Page 53 - High Power Laser Handbook

24 G a s , C h e m i c a l , a n d F r e e - E l e c t r o n L a s e r s Excimer Lasers 25

Window lifetimes of more than 10 billion pulses are achieved
with optimized contamination control systems that enable 24-hour,
7-days-a-week excimer laser operation. Passive contamination con-
trol starts by selecting an enduring electrode material as well as suit-
able materials for the laser tube and its internal components. A
thorough, controlled passivation procedure is applied to build up a
halide layer on all internal parts of the laser tube and to avoid con-
tamination build-up through reaction of the gas mixture’s halogen
components with the laser tube. Active contamination control is sup-
ported by electrostatic particle filtration and, in some cases, by cryo-
genic particle purification. In a typical embodiment, the pressure
gradient generated by the circulation fan directs a fraction of the main
gas flow toward the electrostatic or cryogenic precipitator. Driven by
this pressure difference between intake and exit port of the precipita-
tor, a steady gas flow through the device is achieved without any
additional active fan or gas pump. Corona wires are used to charge
the particles of the incoming laser gas. The gas flow speed within the
precipitator is normally reduced to below 1 m/s to allow the charged
particles to settle on the grounded purifier walls. The cleaned, parti-
cle-free gas is returned to the laser tube near the windows via baffle
boxes, which are constructed like acoustic damping devices; these
boxes are meant to create a turbulent-free gas volume in front of the
laser windows and to prevent shock waves from transporting parti-
cles to the windows. Window lifetimes of 10 billion pulses and more
are standard today in high-performance industrial excimer lasers
with well-designed precipitation systems.
Ideally, halogen gas consumption due to electrode discharging
is compensated for by halogen injections. Advanced self-learning
replenishment algorithms add very small portions of halogen gas to
the laser gas mixture without affecting the laser’s energy stability
during the injection phase. The replenishment rate depends on the
laser’s operating time, input energy, and performance parameters,
such as the high voltage level or the temporal pulse width. The
algorithms maintain the high voltage level and, therefore, keep all
essential beam parameters stable throughout a period of up to one
billion pulses with a single gas fill.

Laser Resonator
The typical resonator configuration for excimer lasers consists of
planar optics. In this configuration, the rear mirror (RM) is a plane
surface with dielectric coating that provides a high reflectivity of
greater than 99 percent. The output coupler (OC) is also a plane mir-
ror surface; the inner surface of the OC provides the reflectivity for
the laser oscillator, whereas the outer surface is coated with a dielectric
antireflection coating for optimum beam output (see Fig. 2.6).
Depending on the excimer’s wavelength and target energy, the OC’s
reflectivity can be as small as a few percent for a high-energy laser