Recently, a research team led by Prof. Zhu Jianqiang and Associate Researcher Jiao Zhaoyang from the Joint Laboratory on High-Power Laser Physics at the Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences, has achieved progress in wavefront Power Spectral Density (PSD) measurement based on knife-edge scanning. The team proposed a knife-edge lateral scanning method for wavefront detection, enabling efficient and automated measurement of the PSD of large-aperture optical components. The related findings were published in Applied Optics under the title "Knife-edge lateral scanning method for automated measurement of wavefront power spectral density."
High-power laser systems place significant demands on high-precision, large-aperture optical components. Such components are typically manufactured using computer-controlled machining techniques. However, existing fabrication technologies like magnetorheological finishing (MRF) or bonnet polishing often introduce mid-to-high spatial frequency errors. These manufacturing errors can lead to beam scattering and nonlinear intensity modulation. PSD is a crucial metric for characterizing these mid-to-high frequency errors on optical surfaces. Currently, the primary method for PSD measurement is interferometry. However, interferometric measurements require high environmental stability and often involve complex optical setups and alignment procedures.
To address this challenge, the research team proposed utilizing a knife-edge scanning and stitching method for wavefront PSD measurement. The knife-edge test is a traditional optical testing method originally used for qualitative assessment of mirror aberrations. This research established an equivalence relationship between wavefront PSD and shade pattern PSD through knife-edge filtering. This allows the direct extraction of wavefront PSD from the shade pattern, eliminating the need for wavefront reconstruction. Furthermore, by employing scanning and stitching, the method removes the interference fringes that affect the original knife-edge measurement process, thereby relaxing the stringent requirements on the knife-edge cutting position. This method offers advantages of simplicity of operation, high efficiency, and cost-effectiveness, making it particularly suitable for in-situ measurement of mid-to-high frequency errors on optical components during manufacturing. This work is of significant importance for improving the iterative efficiency of optical component fabrication and inspection within high-power laser systems.
This research received support from the National Natural Science Foundation of China and other projects.
Fig. 1. (a) Schematic diagram of the principle of the knife-edge lateral scanning method. (b) Measurement effect diagram: from left to right are the loaded phase map,a series of shadowgraphs collected as the knife edge moves from position _{1}$ to _{{n}}$,and the processing result obtained through the algorithm described in the text.
Fig. 2. PSD-1D measurement results in the $y$ direction of the single-frequency samples without frequency aliasing.
