Page 76 - Tunable Lasers Handbook

3 Tunable Excirner Lasers 57

the gas. This is a problem of going from infinite impedance to a value that is
some fraction of an ohm. Maximum transfer efficiency occurs when the imped-
ance of the pulse power matches that of the discharge, and the charging voltage
of the storage system is equivalent to the operating voltage of the steady-state
discharge. In actuality, the discharge operates at a steady-state voltage indepen-
dent of the current within a certain operating range. Thus, a particular pulse
impedance will then define the current density of the discharge.
The decision to construct a particular pulse power impedance is a decision
about how hard we want to pump the discharge volume and it is based on
whether we wish to obtain the best efficiency by pumping at only 5 to 15 J/l atm
or in obtaining a higher energy by pumping harder (typically 30 5/1 atm) but sac-
rificing some inherent efficiency. Long eb al. [75] solved this problem with the
implementation of a high-impedance prepulse. Figure 21 shows a more recent
implementation of this idea where a saturating inductor is being used as a high-
impedance isolator for a low-impedance storage circuit. Here the prepulse must
have sufficient energy to saturate the inductor to allow deposition of the stored
energy. Now the storage circuit can be charged to the much lower operating volt-
age of the discharge and the prepulse circuit is charged to the much higher volt-
age for breakdown. The latter can be very fast since it has very little energy con-
tent. thus, also satisfying the Lin-Levatter fast voltage rise time criterion.
Analysis of pulse compression and prepulse magnetic isolation circuits is dis-
cussed in some detail in an article by Vannini et al. [76].
The type of laser that uses a very fast prepulse generates an extremely stable
discharge and, thus, is capable of long-pulse operation. Another technique that
allows for long-pulse laser oscillation is that of inductive stabilization. As dis-
cussed in the early sections of this chapter, long pulses increase the number of
round-trips in the oscillator and greatly enhance the narrow linewidths of the
laser with frequency tuning elements. A long laser pulse also allows injection
seeding of an amplifier because timing considerations between oscillator and
amplifiers are no longer a problem. This technique uses a segmented electrode
structure with each discharge segment stabilized by an inductance and was
shown capable of sustaining long lasing pulses (90 ns FWHM) in excimer gas
mixture [77,78] of XeC1, XeF, and KrF. Presently, FWHM pulse lengths have
been extended to 250 ns in XeCl and 180 ns in KrF using this technique [79,80].

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t1.V. 3x11 v,

FIGURE 2 1 Circuit used to yield a high-impedance prepulse