Calibration Methods For Long Focal-length Measurement With Talbot Interferometry

Long-focus lens is important in large optical systems, like the Space Optics and the super-power laser system, but there is no effective way to measure its focal length yet. Many methods have been presented for now. Among them, the method based on Talbot interferometry is well-used for high-accuracy measurement. In this paper, accuracy of the method is been analyzed in detail, and the keys for high-accuracy measurement are found, then calibration methods are been proposed. Both simulation and experiments are carried out to prove the advantages of the calibration methods.The calibration methods for long focal-length measurement with Talbot interferometry have two parts. The first one is about the angle of moire fringe and the inclined angle of two gratings, which are the keys for high-accuracy results according to the accuracy analysis. Thus an angle calibration method is applied to reduce the systematic errors of the angles. The results of simulation and experiments show the effectiveness of the angle calibration. The second part is the divergent beam calibration using gratings with different periods. As the collimation of the light is an uncertain parameter, divergent beam calibration and gratings with different periods are used to improve accuracy. The accuracy comparison before and after using the calibration method is been presented, and the results tell that this calibration improves accuracy well in theory.Based on accuracy analysis, the paper proposed calibration methods for long focal-length measurement. Simulation and experiments validate the methods and show their advantages. The experimental results about tests of accuracy, repeatability, reproducibility and stability perform well, and the relative error is lower than0.01%.