An interferometric system for measuring thickness of parallel glass plates without 2π ambiguity using phase analysis of quadrature Haidinger fringes

Abstract

An interferometric system is proposed for measuring the thickness of parallel glass plates by analyzing Haidinger fringes. Although a conventional Haidinger interferometer can measure thickness without 2π ambiguity using positions of peak and valley points in the interferogram, measurement accuracy is directly affected by the number of these points involved in the calculation. The proposed method obtains phase values over the entire interferogram by analyzing the quadrature Haidinger fringes generated by a current modulated laser diode. Therefore, it can achieve high speed measurement and nanometric resolution without mechanical rotation and thickness limitation of specimens. In the experiments, the standard deviation of repeated thickness measurement was evaluated as less than 0.3 nm, and the measured thickness profile of the proposed system agreed with that of a conventional thickness interferometer within ±15 nm. We also discussed the required accuracy of refractive index of specimens to implement the proposed method successfully and presented an exemplary measurement result of a multi-layer coated sample having a discontinuous thickness profile.