## 2 Materials and Methods
### Proposed Algorithm  
Inspection of multilayered structures using the pulsed terahertz technique enables detection and monitoring of separate layers. If there exists any noticeable difference between the dielectric parameters of the utilized polymer resin and fabric, the reflection of the excitation pulse will be observed. The behavior of a single-layer material is presented in Figure 4. The pulse is reflected and transmitted through air-material and material-air interfaces. One can observe some part of the energy in the form of the electromagnetic pulse becoming trapped in the considered layer (resonator). This pulse is repeatedly reflecting off the layer boundaries and, simultaneously, a part of the energy is transmitted out of the layer. Thus, depending on the layer thickness and dielectric parameters, each excited layer can be considered as a source of delayed and attenuated copies of an excitation pulse in both directions (forward-incident and backward-reflected). This effect, in the case of multilayered structure imaging, causes more problematic analysis of the results. Based on this mechanism we can analyze the response of multilayered structures. Each layer in the material acts as resonator, thus the recorded response consists of air-material (FSR), layer-layer and material-air (BSR) interfaces reflections as well as huge amount of delayed and attenuated additional pulses. If the number of layers is rather low (e.g., 3-6), the pulses caused by layer interfaces are clearly visible in the measured signal and the resonators causing additional pulses can be neglected (direct reflection of the layer-layer interface is stronger than the attenuated copy). The exemplary signal acquired in the case of a six-layer material is shown in Figure 5. If the number of layers is higher (e.g., 20) the energy of the main pulse after propagation through many layers is  
Figure 3: Geometries utilized in composite materials evaluation using pulsed terahertz imaging: (**a**) transmission setup; (**b**) reflection “V” setup; (**c**) reflection setup with beam splitter; and (**d**) proposed geometry-double-sided measurement using reflection setup. low (because of reflections of previous layers), becoming comparable with the energy of the resonator effect caused pulses received by the PCA detector. Thus, just reflections from the layers situated close to front surface can be observed with an acceptable signal to noise ratio (SNR). An example of such a situation is presented in Figure 6. Information about the layers situated close to the back surface is not available in this case--it is not possible to measure the layer thickness. Fortunately, the detection of defects like delamination, voids, and inclusions is still possible because of their noticeable difference of permittivity compared with fibers and polymer resin. Figure 4: Interaction of the Gaussian pulse with single dielectric layer; results of the 1D FDTD simulation for various time instances. Figure 5: Signals acquired in case of inspection of layered composite material (basal fiber-reinforced) consisting of six layers: (**a**) A-scan; and (**b**) B-scan; FSR-front surface reflection, BSR—back surface reflection. In order to obtain detailed information about the layers close to back surface of examined material the algorithm of double-sided terahertz inspection is proposed. It is presented in Figure 7. First, a double-sided measurement arrangement, shown in Figure 3d, is performed. The application of this setup is possible using a two-channel terahertz spectroscopy (e.g., Tray 4000 from Picometrix). A more time consuming equivalent of the proposed arrangement is a separate measurement using a reflection setup (Figure 3c) from both sides of the evaluated material. This method of inspection needs an additional data registration process in order to match the obtained results to the same (_x,y_) position. The proposed algorithm can be utilized in both cases. In order to increase the ability of separate layers distinguishing, a deconvolution operation was performed.