Optical thickness measurement with multi-wavelength THz interferometry

We report unambiguous thickness measurement with an all-optical THz source. The optical thickness variation of a test target was measured in a Mach-Zehnder interferometer to approximately 0.5% of the illumination wavelength using an optical parametric THz laser. The frequency of the laser was continuously tuneable, enabling a synthetic wavelength to be produced by sequential illumination at discrete frequencies, thus extending the unambiguous measurement range to half the synthetic wavelength. The all-optical source provides some advantages with respect to opto-electronic and electronic sources, particularly measurement speed and resolution.