Workplan

WP1 Overview of existing principles of THz radars

The first work package provides a starting point and basis for the research project. Basic methods for generating THz waves are well known and have been discussed in detail in several other projects and literature, so we shall capture the requirements and status analysis focused primarily on the available technologies to generate THz waves. We know that the THz equipment mainly photoconductive antennas and lasers are very expensive so we shall propose principles of THz techniques to generate and receive THz waves using as possible with the existing equipment. In the Laboratory for optical systems and optical sensors at the Institute of Automation, Faculty of Electrical Engineering and Computer Science, University of Maribor, we are dealing with micro-machining and processing, optical-fiber management and recently we can change wavelength of the laser diodes by changing shape of the current. The idea of this project is that by utilizing current through the laser diode we can design any form of current, which can be controlled. The goal of the project is to examine the possibility of generating THz waves. Very important to choose the appropriate photo conductive antenna. In the literature, it is observed more possibilities of design of antennas in the photo conductive platform to generate signals. We shall explore the literature and principles of radar techniques for the implementation of THz radar using time of flight, tomography and SAR tomography. We shall examine the literature and review possibilities to generate SAR data on different ways, as shown in the literature. This work package will be base for the project thus it plays an important role in the project structure.

WP2 Laboratory prototype of THz radar

This work package shall implement the THz using existing approaches. The laboratory prototype of THz radar shall be based on interfering of two optical pulses that propagate though fibre Bragg grating, which is used to impart linear chirp on the ultra-short optical pulse. However, the FBG has limited bandwidth which will consequently restrict the bandwidth and time duration of generated chirped electrical pulse, and so limit the time bandwidth product (TBP). This is the major shortcoming of the FBG-based methods. can be demonstrated that linear chirp is imparted by the dispersion compensation fibre without any bandwidth limitation, to generate linearly chirped electrical pulse whose bandwidth may be up to THz. A symmetry of the generated pulse can be controlled by adjusting the power ratio of the optical pulse and the continuous wave.

WP3 Advanced methods for THz signal generation

In this work package, the goal it to theoretically model THz radar system and develop the principles of transmitting and receiving signals using the proposed approach. We shall use the developed THz radar in the WP2, where a Brag grating. The bandwidth of a generated optical chirp can be up to few THz. An advantage of this design in power adjustment of the optical pulse and continuous pulse. In this WP we shall deal with new methods for generating THz waves. We shall observe what is when current through the laser diode is changed and a linear chirp is imparted on an ultra-short optical pulse by the dispersion compensation fibre when Bragg grating is used. We expect that the centre frequency and bandwidth of linearly chirped electrical pulses shall be adjusted. We shall model an optimal shape of the laser diode current.

interferiranim signalom, kadar spreminjamo tok skozi lasersko diodo. Poiskali bomo optimalno obliko toka skozi lasersko diodo.

WP4 Theoretic algorithms for THz data focusing

In this work package, we shall deal with the THz tomography and principles of SAR and ISAR. Advantages of SAR techniques over pulsed techniques shall be observed. There is obvious advantage in the simplified geometry of the scanning object and numerical focusing, which can be applied for more depths. Problem with the curvature, where it is necessary to make the adjustment over the edges of objects shall be researched. We shall review methods of Fourier Back Propagation, Split Step Fourier Back Propagation and Split Step Fourier dependencies for the purposes of correction parabolic curvature. We shall also investigate the Migration phase shift algorithm for SAR data focusing. We main contribution of this WP shall be research on possibilities to implement spotlight focusing mode for THz data.

We shall also investigate the principles THz interferometry. In the quantum cascade laser is part of the wave generated by the radar bounces back and this causes changes in the properties of the laser. With the so-called theory of mixing signals can predict much less crosstalk in the THz quantum cascade laser as in the laser diode and this can give an almost linear relationship between the phase signal and an external laser cavity.

In this work package, we shall examine the generation of THz waves by modulating the signal laser oscillator, where the reflected signal nonlinear mixed with interferometric signal to the laser. With slow modulation frequency lasers, can generate nonlinear interference signal. Morphological characteristics of the signal are defined by the target by target reflection and distribution of spatial reflection. Information about the spatial distribution can be obtained by sequential signal capturing. Using radar theory, we can identify complex reflection ratio of the reflected electric field on the front side of the output mirror. By using SAR principle, a reflected complex electric fields shall be generated that correspond to the spatial location of the receiver with respect to the target. The ISAR algorithm shall reconstruct spatial dependence of the complex reflectivity. ISAR algorithm is limited to a single frequency using a conventional matched filter. We assume that the reflection coefficient of a specific point on the target is modelled as a superposition of electric field projected back to this point. This gives the field of complex numbers where the amplitude is dependent on the reflection coefficient of the target.

WP5 THz Radar with Synthetic aperture

This WP shall develop THz SAR radar. WP1-WP2 shall implement pulse based THz radar and WP3-WP4 shall study principles of SAR THz radar. The implementation of this WP shall be done based on ideas proposed in WP3-4, that means SAR radar, which shall generate chirps in addition to the power settings of the modulated signal and continuous wave signal. Moreover, we will have possibilities to change shape of chirp signal and continuous wave signal. As will be shown in the analysis of the WP3 and WP4 a new THz radar shall be propoed, therefore it is necessary to evaluate the quality of the reconstructed image. In this section, we shall examine the possibility of implementation THz radar in the Spotlight mode, which will be adapted to the geometry of the transmission and recording of signals to the technology spotlight. In this WP we will investigate the possibility of detecting polarization of THz data. From SAR data we know that the polarized data give more information about the surface. For polarization antenna, we use the split loop resonator. It is known that such a structure has a high frequency selectivity and supress well the side bands.