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矢量涡旋光束具有各项异性的空间偏振分布和螺旋相位分布,在傍轴近似条件下,为了精确描述其电场分布,选择左、右旋圆偏振矢量
$\left\{ {{{\boldsymbol{e}}_L},{{\boldsymbol{e}}_R}} \right\}{\text{ = }} \dfrac{1}{{\sqrt 2 }}\left[ {\begin{array}{*{20}{c}} 1\;\;1 \\ {{{ - }}i}\;\;i \end{array}} \right]$ 作为基矢对,矢量涡旋光束可以表示为左、右旋圆偏振涡旋光束的叠加[18]:$$ {\boldsymbol{E}}{\text{ = }}{E_L}{{\boldsymbol{e}}_{\boldsymbol{L}}} + {E_R}{{\boldsymbol{e}}_R} $$ (1) 式中:
$ {E_L}{\text{ = }}A\left( {{r_0}} \right)\exp \left( {i{\delta _L}} \right) $ 、$ {E_R}{\text{ = }}A\left( {{r_0}} \right)\exp \left( {i{\delta _R}} \right) $ 分别表示左、右旋圆偏振涡旋分量,$ {\delta _L} = {l_L}{\theta _0} - \phi /2 $ 和$ {\delta _R} = {l_R}{\theta _0} + \phi /2 $ 分别表示左右旋圆偏振涡旋分量的相位分布,$ {l_L} $ 和$ {l_R} $ 分别表示左右旋圆偏振涡旋分量的相位拓扑荷,$ {\theta _0} $ 为角向坐标,$ \phi $ 表示初始相位,$ A({r_0}) $ 为矢量涡旋光束的振幅分布,具体表示为:$$ A\left( {{r_0}} \right) = {\left( {\frac{{\sqrt 2 {r_0}}}{w}} \right)^{\left| l \right|}}\exp \left( { - \frac{{{r_0}^2}}{{{w^2}}}} \right) $$ (2) 式中:
$ {r_0} $ 为径向坐标;$ w $ 表示束腰半径。结合公式(1)、(2)可以得到矢量涡旋光束入射面的场分布为:$$ {{\boldsymbol{E}}_0}({r_0},{\theta _0}) = {\left( {\frac{{\sqrt 2 {r_0}}}{w}} \right)^{\left| l \right|}}\exp \left( { - \frac{{{r_0}^2}}{{{w^2}}}} \right)\left[ \begin{gathered} \cos (m{\theta _0} + \phi /2) \\ \sin (m{\theta _0} + \phi /2) \\ \end{gathered} \right]\exp (il{\theta _0}) $$ (3) 式中:
$ m = ({l_L} - {l_R})/2 $ 表示矢量涡旋光束的偏振阶数;$ l = ({l_L} + {l_R})/2 $ 表示矢量涡旋光束的相位拓扑荷。柱坐标下,光束通过含光阑复杂光学系统后场分布由柯林斯积分公式给出[19]:
$$ \begin{gathered} {\boldsymbol{E}}(r,\theta ,{{z}}) = \frac{i}{{\lambda B}}\exp ( - ik{{z}})\int_0^{2\pi } {\int_o^\infty {{{\boldsymbol{E}}_0}} } ({r_0},{\theta _0})H({r_0}) \times \\ \exp \left\{ { - \frac{{ik}}{{2B}}\left[ {A{r_0} - 2{r_0}r\cos (\theta - {\theta _0}) + D{r^2}} \right]} \right\}{r_0}{\rm{d}}{r_0}{\rm{d}}{\theta _0} \\ \end{gathered} $$ (4) 式中:
$ k = 2\pi /\lambda $ 为波数,$ \lambda $ 为波长;$ {{z}} $ 为传输方向上的传输距离;$ r $ 、$ \theta $ 分别表示径向坐标和角坐标;$ A、B、D $ 为光学系统的矩阵元,$ H({r_0}) $ 为光阑窗口函数。光阑窗口函数可展开为复高斯函数的有限级数和[20]:$$ H({r_0}) = \sum\limits_{j = 1}^N {{A_j}} \exp \left( { - \frac{{{B_j}{r_0}^2}}{{{a^2}}}} \right) $$ (5) 式中:复常数
$ {A_j} $ 、$ {B_j} $ 分别为展开系数和复高斯函数系数;$ a $ 为光阑孔径。将公式(3)、(5)代入公式(4)可得:
$$ \begin{split} {\boldsymbol{E}}(r,\theta ,{{z}}) = & \sum\limits_{j = 1}^N {{A_j}} \frac{{ki}}{B}\exp ( - ik{{z}})\exp (il\theta )\exp \left( { - \frac{{ikD{r^2}}}{{2B}}} \right)\times\\& \left[ \begin{gathered} \cos (m\theta + \phi /2) \\ \sin (m\theta + \phi /2) \\ \end{gathered} \right] \int_0^\infty {{{\left( {\frac{{\sqrt 2 {r_0}}}{w}} \right)}^{\left| l \right|}}\exp } \left( { - \frac{{{B_j}{r_0}^2}}{{{a^2}}}} \right) \times \\&{J_{\left| l \right|}}\left( {\frac{{kr{r_0}}}{B}} \right)\exp \left( { - \frac{{ikA{r_0}^2}}{{2B}} - \frac{{{r_0}^2}}{{{w^2}}}} \right){r_0}{\rm{d}}{r_0} \end{split} $$ (6) 式中:
$ {J_{\left| l \right|}}\left( \cdot \right) $ 表示第一类贝塞尔函数。利用积分公式
$$ \int_0^\infty {{x^{v + 1}}} {{\rm{e}}^{ - \alpha {x^2}}}{J_v}(bx){\rm{d}}x = \frac{{{b^v}}}{{{{\left( {2\alpha } \right)}^{v + 1}}}}\exp \left( { - \frac{{{b^2}}}{{4\alpha }}} \right) $$ (7) 可得矢量涡旋光束通过光阑-透镜光学系统后的场分布为:
$$ \begin{split} & {\boldsymbol{E}}(r,\theta ,{{z}}) = \sum\limits_{j = 1}^N {{A_j}} \frac{{ki}}{B}\exp ( - ik{{z}})\exp (il\theta )\times\\&\exp \left( { - \frac{{ikD{r^2}}}{{2B}} - \frac{1}{4}{{\left( {\frac{{kr}}{B}} \right)}^2}{{\left( {\frac{{{B_j}}}{a} + \frac{{ikA}}{{2B}} + \frac{1}{{{w^2}}}} \right)}^{ - 1}}} \right) \times\\ & {\left( {\frac{{\sqrt 2 kr}}{{wB}}} \right)^{\left| l \right|}}{2^{ - \left| l \right| - 1}}{\left( {\frac{{{B_j}}}{a} + \frac{{ikA}}{{2B}} + \frac{1}{{{w^2}}}} \right)^{ - \left| l \right| - 1}}\left[ \begin{gathered} \cos (m\theta + \phi /2) \\ \sin (m\theta + \phi /2) \\ \end{gathered} \right] \end{split} $$ (8) 光束的轨道角动量源于光束线动量的角向分量,沿光束传播方向的轨道角动量密度可由径矢
$ {\boldsymbol{r}} $ 叉乘线动量密度$ {\boldsymbol{S}} $ 的角向分量($ \phi $ 分量)得到:$$ {j_z} = \left| {\left( {{\boldsymbol{r}} \times {{\boldsymbol{S}}_\phi }} \right)} \right| $$ (9) 光束线动量密度
$ {\boldsymbol{S}} $ 为坡印亭矢量时间平均值的实部:$$ {\boldsymbol{S}}{\text{ = }}{\varepsilon _0}\left\langle {{Im} \left( {\boldsymbol{E}} \right) \times {Im} \left( {\boldsymbol{H}} \right)} \right\rangle $$ (10) 式中:
$ {\varepsilon _0} $ 为真空介电常数;$ {\boldsymbol{E}} $ 为光束的电场强度矢量,${Im} \left( {\boldsymbol{E}} \right){{ = }}\left( {{\boldsymbol{E}} + {{\boldsymbol{E}}^{\text{*}}}} \right){\text{/}}2$ ;$ {\boldsymbol{H}} $ 为光束的磁场强度矢量,${Im} \left( {\boldsymbol{H}} \right){{ = }} \left( {{\boldsymbol{H}} + {{\boldsymbol{H}}^{\text{*}}}} \right){\text{/}}2$ 。则公式(11)为:$$ \begin{split} {\boldsymbol{S}} = &\frac{{{\varepsilon _0}}}{4}\left[ {\left( {{{\boldsymbol{E}}^*} \times {\boldsymbol{H}}} \right) + \left( {{\boldsymbol{E}} \times {{\boldsymbol{H}}^*}} \right)} \right]= \\ & \frac{{{\varepsilon _0}\omega }}{4}\left( {i\left( {u\nabla {u^*} - {u^*}\nabla u} \right) + 2k{{\left| u \right|}^2}\hat {{z}}} \right) \end{split} $$ (11) 式中:
$ \omega $ 为角频率;$ u $ 为近轴传播下光场的复振幅。线动量密度的角向分量为:
$$ {{\boldsymbol{S}}_\phi }{\text{ = }}\frac{{l{\varepsilon _0}\omega {u^2}}}{{2r}} $$ (12) 公式(12)代入公式(9),可得傍轴近似条件下矢量涡旋光束经光阑-透镜光学系统的轨道角动量密度为:
$$ {j_z}{\text{ = }}\frac{{\omega {\varepsilon _0}l}}{2}{\left| u \right|^2} $$ (13)
Orbital angular momentum and polarization characteristics of vector vortex beam passing through aperture-lens system
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摘要: 与标量涡旋光束不同,矢量涡旋光束同时具有各项异性的空间偏振分布和螺旋相位分布,并携带与相位分布有关的轨道角动量(Orbital angular momentum,OAM)。根据柯林斯衍射积分理论,得到了傍轴近似条件下矢量涡旋光束的OAM密度,实验采集了矢量涡旋光束通过光阑-透镜系统后的光场,详细讨论了光阑截断参数以及光阑-透镜间距等参数对矢量涡旋光场及其OAM密度的影响。结果表明:相比标量涡旋光束,矢量涡旋光束OAM通过光阑系统后随传输距离的衰减更慢,受光阑截断参数影响更小。矢量涡旋光束偏振态不受光阑-透镜系统影响,截断参数大于4.2时,轨道角动量密度和光强不受截断参数影响。在透镜焦点位置处,OAM密度达到最大值。研究结果为矢量涡旋光束OAM特性的应用提供理论依据。Abstract: Different from the scalar vortex beam (SVB), the vector vortex beam (VVB) has both an anisotropic spatial polarization distribution and a spiral phase distribution, and carries the orbital angular momentum (OAM) related to the phase distribution. Based on the Collins diffraction theory, the OAM density under the paraxial approximation is obtained. The light field of the VVB passing through the aperture-lens system was collected experimentally. The effects of the aperture truncation parameters and the aperture-lens spacing on the light field and the OAM density of the VVB are discussed in detail. The OAM density characteristics of the VVB and the SVB through the aperture are compared. The results show that the OAM has a slower attenuation with the transmission distance after the VVB passes through the aperture-lens system and is less affected by the truncation parameters compared with the SVB. The polarization state of the VVB is not affected by the aperture-lens system. When the truncation parameter is greater than 4.2, the OAM density and light field are not affected by the truncation parameter. The OAM density reaches the maximum value at the focal position of the lens. The research results provide theoretical references for the application of OAM characteristics of VVBs.
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图 2 不同截断参数下矢量涡旋光束轨道角动量密度分布。(a)
$ s = 0.5 f $ ,$ s = 0.8 f $ ,$ s = 1.5 f $ 时轨道角动量密度随截断参数的变化;(b)$ s = 0.5 f $ ;(c)$ s = 0.8 f $ ;(d)$ s = 1.5 f $ Figure 2. OAM density distributions of VVB for different truncation parameters. (a) OAM density changes with the truncation parameter at
$ s = 0.5 f $ ,$ s = 0.8 f $ ,$ s = 1.5 f $ ; (b)$ s = 0.5 f $ ; (c)$ s = 0.8 f $ ; (d)$ s = 1.5 f $ 图 3 矢量涡旋光束与标量涡旋光束OAM密度分布。(a)仿真得到的VVB与SVB随传输距离变化;(b)实验得到的VVB与SVB随传输距离变化;(c)仿真得到的VVB与SVB随截断参数变化;(d)实验得到的VVB与SVB随截断参数变化
Figure 3. OAM density distributions of VVB and SVB. (a) Simulation results of VVB and SVB with transmission distance; (b) Experimental results of VVB and SVB with transmission distance; (c) Simulation results of VVB and SVB with truncate parameters; (d) Experimental results of VVB and SVB with truncate parameters
图 5 矢量涡旋光束通过光阑-透镜系统后OAM密度随
$z_0/f $ 的变化。(a) s/f=0.4 ,s/f=0.6 ,s/f=0.8 时轨道角动量密度随截断参数变化的仿真结果;(b) s/f=0.4; (c) s/f=0.6 ; (d) s/f=0.8Figure 5. Variation of OAM density after the VVB passes through the aperture-lens system with
$z_0/f $ . (a) Simulation results of OAM density variation with truncation parameter when s/f=0.4 ,s/f=0.6 ,s/f=0.8; (b) s/f=0.4; (c) s/f=0.6; (d) s/f=0.8图 9 矢量涡旋光束经光阑-透镜系统后光强变化。(a)光强随光阑-透镜间距的变化 (b)光强随截断参数的变化 (c)光强随焦距的变化
Figure 9. The light intensity variation of the VVB passing through the aperture-lens system. (a) Light intensity variation with aperture -lens spacing (b) Light intensity variation with truncate parameters (c) Light intensity variation with focal length
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