Since its development in the last century, the performance of synchrotron light sources has been-increasingly improved, providing a new and efficient platform for research in many fundamental disciplines such as physics, chemistry, materials science, and life sciences, and helping to achieve many cutting-edge results. In synchrotron light sources and X-ray free electron laser devices, grating monochromators and spectrometers are crucial for both beamlines and experimental stations. Monochromators variable-line-spacing (VLS) grating is simple and easily achieve high spectral resolution and transmission efficiency. Thus, VLS gratings have become the dominant
The main methods for measuring grating line densities include interferometry, diffraction, and long trace profiler (LTP) methods. These methods have their advantages and disadvantages. To meet the need of measuring line density of VLS grating, LTP with stitched data is used. In order to complete the Hefei Light Source photoelectron spectral beam line maintenance project, the Hefei Light Source independently developed a VLS grating. In order to characterize the line density more precisely, this paper proposes an improved stitching measurement method using LTP. In particular, the incident angle of each segment is inverted to improve based on the proposed stitching measurement method of LTP. In fact, for previous stitching methods, the angle of incidence at the central of each segment was determined from the angle of the deflector and the relative diffraction angle within each segment, which was based on the angle value given by the LTP detector. However, the angular error of the deflector is not negligible. In this method, first, taking the midpoint of the VLS grating as a reference point, the line density and incidence angle of the reference point is determined. Moreover, with the data of the reference point, the line densities of other positions of this segment are measured. Second, it is to measure the line density distribution of the next segment.
The LTP stitching measurement method is used to test VLS grating parameters using a segmented overlapping data processing method, which avoids angular errors in the deflector and provides a significant improvement in repeatability. The consistency of the test results is better than 1.13×10-6 (RMS) for the same VLS grating using different overlapping rates, and the PV value of the repeatability deviation of the test data decreases significantly with increasing overlapping rate. Therefore, a reasonable selection of step length and the overlapping ratio of two adjacent segments can improve measurement accuracy while suppressing splicing errors and ensuring a certain level of measurement efficiency. However, due to the relative accuracy of the turntable, the deviation of the absolute value of the central density is about 0.1 lp/mm, which needs to be improved by using the relative calibration method. This will be followed by an experimental approach to investigate the effect of central line density error measurements on variable-line-spacing grating parameters and experimental verification.