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图1给出了用于分析的反射式体光栅模型,其中R、S分别为入射光与衍射光振幅大小,表示为
$R({\textit{z}}){{\rm e}^{ - j}}^{\overrightarrow \rho \cdot \overrightarrow r }$ 与$S({\textit{z}}){{\rm e}^{ - j}}^{\overrightarrow \sigma \cdot \overrightarrow r }$ 。$\overrightarrow K $ 为光栅矢量,大小$K{\rm{ = }}\dfrac{{2\pi }}{\varLambda }$ ,$\varLambda $ 为光栅周期,并且满足$\overrightarrow \rho - \overrightarrow K = \overrightarrow \sigma $ ,其中$\overrightarrow \rho $ 和$\overrightarrow \sigma $ 分别为入射光与衍射光的光矢量。$\varphi $ 为光栅矢量倾角,d为光栅厚度。依据Kogelnik耦合波理论[10],对于理想的无吸收型体光栅,其耦合波方程为:
$$ \left\{ \begin{array}{l} {c_R}R' + \alpha R + j\kappa S{\rm{ = }}0\\ {c_S}S' + (\alpha + j\tau )S + j\kappa R = 0 \end{array} \right. $$ (1) 式中:
${c_R}$ 和${c_S}$ 为倾斜因子,$$ \left\{ \begin{array}{l} {c_R} = \cos \theta \\ {c_S} = \cos \theta - K\cos \varphi/\beta \end{array} \right. $$ (2) $\;\beta = \dfrac{{2\pi {n_0}}}{\lambda }$ ,且有$$ \left\{ \begin{array}{l} n = {n_o} + {n_1}\cos \overrightarrow K \cdot \overrightarrow r \\ \alpha = {\alpha _o} + {\alpha _1}\cos \overrightarrow K \cdot \overrightarrow r \\ \end{array} \right. $$ (3) 式中:
${n_o}$ 和${n_1}$ 分别为平均折射率与折射率调制度;${\alpha _o}$ 和${\alpha _1}$ 为平均吸收常数与吸收常数的空间调制度。在理想情况下,${n_0} \gg {n_1}$ 、${k_0}{n_0} \gg {\alpha _0}$ 、${k_0}{n_0} \gg {\alpha _1}$ ,${k_0}$ 为真空波数,${k_0} = \dfrac{{2\pi }}{{{\lambda _0}}}$ ,$\kappa $ 为耦合系数,$$\kappa = \frac{1}{2}({k_0}{n_1} + j{\alpha _1})$$ (4) 耦合系数给出了衍射光与入射光的能量转换关系,若=0,则不存在耦合。
$\tau $ 为失配参数,如果失配参数过大,也将会破坏光栅中的耦合,大小由下式给出:$$\tau = K\cos (\varphi - \theta ) - \frac{{{K^2}}}{{4\pi n}}\lambda $$ (5) 对于反射式体光栅(
${c_S} < 0$ ),边界条件$R(0) = 1$ ,$S(d) = 0$ 。至此可以联立上式得到无吸收(${\alpha _1} = {\alpha _0} = $ $ 0$ )、非倾斜($\varphi = 0$ )的反射式体光栅衍射效率公式为:$$\eta = {\left( {1 + \frac{{1 - \dfrac{{{\xi ^2}}}{{{\psi ^2}}}}}{{\sin {h^2}\sqrt {{\psi ^2} - {\xi ^2}} }}} \right)^{ - 1}}$$ (6) 式中:
$\psi = {n_1}\pi d/\lambda \cos \theta $ ;$\xi = d\tau/2\cos \theta $ 。从公式(6)可以得到不同的参数以及入射角度$\theta $ 会影响体光栅的衍射效率,这体现了体光栅的光谱滤波能力与角度选择特性。且当入射光满足布拉格条件,
$\theta {\rm{ = }}0$ ,$\varphi {\rm{ = }}0$ 时,$$2{n_0}({\lambda _0})\sin (\varphi - {\theta _0})\overrightarrow {{k_0}} = \overrightarrow K $$ (7) 衍射效率能够达到最大,
${\theta _0}$ 为入射光在光栅中与Z轴的夹角。而当入射角度发生变化时,不满足布拉格条件,角度与波长失配,从而会导致体光栅衍射效率下降,与理论值发生偏移。 -
为了验证分析体光栅实际的光谱滤波与角度特性,设计了两块实物体光栅,设计参数与理论仿真参数一致,搭建了测试光路如图8所示。
该测试系统由自由空间宽带光源(FSBLS)、可变光阑(VI)、扩束镜(BE)、准直透镜(CL)、体布拉格光栅(VBG)、光功率计(OPM 1、2)组成。由于光束的发散角会影响体光栅的峰值衍射效率,采用可变光阑、扩束镜、准直镜对光束进行准直以减小发散角的影响。
宽带光源发出宽光谱光束,经由可变光阑、扩束镜、准直透镜、入射到反射式体光栅的反射面上,反射面镀上增透膜,透过率大于99.9%,衍射光由光功率计OPM1接收,透射光由OPM2接收,通过两只功率计的比值,得到体光栅的实际衍射效率。
在测试体光栅的光谱带宽时,将OPM 1替换为日本YOKOGAWA公司生产的高分辨率光谱分析仪AQ6370D。可以通过光谱仪显示直接得到体光栅反射光谱图像,通过光谱仪的运算功能可以得到光谱带宽数值。
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对中心波长λ=1029 nm和λ=1064 nm的两块体光栅分别进行了实验测试,将实验测试数据归一化处理再进行高斯拟合,得到测试曲线,并且与理论仿真曲线进行对比。光谱带宽测试结果如图9所示。
图 9 (a) 1029 nm体布拉格光栅衍射效率测量;(b) 1064 nm体布拉格光栅衍射效率测量
Figure 9. (a) Measurement of diffraction efficiency of VBG at 1029 nm; (b) Measurement of diffraction efficiency of VBG at 1064 nm
可以看出,两块体光栅的中心波长相较于理论值有一定的偏移,光谱带宽有一定的展宽。中心波长的偏移主要是因为实验所用转台精度有限,导致角度不能够和布拉格角完全匹配,光谱带宽的展宽主要是由入射光束的发散角造成,同时波长偏移也会对带宽造成影响。1029 nm体光栅峰值能量大于81%,1064 nm体光栅峰值能量大于80%,理论仿真结果符合较好。
测试结果在1029 nm处的中心波长实测为1029.06 nm,光谱带宽为52 pm,此时的衍射效率为91%。在1064 nm处的中心波长实测为1064.0905 nm,光谱带宽为107.5 pm,此时的衍射效率为92%。相较于传统的光学滤波器件(透过率70%,光谱带宽1 nm左右),体光栅衍射效率更高,光谱带宽更窄。
实验值与理论值的数据对比由表1给出。
表 1 体布拉格光栅实测数据对比
Table 1. Comparison of VBG measurement data
Center wavelength/nm Measured wavelength/nm Diffraction efficiency theoretical
value/measured valueTheoretical value/measured
value of spectral bandwidth1029 1029.06 97%/91% 20 pm/52 pm 1064 1064.0905 96%/92% 80 pm/107 pm 从实测数据可得,衍射效率和光谱带宽均与理论设计值(即前文中仿真结果)相差较小,实验光路设计合理。并且,当使用准直性与单色性更好的激光光源时,体光栅的衍射效率与光谱带宽将会进一步提升。说明该实验能够有效地测试体光栅的光谱滤波性能,也体现了体光栅在实际应用时拥有良好的光谱滤波性能。
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对上述两块体光栅的角度特性进行了测试。实验根据布拉格衍射条件,测试了体光栅在较大角度变化时,布拉格角发生变化导致中心波长的改变。
实验光路如图7所示,通过角度转台记录体光栅入射角度,使用光谱仪得到不同角度入射时的衍射光中心波长。这种特性使得将体光栅应用于可调谐滤波器件成为可能。图10给出了测试的角度曲线,并且与理论曲线进行了对比。
图 10 (a) 1029 nm体布拉格光栅的角度特性;(b) 1064 nm体布拉格光栅的角度特性
Figure 10. (a) Angle characteristics of VBG with center wavelength at 1029 nm; (b) Angle characteristics of VBG with center wavelength at 1064 nm
从图中可以看出,1029 nm体光栅在−2°附近与理论值重合较好,1064 m体光栅角度特性测量值基本符合理论值。0°附近与理论值重合,随着角度的增加,自由空间光路的散射以及反射将会产生影响,相对于理论值有部分偏离。这是由于体光栅有最佳工作波长,随着角度增加,自由空间光路的散射以及反射导致布拉格衍射条件被破坏,从而影响体光栅最佳的工作性能。在允许的波长范围内,通过旋转体光栅以得到不同中心波长的窄带衍射光,该结果可以用来指导体光栅用于可调谐滤波相关领域。
对于反射式体光栅,不满足布拉格衍射条件的入射光会经由体光栅端面透射,从而大大减少反射光对于衍射光的干扰,可显著提高衍射光信号的信噪比。
通过实验分别测试了体光栅的衍射效率、光谱宽度、角度特性,并且与理论仿真的结果进行了对比,分析了实际与理论的误差来源及如何提高体光栅的滤波性能。
Characteristics of volume Bragg grating spectral filter
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摘要: 高透过率、窄光谱带宽、高抑制比的滤波器件是提升光学系统光谱纯度的核心部件,也是近年来新型光学器件领域研究的热门方向。传统的滤波器件不能够同时满足透过率、光谱带宽以及带外抑制的要求,而文中研究的体光栅作为一种新型的光栅,衍射效率大于90%,光谱带宽在100 pm左右,并且拥有一定的角度滤波特性。依据耦合波理论,理论仿真了各个光栅参数(光栅厚度、折射率调制度、光栅周期、光栅倾角)对光栅衍射效率及光谱带宽的影响,定量地给出了参数影响的大小,为体光栅的设计提供了理论指导。设计了体光栅测试系统,通过实验验证了体光栅的衍射效率、光谱带宽、角度选择性,实验结果与理论较为符合,为体光栅的实际应用提供了实验数据支持。Abstract: The filter element with high transmittance, narrow spectral bandwidth and high suppression ratio is the core component to improve the spectral purity of an optical system, and it is also a hot research direction in the field of new optical devices in recent years. The traditional filter elements can not meet the requirements of transmittance, spectral bandwidth and out-of-band suppression simultaneously. As a new type of grating, the diffraction efficiency of the volume grating is usually greater than 90 %, the spectral bandwidth is about 100 pm, and it has angle filtering characteristics. Based on the coupled-wave theory, the influence of grating parameters (grating thickness, refractive index modulation, grating period, grating inclination angle) on the diffraction efficiency and spectral bandwidth of the grating was theoretically simulated, and the influence of parameters was quantitatively given, which provided theoretical guidance for the design of volume grating. The volume grating test system was designed, and the diffraction efficiency, spectral bandwidth and angle selectivity of the volume grating were verified by experiments. The experimental results are in agreement with the theory. The diffraction efficiency is greater than 90% , and the spectral bandwidth is about 100 pm, which provides support for the practical application of the volume grating.
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Key words:
- spectral filter /
- bandwidth /
- diffraction efficiency /
- volume Bragg grating
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表 1 体布拉格光栅实测数据对比
Table 1. Comparison of VBG measurement data
Center wavelength/nm Measured wavelength/nm Diffraction efficiency theoretical
value/measured valueTheoretical value/measured
value of spectral bandwidth1029 1029.06 97%/91% 20 pm/52 pm 1064 1064.0905 96%/92% 80 pm/107 pm -
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