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考虑硅基QPD结构,根据能量守恒方程以及传统热传导方程,在硅基QPD单一象限受激光辐照后温度尚未达到相变温度的实际条件下,给出不包含相变的热传导方程,如公式(1)所示:
$$\begin{split} &k\dfrac{{{\partial ^2}T(x,y,{\textit{z}},t)}}{{\partial {x^2}}} + k\dfrac{{{\partial ^2}T(x,y,{\textit{z}},t)}}{{\partial {y^2}}} + k\dfrac{{{\partial ^2}T(x,y,{\textit{z}},t)}}{{\partial {{\textit{z}}^2}}}+\\ & {Q_L} + {Q_J} = \rho C\dfrac{{\partial T(x,y,{\textit{z}},t)}}{{\partial T}} \end{split} $$ (1) 式中:
$T\left( {r,x,y,{\textit{z}},t} \right)$ ,$\rho $ ,$C$ ,$k$ 分别表示$t$ 时刻的温度场分布、材料密度、比热容、热传导率;${Q_L}$ 为激光制热热源;${Q_J}$ 为焦耳热源[2]。$${Q_L}(T,x,y,{\textit{z}},t)\! =\! {I_0}(1 \!-\! R(T))\alpha (T)f(x,y)g(t)\exp ( - \alpha (T){\textit{z}})$$ (2) 式中:
${I_0}$ 为激光光斑峰值功率;$R(T)$ 为硅的反射系数;$\alpha (T)$ 为硅的吸收系数[2];$f\left( {x,y} \right)$ 为激光光束的空间分布;$g(t)$ 为激光光束的时间分布。$${I_0} = \frac{{{I_{Avg}} \cdot \pi {r_0}^2}}{{ \displaystyle\int_0^{{r_0}} {f(x,y)2\pi \sqrt {{x^2} + {y^2}} {\rm{d}}x{\rm{d}}y} }}$$ (3) 式中:
${I_{Avg}}$ 为激光均值峰值功率;${r_0}$ 为激光光斑半径。$$f(x,y) = \exp \left( - \frac{{2\left( {{x^2} + {y^2}} \right)}}{{r_0^2}}\right)$$ (4) $$g(t) = \left\{ {\begin{array}{*{20}{l}} {1,0 < t < \tau } \\ {0,t > \tau } \end{array}} \right.$$ (5) 式中:
$\tau $ 为激光的脉宽。$$\alpha (T) = 1\;023 \times {\left( {\frac{T}{{273}}} \right)^4}[1/m],273\;{\rm K} \leqslant T \leqslant 1\;687\;{\rm K}$$ (6) $$R(T) = 0.33,273\;{\rm K} < T < 1\;687\;{\rm K}$$ (7) 初始条件为:
$${T_0} = 300\;{\rm K}$$ (8) 边界条件:
$$ - k\frac{{\partial T\left( {x,y,{\textit{z}},t} \right)}}{{\partial x}}\left| {_{x = \pm 2.45\;{\rm{mm}}}} \right. = 0$$ (9) $$ - k\frac{{\partial T\left( {x,y,{\textit{z}},t} \right)}}{{\partial y}}\left| {_{y = \pm 2.45\;{\rm{mm}}}} \right. = 0$$ (10) $$ - k\dfrac{{\partial T\left( {x,y,{\textit{z}},t} \right)}}{{\partial {\textit{z}}}}\left| {_{{\textit{z}} = 282\;{\text{μ}} {\rm{m}}}} \right. = 0$$ (11) $${Q_J} = E(T,x,y,{\textit{z}},t){J_L}(T,x,y,t)$$ (12) 式中:
$E(T,x,y,{\textit{z}},t)$ 、${J_L}(T,x,y,t)$ 为光生电流密度。基于理论研究,针对温度场进行数值计算,获得激光光斑作用上表面中心点温升变化情况,以0.5 ms计算结果为例,如图1所示。随着能量密度增加,上表面光斑作用中心点温升越快,温度峰值越高。随着能量密度超过探测器性能损伤阈值并继续增大,可造成的损伤范围越大,损伤半径和损伤深度不断増大,则掺杂离子在硅熔融阶段,向象限1内部扩散越深,范围越大,同时产生的缺陷也越来越多,象限1性能逐渐下降,并逐渐产生形貌损伤。
图 1 不同能量密度下,上表面光斑中心对应点温度随时间演化仿真关系图
Figure 1. Simulation diagram of temperature evolution with time at the corresponding point of the upper surface spot center under different energy densities
激光辐照单一象限过程中,QPD受激光辐照吸收能量,形成内部缺陷,激光强度逐渐增加,可造成探测器性能下降以及形貌损伤。因此,针对不同激光辐照QPD损伤面积、形貌展开研究。
Experimental study on damage area and morphology of silica-based QPD induced by long pulse
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摘要: 为了研究硅基QPD在不同能量密度、不同脉宽激光辐照下的损伤面积、形貌,基于二维显微测量技术,测量了硅基QPD单一象限的损伤面积、形貌随激光能量密度和脉宽的变化。结果表明,在毫秒脉冲激光作用下,硅基QPD产生表面剥落、褶皱、裂纹、熔坑等损伤效果,且主要受入射激光功率密度影响,损伤面积随激光能量密度逐渐增加,随脉宽增加逐渐降低。通过实测分析,得出了不同激光脉宽下,硅基QPD表面形貌损伤阈值。激光脉宽为0.5 ms,能量密度为15.79 J/cm2时,硅基QPD出现熔融损伤;而脉宽为1.0、1.5、2.0、3.0 ms时,硅基QPD出现表面剥落的能量密度值为14.12、33.94、39.76、47.62 J/cm2。Abstract: Based on two-dimensional metallographic microscopic measurement technology, the damage area and morphology of the silicon-based quadrant photo-detector(QPD) were studied under different laser energy fluences and pulse widths. The damage area and morphology of silicon-based QPD with single cell change with laser energy fluences and pulse width were measured. The results showed that, the QPD produced surface pooling, folding, cracks, ablation areas and other damage effects under the action of a millisecond pulse laser. The damage area mainly affected by the incident laser energy fluences, and the damage area gradually increased with the laser energy fluences and decreased with the increase of pulse width. The damage thresholds of a silicon-based QPD with different laser pulse widths were obtained. At 0.5 ms, and the energy fluences was 15.79 J/cm2, the silicon-based QPD produced melting damage, and the energy fluences values of surface-damaged thresholds in the silicon-based QPD with pulse widths of 1.0, 1.5, 2.0 and 3.0 ms are 14.12 J/cm2, 33.94 J/cm2, 39.76 J/cm2 and 47.62 J/cm2.
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Key words:
- long pulse /
- silicon-based quadrant photo-detector /
- damage morphology /
- damage threshold
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