Research On Several Issues Of Femtosecond Pulse Coherent Grouping

In recent years,there is a growing interest in creation of ultrashort ultrahigh intensity laser systems.Both the basic researches and application researches expect the laser facilities to offer higher peak power and higher focal intensity.The ultrarelativistic intensity conditions(>1023 W/cm2)in these systems provide great opportunity for investigation and application of many research fields,such as laser-plasma electron and ion acceleration,hard X-ray and radiation generation,relativistic self-focusing and so on.Owing to that the peak-power of single laser source is restricted by optical element aperture,damage threshold of materials,thermo-optical issue and detrimental nonlinear effect,it is difficult to further increase the peak intensity of a single channel laser system greatly.Coherent beam combining(CBC)is the most promising way to scale up the available peak power while preserving beam quality of individual laser systems.Many important techniques for efficient CBC have been developed.However,there is still a great challenge to realize efficient CBC for large aperture ultrashort ultrahigh intensity pulse laser systems,including coherent combining theories,error measurements and controls.This paper aims at the physical problems and key technologies of CBC.Both theoretical and experimental researches are made,and some results are achieved.1.Spectral phase effects are important issues and have significant impact on the performance of CBC.In order to understand spectral phase effects and get the control requirements of CBC for ultrashort ultrahigh intensity laser systems,we analyzed the fundamental physical process of CBC systematically in temporal-spectral domain.It is demonstrated that different orders of dispersion influence intensity shape of the combined beam,and high order dispersions affect the temporal contrast of the combined beam,while the number of the channels to be combined has little impact on the temporal Strehl ratio(SR)of the combined beam.Efficient CBC of ultrashort ultrahigh intensity laser systems requires that residual dispersions of pulses should be compensated effectively.In view of the current state of the art situation,the core and difficulty of the control requirement in real systems is the group delay,which is influenced by many environmental factors,while GDD,TOD and FOD could be compensated and controlled statically.2.Longitudinal chromatism is another important factor for CBC in large-aperture ultrashort ultrahigh intensity laser systems.However,it has not been discussed systematically in previous works.In a large aperture ultrashort ultrahigh intensity laser system with pulse duration of 100 fs or larger,lenses are used as spatial filters in amplifier section to image-relay and clear the beam of detrimental spatial frequency modulations.In order to reveal the influence process and acquire the PTD control requirement for realizing efficient CBC,the impact of PTD on the pulse temporal characteristics and spatial distribution at the focus are analyzed based on Fourier-optics.We investigated the space-time effects on coherent combining results.For CBC of two beams longer than 100 fs,the PTD should be less than 0.7 times of the pulse duration to realize efficient beam combining while the PTD must be less than 0.4 times of the pulse duration for more channels combining.When GVD is not neglected for pulse duration shorter than 100 fs,the PTD should be less than 0.3 times of the pulse duration for efficient CBC.To realize efficient CBC of ultrashort ultrahigh intensity laser systems,longitudinal chromatism should be compensated or avoided effectively for different pulse durations.It should be pointed out that for laser systems with pulse duration less than 10 fs,only reflective telescopes for the signal beam are employed.This paper will provide a reference for designing ultrashort ultrahigh intensity laser systems based on CBC.3.According to the urgent requirement of pulse timing in CBC,a method of timing measurement and control with wide detection range and high precision is proposed.It is based on the temporal modulation of combined pulse caused by the relative time delay between individual pulses.To illustrate this technique,a proof-of-principle experiment is demonstrated based on two 237 fs pulses.We have characterized the timing jitter of two pulses with attosecond precision and femtosecond range using optical cross correlation and electrical energy interferometry.The timing of the two pulses at femtosecond level is adjusted based on the results of the optical cross correlation.The electrical energy interferometry permits the measurement and control of the timing jitter at attosecond precision between the two pulses in different channels by a closed loop.The peak-to-valley(PV)timing jitter of the two pulses to be combined is less than 700 attoseconds in one hour and the average efficiency of coherent beam combining(CBC)could reach to 91.6%.This timing jitter measurement and control technique is beneficial to efficient short pulse especially Petawatt laser system CBC in the future.4.Optical parametric chirped pulse amplification(OPCPA)is a well-known amplification technology in ultrashort pulse regime.And it is most promising technology for amplification systems of coherent combining of ultrashort ultrahigh intensity laser pulses.We focus on the spatial beam quality of OPCPA.In order to simplify correction systems and improve performance of CBC,optical components should be shared by different channels as many as possible.Most of OPA amplification stages are shared by different channels.We only simulated last independent OPA amplification stage because wavefront difference of different channels has greater impact on CBC performance.A model is built to understand wavefront distortions of OPA.We investigate wavefront distortions of OPA used in coherent beam combining of ultrashort ultrahigh intensity laser systems.The factors include wavefront,energy,spatial intensity distribution of pump pulse,interaction length,noncollinear angle and so on.Diffraction and walk-off have limited influence on wavefront of amplified signal.Some requirements and changing trends of requirements of OPA is proposed.It's meaningful for practical large aperture ultrashort ultrahigh intensity laser beam coherent combining systems.