Page 60 - Tunable Lasers Handbook

3 Tunable Excirner Lasers 41

2.2 Rare Gas Excjmer Lasers

The IC: -12; transitions in the noble gases (Ar,, Kr,, Xe,) provide VUV
laser radiation. They all exhibit continuum emission.-The low stimulated emis-
sion cross sections and short lifetimes of the upper states require high pump
rates. which necessitates an electron beam generator as a pumping source. The
expense and cumbersome nature of such systems have unfortunately limited
their availability to relatively few laboratories. Despite the dearth of low-loss
and damage-resistant optical materials in the VUV, there has been considerable
progress in line narrowing and tunability of these three laser media. The perfor-
mance of these lasers is listed in Table 4.

3. TUNING OF DISCHARGE AN5 ELECTRON BEAM
PUMPED EXCIMER LASERS

The avalanche discharge excimer laser is the most common format that is
readily available to the researcher. These devices are relatively compact and
occupy a fraction of the space of an optical table. In terms of frequency tunabil-
ity. they can potentially access the full bandwidth of the excimer laser transi-
tions, which, as we have seen in the previous sections, vary from molecule to
molecule. For a typicall homogeneously broadened single broadband transition
the full-width half-maximum bandwidth is of the order of 200 cm-1.
Typically a narrowband tunable oscillator is developed that is then amplified
in single-pass. multiple-pass, or regenerative amplifier configurations to obtain
high powers (Fig. 4), Often the amplifier may be an electron beam pumped or
electron beam sustained discharge laser. These lasers are generally low-gain,
large-volume devices with temporal gain times of a factor of 10 to 20 longer
than the commercially available avalanche dischxge lasers.

TABLE 4 Performance of Rare Gas Excimer Lasersa

Wavelength Linewidth output
Laser (nm) (nmJ Tuning elements poiier t MW) Reference

Ar2* 124.5-127.5 0.3 Prism 2 [57]
123.2-1 27.4 0.6 Grating 0.001 tjgl
126 16 [W
Kr,' 115.7 0.8 [6@1
xe2- 170-175 0.13 Prism 0.7 t611

JAdapted from Hooker and Webb [56]