasing species in an excimer laser. 
These and other objects of the invention are achieved by an excimer laser 
including excimer lasing species and means for exciting or pumping said 
lasing species with microwave energy and the method of exciting or pumping 
said lasing species with microwave energy.

Referring to FIG. 1 the laser includes a resonator cavity including a 
quartz tube 11 containing the lasing species and sealed with windows 12 
and 13, disposed at Brewster angles. External mirrors 16 and 17 further 
define the cavity. Suitable means are provided for coupling the microwave 
energy into the lasing species to pump or excite the species. Microwave 
energy from a suitable source such as a magnetron is coupled or supplied 
to one end of the primary waveguide 18. The other end 19 of the waveguide 
is suitably terminated to prevent reflections. The microwave energy is 
coupled from the primary waveguide into the secondary waveguide 21 which 
contains the quartz tube 11. Typically, tubes having a 3 mm .d. and an 
active length of 40 cm are used. The waveguides are pressurized with 
SF.sub.6 to prevent breakdown at high microwave powers. The microwave 
coupler consists of a series of slots in the common broad wall of the two 
guides. The size and spacing of the slots are adjusted to provide nearly 
uniform transfer of energy along the length. Using such techniques 80-90% 
of the input microwave energy can be absorbed in the gas mixture over a 
wide range of mixtures and pressures. For low power microwave inputs the 
discharge is more stable and reproducible if it is initiated by a small 
amount of preionization. This was accomplished by placing a sealed quartz 
tube 22 containing .about.1 torr of Xe in the secondary waveguide 21. This 
low pressure "flashlamp" breaks down early in the microwave pulse 
providing simple, self-timed UV preionization of the laser mixture. At 
input powers above 0.7 MW the preionization is not necessary. In either 
case, once the discharge is initiated the power reflected to the source is 
insignificant An all-stainlesssteel closed loop system including pipes 25 
and 24 and pump 26 is used to circulate the gas mixture through the plasma 
tube. 
In one example the available microwave power was limited to about 600 kW 
and the secondary guide consisted of a standard WR-90 waveguide containing 
a 3 mm i.d. quartz tube. Laser action in XeCl* was observed at several 
wavelengths centered at 308 nm. FIGS. 2A, 2B, and 2C show the relative 
time behavior of the microwave, spontaneous fluorescence, and laser pulses 
for a mixture of 0.3% Xe, 0.05% HCl, and 99.6% Ne at a total pressure of 2 
atm. The long spontaneous emission time, over 500 ns, confirms the ability 
of microwaves to provide stable, long-term excitation of high pressure 
mixtures. While the observed laser pulse length of 150 ns is 10 times 
longer than those of discharge systems, it is surprisingly short relative 
to the fluorescence pulse length. We have not yet determined the cause of 
this behavior; some possible mechanisms include buildup of a transient 
loss, kinetic bottlenecks, inhomogeneous excitation, and thermal or 
acoustic distortions of the optical path. 
The maximum roundtrip gain in this system was about 20%. Using 5% output 
coupling the peak laser output was about 500W, representing an efficiency 
of 0.1%. The normal repetition rate was 10 Hz, but rates up to 400 Hz were 
possible. It is interesting to note that our optimum gas mixture differs 
significantly from those normally used for avalanche discharge lasers. The 
insensitivity of the microwave excitation to details of the gas 
composition has permitted us to choose gas mixtures on the basis of basic 
laser performance rather than for discharge behavior. 
In another example a cell was designed in which the secondary guide area 
was reduced by decreasing the waveguide height to about 5 mm, the plasma 
tube outside diameter. An exchangeable coupling plate was used to permit 
optimization of microwave power deposition into the gain medium. Best 
results were obtained with a slot structure over the input half of the 
discharge length, followed by a completely open region between the two 
guides. No preionization was used. FIGS. 3A, 3B and 3C show the behavior 
of this system. Following the initial breakdown, the microwave absorption 
was 100% for the first 500 ns of the pump pulse, and then decreased to 50% 
for the remainder of the pulse. Laser action occurred at the time of 
maximum fluorescence which coincided with the drop in microwave 
absorption. Compared to the other experiment we observed somewhat longer 
pulses, .about.200 ns but reduced peak powers, .about.250W. The net gains 
were higher than before, .about.40% per roundtrip, so that larger output 
coupling could improve powers and efficiency. At this higher excitation 
density the maximum pulse rate was limited to 190 Hz. 
The microwave absorption was studied up to total pressures of 5 atm. 
Although the temporal behavior of the absorption was approximately 
constant, the effects of finite skin depth became evident as a ring of 
bright fluorescence at the circumference of the plasma tube. Our 
qualitative observations indicate that as the pressure increases skin 
depths decrease and excitation becomes non-uniform. Even at a total 
pressure of 2 atm., a 1 mm wide bright ring was visible. However, the 
temporal development of this non-uniformity has not been studied and it 
seems likely that it develops late in the pulse and corresponds to the 
observed 50% decrease in absorption. Nevertheless, small skin depths and 
non-uniform electron density distributions across the plasma tube could 
ultimately limit the deposition of microwave power into high pressure 
gases. 
Although only one lasing species was tested it will be apparent to one 
skilled in the art that other excimer species such as those described 
above can be used without departing from the spirit and scope of this 
invention. 
It is seen that microwave pumping is a simple, practical technique for 
producing relatively long-pulsed excimer lasers. Pulse lengths are 
sufficient to allow mode-locking, good spatial mode control, and/or 
narrow-band frequency tuning. Such a system should be useful for 
generating well-controlled pulses for injecting into a high power 
amplifier chain. The laser can also serve as a useful stand-alone source 
of moderate UV power for spectroscopy, photochemistry, dye laser pumping, 
etc. In addition, the device is easily constructed with high-vacuum, 
halogen-compatible materials. The metal and quartz construction, and the 
absence of electrodes with their sputtering problems, along with the 
mature, reliable microwave technology should result in long system 
lifetimes and reliable hands-off operation.