Study On Propagation Of Ultrashort Pulses In Large Numerical Aperture Lens Systems

Ultrashort pulse is widely used in industry,information and other fields because of its ultra-high peak power and narrow time width.Through the study of ultrashort pulse transmission characteristics,the transmission process of ultrashort pulse can be optimized to obtain the ultrashort pulse with higher peak value and narrower time width,which makes the application prospect of ultrashort pulse in transient measurement,chemical reaction dynamics,optical frequency comb and so on more broad.Based on the tracking function of ZEMAX,and the joint calculation of ZEMAX and Mathematica,the focusing and transmission characteristics of ultrashort pulse passing through large numerical aperture lens system are studied.A method for calculating the transmission characteristics of ultrashort pulse with arbitrary waveform passing through any complex optical system is proposed.Firstly,we write the macro program in ZEMAX,and the phase distribution of the ultrashort pulse on the exit pupil of the optical system is obtained by ray tracing;According to Sommerfeld diffraction integral and Fresnel diffraction integral,five kinds of spatial propagators for light wave propagation calculation are compiled,and the influence of spatial sampling on propagation results is analyzed.Combined with ray tracing and space propagator,the program for calculating pulse propagation is compiled;The program is used to study the focusing characteristics of ultrashort pulse passing through an ideal lens.It is found that the wavefront of the pulse is a perfect spherical wave,and the distribution of the focus is symmetrical,which is consistent with the theory.The correctness of the program is verified.Secondly,we studied the focusing characteristics of ultrashort pulse passing through a single lens system with large spherical aberration.By analyzing the spatial evolution of the pulse in the focus region,it is found that the strong coherence effect in the region before the focus comes from the influence of spherical aberration.The paraxial light is mainly focused in the focus position,and the light is divergent in the region after the focus,and the coherence phenomenon disappears;The influence of optical system aperture on focusing is studied.It is found that there is an optimal entrance pupil diameter,which makes the pulse have high peak intensity and good waveform distribution at the focus;It is found that the position dispersion is another factor affecting the focusing of ultrashort pulse.Finally,we studied the lens system with large axial color differenc.In this system,the spatial evolution of pulses is similar to that of ideal lens focusing,and the wavefront of pulse is symmetrical about focus;The coherence effect caused by position color difference is different from that of spherical aberration,which is characterized by coherence in the front and back of focus area,and spherical difference only causes coherence effect in the area before focus.The research on focusing with pulse width further supports this conclusion;The research on the influence of aperture size on focusing shows that the influence of spherical aberration can still be observed in the system with small ball difference,but its influence is weak,which can only be observed under certain conditions.At the same time,there is no optimal pupil diameter in the lens system,and the pulse light spot increases only with the decrease of pupil.