The Second Harmonic Frequency-resolved Optical Gating Method Measuring The Ultrashort Pulse

Ultrashort pulse has been highly concerned by researchers since its birth because of its extremely short pulse width and extremely high peak power.In the 1970s,researchers created the pump-probe technology,which achieved femtosecond time-resolution by adjusting the relative delay between the probe pulse and the pump pulse via a precise displacement platform.In the 1980s,the advent of femtosecond chemistry opened a new chapter in the study of chemical reactions.Subsequently,ultrashort pulses are widely used in condensed matter physics,chemistry,biology,nanomaterials and other fields.In the 21st century,the development of free electron laser technology has promoted the deeper exploration of atoms and molecules,while the maturity of ultra-fast optical fiber technology has promoted the development of ultra-fast optics from the laboratory to the market.Although ultrashort pulse has a wide prospect,the characterization technology restricts its further development to some extent.The phase plays a very important role in the optical pulse,which affects the peak power,pulse width and other important parameters.The details of the pulse also affect some experimental results,especially the chirped pulse,which plays an important role in the study of molecular vibration.The main methods of measuring pulse width are direct measurement and autocorrelation method.The direct measurement method is limited by the response time of photodetectors and cannot directly measure picosecond or femtosecond pulses.The autocorrelation method converts the time resolution to the spatial resolution,and can obtain the pulse width information to some extent,but not the pulse detail information,not to mention the phase information,and it is difficult to distinguish the noise,and even leads to the wrong result.Frequency resolved optical gating(FROG)is improved based on autocorrelation method,which converts one dimensional time-domain information into two dimensional time-frequency information,and use Fourier transform iteration algorithm to theoretically uniquely reconstruct the pulse phase.There are three parts of the main work.First An analysis of non-collinear phase matching.The most important step in building the SHG-FROG system is to produce non-collinear sum-frequency light.However,there is nosystematic and intuitive analysis of non-collinear phase matching.In this paper,BBO which is uniaxial crystal is taken as an example to analyze the phase matching problem of main section,arbitrary section and non-collinear incidence with crystal rotationSecond Set up SHG-FROG system and code control program.This paper introduces the structure and operation method of FROG measurement system,discusses optical path collimation and second harmonic frequency light in detail,and analyzes the theoretical basis.Then introduce the control program,explain the program structure and its functions,and illustrate the specific problems such as serial port communication,spectrometer control and displacement control in detail.Finally,summarize the shortcomings and the improvement projects.Third Algorithm analysis and pulse reconstruction.Firstly,introduce the basic principle of reconstruction algorithm,including generalized projection algorithm and principal component generalized projection algorithm.Then discuss the convergence result of the trace with noise including natural noise,background noise and local noise by using the generalized projection algorithm.Finally,the FROG system was used to measure the pulse emitted by the nonlinear polarized rotationally mode-locked fiber laser doped with Yb:YAG,and pulse was reconstructed by the generalized projection algorithm.Other relative parameters of the laser are also measured.