An asymmetric polarization-based frequency scanning interferometer : design principle

Usage of transparent objects has been increased with technological development of optical structure in display industries and micro optical component in MEMS industries. Their optical characteristics highly depend on the materials and the micro structures. When the optical measurement methods are used for dimensional quality control in their manufacturing, polarization change problem causes the measurement difficulties due to low sensitivity to measurement signals and high sensitivity to noise signals. Interferometry is one of the most promising optical surface measurement techniques. In conventional symmetric interferometers, as the intensities of the reflected lights from the reference mirror and the object are much different, it results in low contrast of interference fringes and low accuracy of dimensional measurement. In this paper, to solve this problem, an asymmetric PFSI(Polarization based Frequency Scanning Interferometer) is proposed using asymmetric polarimetric method. The proposed PFSI system controls the polarization direction of the beam using polarizer and wave plate with conventional FSI system. By controlling the wave plate, it is possible to asymmetrically modulate the magnitude of object beam and reference beam divided by PBS. Based on this principle, if target object consists of transparent parts and opaque parts with different polarization characteristics, each of them can be measured selectively. After fast Fourier transform of the acquired interference signal, the shape of object is obtained from OPD(Optical Path Difference) calculation process. The proposed system is evaluated in terms of measurement accuracy and noise robustness through a series of experiment to show the effectiveness of the system.