Page 60 - High Power Laser Handbook

32 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 33

Master oscillator OC
Line Electrodes M1
narrowing Laser gas
module

Laser window

Power amplifier
RP BS

M2
Figure 2.13 Typical dual-stage 193-nm laser for microlithography using a
ring architecture. OC: output coupler; M1, M2: mirrors; BS: beam splitter;
RP: retarder prism.

combined with a blazed grating in Littrow condition. The low output
of this oscillator is then amplified in the second chamber to reach
output powers of 90 W at a repetition rate of 6 kHz. The specific com-
bination of the two chambers can be a straight master oscillator power
amplifier (MOPA) configuration or it can use a ring geometry in the
amplifier stage (see Fig. 2.13).
The master oscillator emits a low-power beam with a narrow line
width, as determined by the line-narrowing module. The oscillator’s
output is then directed toward the amplifier by mirror M2 and gets
amplified. The retarder prism (RP) deflects the beam, which then gets
amplified to the full output. The beam splitter redirects a small por-
tion of the output beam into the amplifier for a second and third loop
of amplification. 6,7

2.3.3 LASIK
LASIK (laser-assisted in situ keratomileusis) is the dominant photo-
refractive procedure employed worldwide to correct vision by direct
ablation of corneal tissue. In 1983, Trokel and Srinivasan of IBM
started their pioneering work on photorefractive surgery using the
8
193-nm excimer laser. Ever since, this application has driven excimer
laser development to provide extremely compact lasers that (1) are
optimized for ArF operation at 193 nm, (2) meet all stringent require-
ments of the medical device regulations, and (3) provide long,
maintenance-free operation and simplicity as demanded by medical
applications. At the early stage of photorefractive eye surgery, larger
lasers with higher energy were used. Today’s trend, however, is to
use small energies in the range of 3 to 5 mJ/pulse and repetition rates
between 200 and 1000 Hz; these ranges, in combination with high-
speed precision scanners, provide a short treatment time. A typical