激光增益材料的光诱导损伤特性分析

Photoinduced Damage Characteristics of Laser Gain Materials

通过激光放大器获得高功率的激光输出时,增益材料的损伤特性决定了激光器的使用寿命。以激光二极管(LD)端面泵浦的单片钕玻璃激光放大器为例,对泵浦过程增益材料内部的光场特性和热效应引起的端面应变进行了研究。结合电子增殖理论,建立了一个激光增益材料场致损伤特性分析模型。研究了增益材料内部的雪崩电离速率和多光子电离速率的变化规律,并根据临界自由电子数密度确定了材料发生损伤的具体位置。研究结果表明,激光增益材料发生损伤的位置受到信号光初始光强值、脉冲宽度和泵浦光功率密度影响。当初始光场能量一定,脉冲宽度从10 ns增加到13 ns时,损伤点向入射端方向移动大约14 mm。泵浦光功率密度越大,端面热效应越明显,材料更容易发生损伤。

Objective The laser amplifier is an important part of a high-power solid laser system and a key link for the system to achieve high-power output.Two main problems deserve special attention in the research of laser amplifiers.One is to optimize the characteristics of the gain material according to the requirements of the system on energy gain.The other is to analyze the optical load capacity of the gain material and optimize the structure design accordingly.Once the optical field inside the material exceeds its load capacity,amplifier damage leaves the whole system unable to operate normally,which necessitates the study of the damage characteristics of the amplifier.Methods Taking the laser diode(LD)end-pumped monolithic neodymium glass laser amplifier as an example,this paper investigates the characteristics of the optical field inside the gain material in the pumping process and the end-face strain caused by the thermal effect.Drawing on the theory of electron proliferation,the paper constructs a model for analyzing the field-induced damage characteristics of a laser gain material under ideal and thermal conditions.It further explores the rates of avalanche ionization and multi-photon ionization in the gain material and determines the specific location of damage according to the critical free electron number density.Results and Discussions The energy and pulse width of the incident laser can be reasonably optimized to fully exploit the amplification performance of the gain medium.As the incident laser energy and laser pulse width increase,the location of damage moves toward the incident end[Fig.4(b)].When the pulse width is increased from 10 ns to 13 ns,the damage point moves by approximately 14 mm.Within the range with an optical field value of 4X10*V/m,the movement range of the damage point is 14%of the material thickness.Within the range with a pulse width of 3 ns,the movement range of the damage point reaches 35%of the material thickness.Therefore,when the signal light is amplified,the laser pulse width should be smaller than 10 ns,and the initial optical field value should be lower than 3.3035X10 V/m if the thickness of the neodymium glass is 40 mm.In this case,the damage inside the neodymium glass can be avoided.Moreover,due to the influence of the thermal deformation of the material on its damage characteristics,the material is affected by both gain and the thermal effect under different pump power densities,and the damage location is closer to the incident end than that in the ideal case(Fig.8).Specifically,when the pump power density is 1X104 W/cm²and 1X105 W/cm²,the internal damage positions is at 22.51 mm and 6.43 mm,respectively.Conclusions This paper builds an analysis model for gain material damage by taking the LD end-pumped monolithic neodymium glass laser amplifier as an example.Then,it studies the rates of avalanche ionization and multi-photon ionization in the gain material and determines the specific damage location in the material under the two conditions according to the critical free electron number density.The calculation model is further extended according to the actual situation.Effective measures are put forward to prevent the gain material from damage and prolong its service life.The results show that under the influences of the thermal effect and pump power density on the material gain,no damage actually occurs in the material when the pump power density is 1 X 1o3 W/cm²because the material gain is small.In contrast,when the pump power density is 1 X 10*w/cm²and 1 X 105 W/cm,the field-induced damage in the material occurs at the positions of 22.51 mm and 6.43 mm,respectively.This can be attributed to different modulation of the optical field of the incident signal light caused by the different influences of the thermal effect on the gain material and the different degrees of end-face deformation.A larger pulse width of the incident laser corresponds to a smaller damage threshold of the material and damage closer to the incident end.When the initial optical field value is constant and the pulse width increases from 10 ns to 13 ns,the damage point moves forward by about 14 mm.If the thickness of the neodymium glass is 4 mm,the peak power of the pump light should be lowered to reduce the impact of the thermal effect on the end face on the initial signal light.In addition,the laser pulse width should be smaller than 10 ns,and the initial optical field value should be lower than 3.35 X 1os V/m.In this way,the damage inside the glass can be avoided.Therefore,after the thickness of the neodymium glass is determined,the initial optical field value and the pulse width of the incident laser and the pump power density can be adjusted to avoid the material damage as a result of excessive modulation of the optical field caused by end-face deformation or excessive amplification of laser energy in the material and ultimately improve the service life of the gain material.