Page 55 - High Power Laser Handbook

26 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 27

Beam profile, long axis
120
100
Intensity [%] 60
80
40
20
0
0 20 40 60 80 100 120 140
Position (mm)
Beam profile, short axis
120
100
80
Intensity [%] 60
40
20
0
0 20 40 60 80 100 120
Position (mm)
Figure 2.7 Beam profile of high-energy excimer laser at 248 nm and energy of
1000 mJ measured by CCD camera. Two-dimensional color display; one-dimensional
intensity profile.
low-divergence resonators with curved optics have been developed
and are used in various technical variants. In the basic concept, the
resonator is made up of spherical curved mirrors that form a Casseg-
rain telescope with magnification M. The beam is expanded by the
magnification factor within the resonator, and the output divergence is
reduced (see Fig. 2.8). Practical values of M are in the range of 5 to 15,
which leads to an increase in brightness of up to 2 orders of magnitude.
2
Energy densities of more than 10 kJ/cm are achieved in the focal spot.
Variants of the low-divergence resonator use cylindrical optics that
expand the beam only in one dimension and therefore reduce the laser
beam divergence only for one desired beam axis. This is particularly
useful for equalizing the beam parameter product of the laser for both
axes or for achieving a highly focusable beam in one axis while leaving
the other axis with high divergence.

RM OC

Electrodes Laser gas

Laser window
Figure 2.8 Low-divergence resonator. RM: rear mirror; OC: output coupler.