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直接吸收光谱技术的基本原理是入射光通过一定长度的气体池,由于待测物质的部分吸收,透射出来的光强度会有一定程度的减弱,假设光的散射可以忽略,那么透射光相对于入射光的衰减就被定义为吸收。通过记录透射光强随波长的变化曲线就可以得到样品的吸收光谱,用Beer-Lambert定律[15]表达可以写成:
$ I = {I_0}\cdot{\rm{exp}}( - \alpha L) $
(1) 式中,I为入射光强;I0为透射光强;L为气体池长度(单位为cm),也就是待测气体的有效吸收光程;α为吸收系数(单位为cm-1),所有的定量吸收光谱都是以Beer-Lambert定律为基础。在进行数据处理时,通常令吸收度A为:
$ \left\{ \begin{array}{l} A = - {\rm{ln}}\frac{I}{{{I_0}}}\\ \alpha = \frac{A}{{{L_{{\rm{eq}}}}}} \end{array} \right. $
(2) 式中,Leq表示腔内介质有效吸收长度。
腔增强吸收光谱中的谐振腔是由两块分别置于气体池两端的壁上平凹高反镜组成,故腔长即为气体池长度L。而L与腔镜的曲率半径r之间满足关系[16]:0 < L < r或r < L < 2r。当光腔调节好后,通过扫描腔长,将激光耦合到谐振腔内,使其在两腔镜之间多次来回反射,谐振腔的精细度越高,激光在腔镜间反射的次数越多。激光透过谐振腔的强度随时间t变化可以表示成[17]:
$ {I_{\rm{t}}} \propto \int_0^\infty {{I_0}(\nu )} {\rm{exp}}\left[ { - t/\tau \left( v \right)} \right]d\nu $
(3) 式中,It为透射光强;I0为入射光强;ν表示频率;τ(ν)则是谐振腔的衰荡时间:
$ \tau \left( \nu \right) = \frac{L}{{c(\left| {{\rm{ln}}R} \right| + \alpha L)}} $
(4) 式中,c表示光速;L表示谐振腔的腔长;|lnR|表示谐振腔的腔镜损耗和腔内介质的损耗之和,其中,谐振腔的损耗主要包括腔内介质的吸收损耗以及腔镜各种损耗;R为腔镜反射率。
由于腔增强吸收谱光谱技术中,所用的高反镜反射率R一般都在99%以上,所以|lnR| ≈(1-R)。那么,(4)式就可以写成:
$ \tau \left( \nu \right) = \frac{L}{{c\left[ {\left( {1 - R} \right) + \alpha L} \right]}} $
(5) 从(5)式中可以看出,腔镜反射率R、腔长L及腔内介质的吸收系数α是影响谐振腔的衰荡时间τ(ν)的3个主要参量。如果用谐振腔的衰荡时间τ(ν)与光速c的乘积来表示腔内介质对光的实际吸收路径,则:
$ {L_{{\rm{eq}}}} = \frac{L}{{\left( {1 - R} \right) + \alpha L}} $
(6) 如果腔内介质的单程吸收损耗远远小于腔镜损耗,即αL≤1-R,则(6)式可以写成:
$ {L_{{\rm{eq}}}} = \frac{L}{{1 - R}} $
(7) 相对于传统的直接吸收光谱来说,当气体池长度相同时,腔增强吸收光谱的有效吸收路径[18]被扩大了1/(1-R)倍。如果腔镜反射率在99%以上,那么有效吸收路径会增加1000倍甚至10000倍,这是其它吸收光谱技术所不能比拟的,同时这也使得腔增强吸收光谱具有很高的探测灵敏度。
NH3的腔增强吸收光谱检测技术
NH3 measurement based on cavity enhanced absorption spectroscopy
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摘要: 为了研究腔增强吸收光谱技术是否能用于NH3气体浓度的检测,采用扫描腔长的方法,以分布反馈式可调谐半导体激光器作光源,用两块高反射率平凹透镜(反射率约为99.9%,曲率半径约为1m)组成的光学谐振腔作吸收池,搭建腔增强吸收光谱装置。在34cm长的吸收池内测量NH3气体分子在1.5μm附近的弱吸收谱线;通过不断增加NH3气体浓度来改变腔内压强,每充入一次NH3都测量并保存一次吸收光谱。通过数据处理,分析谱线宽度随气体浓度变化的关系以及吸收度随腔内压强增加的变化情况,发现都能呈现出良好的线性关系,并对残差噪声进行统计分析,得到了3.3×10-8cm-1的最小探测灵敏度。结果表明,高探测灵敏度的腔增强吸收光谱技术,可以实现NH3气体浓度的测量,并能得到较好的探测精度。Abstract: In order to study whether the cavity-enhanced absorption spectroscopy can be used for the detection of NH3 gas concentration, a cavity-enhanced absorption spectroscopy system was built up. In the system, a tunable distributed feedback diode laser was used as the light source, and a plano-concave optical cavity, consisting of two mirrors with high reflectivity (reflectivity about 99.9% and radius of curvature about 1m), was used as the absorption cavity. Weak absorption spectra of NH3 gas molecules near 1.5μm were measured in the 34cm absorption cell by using cavity length scanning method. The pressure in the cavity was changed by increasing the concentration of NH3 gas. The absorption spectra were measured and stored after charging NH3 each time. By data processing, the relationship between spectral line width and gas concentration was analyzed. And the change of absorbance with the increase of cavity pressure was studied. The results show a good linear relationship. The minimum detection sensitivity of 3.3×10-8cm-1 is obtained by the statistical analysis of the residual noise. The experimental results show that NH3 concentration detection with good detection precision can be realized by using high sensitivity absorption spectroscopy technology.
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Key words:
- spectroscopy /
- NH3 /
- cavity enhanced absorption spectroscopy /
- sensitivity
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