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B.Schrader和G.Bergmann等人基于Kubelka-Munk理论在一维近似情况下分析了光与浑浊介质相互过程[15],并推导出激光反射(
${I_\rm R}$ )与透射强度(${I_\rm T}$ ),拉曼透射与反射强度与样品散射系数($r$ )、吸收系数($a$ )、厚度($d$ )等因素之间的关系,如图1所示。$$ {\rm d} i=-(r+a) i {\rm d} x+r j {\rm d} x $$ (1) $$ {\rm d} j=-(r+a) j {\rm d} x+r i {\rm d} x $$ (2) $$ k^{2}=2 r a+a^{2} $$ (3) $$ I_{{\rm{T}}}=I_{0} \frac{k}{(a+r) \sinh k d+k \cosh k d} $$ (4) $$ I_{{\rm{R}}}=I_{0} \frac{r \sinh k d}{(a+r) \sinh k d+k \cosh k d} $$ (5) 令激光强度
${{{I}}_0}{\text{ = }}1$ ,样品吸收系数$a = 0.001$ ,样品散射系数$ r = 10 \;{{\rm{cm}}^{ - 1}} $ ,代入公式(4)和(5)可得出透射及反射光强随样品厚度$d$ 的变化情况,如图2所示。图 2 透射与反射光强随样品样品厚度变化
Figure 2. Transmitted and reflected light intensity varies with sample thickness
激光作用样品时,除了产生与入射光频率相同的弹性散射光以外,还有频率分量与入射光不同的非弹性散射即拉曼信号。关于激光作用样品产生拉曼信号强度可由公式(6)表示:
$$ {I_i} = k\varOmega \frac{{\partial \sigma }}{{\partial \varOmega }}{n_i}V{I_0} $$ (6) 式中:
${I_i}$ 为拉曼信号强度;$k$ 为样品散射系数;$\varOmega$ 为收集立体角,$V$ 为激光与样品相互作用体积;$\dfrac{{\partial \sigma }}{{\partial \varOmega }}$ 为样品的微分散射截面,${n_i}$ 为样品粒子数密度,${I_0}$ 为激发光强。通过公式(6)可以看出,通过增加激发光强、有效作用体积,增大收集立体角等手段可提高拉曼光谱的探测灵敏度。为了解决传统拉曼探头粉末样品探测中存在采样体积小、灵敏度低的问题,在探头顶端引入石英管(如图3所示)。光束通过传输光纤耦合至石英管中,与粉末样品相互作用产生拉曼信号。探头顶端的石英管一方面可增加光束与粉末样品的有效作用体积,另外可减少粉末对拉曼系统收集视场遮挡。综上分析,在传统拉曼光谱探头中引入石英管可提高拉曼光谱技术对粉末样品的探测灵敏度。
Quartz tube enhanced Raman fiber probe for powder detection
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摘要: 拉曼光谱技术以其多组分同时探测、分子指纹特性等优点被广泛应用于多个领域,但其较低探测灵敏度严重限制了此技术的进一步发展。为了提高拉曼光谱技术粉末样品原位分析能力,提出了一种基于石英管增强的高灵敏度拉曼光纤探头。探头采取内镀金属空芯光纤用于光信号的传输,有效减小了背景信号对拉曼光谱的影响;探头底端采用石英管设计,增大采样体积和收集效率,提高了拉曼光谱的探测灵敏度。理论分析了石英管拉曼光谱技术可提高粉末样品的探测灵敏度,详细介绍了探头的设计和实现,进一步评估探头的性能,结果表明:相较于球透镜拉曼探头,拉曼信号强度(NaHCO3)增强2.92倍。为模拟实际应用场景,采用光纤拉曼探头成功获得了容器中不同深度粉末样品(Na2SO4和NaHCO3)的拉曼光谱信息。文中设计的石英管增强拉曼探头具有外径尺寸小(外径仅为2 mm)、灵敏度高等优点,可对深层次粉末样品进行探测,为现场粉末样品原位分析提供了一种新途径。Abstract: Raman spectroscopy has been widely used in many fields due to its advantages of multi-component simultaneous detection and molecular fingerprinting characteristics, but its low detection sensitivity severely limits the further development. In order to improve the in-situ analysis capability of powder samples by Raman spectroscopy, a high-sensitivity optical fiber Raman probe based on quartz tube enhancement is proposed. The metal-coated hollow-core fiber is used for optical signal transmission, which effectively reduces the influence of background signal on Raman spectrum. The bottom of the probe is designed with a quartz tube, which increases the sampling volume and collection efficiency, it improves the detection sensitivity of Raman spectrum. Firstly, theoretical analysis of quartz tube Raman spectroscopy can improve the detection sensitivity of powder samples. Secondly, the design and implementation of the probe are introduced in detail. Lastly, the performance of the probe is further evaluated. The results show that the Raman signal intensity (NaHCO3) increased by a factor of 2.92 compared with the spherical lens Raman probe. In order to simulate practical application scenarios, the Raman spectral information of powder samples (Na2SO4 and NaHCO3) at different depths in the container is successfully obtained by using the fiber Raman probe. The quartz tube-enhanced Raman probe designed in this paper has the advantages of small outer diameter (only 2 mm) and high sensitivity. It can detect deep powder samples and provide a new way for in-situ analysis of powder samples on site.
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Key words:
- Raman spectroscopy /
- fiber probe /
- quartz tube /
- powder detection /
- signal enhancement
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图 11 不同深度粉末样品的探测示意图及结果。 (a) 实验装置示意图; (b) 探头距试管底端0.5 cm处的探测结果;(c) 探头距试管底端3.5 cm处的探测结果; (d) 探头距试管底端6.5 cm处的探测结果
Figure 11. Experimental setup and results of powder samples detection at different depths. (a) Experimental setup; (b) Probe-to-bottom=0.5 cm; (c) Probe-to-bottom=3.5 cm; (d) Probe-to-bottom=6.5 cm
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