Spatial frequency controlled by a phase carrier in spatial periodic modulation

An approach to controlling the spatial frequency of spatial periodic modulation (SPM) on focal plane has been presented. This approach is based on the application of phase carrier in the near-field beam. The phase amplitude of phase carrier is able to modulate the intensity of the spatial frequency of SPM from the maximum to 0. In addition, the period T of phase carrier determines the position of the strongest spatial frequency of amplitude SPM. When the period is increased, the position of the strongest spatial frequency is closer to the center. In order to avoid introducing new modulation to the output near-field beam, it is necessary to choose appropriate value of T to ensure that the strongest spatial frequency is beyond the cutoff frequency of pinhole. The feasibility of this method is verified by theoretical analysis and experimental results. This method provides a different perspective to solve the problems that spatial filter encounter, such as the pinhole closure and the damage to the material of pinhole filter periphery, in high power laser systems, as well as improving the quality of output near-field beam.