The contact-type thickness measuring machine was developed as a standard instrument in Korea. The major target of this instrument is silicon wafers. The measurement range is 5 mm with a measurement unceratinty of 100 nm.
The optical comb of the femtosecond pulse laser can determine abosulte distances without a phase ambiguity problem. By analyzing the interference spectra, the absolute distance can be measured with a rate of up to 1 kHz and a resolution of 10 nm in the measurable range of several meters.