| Optical-controlled RF radar technology is one of the key technologies to improve the performance of radar due to its large bandwidth,high operating frequency,low transmission loss,and immunity to electromagnetic interference.The RF signal is modulated onto the light wave,processed and controlled by the optical device,and the optical-controlled distributed radar structure is used,so that the system can further obtain the spatial diversity advantage,thereby further improving the signal-to-noise ratio through the coherent accumulation.Optical true time-delay,the key technology of optical-controlled distributed coherent radar,aims to provide stable,flat and adj ustable delay/phase shift for each phased array element of the optical-controlled radar.For the entire optical-controlled radar system,a transmission link which can achieve transparent and distortion-free transmission is required,and meanwhile provide large-bandwidth delay/phase shift,frequency conversion and other functions,and ultimately achieve multi-dimensional detection applications.This paper focuses on the scientific problems and key technologies of the optical true time-delay system in the optical-controlled distributed coherent radar,and carries out in-depth theoretical modeling,simulation and experimental research on the transmission and processing of RF radar signals in the optical-controlled distributed radar system.Based on the theory of microwave photonics,electromagnetic field and electromagnetic wave,the anti-dispersion phase-tunable microwave mixer based on a dual-drive dual?parallel Mach-Zehnder modulator is proposed,and the RF signal transmission and experimental platform with no power-selective fading is built.This paper further research the problem of RF-vortex-wave superposition state and reception of deflection state in optically controlled phased array systems,for the aim of multi-beam forming and RF-vortex-wave transmission and reception in multi-dimensional detection applications.The innovations and main work of the paper are as follows:1.Aiming at the problem of frequency-selective power fading caused by dispersion in long-distance transmission scene in optical-controlled distributed coherent radar,an anti-dispersion phase-tunable microwave mixer based on a dual-drive dual-parallel Mach-Zehnder modulator is proposed.The scheme is characterized in that the two sub-MZMs of the DDDP-MZM are used to realize the single-sideband phase-shift modulation of the local oscillator and the double-sideband modulation of the input microwave signal respectively.The advantage of this scheme is that the anti-dispersion function is realized by the single-sideband local oscillator modulation,and the phase shift of the output frequency conversion signal is realized by electrically shifting the phase of local oscillator signal at the single frequency point,thereby realizing a large bandwidth.This method has been experimentally verified,showing good accuracy and system stability.The experiment proves that after 51 km single-mode fiber dispersion link,the local oscillator sideband modulation used in this scheme is not subject to the power fading caused by dispersion,which is consistent with the theoretical analysis.For a one-dimensional phased array radar operating at 20 GHz with half-wavelength element spacing,the beam position angle fluctuation of this scheme is as low as 1.59°,which is equivalent to an optical delay fluctuation of 0.69 ps.2.The beamforming scheme based on optical frequency-domain processor is proposed to solve the multi-dimensional detection problem of optical-controlled distributed coherent radar.Based on this scheme,a two-dimensional multi-beamforming structure of optical-controlled pencil wave based on optical spectrum processor is proposed,and the transmission experiment of an optical-controlled RF-OAM electromagnetic wave carrying an IQ signal was conducted for the first time.Typical multi-beam antenna technologies currently include substrate integrated waveguide technology(SIW),feed network technology,and so on.The key problems of SIW are the complex system,high crossover level,and beam deflection under large bandwidth conditions.The optical-controlled system can bring the traditional advantages based on microwave photonics(solving the beam squinting problem in large bandwidth conditions and immunity to electromagnetic interference).By using an optical true time-delay(OTTD)structure based on optical frequency domain processing,the antenna array becomes an active phased array,and each element is wideband adjustable,so special functions such as scanning,tracking,and vortex beam generation can be realized,to increase the radar's performance.3.Measurement method of a double superimposed RF vortex wave is proposed,to solve the problem of complicated superimposed OAM wave front.This method is based on the principle of circular phase gradient.By using the circular gradient method,all the phase information in the receiving circle can be collected to get the circular gradient function,then calculate the average value of the annular phase gradient function to obtain the smaller mode,finally the peak factor can give us the difference of two modes.The traditional phase gradient method can measure the RF-vortex wave with a single vortex mode,but for the superimposed vortex wave,since the gradient function is a variable on the receiving circle,it cannot be directly measured by the phase gradient method.Thus,the method proposed in this paper breaks through this bottleneck,and improves the efficiency and performance of the OAM-related radar applications.In this paper,a verification experiment was carried out,and the vortex wave with the +1 and-2 superposition states was successfully measured by this method.4.Misalignment measurement method of a tilted RF vortex wave is proposed,based on OAM spectrum analysis.The proposed method can acquire the position of tilted singularity point with mean OAM value distribution,and the position of original pivot point with OAM variation distribution.Thus,the misalignment measurement can be realized through simple image processing algorithms.Compared with feedback method,the proposed method has the advantage of not requiring repeated measurement and correction steps.This paper analyzed the principle,and conducted measurement experiment on a 10°tilted RF vortex beam,with an error of only 0.39°.Through this research,the problem of incorrect reception in RF vortex radar applications is partially solved. |
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