Scale length of optics low-frequency phase aberration

The scale length is an important parameter of the low-frequency phase aberration. The mathematics model to get t he scale lengths of the phase aberration caused by rectangular optics is established and the scale lengths of some Nd-glass amplifiers are calculated. And the characteristic values of slab-amplifiers are got. The relationship between the scale length and the spot size is studied. The change of the scale length in propagation is also analyzed.