Dual-Frequency Terahertz Heterodyne Interferometry for Nondestructive 3D Profiling of Large Step Heights

We present a dual-frequency Terahertz (THz) heterodyne interferometry system for nondestructive 3D profiling of large step heights without phase unwrapping. The system employs photonic generation of two THz frequencies around a center frequency of 306 GHz with tunable frequency spacing from 8 to 11.5 GHz, corresponding to synthetic wavelengths of 37.5 to 26.1 mm. Through phase-sensitive heterodyne detection, minimum detectable height differences of 6.7 μm at 10 GHz and 19.7 μm at 8 GHz were achieved. This allows for direct measurement of millimeter-scale discontinuities within a single 2π phase cycle. Furthermore, experimental validation in 3D-printed polylactic acid (PLA) samples demonstrates accurate measurements up to 27 mm with the measured refractive index of 1.638. Taking into account a frequency modulation of 8 GHz and the refractive index of PLA, step heights up to 29.3 mm are measurable within a single 2π phase cycle. The system also enables non-destructive profiling, passing through a 200-μm paper envelope that is an optically opaque material, and images embossed structures. This technique facilitates non-contact metrology for composite material inspection, automotive components, and multilayer coatings where optical methods fail due to material opacity.