Abstract: The KH$_2$PO$_4$ crystal is a key component in optical systems of inertial confinement fusion (ICF). The microwaviness on a KH$_2$PO$_4$ crystal surface is strongly related to its damage threshold which is a key parameter for application. To study the laser induced damage mechanism caused by microwaviness, in this paper the near-field modulation properties of microwaviness to the incident wave are discussed by the Fourier modal method. Research results indicate that the microwaviness on the machined surface will distort the incident wave and thus lead to non-uniform distribution of the light intensity inside the crystal; in a common range of microwaviness amplitude, the light intensity modulation degree increases about 0.03 whenever the microwaviness amplitude increases 10 nm; 1 order diffraction efficiencies are the key factors responsible for light intensity modulation inside the crystal; the light intensity modulation is just around the microwaviness in the form of an evanescent wave, not inside the crystal when the microwaviness period is below 0.712 $\mu$m; light intensity modulation degree has two extreme points in microwaviness periods of 1.064 $\mu$m and 1.6 $\mu$m, remains unchanged between periods of 3 $\mu$m and 150 $\mu$m, and descends above the period of 150 $\mu$m to 920 $\mu$m.

Key words: KH₂PO₄ crystal, laser induced damage threshold, microwaviness Fourier modal method, modulation degree