Nonlinear error correction for Terahertz FMCW System by a new beat frequency estimation method.

Terahertz (THz) frequency modulated continuous wave (FMCW) technology is a means of nondestructive testing. The signal's nonlinearity is an unavoidable problem in the daily application of THz FMCW technology. The signal's nonlinearity will lead to the spectrum broadening of the FMCW's beat frequency (BF) signal, which degrades the range resolution and result in distance-measuring error. Traditional methods require additional hardware or require a lot of computation, which are not conducive to the miniaturization of the system and real-time measurement. A novel method for correcting the nonlinear error of THz FMCW technology has been proposed and demonstrated in this article. In the proposed method, the windowed Fourier transform (WFT) is introduced to estimate the BF corresponding to the measured target, according to the linearity distribution of voltage-controlled oscillator (VCO). In this way, the measured target's BF can be accurately estimated from the unprocessed BF signal with a poor linearity. From the estimated BF of the reference target, the non-linear compensation coefficients are calculated. With the non-linear compensation coefficients, the non-linearity of the output BF signal can be calibrated. The results of simulations and experiments show that the proposed method allows the range resolution of an FMCW system to reach the theoretical limit.

Method for defect contour extraction in terahertz non-destructive testing conducted with a raster-scan THz imaging system.

In this paper, a neoteric algorithm based on a two-dimensional continuous wavelet transform is developed to get the defect contour in the terahertz (THz) non-destructive testing result obtained from a raster-scan frequency-modulated continuous-wave (FMCW) THz imaging system. In order to prove the method's validity, an experiment is carried out. The result of the experiment shows that the method allows one to extract the defect contour from the THz FMCW interference with severe stripe noises. Moreover, the relative error of defect area between the actual value and that obtained from the extracted defect contour is no more than 3.03%. This means that the method provided an effective and exact way to extract a defect contour in THz FMCW interference.