Research On Beam Smoothing Technology Of High Power Nd: Glass Laser

Inertial Confinement Fusion(ICF)is of great significance for energy development.The key to its research is to build a laser system with high power and high beam quality.In the ICF experiment,the irradiation on the target surface has to be very uniform,thus a number of beam smoothing techniques have been developed.The existing beam smoothing techniques can be divided into two categories: spatial techniques and temporal techniques.The theoretical and experimental explorations have indicated that a scheme combining the temporal technique of smoothing by spectral dispersion(SSD)and the spatial technique of lens array(LA)or continuous phase plate(CPP)is feasible.In the laser driver using the SSD technique,the spectral dispersed laser beam propagates in the following optical path through the multi-stage amplifier and spatial filter.The amplifier amplifies step by step the seed beam which is injected into it and at the end the required energy level can be reached on the target surface.The amplifier is one of the most important parts of the laser system in that it not only accounts for a large proportion of the total cost but also determines the overall performance of the system to a large extent.Thus,the gain characteristic of the amplifier is an important factor that should be considered in the study on beam smoothing techniques.Neodymium glass amplifiers have been widely used in ICF solid-state laser systems because of their high gain bandwidth,high saturation flux,long lifetime,feasibility in large size,etc.For example,in the National Ignition Facility of the United States and the SG laser facilities of China,plate neodymium glass amplifiers pumped by xenon lamps are employed.In this paper,it is investigated the influence of the gain characteristic of the neodymium glass amplifier on the performance of smoothing by spectral dispersion in the high power laser system.Firstly,we analyze the effects of SSD parameters on the smoothing performance when there is no amplifier.Then,the intensity distribution of the target focal spot in the SSD+LA scheme and in the SSD+CPP scheme is respectively analyzed based on two-dimensional numerical calculations when the amplifier is included.After the SSD unit is added to the optical path,the irradiation uniformity of the focal spot is obviously improved,and within a certain range the greater the bandwidth,the better the uniformity.The numerical results show that,when the central wavelength of the incident beam is different from the central wavelength of the amplifier gain curve,intensity distribution of the target spot will be affected in a degree depending on the amplification factor;while these two wavelengths are very close or identical,variation in the intensity distribution is trivial.The irradiation uniformity of the target focal spot depends mainly on the bandwidth of the phase modulated laser beam,and the gain characteristic of the amplifier has no obvious effect on the power spectrum and uniformity.The conclusion of this paper provides an important theoretical reference for the practical application of the technique of smoothing by spectral dispersion in laser system.