
332 Ultrashort Sources I: Fundamentals
will recede towards the focus A. The incident beam parameters can be adjusted
such as to create a tight round focal spot.
To assess the importance of astigmatism, let us consider a simple ring cavity
with two beam waists. We assume that one beam waist is formed by two lenses
of 15 mm focal distance, spaced by 30 mm. The other waist is formed by two
mirrors of focal distance f = 25 mm separated by a distance d. The distances
between the two waists are1mand3mina4mperimeter ring cavity. From
the expression for the ABCD matrix for this cavity one finds that the stability
range is f < d < 1. 01f . If astigmatism because ...