Page 72 - Tunable Lasers Handbook

3 Tunable Excimer Lasers 53

76 cm it takes greater than 15 ns for the diffraction-limited mode to develop-
even at magnifications as high as 20 to 30. Thus, for a typical discharge excimer
laser gain time of 15 to 25 ns, very little time is left to extract diffraction-limited
energy in the unstable resonator.
The temporal development of the lasing beam quality in unstable cavities has
been studied in copper vapor lasers and is shown to be continuously improving
until it reaches the diffraction limit. Even if the cavity separation is halved in the
preceding example to Lo = 38 cm, Eq. (9) shows it still takes some 8.6 ns for the
diffraction-limited mode to form. In the case of injection locking of the unstable
resonator as a regenerative amplifier, the primary concern is to pick the magnifi-
cation so that the gain is below the critical gain value so that the unstable cavity
cannot go into spontaneous oscillation. This criterion, however, is different for
different excimer gases and for different pulse lengths of the injection oscillator
seed source. For a system such as XeCl where there are five broad lines lasing
into different lower states and where all the transitions cannot be treated as a
homogeneously broadened source, the injection source tuned to a frequency in
one of the transitions only lowers the gain at other frequencies within that transi-
tion. The other transitions still retain their small-signal gain. Therefore, high mag-
nification is required to keep those transitions from oscillating.

4. DISCHARGE EXCIMER LASERS

The development of dependable, long-lived excimer laser systems requires
one to address among other questions that of pulse power, gas cleanup, and gas
flow. We proceed now with a discussion of pulse power techniques that have
been used LO obtain lasing of the rare gas halide lasers in avalanche discharges.
Improved pulse power techniques are the most important key to the devel-
opment of reliable commercial laser systems because the possibilities of manip-
ulating pulse lengths, the elimination of streamer arc formation, and the reduc-
tion or elimination of high-current, fast-pulse-power circuits affect other issues
of component lifetimes, gas lifetimes, etc.
The engineering of pulse power in commercial lasers today is fundamen-
tally governed by the limited stable discharge times of the electronegative rare
gas halide gas mixtures in avalanche discharges. The stable discharge time for a
UV preionized laser system is dependent on gas pressure and electrode gap sep-
aration. Typically, for a 3-atm, 3-cm gap laser, this time is of the order of 30 to
40 ns. Thus, the problem becomes one of depositing almost all stored energy
within this time. Energy deposited subsequently goes into streamer arcs, which
do not provide lasing, and greatly shortens the gas lifetime.
The first application of this technique is by Burnham and Djeu [69] when
they separated the timing of the UV preionization surface discharge to the main
discharge in a very fast L-C inversion circuit used by Tachisto, Inc., for their