Pulse Shaping Based On New Phase Modulation Technology

Inertial confinement fusion(ICF)can provide clean and clean energy for human beings and solve the problem of energy shortage in the future.However,in the study of inertial confinement fusion,the interaction between the incident laser and the plasma produces a large plasma density gradient,which results in stimulated Brillouin scattering(SBS),stimulated Raman scattering(SRS)and laser filamentation etc.To efficiently suppress laser induced parametric instability(PI),various beam-control techniques have been proposed and adopted,such as smoothing by spectral dispersion(SSD),polarization smoothing and phase plates smoothing.Previously,nanosecond high-energy pulses were used for final heating the target in laser driven ICF.However,these nanosecond pulses have a much longer time delay compared with these nonlinear PIs' relaxation lifetimes,leading to easy accumulation of PI effects.To that end,B.Afeyan et al.recently proposed using spike trains of uneven duration and delay(STUD)pulses to modulate the laser drive beams and thereby tame PI effects.STUD pulses may be implemented as a sum of localized super-Gaussian spikes with a super-Gaussian exponent of 8-10.One of the two main characteristics is that the width of a single pulse of the STUD pulse train is about 10 picoseconds and the rising or falling edge of the pulse is less than 10 picoseconds to prevent the accumulation effect of the laser plasma,thereby suppressing the laser plasma Parametric instability.The other feature is the need for STUD single pulse interval between 10-20 picoseconds of zero to prevent electronic cloud density disturbance,thus undermining the accumulation of laser plasma wave,to suppress the instability of the laser plasma parameters.Based on the above principle,this paper respectively uses the time grating system and the time lens system to generate this STUD pulses.In the case of using the time grating system to generate the STUD pulses,the characteristics of the pulse sequence under different modulation conditions are theoretically studied.This time grating scheme,which is a temporal analogy of spatial grating,introduces great degree of freedom for controlling the output pulse characteristics(pulse width,repetition rate,pulse shape,etc.)through simply tuning the electronics elements and the programmable phase modulation function.The narrowest pulse width is highly determined by the modulation parameters and the branch number N,and the numerically obtained value is around tens of femtoseconds in the current case.When super-Gaussian pulses are modulated with an optimized and modified trapezoidal function,the pulse rising/falling edge can be greatly compressed to form a clean nearly-square wave(with edges less than 10 fs).STUD pulses generated with this time grating system have high-degree controllability and are very beneficial for suppressing parametric instabilities in laser driven inertial confinement fusion.In the case of using the time lens system,we propose a fiber-loop based time lens system to realize STUD pulses,and theoretically study their features under various parameter circumstances.This time lens system,analogous to spatial lens,introduces great controllability for the output pulse characteristics through tuning both the phase modulation device and the dispersion-compensating setup.To achieve high-quality STUD pulses(with steep pulse edges),several factors including phase modulation function,modulation strength(spectral broadening width)and dispersion compensating status must be optimized.With the parabolic phase modulation function,the STUD pulse duration can be narrowed to ~10 ps with edge widths of ~3 ps,which is very desirable for suppressing parametric instabilities in laser driven inertial confinement fusion(ICF).The two methods of generating STUD pulse without mode-locking in this paper are simple in structure,easy to operate and highly controllable.It is very suitable for suppressing parametric instability in laser driven inertial fusion.