Abstract
Unstable resonators are typically employed to obtain reasonable beam quality from copper laser oscillators. However, due to relatively long round-trip times in the resonator (~15 ns) relative to the inversion time (~50 ns) only three to four round trips are possible. The angular divergence of a copper laser pulse starts at the aspect angle of the resonator and reduces by the magnification M on each succeeding round trip.1 For our copper lasers this initial full angle divergence is θl ≃ 103 × (2.44λ/D). With an M= 15 unstable resonator it is not until the third round trip that the beam divergence approaches the diffraction limit. Since the gain is dropping by the third round trip in the resonator, the resulting laser pulse has most of its energy in highly diverging components and only a small fraction in the later lower divergence component. Propagation of this beam through a relatively small f/No. optical system causes nearly three-fourths of the pulse energy and two-thirds of the pulse length to be lost from the transmitted beam.
© 1986 Optical Society of America
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