A single-element interferometer for measuring parallelism and uniformity of transparent plate

Abstract

The transparent plates (such as organic glass, plastic plate) are widely used in the construction industry, high-tech products and scientific research applications, and its parallelism and uniformity measurement in the manufacture and quality control become more and more inevitable. Interferometer is a label-free, high-precision, and high-efficient device that can be used in many fields. According to a single-element interferometer, we demonstrate a measurement for the parallelism and uniformity of transparent medium. Beam-splitter cube is a key component. Half of plane wave laser source passes through the measured medium and the remaining half directly passes through the air, then these two halves with different optical paths meet in the beam-splitter cube. The parallelism or uniformity is determined by calculating interference fringe shift number during rotating the measured sample. The coherent beam is divided into two parts by a beam-splitter, one passes through the lens and then arrives at a photoelectric counter, and the other arrives at the observation plane of the charge-coupled device. The photoelectric counter is used to count the integer part of fringe shift number during rotating the sample; and the decimal part can be detected by calculating the phase difference of the two interferograms captured before and after rotation. The measurement principle of the proposed device is analyzed in detail, and the numerical simulations of the fringe shift number and the gray level changing with the sample rotation angle, the thickness and the refractive index of the sample are carried out. The simulation results show that the bigger the rotation angle, thickness and refractive index of the sample, the greater the fringe shift number will be. Therefore, the measurement accuracy can be improved by increasing the rotation angle and the thickness of the sample. In addition, we also simulate the measurement processes of two kinds of samples, which are unparallel and inhomogeneous transparent plates. The simulation results prove the feasibility and high accuracy of the proposed method. Finally, the optical experiment is conducted to demonstrate the practicability of the present device. The parallelism of a cuvette used for more than one year, is tested by our device. The results show that the difference in thickness between the cuvettes is on a micron scale, the peak-valley (PV) value is 9.92 m, and the root mean square (RMS) value is 2.2 m. And the difference between the contrast test results and the results from the proposed method is very small, the PV value is 0.569 m, and the RMS value is 0.131 m. The stability and repeatability of the proposed setup are tested in the experimental condition. The mean value and standard deviation of the fringe shift number during 30 min are 0.0012 and 0.0008, respectively. These results further testify the high accuracy and stability of our method. In conclusion, the performance of our measurement method is demonstrated with numerical simulation and optical experiment.