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首先对激光在大气介质中的传输行为进行表征。激光束是一种特定频段的电磁场,其运动规律可以用Maxwell方程组和电磁本构方程描述,基于已有理论可推导出激光的电场强度$ \vec E $满足如下的波动方程[16]:
$$ {\nabla ^2}\vec E - {\mu _0}\sigma \frac{{\partial \vec E}}{{\partial t}} - {\mu _0}{\varepsilon _0}\frac{{{\partial ^2}\vec E}}{{\partial {t^2}}} - {\mu _0}\frac{{{\partial ^2}\vec P}}{{\partial {t^2}}} = 0 $$ (1) 针对无耗介质,即$ \sigma = 0 $ (大气未被电离而产生等离子体可认为是无耗介质),公式(1)可进一步可简化为:
$$ {\nabla ^2}\vec E - \frac{1}{{{C^2}}}\frac{{{\partial ^2}\vec P}}{{\partial {t^2}}} - \frac{1}{{{\varepsilon _0}{C^2}}}\frac{{{\partial ^2}\vec P}}{{\partial {t^2}}} = 0 $$ (2) 式中:$ {C^2} = \dfrac{1}{{{\varepsilon _0}{\mu _0}}} $。由此可以求解出稳态激光场所满足的矢量亥姆霍兹方程:
$$ {\nabla ^2}\vec E + {k^2}{n^* }\vec E = 0 $$ (3) 式中:k为真空中光波的波数,$ k = \dfrac{{2\pi }}{{n^* \lambda }} $;$n^* $表示复折射率,$n^* = \sqrt {1 + \chi }$。
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针对激光电场控制公式(3)进行求解。假设有沿着x轴方向传输的光场,其光场的解可以表示为:
$$ \vec E\left( {x,y,z} \right) = \varPsi \left( {x,y,z} \right)\exp \left( {ikx} \right) $$ (4) 将公式(4)代入公式(3),得到:
$$ \frac{{{\partial ^2}\varPsi }}{{\partial {y^2}}} + \frac{{{\partial ^2}\varPsi }}{{\partial {z^2}}} + 2ik\frac{{\partial \varPsi }}{{\partial x}} + \frac{{{\partial ^2}\varPsi }}{{\partial {x^2}}} = 0 $$ (5) 由于激光具有很强的会聚性且是典型的高斯光束,满足傍轴近似条件,对公式(5)进一步求解,可得到激光电场的解:
$$ \begin{split} E\left( {x,y,z} \right) =& {E_0}\frac{{{w_0}}}{{w\left( x \right)}}\exp \left( { - \frac{{{y^2} + {z^2}}}{{{w^2}\left( x \right)}}} \right)\exp \left( { - ik\frac{{{y^2} + {z^2}}}{{2R\left( x \right)}}} \right)\cdot\\ & \exp \left( { - i\left( {kx - \eta \left( x \right)} \right)} \right) \end{split}$$ (6) 其中,在x点处的光斑半径:
$$ w\left( x \right) = {w_0}\sqrt {1 + {{{x^2}}}/{{x_0^2}}} $$ (7) 式中:$ {x_0} = \pi {n}w_0^2/\lambda $为瑞利长度,n为介质的折射率。
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高速流场存在激波、边界层、湍流等复杂结构,对激光的影响作用体现在流场温度、压力以及密度变化对其折射率产生影响,进而影响电场分布特征,导致激光传播方向的改变和能量的衰减。
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在非均匀介质中,光的传播轨迹比较复杂,与非均匀介质的状态有密切关系。光波在非均匀大气中传输时,大气温度和气压引起其折射率随空间和时间的随机起伏。在非均匀介质情况下,折射率n不是常数,而是空间位置的函数。在强光作用下,介质中的非线性电极化效应不能被忽略,导致折射率依赖于光强。非线性折射率的表达式为:
$$ n = {n_0} + \gamma I $$ (8) 式中:n0为参考状态气体折射率;γ为非线性折射率系数;I为光强度。激光束的光强与激光电场强度有如下关系:
$$ I = \frac{1}{2}{\varepsilon _0}c{n_0}{\left| E \right|^2} $$ (9) 故可得出折射率为:
$$ \begin{split} n =& {n_0} + \gamma \frac{1}{2}{\varepsilon _0}c{n_0}\left| {E_0}\frac{{{w_0}}}{{w\left( x \right)}}\exp \left( { - \frac{{{y^2} + {z^2}}}{{{w^2}\left( x \right)}}} \right)\cdot \right.\\ &\left. \exp \left( { - ik\frac{{{y^2} + {z^2}}}{{2R\left( x \right)}}} \right)\exp \left( { - i\left( {kx - \eta \left( x \right)} \right)} \right)\right|^2 \end{split}$$ (10) 可以看出,在高速流场中,气体密度、温度和压力进一步对折射率产生影响。
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激光在气流层中的传输特性是由光束在通过气流层空间范围时所引起的光学性质的变化决定的。当光束穿过高速流场中时受到某种方式的扰动,与无扰动时相比,光线偏离原来的传播方向。首先,折射率与流场密度之间的关系可通过Lorenz-Lorentz公式给出:
$$ \left( {\frac{{{n^2} - 1}}{{{n^2} + 2}}} \right)\frac{1}{\rho } = \frac{2}{3}{K_{{\rm{GD}}}} $$ (11) 式中:KGD为Gladstone-Dale系数(比折射度),m3/kg,其表达式为:
$$ {K_{{\rm{GD}}}} = \sum {{K_i}} \frac{{{\rho _i}}}{{\bar \rho }} = \frac{{{K_h}}}{{\bar \rho }} $$ (12) 折射率随温度的变化可以表示为:
$$ n = 1 + \left( {{n_c} - 1} \right)\frac{{\overline T }}{T} $$ (13) 气体在可压缩过程中,由于压力变化引起密度变化,使局部气体折射率变化,对于绝热变换有:
$$ n = 1 + {\delta _0}{\left( {{p}/{{\overline p}}} \right)^{\tfrac{1}{\gamma }}} $$ (14) 式中:γ=cp/cv为比热比,cp为定压克分子热容量,cv为定容克分子热容量。基于公式(10)、(11)、(13)和(14)可得到折射率与流场密度、温度、压力之间满足:
$$ \frac{{{{(n - 1)}^\gamma }({n^2} + 2)}}{{n + 1}} = \frac{{3R\overline \rho \overline T{\delta _0}^\gamma }}{{2{K_h}\overline P}}({n_0} - 1) $$ (15) 由公式(15)可知,折射率与流场的密度、温度、压力呈非线性关系。
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电磁波传播时即有能量传播,单位时间内穿过与传播方向垂直的单位表面的能量称为功率流。功率流是一个矢量,其方向是该点能量流动的方向。从微观角度来说,当激光束在大气中传输时,可以认为电磁场的电场分量作用于物质的微观结构,令结构中的带电粒子受到外力做受迫振动,该作用使得电磁场的一部分能量转化为热能,宏观就体现为入射光的能量发生了损耗,称为光被吸收。一束入射光通过某一介质层,其光通量由于受到介质的散射和吸收,使得通过介质层以后的原光路上的出射光受到衰减,即大气消光。研究大气的消光特性时是从入射光与出射光为同轴光的角度出发的。光强因散射和吸收而引起的衰减遵从比尔规律,即:
$$ I = {I_0}{{\rm{e}}^{ - \mu \left( \lambda \right)l}} $$ (16) 式中:I0为经过大气衰减前的光强;I为经过大气衰减后的光强;l为传输距离;μ (λ)为波长为λ的激光的大气衰减系数。公式(16)表示大气对光的消光特性是随着介质层的厚度呈现指数规律减弱的。功率和光强之间的关系满足$P = SI$。由公式(6)和公式(16)可得:
$$\begin{split} P =& \frac{1}{2}S{\varepsilon _0}c{n_0}\left| {E_0}\frac{{{w_0}}}{{w\left( x \right)}}\exp \left( { - \frac{{{y^2} + {z^2}}}{{{w^2}\left( x \right)}}} \right)\exp \left( { - ik\frac{{{y^2} + {z^2}}}{{2R\left( x \right)}}} \right)\cdot\right. \\ &\left. \exp \left( { - i\left( {kx - \eta \left( x \right)} \right)} \right) \right|^2{{\rm{e}}^{ - \mu (\lambda )l}} \end{split} $$ (17) 公式(17)描述了激光在流场中的能量衰减行为。
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为验证所提出强激光在高速流场中的传输及衰减率理论模型,将计算结果与风洞试验结果进行对比。风洞试验是由激光发射器发射功率为713 W的激光,穿过风洞一侧K9玻璃进入风洞,然后透过风洞中部的有机玻璃,再穿过风洞另一侧的K9玻璃,并对其光斑半径和功率进行测量,激光在流场中传输的模型示意图如图1所示。
风洞中的通入气流的马赫数分别为0、0.85、3,风洞两侧K9玻璃之间的距离为0.6 m,靶材玻璃处在0.3 m位置处。具体试验结果如表1所示。
表 1 风洞试验参数及结果
Table 1. Wind tunnel test parameters and results
Mach number, Ma P0/W P2/W d0/cm d2/cm L/m 0 713 240 4.4 8.63 0.6 0.85 713 258 4.4 8.69 0.6 3 713 308 4.4 - 0.6 通过所提出的模型计算了激光在流场中的光斑半径变化和电场分布特征,如图2~图4所示,并与试验结果对比了到靶光斑半径d2和功率P2。模型所采取的参数见表2。
图 2 Ma=0,P0=713 W时激光在流场中的(a)光斑半径变化、(b)电场分布特征及(c)激光表面分布特征
Figure 2. Laser in the flow field (a) changes in spot radius, (b) the electric field distribution, and (c) the laser surface distribution at Ma=0, P0=713 W
图 3 Ma=0.85,P0=713 W时激光在流场中的(a)光斑半径变化、(b)电场分布特征及(c)激光表面分布特征
Figure 3. Laser in the flow field (a) changes in spot radius, (b) the electric field distribution, and (c) the laser surface distribution at Ma=0.85, P0=713 W
图 4 Ma=3,P0=713 W时激光在流场中的(a)光斑半径变化、(b)电场分布特征及(c)激光表面分布特征
Figure 4. Laser in the flow field (a) changes in spot radius, (b) the electric field distribution, and (c) the laser surface distribution at Ma=3, P0=713 W
表 2 计算所选取参数
Table 2. Selected parameters for calculation
Mach number, Ma P0/W d0/cm L/m 0 713 4.4 0.6 0.85 713 4.4 0.6 3 713 4.4 0.6 从图2~图4可以看出,所提出模型计算得到靶光斑半径和能量衰减与风洞试验结果一致,验证了所提出模型的准确性和可靠性。试验中,由于风洞两侧有K9玻璃,风洞中靶材为有机玻璃,其对光斑半径和能量衰减影响较大,在计算中考虑了玻璃的影响,其作用体现在对扩散系数$\;{\beta ^l}$和衰减系数$\; \mu (\lambda )$的变化。从图中可以看出,光斑半径随着传输距离的增加而逐渐增大,说明流场和玻璃对光束具有发散作用;电场强度随着传输距离的增加而逐渐减弱,说明流场和玻璃对光束能量具有削弱作用;激光是典型的非线性光,在光束中心位置强度最大,自内至外逐渐减小。
Laser transmission and attenuation characteristics in high-speed flow field
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摘要: 基于Maxwell方程组构建了描述激光在大气中传播行为的理论模型,并通过求解得到了具有高斯分布特征的激光分布解析解;同时,基于流场控制方程计算了不同马赫数的流场分布特征;在此基础上,通过建立非线性折射率模型,考虑流场密度、温度、组分以及压力的影响,研究了各因素对激光折射率的影响规律;最后,基于激光电场分布特征、流场分布特征以及比尔定律,建立了描述激光在高速流场中的能量衰减模型,揭示了高速流场对激光折射和衰减的影响规律。Abstract:
Objective When a high-energy laser passes through the target airflow boundary layer, its optical path transmission path will change, and its energy density will also decay. It is the key to study the influence mechanism of the target gas boundary layer on the laser transmission performance for the application of laser as energy carrier in military field. The laser passing through the high-speed flow field to reach the target material is a nonlinear dynamic problem involving the coupling of electric fields, magnetic fields, temperature fields, and flow fields. In order to study the transmission and attenuation characteristics of laser, it is necessary to accurately describe and characterize the interaction between laser and flow field. Methods Based on the classical electrodynamics theory, a theoretical model describing the laser propagation behavior in the atmosphere is constructed. At the same time, the flow field distribution characteristics of different Mach numbers are calculated based on the flow field governing equation. On this basis, considering the influence of flow field density, temperature, velocity and pressure, the nonlinear refractive index model is established, the influence of each factor on laser refractive index is studied. Finally, based on the laser transmission and attenuation characteristics, the influence of the gas boundary layer on the laser-to-target parameters is revealed. Results and Discussions The radius of the laser spot gradually increases with the increase of transmission distance, which indicates that the flow field and glass have a divergent effect on the beam (Fig.2-4). Under the action of glass and flow field, the power to target is in good agreement with the experimental data. The electric field intensity decreases gradually with the increase of transmission distance, which indicates that the flow field and the glass have a weakening effect on the beam energy. The loss of laser transmission power is directly proportional to density and pressure, and inversely proportional to temperature (Fig.19). At the same Mach number, the influence of the flow field on the laser mainly comes from the refractive index of the flow field, which is closely related to the flow field density. The smaller the flow field density is, the greater the attenuation rate of the laser passing through the flow field is. The smaller the pressure in the flow field is, the thinner the air is, which affects the refractive index of the gas. Conclusions The distribution characteristics of laser cross section always follow the middle electric field value to the maximum, and then gradually decrease to the surrounding, which shows that laser is a typical nonlinear light. Different Mach numbers correspond to different gas densities. From air to high-speed flow field, there will be a sudden change in gas density. Under the same transmission distance, the power at high Mach number is greater than that at low Mach number. When the Mach number is fixed, the corresponding power decreases approximately linearly with the increase of transmission distance. -
表 1 风洞试验参数及结果
Table 1. Wind tunnel test parameters and results
Mach number, Ma P0/W P2/W d0/cm d2/cm L/m 0 713 240 4.4 8.63 0.6 0.85 713 258 4.4 8.69 0.6 3 713 308 4.4 - 0.6 表 2 计算所选取参数
Table 2. Selected parameters for calculation
Mach number, Ma P0/W d0/cm L/m 0 713 4.4 0.6 0.85 713 4.4 0.6 3 713 4.4 0.6 -
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