Key Technologies Research On Synthetic Aperture Ladar

Synthetic Aperture Ladar(SAL),since its inception,has grasped a lot of attention because of its high detection accuracy.SAL is also the only means of achieving centimeter level detection accuracy within a few kilometers.Meanwhile,thanks to the emergence of the large tunable bandwidth laser,SAL also has an important development prospect in the field of microscopic detection.However,in practical applications,due to the different emission mechanisms of light wave from microwave,many factors have restricted the development of SAL,so it has not been practically used as the Synthetic Aperture Radar(SAR).In this dissertation,we study the key technologies which restrict the development of SAL.1.The emission source of SAL is the laser,the laser linewidth determines the coherent length of the outgoing laser,it also determines the actual detecting range of the SAL.The coherent length of ordinary laser is very short,so it is difficult to meet the requirement of SAL.To solve this issue,the dissertation has studied the single longitudinal mode narrow linewidth laser.Through the analysis of single longitudinal mode frequency selection technique and linewidth narrowing technique,the requirements of SAL in practical applications are also considered.A novel single longitudinal mode narrow linewidth fiber laser which based on phase-shift fiber grating and saturable absorber is proposed and implemented.After testing the key technical characteristics such as linewidth,power and stability of the fiber laser,it is shown that this laser could meet the needs of SAL applications in future.2.Normally,the linear frequency modulation laser is used as the emission source for SAL,and the distance resolution of SAL depends on the linearity of the linear frequency modulated laser.In order to compensate the linearity of the frequency modulated laser,the SAL system need to analyze the data of the reference channel and compensate the distance from SAL to the phase error in the post processing.In this way,the difficulty of data processing in the back-end is increased,and the real-time monitoring and real-time compensation of the frequency modulated laser linearity can't be realized,which makes the SAL imaging effect worse.This paper proposes a new method to monitor the linearity of linear frequency modulated laser in real time.This method uses Hilbert Transformation to process the reference channel heterodyne intermediate frequency signal in real time,which can reflect the change of signal frequency in time.Furthermore,the linearity of the frequency modulated laser can be compensated in real time.3.To compensate for the nonlinear errors of SAL linear frequency modulation light source,a programmable gate array(FPGA)is used as a nonlinear compensation system processor in this system.The reference channel heterodyne intermediate frequency signal is compared with a standard signal source within the FPGA,and a phase-locked loop is used to lock the intermediate frequency signal to the standard signal.The output signal of the phase-locked loop is converted into the driving signal of the laser frequency modulation element after FPGA processing,thus can be compensated the nonlinearity of the SAL frequency modulated laser source.Firstly the dissertation use Hilbert Transformation method to monitor the linearity of the frequency modulated laser.The difference of linearity is very obvious in the case of adding and without adding compensating system.Subsequently,the nonlinear compensation system is used to test the imaging effect of SAL range profile.By using the nonlinear compensation system,the resolution of the range profile has been greatly improved,and the ranging accuracy improvement is about 26 times as much as the case that of without compensation system.4.This dissertation also extends the application of linear frequency modulation laser technology in SAL to microwave photonics.A novel light generation microwave method based on large bandwidth frequency modulated laser and double Mach-Zehnder Interferometers(MZI)are proposed and implemented,it is also expected to be a SAL based microwave photonic radar solution in the future.This method uses a heterodyne beating signal which generated by an ultra-long fiber delay line of one MZI to obtain microwave signals.On the other hand,MZI with the shorter fiber delay line is used as a reference channel to compensate the linearity of the chirp laser,it also can improve the phase noise of the generated microwave signals.In the test,the optimized phase noise index is improved by 31.34 dB.At the same time,by changing the frequency modulation rate of the chirp laser,the system realize the continuous coverage of microwave signals from 1.743 GHz to 5.134 GHz.Due to the limitations of the bandwidth of photodetector,the experimental result is unable to obtain higher frequencies of microwave.Nevertheless,the frequency of microwave signals generated by this method could be expected to exceed 100 GHz.In this dissertation,two kinds of problems which restrict the development of SAL are discussed:the narrow linewidth of emitting optical source and the nonlinear problem of the chirp optical source.The corresponding solutions are proposed,and a single longitudinal mode narrow linewidth fiber laser system and a linear frequency modulation laser source with the nonlinear real-time compensation system are respectively implemented under laboratory conditions.Through the actual test,these two systems have reached the desired performance.In addition,the dissertation applies the nonlinear compensation technique of linear frequency modulated laser to the emerging field of microwave photonics.A new method of microwave generation is proposed and realized.Compared with other known microwave generation methods,this method has many advantages,such as small size,low cost and easy realization.It provides a new way for the development of microwave photonics and microwave photonic radar in the future.