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Nd∶YAG作为一种激光晶体具有典型的四能级结构,其能级结构如图 1中蓝色部分所示。图中还给出了Nd∶YAG主要能级相关的Stark能级分布,R与Y表示F和I的正能级。以Nd∶YAG晶体红外区域790nm~830nm波长范围内的吸收带为例,在基质中由于晶体场的作用,Nd3+离子的单个能级将分裂为若干个Stark子能级,因此,Nd∶YAG晶体的吸收带是由多个子吸收峰组成的。图 1中红色部分描述了Nd∶YAG固体激光器的四能级结构的工作状态。光将离子由基态E0抽运到抽运带,大部分的激发态离子通过无辐射跃迁到亚稳态能级E2,然后输出1064nm的激光到达终态能级E1,最后,离子通过迅速的无辐射跃迁回到基态能级。
假设从抽运带到上能级的跃迁过程非常迅速,抽运带的粒子数密度n3≈0。则典型的激光四能级速率方程如下[23]:
$ \frac{\mathrm{d} n_{3}}{\mathrm{~d} t}=W_{\mathrm{p}} n_{0}-W_{\mathrm{nr}} n_{3} $
(1) $ \frac{\mathrm{d} n_{2}}{\mathrm{~d} t}=W_{\mathrm{nr}} n_{3}-\left(n_{2}-\frac{g_{2}}{g_{1}} n_{1}\right) c_{n} \sigma \varphi-\left(\frac{n_{2}}{\tau_{21}}+\frac{n_{2}}{\tau_{20}}\right) $
(2) $ \frac{\mathrm{d} n_{1}}{\mathrm{~d} t}=\left(n_{2}-\frac{g_{2}}{g_{1}} n_{1}\right) c_{n} \sigma \varphi+\frac{n_{2}}{\tau_{21}}+\frac{n_{3}}{\tau_{31}}-\frac{n_{1}}{\tau_{10}} $
(3) $ n_{\mathrm{t}}=n_{0}+n_{1}+n_{2}+n_{3} $
(4) 式中,Wp是抽运速率;Wnr是能级3和能级2之间的无辐射跃迁速率;g1和g2表示能级1和能级2的简并度; n0, n1, n2, n3是各能级的粒子数密度,nt表示各密度之和;τij是能级i到能级j之间的跃迁寿命;σ是受激辐射截面面积;cn是光在折射率为n的介质中传播的速度;φ是光子数密度。
由于吸收带包含多个Stark子能级,在忽略抽运带中子能级之间的跃迁的情况下,(1)式和(2)式中的某些参数将发生如下的变化。
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用LED抽运光源的功率和激光工作物质的吸收系数来描述抽运速率,将LED的发射光谱和增益介质的吸收光谱引入速率方程来模拟LED带抽运Nd∶YAG激光器将提高仿真的精度。
在LED带抽运中,输入功率Pin是波长λ的函数,由LED抽运光源决定。吸收系数也是波长的函数,因此,被增益基质吸收的抽运功率也应为波长的函数,其表达式如下所示:
$ P_{\mathrm{abs}}(\lambda)=P_{\mathrm{in}}(\lambda)\left[1-\mathrm{e}^{-\alpha(\lambda) L}\right] $
(5) 式中,α(λ)表吸收数,L是增益介质长度。用(5)式来描述带抽运的抽运速率:
$ W_{\mathrm{p}}=\frac{\eta \int_{\lambda_{1}}^{\lambda_{2}} P_{\mathrm{in}}(\lambda)\left[1-\mathrm{e}^{-\alpha(\lambda) L}\right] \lambda \mathrm{d} \lambda}{n V h c_{0}} $
(6) 式中,V是体积,h为普朗克常数,η是量子效率,c0是真空中的光速,(λ1, λ2)是LED抽运光源的波长范围。
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无辐射跃迁出现在几乎每一个能级上,无辐射跃迁速率主要受晶体主晶格和能级间隔影响,可以表示为(7)式所示的形式。和抽运速率不同的是,无辐射跃迁速率是一个离散的值,因此,在带抽运的速率方程计算中,用∑Wnr来表示无辐射跃迁速率,具体公式如下:
$ \begin{aligned} W_{\mathrm{nr}}(T, \Delta E)=& W_{0}\left\{\frac{\exp [\hbar \omega /(k T)]}{\exp [\hbar \omega /(k T)]-1}\right\} \times \\ & \exp (-\gamma \Delta E) \end{aligned} $
(7) 式中,W0表示温度T=0K时的多声子弛豫几率,ΔE为发生跃迁的能级间隔,$ \hbar $表示约化普朗克常数,ω为角动量,k为玻尔兹曼常数,γ为相互作用耦合参量,可以表示为[24]:
$ \gamma=\hbar \omega_{\mathrm{m}}^{-1}\left[\lg \left(\frac{N}{S_{0}(\bar{n}+1)}\right)^{-1}-1\right] $
(8) 式中, ωm是晶体基质的最高声子频率,N为声子数目,n为玻尔兹曼分布,S0是与基质有关的常数。
此为单一能级的无辐射跃迁几率的实验所得带隙公式。由于LED带抽运中,抽运带内含有多个吸收峰,对于无辐射跃迁来说,每一个吸收峰对应的跃迁都含有一个对应的非辐射跃迁。
综上所述,可以得到如下形式的速率方程:
$ \begin{array}{c} \frac{{{\rm{d}}{n_3}}}{{{\rm{d}}t}} = \frac{{\eta \int_{{\lambda _1}}^{{\lambda _2}} {{P_{{\rm{in }}}}} (\lambda )\left[ {1 - {{\rm{e}}^{ - \alpha (\lambda )L}}} \right]\lambda {\rm{d}}\lambda }}{{nVh{c_0}}}{n_0} - \\ {n_3}\sum {{W_{{\rm{nt}}}}} \end{array} $
(9) $ \begin{gathered} \frac{\mathrm{d} n_{2}}{\mathrm{~d} t}=\frac{\eta \int_{\lambda_{1}}^{\lambda_{2}} P_{\mathrm{in}}(\lambda)\left[1-\mathrm{e}^{-\alpha(\lambda) L}\right] \lambda \mathrm{d} \lambda}{n V h c_{0}} n_{0}+ \\ n_{3} \sum W_{\mathrm{nr}}-\left(n_{2}-\frac{g_{2}}{g_{1}} n_{1}\right) c_{n} \sigma \varphi-\left(\frac{n_{2}}{\tau_{21}}+\frac{n_{2}}{\tau_{20}}\right) \end{gathered} $
(10) $ \frac{\mathrm{d} n_{1}}{\mathrm{~d} t}=\left(n_{2}-\frac{g_{2}}{g_{1}} n_{1}\right) c_{n} \sigma \varphi+\frac{n_{2}}{\tau_{21}}-\frac{n_{1}}{\tau_{10}} $
(11) $ n_{\mathrm{t}}=n_{0}+n_{1}+n_{2}+n_{3} $
(12) 将参数调整后的表达式带入到速率方程中,并按照一般解法求解速率方程,仿真部分均基于上述经调整后的速率方程。
LED带抽运Nd∶YAG激光器
LED band pump Nd∶YAG laser
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摘要: 为了研究抽运光源光谱与增益介质吸收光谱对发光二极管(LED)带抽运激光器输出效率的影响,进一步提高输出效率,将光谱信息引入激光速率方程中,建立了LED带抽运速率方程, 采用该方法对LED带抽运Nd∶YAG激光器进行了理论分析和实验验证。结果表明,利用红外LED对Nd∶YAG激光器进行侧面抽运,当抽运能量为9.1mJ时,取得了能量为607μJ的1064nm激光输出,达到实验中最高的倾斜效率15.5%,此时光转换效率为6.67%;速率方程的计算求解和实验的输出能量二者基本吻合。这一结果对研究提高LED带抽运激光器的输出效率是有帮助的。Abstract: In order to study the influence of the pump light source spectrum and the absorption spectrum of the gain medium on the output efficiency of light-emitting diode(LED) band pump laser, and to improve the output efficiency, the spectral information was introduced into the laser rate equations to establish the rate equations of the LED band pump Nd∶YAG laser. Theoretical analysis and experimental verification of LED band pumped Nd∶YAG laser were carried out. The Nd∶YAG laser was side-pumped by infrared LEDs. Under the pump energy of 9.1mJ, a 1064nm laser output with an output energy of 607μJ was obtained. The slope efficiency of this LED band pump Nd∶YAG laser was 15.5%, and the corresponding optical-to-optical efficiency was 6.67%. The results show that the calculated solution of the rate equations was basically consistent with the experimental output energy, which is helpful for the study of improving the output efficiency of LED band pump laser.
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Key words:
- lasers /
- band pump /
- laser rate equations /
- light-emitting diode
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