Page 69 - Tunable Lasers Handbook

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

-- -
- UNIFORM BEAM CASE a
QAUSSIAN BEAM CASE a -
-

-
-
----

1.0 - I I I I
I
I- - GAUSSIAN BEAM CASE a -
9 0.0 - --- QAUSSIAN BEAM CASE b
5
2 -
-J 0.6 -
0
W -
0.4 -
J
a
-
I ---- ---_____
a 0.2 -
0
z
0 I I I I
N
FIGURE 14 Normalized linewidth as a function of N assuming a Gaussian intensity distribu-
tion for (a) Case a and (bj Case b (from Sze et al. [lS]j.

3.3 Unstable Resonator Configurations
The use of unstable resonators in high-gain, short-pulse systems is dis-
cussed by Isaev et al. [67] and by Zemskov et al. [68]. Their conclusions are
summarized by Car0 et al. [4]. To understand the formation of diffraction-
limited beams in excimer laser systems, one needs to consider how the diffrac-
tion-limited mode is developed in unstable resonators. Although the power
extracted from the gain medium is accomplished by the expanding beam in the
unstable resonator, the diffraction-limited seed is developed from ASE by the
oppositely propagating converging beam. The time required for the converging
beam to reach the diffraction limit must be short compared to the gain time of