Improvement of spectrally resolved interferometry without direction ambiguity and dead zone

Abstract

Spectrally-resolved interferometry (SRI) is a very useful technique to measure distances and surface profiles based on the analysis of the spectral interferogram. The most attractive feature of SRI is to obtain the spectral phase to extract the measuring distance at once without any scanning mechanism opposed to the low coherence scanning interferometry although phase shifting techniques can be involved in SRI to improve the measurement accuracy in some cases. However, the measurement range of SRI is relatively small because of the fundamental measuring range limitations such as the maximum measurable range and the minimum measurable range. Moreover, the important issue in SRI is the direction ambiguity because it always provides the positive values, regardless of the direction. In case of measuring optical path difference (OPD) when the reference path is longer than the measurement path, the measurement result of SRI is the same as the distance in the opposite case. Then, SRI only uses one direction to measure distances or surface profiles for the linearity of the measurement results due to these fundamental characteristics although its whole measuring range is two times longer. In this investigation, we propose a very simple and effective technique to eliminate the direction ambiguity and the dead zone, which limit the measurable range in SRI. By using a dispersive material, the nonlinear spectral phase caused by the dispersion can provide useful information and determine the direction of measuring distances. In addition, the dead zone can be successfully removed by two complementary measurement results in dichroic SRI.