Selection of Spectral Range for Screening Systems in the Terahertz Range
V. V. Startsev, V. K. Popov, V. I. Voitov,
EDB ASTRON JSC,
Lytkarino, Moscow Region, Russia
A complex for detecting prohibited substances when they are hidden in the elements of clothing has been developed. The complex operates in the visible, IR and THz ranges. Spectral studies of the transmittance of THz radiation were carried out with samples of packaging materials and fabrics: cotton, denim, wool, pigskin, polyethylene (about 0.2 mm thick), paper (about 0.2 mm thick). The frequency range in which all samples of packaging materials and clothing fabrics are most transparent is determined.
Article received for editing 07.05.2019
A number of publications on the results of studies of spectral transmittances of terahertz (THz) radiation by tissues and packaging substances are known [1–3]. However, the data obtained in different laboratories are different from each other, so the task of their experimental verification arose. Specialists of EDB Astron JSC carried out such tests at the stage of development of a complex for detecting prohibited substances when they were hidden in clothes. The complex is designed to work in the visible, IR and THz ranges. These results were presented in a review of the development of THz detection systems . This article is devoted to the choice of the spectral range that is optimal for the operation of the THz-imaging system.
By the time research began, the unambiguous view of the spectral transmittance of THz radiation by the atmosphere was not known. Therefore, in order to select the transparency window that is optimal for constructing a THz-imaging system, the task was to experimentally determine the transmittance of THz radiation by the atmosphere layer.
The choice of the spectral range that is optimal for the operation of the THz imaging system in the frequency range from 0.02 to 10.00 THz was made on the basis of spectral studies of the transmittance of the THz radiation from clothing fabric, packaging materials, and an atmosphere layer.
When selecting a spectral range, a set of experimental and theoretical studies was conducted:
- spectral studies of the transmittance of THz radiation by various packaging materials and clothing fabrics were carried out in order to determine the frequency domain in which the listed media are most transparent;
- the contribution of the THz-radiation scattering process in clothing fabrics to the total transmittance of THz-radiation by the medium was evaluated
the method [4, 5] of the mathematical description of the process of THz-radiation scattering in clothing fabrics was applied; the establishment of a relationship between the spectral transmittance and the structure of the material of clothing;
- the spectral transmittances of THz radiation by the atmosphere layer were investigated in order to select the optimal atmospheric transparency window for constructing a THz imaging system;
- analysis of the results of experimental and theoretical studies was carried out and the choice of the frequency range optimal for the construction of a passive THz-imaging system was made.
Equipment and methods of measurement
To achieve the above objectives, spectral transmittances of THz radiation intensity were investigated in a wide spectral range from 0.02 to 10.00 THz. THz spectrometer in the time domain (terahertz time-domain spectrometer) «zOmega» (Fig. 1) and FT-VIRTEX 70V FT-IR spectrometer (Fig. 2) were used as equipment. The zOmega THz spectrometer allowed us to record spectral characteristics in the frequency range from 0.02 to 3.00 THz, and the VERTEX 70V FTIR spectrometer allowed us to conduct studies in the frequency range from 1.50 to 10.00 THz. It is important to note that the spectral ranges of operation of both THz spectrometers overlap, making it possible to cover the entire necessary region of the spectrum of electromagnetic waves.
Spectral studies of the transmittance of THz radiation by intensity were carried out with the following samples of packaging materials and fabrics: cotton fabric, denim fabric, wool fabric, wool sweater, wool trousers, wool coat, pigskin, polyethylene (about 0.2 mm thick), paper (about 0.2 mm thick). Based on the results of spectral studies, a frequency range was determined in which all samples of packaging materials and clothing fabrics are most transparent.
The method of mathematical description of the THz radiation scattering process in clothing fabrics made it possible to analyse the influence of the THz radiation scattering process in the medium on the recorded spectral characteristics. The studies of the spectral transmittance of THz radiation by the atmosphere layer made it possible to determine the frequency range in which the atmosphere is most transparent.
On the basis of the obtained results of spectral studies, the results of studying the process of THz-radiation scattering by matter, the optimum frequency range was established for constructing a THz-imaging system.
Terahertz spectroscopy in the time domain is driven by probe study of the object under study with a short THz radiation pulse with a duration of only 1.00–2.00 ps [1, 2]. A signal passing through an object or reflected from an object (time dependence of the electric field intensity) is recorded by a detector with a high temporal resolution of up to 50–100 fs. A short THz radiation pulse contains spectral components in the frequency range from 0.01 to 3.5 THz, which makes it possible, using a Fourier analysis apparatus, to record complex amplitude reflectances or transmittances of THz radiation by matter .
If we carry out the procedure of recording signals and processing the recorded data in accordance with the method defined for this purpose , then using THz spectroscopy it is possible to restore the spectral dependence of the complex sample refractive index  the depth coordinates are the profile of the optical characteristics of the sample .
Both the generation and detection of the THz-electric field with high temporal resolution use ultrashort laser pulses of the optical range. The generation and detection of THz pulses can be carried out using the photo-switching effect in a semiconductor. Sources and detectors of THz-fields in this case are photoconductive antennas. The generation of THz pulses can be carried out using the nonlinear effect of optical rectification in electro-optical crystals, and detection using the Kerr effect in electro-optical crystals. These effects are close to each other, since they are determined by the same coefficients of the nonlinear dielectric susceptibility tensor of a substance. There are methods for generating and detecting a THz field in an air plasma. These methods are discussed in detail in [1,2,9].
A schematic diagram of the zOmega terahertz spectrometer is shown in Fig. 3. In this spectrometer, the generation of THz pulses is carried out in a photoconductive antenna pumped by femtosecond pulses of a fibre Yb laser, and the detection of a THz electromagnetic field is performed in an electro-optical detector built on the basis of a ZnTe crystal. We briefly describe the principle of operation of the THz spectrometer.
The duration of ultrashort optical pulses of a fibre laser is 80 fs, and their repetition rate is 50 MHz.
The beam of a femtosecond fibre laser (FSLV) falls on a beam splitter (BS), splits into two parts: the pump beam and the test beam, and the pump beam has a higher intensity [4,5].
Ultrashort pulses of the pump beam fall on the dielectric substrate of the photoconductive antenna (PCA). Each optical pump pulse is involved in the generation of the corresponding THz radiation pulse. A typical view of a THz pulse obtained using a GaAs-based photoconductive antenna and registered with an electro-optical detector is shown in Fig. 4a, and its amplitude Fourier spectrum is shown in Fig. 4b.
Discussion of the results
Let's consider the results of spectral studies of samples on the example of cotton fabric. To visualize the recorded graphs of spectral transmittances, an application package was used to solve the problems of technical calculations MATLAB.
Fig. 5–7 show the spectral transmittance of THz radiation intensity by samples of cotton fabric. Fig. 5 details the graph of the spectral transmittance in the region from 0.02 THz to 0.50 THz.
The results of spectral studies of samples of cotton fabrics show that this type of fabric transmits more than 50% of THz radiation power in the frequency range from 0.02 to 1.00 THz. Above 1.00 THz-spectral transmittance decreases strongly, and from 3.00 to 10.00 THz its value is close to zero.
In a similar way, other packaging materials were studied. The results of spectral studies and mathematical modelling showed that THz radiation is strongly scattered in clothing. For most of the considered packaging materials and clothing fabrics, the spectral transmittance of THz radiation in intensity approaches zero at frequencies above 1.50–2.50 THz. The THz radiation is most strongly attenuated when it passes through a layer of pigskin and also through woollen clothes.
It was determined that the wool sweater transmits more than 40% THz radiation intensity in a narrow spectral region from 0.02 to 0.10 THz, and at frequencies less than 3.5 THz, about 5% THz radiation, being the most opaque from studied environments Isolated pigskin skin transmits about 40% of the radiation at a frequency of less than 0.40 THz and has a negligible transmission in the spectrum of up to 3.00 THz.
Based on experimental and theoretical studies, we found that attenuation of THz radiation in packaging materials and clothing is an obstacle and will not allow detecting a hidden prohibited object at frequencies above 0.35 THz.
It was determined that the operating frequency of the THz-picture system should be in the range of 0.02 to 0.35 THz.
There are other factors that determine the choice of operating frequency THz-imaging system. Among them are the achievable resolution of the system, as well as the limited elemental base of THz radiation sources and receivers.
It is obvious that the limiting resolution (the size of the minimum resolvable parts of the object) of the THz-imaging system is directly proportional to the operating frequency of the THz-imaging system. In order to be able to detect small parts of the object under investigation, it is first necessary to select the maximum operating frequency of the system from the interval of 0.02–0.35 THz.
In order to select the operating parameters of the THz-imaging complex of the control of hidden transport in the details of the clothes of prohibited substances, links have been established between the spectral transmittance of the THz-radiation and the structure of the material of the clothes. Of the materials studied, a wool sweater was found to be the opaquest medium for THz radiation: it transmits more than 40% THz radiation in intensity in a narrow spectral region from 0.02 to 0.10 THz, and at frequencies less than 3.5 THz, about 5% of radiation. Isolated pigskin skin transmits about 40% of the radiation at a frequency of less than 0.40 THz and has a negligible transmission in the spectrum of up to 3.00 THz. It was determined that the operating frequency of the THz-imaging system should be in the range of 0.02 to 0.35 THz.