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A method for removing spurious fringes based on characteristic spectrum band-stop filter

In the interference test process of a parallel flat optical element, the light beam after multiple reflections on the front and rear surfaces of the element with the wavefront to be measured often cause multi-surface interference, forming spurious fringes on the interferogram, which will introduce large errors to the wavefront measurement. Spurious fringes mainly have a great impact on the parameters such as wavefront gradient root mean square (GRMS) and mid-spatial-frequency power spectral density (PSD). The RMS value of the wavefront containing spurious fringes is generally significantly larger than the true value, which will affect the accurate measurement and evaluation of the wavefront quality of optical components. Existing spurious fringes suppression methods often have the disadvantages of multiple adjustment steps in the test process or high hardware requirements, and it is difficult to match the requirements for rapid and high-efficiency test of high-precision optical flat components. This paper proposes a method for removing spurious fringes in interference test based on characteristic spectrum band-stop filter, which can achieve accurate removal of spurious fringes. First, by using the ZoomFFT algorithm to zoom up the spectrum of the wavefront data points, the effective identification of the spectral characteristics of the spurious fringe is realized; then the band-stop filter in a specific area is used according to the spectral characteristics of the spurious fringes, only the frequency spectrum of the spurious fringes is removed, and the wavefront data of the component to be tested is completely retained without changing its own shape; the Quad-Flip operation and error function filter window are used for spectrum filtering, which effectively reduces Gibb's noise in the frequency domain due to the sudden truncation of the input data edge during FFT transformation. The transmitted wavefront of a fused silica element with a diameter of Φ100mm was tested on the ZYGO static interferometer, and the test results contained a large number of spurious fringes. After processing by this method, the spurious fringes were removed. The mid-spatial-frequency wavefront RMS of the component is reduced from 5.365nm to 3.678nm. The method does not need to add additional hardware and tedious measurement and adjustment operations, the calculation is fast, and the spurious fringes removal is accurate.