Page 52 - 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

The position of the electrodes—anode (a) and cathode (c)—is
shown for reference in Frame 2 (Fig. 2.5). As shown in the figure, an
expanded beam from a green helium-neon (HeNe) laser was directed
longitudinally through the discharge; the picture frames were then
recorded which synchronized with the discharge. The laser repetition
rate was fixed to 4 kHz. The picture shows the variation of the gas
speed with the driving motor set to frequencies of 40 Hz, 50 Hz,
60 Hz, and finally 70 Hz. The discharge region appears black because
the change in its refractive index by the heated gas optically “blocks”
the light. The 40-Hz setting (Frame 1) corresponds to a slow gas
exchange speed, the gas volume of the discharge that is the 4th pulse
and the leading pulses 3, 2, and 1 are seen. Because the spacing
between the gas volumes is small, the laser action is affected by this
disturbance of the gas and becomes unstable. With increasing gas
flow speed, the clearing increases until, at 60 Hz, sufficient clearing
and stable operation of the laser is observed. The typical gas circulation
speed is about 25 m/s for high-energy lasers and up to 50 m/s for high-
power and high-repetition-rate industrial excimer lasers. For high-
power excimer lasers that use large-discharge cross sections and high
repetition rates, clearing the gas volume between consecutive laser
pulses becomes a demanding task. The built-in flow-loop system,
which resembles a flow nozzle, optimizes the flow in the discharge
region to avoid nonuniformity and flow separation on the electrode
surfaces. Careful design of the gas flow-loop system using wind tun-
nel simulation allows gas speed and flow uniformity to be optimized
to enable large cross sections and high repetition rates.
Excimer lasers typically operate with 2 to 4 percent conversion
efficiency between the electrical input power and the UV output
power. The surplus energy is removed efficiently as excess heat. The
forced circulation in the laser tube brings the laser gas, heated by the
laser discharge, to a heat exchanger, where it is recooled to the correct
operating temperature. As with all gas laser cooling systems, efficient
heat transfer between the laser gas and the heat exchanger represents
a challenge. The heat exchanger, which in most designs uses water as
a cooling medium in a closed- or open-loop system, needs sufficient
contact area to provide high temperature stability, especially at high
pulse repetition rates. On the other hand, the gas flow resistance
across the heat exchanger must be small in order to be compatible
with the cross flow fan characteristic. For optimum output, the laser
tube windows must be protected against contamination from electro-
chemical erosion processes in the discharge. The laser tube window’s
outside surface is usually purged by dry pure nitrogen to remove all
gaseous contaminations and impurities present in the environmental
air. Consequently, purge systems have become a standard feature on
all high-power and high-repetition-rate excimer lasers. In addition,
active and passive contamination controls are necessary to keep the
inside of the tube windows clean.