Real-Time Signal Processing And Phase Retrieval Technique Of Time-Stretched Femtosecond Ranging Lidar

High-precision range detection is of great significance to satellite networking formation,three-dimensional topography,device calibration,high-end industrial manufacturing,security and autonomous driving.Femtosecond laser plays an important role in the field of high-speed precision ranging due to its short pulse width,wide spectrum and high repetition rate.It is difficult for traditional femtosecond ranging methods to achieve nanometer precision measurement at nanosecond detection time.To pursue high-speed and high-precision distance measurement,an interferometric ranging method based on real-time Fourier transformation,also known as dispersive time-stretching interferometry,has been developed.With further development,femtosecond laser time-stretching technology has become a powerful tool in optical signal processing,atmospheric detection,high-speed imaging,biomedical detection and high-energy physics.Based on femtosecond time-stretched interferometric ranging method,the problem of real-time storage and real-time processing of high-speed,high-frequency signals with the processing capavility of existing electronic devices are analyzed.The problem that the measurement accuracy of this method is limited by the frequency stability of femtosecond laser repetition rate and offset frequency in nanometer level is also analyzed.The solutions using microwave photonic signal processing and absolute frequency locking combined with phase retrieval scheme are porposed.The main work of the thesis is as follows:1.The development of ultrafast optics is briefly reviewed.For distance measurement applications,the current mainstream femtosecond ranging methods and the application development of time-stretching technology are introduced.It is pointed out that the time-stretching femtosecond laser ranging method has the potential to realize high-speed and high-precision detection.2.The time-stretching femtosecond laser interferometric ranging system adopts allfiber Mach-Zehnder interference structure,in which the detection arm contains the distance to be determined.After time-stretched by dispersive fiber,the change of distance is converted into the frequency variation of time-domain interference signal.In order to deal with the challenges of storage and real-time processing of high-frequency temporal interferogram,the microwave photonic real-time signal processing method is introduced.The system only needs to detect the light intensity which reduced the hardware requirements of the measurement instruments.The detector bandwidth requirement is reduced from 20 GHz to 350 MHz which demonstrates the capability of real-time detection.With the calibrated distance-transmittance mapping,the corresponding distance value can be obtained by detected transmittance.In the experiment,the range accuracy of micron meters and detection update rate of 0.1 MHz in dynamic range of 15 mm and 45 mm are realized.3.In addition to the real-time performance,another aspect is detection accuracy.The stability of the repetition rate and the offset frequency of femtosecond laser will affect the final ranging accuracy.Experiments demonstrate that the detected temporal interferogram is phase sensitive.In order to pursue higher detection accuracy,the femtosecond laser is absolutely frequency locked through feedback loop to realize phase-stable temporal interferogram.To avoid the chirp of the temporal interferogram caused by high-order dispersion,the detected timedomain interference signal is converted into a chirp-free frequency interferogram by establishing a time-frequency mapping.The original signal processing method is optimized and improved in this work and the phase retrieval method is used to calculate the corresponding distance.The accumulated phase of the frequency interferogram is linearly fitted and the distance value can be obtained by its slope.This distance retrieval algorithm is simple to process,insesnsitive to pulse shape and capable of high-speed nanoscale detection in millimeter-level dynamic range.4.Based on broad-spectrum property of femtosecond laser,an idea of multichannel femtosecond time-stretched Lidar is proposed.Combined with various beam steering techniques,the application of femtosecond time-stretched ranging Lidar in 3-dimensional imaging is prospected.