Polarization-sensitive white light interferometer with an autofocus device

A polarization-sensitive white light interferometer (PSWLI), which is a promising technique that can be used to measure the stress induced birefringence, is developed. The use of wide-spectrum light source brings to PSWLI a resolution in the micron range, but the difficulty arises when searching for the best fringe contrast within the extremely short coherence length, especially for the Linnik interference configuration. To tackle this problem, an autofocus device based on the improved astigmatic method is embedded in the PSWLI system to firstly automatically determine the best foci of the reference mirror and the test sample. Then the minimization of optical path difference (OPD) of two interference arms are implemented by the root mean square fringe contrast function combined with a 4×4 pixel binning of the CCD camera. The autofocus time is no more than 0.3 second and the minimization of OPD has a speed of 2.2 min/mm. Finally, the developed PSWLI system is calibrated by the Berek compensator and the birefringence measurement result is obtained by simultaneously calculating the phase retardation, the optical axis orientation, the reflectance and the stress map of a stress induced birefringence sample.