Recently, a research team led by Professor Zhu Jianqiang and Associate Professor Jiao Zhaoyang from Shengguang II facility, has made progress in measuring the Power Spectral Density (PSD) of large-gradient wavefronts. The team proposed a wavefront detection method based on knife-edge scanning filtering, which enables precise detection of mid-to-high spatial frequency errors in wavefronts with large gradients. The related findings have been published in Optics and Laser Technology under the title "Knife-edge scanning filtering method for mid-spatial frequency errors detection in large-gradient optics."

Large-gradient phase-type optical components, represented by Continuous Phase Plates (CPP), are widely used in high-power laser systems. Their mid-to-high spatial frequency manufacturing errors must be strictly controlled to ensure downstream beam quality. PSD is a key metric for evaluating these mid-to-high frequency errors. Conventional detection methods rely on interferometers or profilers for wavefront measurement, followed by PSD analysis. However, large wavefront gradients often prevent standard interferometric methods from generating valid interference fringes or successfully demodulating phase information, thereby hindering effective PSD detection.

To address this bottleneck, the research team proposed the knife-edge scanning filtering method for PSD detection of large-gradient phase components. This method involves two scans in opposite directions across the wavefront's spectral plane using a knife-edge. By summing the series of knife-edge shadow patterns for each scanning direction and calculating their difference, the wavefront phase gradient is extracted. The method inherently offers a large dynamic range and directly computes the wavefront PSD from the phase gradient, thereby bypassing the need for wavefront reconstruction. This work is expected to provide a novel testing solution for mid-to-high frequency detection of high-end components that is both low-cost and simple to operate. It holds significant importance for the efficient manufacturing and iterative inspection of large-gradient phase optics.

Fig. 1. Optical module of the knife-edge scanning filtering method.

Fig. 2. Measurement results of CPP. (a) Phase gradient measurement results along the x-direction of two measured regions. (b) One-dimensional PSD measurement results.

The related research was supported by projects including the Chinese Academy of Sciences Strategic Priority Research Program and the Youth Innovation Promotion Association.