Self-Calibrated Absolute Thickness Measurement of Opaque Specimen Based on Differential White Light Interferometry

In this article, we present a self-calibrated absolute thickness measurement method for opaque specimens based on fiber-optic white light interferometry. A new type of self-reference probe, which is composed of a fiber ferrule, an integrated beam splitter, and a gradient index lens, is presented. In our setup, two probes of this kind are fixed facing each other, and the distance between them can be measured. Then we insert a specimen between the probes and measure the distance between the specimen surfaces and probes on both sides. This way, a self-calibrated measurement of specimen thickness can be realized without the traditional calibration process using a standard thickness sample. Thus, the uncertainty component introduced by the standard thickness sample can be eliminated. The performance of the proposed method is evaluated by measuring standard thickness samples calibrated by the National Institute of Metrology of China, and the results show a good agreement with the nominal values. Also, we analyzed several key uncertainty components, as well as the reliability of our method. The calculated extended measurement uncertainty (k = 2) is 0.06 μm for Cr specimen with a nominal thickness of 10.56, and 0.08 μm for Ni specimen with a nominal thickness of 31.46 μm.