Photonics-based Microwave Signal Generation And Transmission For Distributed Coherent Aperture Radar

The detection sensitivity of radar is proportional to the square of the antenna aperture.As the demand on the detection sensitivity increases,the size of the antenna increases.However,radars with large apertures not only suffer from poor mobility but also have many difficulties in fabrication and maintenance.To avoid these problems,distributed coherent aperture radar is proposed by the Lincoln Laboratory,in which multiple radars with small apertures could be regarded as one equivalent radar with much larger aperture by achieving coherent transmission and receiving.To achieve distributed coherent aperture radar,the precision synchronization of local oscillators(LOs)is a crucial prerequisite.However,there is no effective solution to meet the synchronization requirement till now.Aiming at achieving highprecision synchronization of the LOs,we focus on the research of the generation of microwave signals with low phase noise and its distribution with stable phase in this thesis.Here we adopt the opticalmicrowave phase detector to extract microwave signals from mode-locked lasers(MLLs),combining with baseband frequency synthesis and image-rejection frequency mixing techniques,and develop a broadband,frequency-agile and ultralow-phase-noise radar signal source.Then microwave signals are distributed over fiber with stable phase by taking advantage of conjugate frequency-mixing,as well as digital phase-locked loop.At last,the coherent combination of four distributed apertures is achieved using the above-mentioned techniques in the experiment,which could be directly used in the distributed coherent aperture radar in the near future.The optical-microwave phase detector based on polarization modulation is proposed in the thesis.By numerical simulation and experiment,the characteristic of the phase detector is studied as well as the parameters of the devices and the system that may affect its performance.A phase-locked loop is constructed based on the optical-microwave phase detector to lock a voltage-controlled oscillator(VCO)to an MLL to attain a stable microwave LO signal.By simulation,we get to know the phase noise of the microwave signal is determined by three noise sources.They are the frequency noise of MLL,the residual noise of the optical-microwave phase detector and the phase noise of the free-running VCO separately from low to high offset frequency.The experimental results verify the conclusion.To improve the phase noise of microwave signal furthermore,we study the occurrence mechanism of MLL's noise and effectively reduce the close-to-carrier phase noise by using optical delay line as the reference to stabilize the repetition rate of the MLL.We obtain a 10 GHz signal with phase noise of-80 d Bc/Hz@10 Hz,-145 d Bc/Hz@10 k Hz and-165 d Bc/Hz@10 MHz.In addition,since the frequency-hopping technique and frequency diverse technique are often used in radar,LOs with different frequencies are required.By regarding the locked MLL as the frequency reference and using the optical-microwave phase detector to extract 10 GHz signal,together with the direct digital synthesis technique and image-rejection mixing technique,we design and implement an X band ultralow-phase-noise and frequency-agile radar signal source.The signal source output signals between 9~11 GHz,the frequency tuning resolution and switching time is 135 p Hz and 135 ns separately,the spur suppression ratio is about 58 d B and all output frequencies have the same phase noise within 1 k Hz offset frequency,that is,-80 d Bc/Hz@10 Hz and-125 d Bc/Hz@1 k Hz,the integrated timing jitter range is 7.6~9.1 fs with integration range of 10 MHz and 10 Hz.At last,a scheme based on the conjugate frequency mixing for microwave signals' transmission over fiber with stable phase is proposed in which only two stages of frequency mixing are required.By using the scheme,we experimentally limit the signal's phase vibration in 2.86? after transmitted over 20-km fiber.Furthermore,the scheme is optimized.By adopting the pre-distortion technique together with the conjugate frequency mixing technique to achieve the stable transfer of microwave signals,no auxiliary signal source is needed anymore.By using the optimized scheme,we experimentally limit the signal's phase vibration in 2.52? after transmitted over 20-km fiber.Lastly,by using optical-microwave phase shifter and optical tunable delay line as actuators,we propose another scheme based on the phase-locked loop to achieve the stable transfer of microwave signal.We experimentally limit the signal's phase vibration in 2? after transmitted over 6-km fiber.By adopting the above-mentioned technique,the coherent combination of four apertures is achieved and the combined beam stays stable in the observation time.