Vladislav V. Trofimov

Possible way for increasing the quality of imaging from THz passive device

Using the passive THz imaging system developed by the CNU-THz laboratory, we capture the passive THz image of human body with forbidden objects hidden under opaque clothes. We demonstrate the possibility of significant improving the quality of the image. Our approach bases on the application of spatial filters, developed by us for computer treatment of passive THz imaging. The THz imaging system is constructed with accordance to well known passive THz imaging principles and to the THz quasi-optical theory. It contains a scanning mechanism, which has a detector approximately with 1200μm central wavelength, a data acquisition card and a microcomputer. To get a clear imaging of object we apply a sequence of the spatial filters to the image and spectral transforms of the image. The treatment of imaging from the passive THz device is made by computer code. The performance time of treatment of the image, containing about 5000 pixels, is less than 0.1 second. To illustrate the efficiency of developed approach we detect the liquid explosive, knife, pistol and metal plate hidden under opaque clothes. The results obtained demonstrate the high efficiency of our approach for the detection and recognition of the hidden objects and are very promising for the real security application.

Possibility of the detection and identification of substance at long distance at using broad THz pulse

The spectral properties of THz pulses containing a few cycles reflected from a flat metallic mirror placed at long distance about 3.5 meters from the parabolic mirror are investigated. The samples for analysis were placed before this mirror. Measurements were provided at room temperature of about 18-20° C and humidity of about 70%. The aim of investigation was the detection of a substance under real conditions. At the present time our measurements contain features of both transmission and reflection modes. This leads to a strong modulation of the spectrum and makes difficulties for identification. As samples for our current research we used several neutral substances: paper layers, a thick paper bag, chocolate and cookies. The first problem deals with the detection of common and mismatched spectral properties of samples with paper layers, a thick paper bag and explosives. HMX, PETN and RDX were used as explosive samples. The dependence of the accuracy of identification of samples with paper layers and a thick bag is studied when using short transmitted THz signals with opposite absolute phases as calibration signals. Common and mismatched spectral features of neutral substances: chocolate, cookies and drugs MA, MDMA were investigated by modified integral criteria as well.

High effective SHG of femtosecond pulse with ring profile of beam in bulk medium with cubic nonlinear response

Possibility of high intense second harmonic formation with beam profile and pulse shape, which are similar the Gaussian ones, under the cascading SHG with taking into account cubic nonlinear response of medium is shown. Laser pulse on doubling frequency has intensity that is comparable or greater than the one of basic harmonic. The duration of second harmonic pulse can be many times less than the first harmonic pulse duration. We take into account dispersion and diffraction of laser pulses and group velocity mismatching.

Transmission of THz pulse with a few circles through opaque samples placed at long distance (4-6 metres)

Nowadays, the detection and identification of dangerous substances at long distance (several metres, for example) by using of THz pulse reflected from the object is an important problem. The main problem with this technique is the absorption of THz energy by water vapor. However, using THz pulsed radiation is possible at distance of some metres as it is well-known. Below we demonstrate possibility of THz signal measuring reflected from a flat metallic mirror placed about 4 metres from the parabolic mirror. Investigated object is placed before this mirror. Therefore, at present time our measurements contain features of both transmission and reflection modes. The reflecting mirror is used because of weak averaged power of femtosecond laser. This power is about 1 W. Moreover, the laser beam splits many times. Therefore, the averaged power falling on the THz emitter decreases at least 8 times. The pulse duration generated by the femtosecond laser is equal to 68 fs. Measurements were provided at room temperature and humidity about 70%. The aim of investigation was the detection of a substance in real conditions. We discuss new features of the detection of a substance covered under various ordinary materials and possible way for their influence deleting on the detection using reflected THz pulse. We discuss also details of action of THz pulse with a few circles on media. The main feature of such interaction is its dependence from absolute phase of the THz pulse. We demonstrate results of computer simulations as well as physical experiment results for propagation of such laser pulses.

Peculiarities of the detection and identification of substance at long distance

Nowadays, the detection and identification of dangerous substances at long distance (several meters, for example) by using of THz pulse reflected from the object is an important problem. In this report we demonstrate possibility of THz signal measuring reflected from investigated object that is placed before a flat metallic mirror. A distance between the flat mirror and the parabolic mirror this mirror is equal to 3.5 meters. Therefore, at present time our measurements contain features of both transmission and reflection modes. The reflecting mirror is used because of weak average power of used femtosecond laser. Measurements were provided at room temperature and humidity about 60%. The aim of investigation was the detection of a substance in real condition. Chocolate and Cookies were used as samples for identification. We also discuss modified correlation criteria for the detection and identification of various substances using pulsed THz signal in the transmission and reflection mode at short distances of about 30-40 cm. These criteria are integral criteria in time and they are based on the SDA method. Proposed algorithms show both high probability of the substance identification and a reliability of realization in practice. We compare P-spectrum and SDA- methods in the paper and show that P-spectrum method is a partial case of SDAmethod.

Formation of Intensive Pulses with Super Short Duration Under the Cascading SHG in Bulk Medium

Using computer simulation, we investigate cascading SHG under the conditions of action of cubic nonlinearity and group velocity mismatching with the aim of high intense formation of fundamental and doubling frequency waves. The phase mismatching in our computer experiments corresponds to self-focusing of laser pulses. Various scenario of laser pulse formation on doubling frequency with intensity, that is comparable with the one of fundamental harmonic, are found. It should be emphasized under some conditions the centers of both pulses do not change their positions despite on group velocity mismatching and both pulses propagate as in the case of absent of group velocity mismatching. We take into account dispersion and diffraction of laser pulses as well

New algorithm for the passive THz image quality enhancement

We propose a new approach for THz image quality enhancing using correlation function between the image under consideration and a standard image. The standard image moves in two directions along a image under analysis. As a result, 2 D correlation function is obtained. Multiplying this function by color number belonging to a grey scale, we restore the image under the analysis. This allows to suppress a noise on a new image. This method allows to see the person clothes details that it means multi-times increasing of the passive THz camera temperature resolution. We discuss a choice of standard image characteristics for an achievement of correlation function for high contrast. Other feature of our approach arises from a possibility of a person image coming to the THz camera by using a computer processing of the image only. It means that we can “decrease” a distance between a person and the passive THz camera. This algorithm is very convenient for using and has a high performance.

Increasing the instrumental resolution of a commercially available passive THz camera due to computer treatment of image

We demonstrate the improvement of the both quality of the image captured by TS4 - the commercially available THz passive camera manufactured by ThruVision Systems Ltd. - and instrumental resolution of this camera. The measurements range of this device reaches 10 meters. Our approach bases on application of novel spatial filters and algorithms, developed by us for computer processing of passive THz images produced by the camera.

Real-time computer processing of image from THz passive imaging device for improving of images

We demonstrate real-time computer code improving significantly the quality of images captured by the passive THz imaging system. The code is not only designed for a THz passive device: it can be applied to any kind of such devices and active THz imaging systems as well. We applied our code for computer processing of images captured by various passive THz imaging devices manufactured by different companies. The performance of current version of the computer code is greater than one image per second for a THz image having more than 5000 pixels and 24 bit number representation. Processing of THz single image produces about 20 images simultaneously corresponding to various spatial filters. The computer code allows increasing the number of pixels for processed images without noticeable reduction of image quality. The performance of the computer code can be increased many times using parallel algorithms for processing the image. We develop original spatial filters which allow one to see objects with sizes less than 2 cm. The digital imagery is produced by passive THz imaging devices which captured the images of objects hidden under opaque clothes. For images with high noise we develop an approach which results in suppression of the noise after using the computer processing and we obtain the good quality image. The results demonstrate the high efficiency of our approach for the detection of hidden objects and they are a very promising solution for the security problem.

Concealed object detection using the passive THz image without its viewing

We propose a real-time algorithm of computer processing for the passive THz image with the aim of concealed object detection without looking through this image. The algorithm uses a correlation function between the THz image and a standard image. The sound alarm takes place if a correlation function value is greater than its certain value. This algorithm allows us to do a conclusion about presence of forbidden objects on the human body by using computer analysis.