High-speed Terahertz Time domain spectroscopy (High-speed THz-TDS) system is developed on the basis of traditional THz-TDS. It can effectively compensate for the shortcomings of traditional THz-TDS. High-speed THZ-TDS features fast spectrum scanning speed and High spectrum resolution. But it needs a stable local clock signal to lock the repetition rate of the laser in the system. In this paper, a method of taming OCXO with China's BeiDou Navigation Satellite System (BDS) is proposed to generate stable clock signals. Using this signal as the local clock for the high-speed THZ-TDS. The cost of this method is 2,600 yuan, less than one-tenth of the price of rubidium atomic clocks, without significantly reducing stability. This method provides a high stability, low cost clock signal for high-speed THZ-TDS system, and has long term stability.
Terahertz imaging has a wide range of applications in the field of nondestructive testing. In this paper, a novel thickness-testing method based on active terahertz imaging with a non-cooled focal plane array terahertz camera is proposed, which uses the poly tetra fluoroethylene (PTFE) as the object to study. Firstly, we capture the active terahertz images of PTFE wafers of different thickness using IRXCAM-THz-384 camera. Second, by using the curve fitting method, interpolation method, grey theory model, neural network and support vector machine, the relationship between the thickness of PTFE wafers and grayscale is predicted. Then the accuracy of different methods is compared. Finally, we establish the thickness-grayscale relationship model. The experimental results show that the predicted greyscale data is close to the actual data, and the error range can be controlled within 1%, which verifies the accuracy of the model and meets practical engineering application standards.
Chinese traditional lacquerwares, ornamental objects decorated by multiple, composite layers of lacquers, are one of the most significant expressions of China art. Existing X-ray radiography and other detection technologies will cause irreparable damage to the lacquerware due to their high photon energy. Terahertz time-domain spectroscopic reflectometric imaging (THz-TDSRI) system being capable of highlight interfaces between layers in a stratigraphic buildup, could be a complementary technique for obtaining structural information about lacquered objects. In this study a Chinese lacquerware has been investigated by terahertz (THz) reflectometric imaging. The investigated lacquerware belongs to Palace Museum. The lacquerware is a decorative work made of a wood panel covered with several layers of lacquers to depict motifs. In order to detect lacquerware, the THz-TDSRI system was built by us. For imaging applications, the source of contrast is the optical density of materials and in reflection geometry the back-reflected THz pulse is analyzed for reflections originating from the various interfaces present between the various sample layers. The value of the electric field measured for each spatial coordinate (X, Y) of the scanned areas has been used for the bidimensional visualization of the lacquerware. The displaying method used in THz imaging application is pseudo coloring. Utilizing THz-TDSRI system, we observe non-invasively buried layers of the lacquerware, including faults in the wood layer, with the lacquerware was not injured. This shows that THz time-domain spectroscopic reflectometric imaging is a non-destructive inspection method for lacquer ware and has great potential in the future.
We analyzed the reasons why ZnTe generates terahertz echo. Based on the Gaussian beam, the terahertz signal with echo is produced by simulation. In this paper, the deconvolution algorithm is further improved. The linear assumption that sample absorption is proportional to frequency is abandoned. We studied the method of echo based on Echo State Network. Set 200 neural units in each layer of the network. We specify 250 rounds of training. To prevent the gradient from exploding, set the gradient threshold to 1. Specify an initial learning rate of 0.005. After 125 rounds of training, multiply it by a factor of 0.2 to reduce the learning rate. This method largely removes echo. But it has not been completely removed, and there is still less than 5% of the echo signal remaining. We consider the reason may be that the number of neurons, the number of iterations, weight setting and other parameters have yet to be optimized. Although this method does not completely remove the echo, but provides a new method for removing the echo. By optimizing parameters such as the number of neurons, the number of iterations, weight setting and increasing the amount of training data, this method is expected to become a new method for echo removal.
The light field includes the direction information and location information. Light field imaging can capture the whole light field by single exposure. The four-dimensional light field function model represented by two-plane parameter, which is proposed by Levoy, is adopted in the light field. Acquisition of light field is based on the microlens array, camera array and the mask. We calculate the dates of light-field to synthetize light field image. The processing techniques of light field data include technology of refocusing rendering, technology of synthetic aperture and technology of microscopic imaging. Introducing the technology of light field imaging into THz, the efficiency of 3D imaging is higher than that of conventional THz 3D imaging technology. The advantages compared with visible light field imaging include large depth of field, wide dynamic range and true three-dimensional. It has broad application prospects.
The terahertz monolithic integrated device is to integrate the pumping area of the terahertz generation, the detection area of the terahertz receiving and the metal waveguide of terahertz transmission on the same substrate. The terahertz generation and detection device use a photoconductive antenna structure,the metal waveguide use a microstrip line structure. The evanescent terahertz-bandwidth electric field extending above the terahertz transmission line interacts with, and is modified by, overlaid dielectric samples, thus enabling the characteristic vibrational absorption resonances in the sample to be probed. In this device structure, since the semiconductor substrate of the photoconductive antenna is located between the strip conductor and the dielectric layer of the microstrip line, and the semiconductor substrate cannot grow on the dielectric layer directly. So how to prepare the semiconductor substrate of the photoconductive antenna and how to bond the semiconductor substrate to the dielectric layer of the microstrip line is a key step in the terahertz monolithic integrated device. In order to solve this critical problem, the epitaxial wafer structure of the two semiconductor substrates is given and transferred to the desired substrate by two methods, respectively.
Since many vibrations and rotational levels of biomolecules fall within the THz band, THz spectroscopy can be used to identify biological samples. In addition, most biomolecules need to maintain their biological activity in a liquid environment, but water as polar substance has strong absorption to the THz wave. Thus, it is difficult to detect the sample information in aqueous solution using THz wave. In order to prevent the information of biological samples were masked in the solution, many research methods were used to explore how to reduce the water absorption of terahertz. In this paper, we have developed a real-time chemical methodology through transmission Terahertz time-domain spectroscopy (THz-TDS) system. The material of Zeonor 1020r is used as substrate and cover plate, and PDMS as channel interlayer. The transmission of the empty microfluidic chip is more than 80% in the range of 0.2-2.6 THz by THz-TDS system. Then, experiments were carried out using chips, which were filled with different volumes of 1, 2- propanediol, and it has been proved that the microfluidic chip could reduce the water absorption of terahertz. Finally, in order to further explore the reduction of terahertz to water absorption, we inject different concentrations of electrolyte to the chip. The results show that with the addition of different electrolytes, terahertz transmission line has evident changes. It can be taken into account that the electrolyte has different effects about the hydrogen bonds in the aqueous solution. Some of them can promote water molecules clusters, while others destroy them. Based on the basis of microfluidic chip, the discovery of this phenomenon can provide a way that reduces water absorption of terahertz. This work has laid a solid foundation for the subsequent study in reducing water absorption of terahertz.
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