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.

Chondroitin sulfate proteoglycans impede myelination by oligodendrocytes after perinatal white matter injury.

Hypomyelination is the major cause of neurodevelopmental deficits that are associated with perinatal white matter injury. Chondroitin sulfate proteoglycans (CSPGs) are known to exert inhibitory effects on the migration and differentiation of oligodendrocytes (OLs). However, few studies describe the roles of CSPGs in myelination by OLs and the cognitive dysfunction that follows perinatal white matter injury. Here, we examined the alterations in the expression of CSPGs and their functional impact on the maturation of OLs and myelination in a neonatal rat model of hypoxic-ischemic (HI) brain injury. Three-day-old Sprague-Dawley rats underwent a right common carotid artery ligation and were exposed to hypoxia (6% oxygen for 2.5h). Rats were given chondroitinase ABC (cABC) via an intracerebroventricular injection to digest CSPGs. Animals were sacrificed at 7, 14, 28 and 56days after HI injury and the accompanying surgical procedure. We found that the expression of CSPGs was significantly up-regulated in the cortical regions surrounding the white matter after HI injury. cABC successfully degraded CSPGs in the rats that received cABC. Immunostaining showed decreased expression of the pre-oligodendrocyte marker O4 in the cingulum, external capsule and corpus callosum in HI+cABC rats compared to HI rats. However HI+cABC rats exhibited greater maturation of OLs than did HI rats, with increased expression of O1 and myelin basic protein in the white matter. Furthermore, using electron microscopy, we demonstrated that myelin formation was enhanced in HI+cABC rats, which had an increased number of myelinated axons and decreased G-ratios of myelin compared to HI rats. Finally, HI+cABC rats performed better in the Morris water maze task than HI rats, which indicates an improvement in cognitive ability. Our results suggest that CSPGs inhibit both the maturation of OLs and the process of myelination after neonatal HI brain injury. The data also raise the possibility that modifying CSPGs may repair this type of lesion associated with demyelination.

[Identification of Chinese herbal medicines based on terahertz spectroscopy analysis].

In order to study the feasibility of the identification of Chinese herbal medicines based on terahertz spectroscopy, the optical characteristic of astragalus, angelica, eucommia and three kinds of astragalus samples with different impurities in the frequency range 0.2-2.2 THz were researched by terahertz time-domain spectroscopy (THz-TDS), and their time-domain spectra, the frequency spectra and the absorption spectra were obtained at room temperature. The results indicated that the time-domain spectra, frequency-domain spectra and absorption spectra of astragalus, angelica, and eucommia have large differences in such a frequency range, the frequency-domain spectra and absorption spectra of three kinds of astragalus samples with different impurities are similar but there exists distinct difference. These researches proved that it is feasible to use terahertz time-domain spectroscopy to identify Chinese herbal medicine and provided a new method for Chinese herbal medicine quality control.