Knife-Edge Scanning Filtering Method for Mid-spatial frequency errors Detection in Large-Gradient Optics

Large-gradient phase components, such as continuous phase plates (CPPs), are widely used in high-power laser systems. Their mid-frequency surface errors must be strictly controlled to maintain beam quality. Power spectral density (PSD) is a key metric for evaluating these errors. However, existing PSD measurement methods rely on wavefront reconstruction, which becomes challenging when testing large-gradient components. For example, conventional interferometry often fails to produce effective interference fringes under such conditions. A knife-edge scanning filtering method was proposed for PSD measurement. Two scans in opposite directions are performed at the wavefront frequency plane. The gradient of the wavefront phase is extracted by summing the shadowgraphs from each scanning direction and computing their difference. This method directly calculates the wavefront PSD from the phase gradients, thereby avoiding wavefront reconstruction and eliminating cumulative errors from integration. It inherently ensures a large dynamic range. An integrated knife-edge module was developed for rapid alignment of the optical axis. It removes the need for manual calibration between scans and simplifies the procedure. Experimental results show that the relative errors of the measured PSD characteristic peaks are within 7%. This method is of significance for the efficient fabrication and testing of large-gradient phase components, thereby supporting the stable operation of high-power laser systems.