Page 48 - High Power Laser Handbook

20 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 21

f

e

d
a

c
b

Figure 2.2 Example of an excimer laser tube with surface corona
preionization. (a) Circulation fan, (b) electrostatic filter, (c) laser tube
(pressure vessel), (d) heat exchangers, (e) electrodes, (f ) surface corona
preionization.

excitation. This method provides outputs of up to several joules and
repetition rates in the kHz range. The discharge unit is integrated into
the laser tube, which is designed as a high-pressure gas vessel (Fig. 2.2).
The laser gas mixture in the laser tube consists of a 0.05 to 0.50
percent halogen component, a 3 to 10 percent inert gas component,
and the buffer gas (helium or neon) at a pressure of 3 to 6 × 10 Pa.
5
Excimer lasers use short excitation pulses that terminate the discharge
before the onset of instabilities, which leads to the typical short laser
pulses of 10 to 30 ns.

Discharge Circuit
The technique to produce and control the homogeneous gas discharge is
crucial for the perfor mance of an excimer laser. The most important parts
of this technique are the preionization of the laser gas, the discharge elec-
trodes, the gas flow system, and the high-power discharge circuit.
In the context of an excimer laser, the term preionization means
uniformly seeding the discharge volume with electrons and ions
before initiating the main discharge. Sufficient electron density of
9
–3
10 to 10 cm is required to achieve a uniform glow discharge and
7
to avoid instabilities. The electrode structures and preionization
techniques used for excitation of the laser determine the cross sec-
tion and quality of the discharge, as well as the laser’s energy out-
put and efficiency. Commercial high-power industrial excimer
lasers usually employ either spark discharges or surface corona