Page 61 - Tunable Lasers Handbook

42 R. C. Sze and D. G. Harris

DISPERSIVE OSCILLATOR AMPLIFIER
ELEMENTS GAIN MEDIUM GAIN MEDIUM
I I

NARROWBAND TUNED OSCILLATOR AND SINGLE PASS AMPLIFIER

DISPERSIVE OSCILLATOR AMPLIFIER
ELEMENTS GAIN MEDIUM GAIN MEDIUM

NARROWBAND TUNED OSCILLATOR WITH REGENERATIVE AMPLIFIER
FIGURE 4 Generalized oscillator-amplifier configurations. Amplifier stages incorporating
unstable resonator optics can also be known as forced oscillators.

The temporal characteristics of the oscillator must meet a number of
requirements in terms of obtainable linewidths and in terms of compatibility
with the temporal characteristics of the amplifier. The narrowness of the line-
width using a dispersive element, such as a grating or multiple-prism arrange-
ment, is typically improved by an order of magnitude or more over single-pass
linewidths when many round-trips are available in the oscillator [62]. Thus, the
gain time in the oscillator is an important factor in the achievable linewidth of an
excimer laser system. The gain time of the oscillator must also be compatible
with the gain time of the amplifier system. It is, however, possible to have oscil-
lator gain times that are shorter than the amplifier system and still extract energy
from the amplifier for the full gain time of the amplifier.
In single-pass and multiple-pass configurations, this can be done by beam-
splitting the oscillator pulse and restacking the pulses with appropriate time
delays so that the total pulse length matches the total gain time of the amplifier. In
a regenerative amplifier configuration, a short-pulse oscillator can control the
total gain time of the amplifier if the reflected field of the amplified oscillator
pulse from the first pass is sufficient LO control the frequency output of the second
pass and so forth. Generally, the degradation of the narrow frequency field is such
that the technique is not effective when factors of 10 in gain times between the
oscillator and amplifier are involved. The success of the latter method is generally
based on the conservatism of the regenerative amplifier design. In general, care
should be taken to ensure the magnification is large enough so that the amplifier
is incapable of going into oscillation without the injected oscillator pulse.
Remember that the wavelength purity of the amplified pulse cannot be better than
the ratio of the injected oscillator intensity over the amplified spontaneous emis-
sion (ASE) in the amplifier radiated into the solid angle of the oscillator beam. It