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自发喇曼散射的信号强度I与激发光光强、激发光波长和收集立体角等参量有关。公式如下:
$ I=I_{\text {laser }} n l_{\text {eff }} \frac{\mathrm{d} \sigma_{\mathrm{R}}}{\mathrm{d} \mathit{\Omega}} \Omega $
(1) 式中,I是信号强度,Ilaser是激发光光强,n是气体分子密度,leff是激发光作用长度,$\frac{\mathrm{d} \sigma_{\mathrm{R}}}{\mathrm{d} \mathit{\Omega}} $是微分喇曼散射截面,Ω是收集立体角。喇曼散射截面与激发光波长有关,关系是: ${\sigma _{\rm{R}}}(\lambda ) \sim {\left( {\frac{1}{\lambda }} \right)^4} $。
基于半导体激光器共振外腔的喇曼散射实验装置如图 1所示。该装置包括腔增强装置与喇曼光谱检测系统。试验中,采用90°构型收集喇曼散射光。使用的激光器是无减反膜处理的半导体激光二极管(laser diode,LD)(CLD405500K, NICHIA),波长在408nm左右,线宽为0.9nm, 最大功率为500mW。激光被准直透镜(C671TME-405, Thorlabs)耦合入外腔,焦距为4.01mm。外腔由两块球面镜组成,曲率半径为50mm,R1的反射率为96.5%,R2的反射率为99.5%,两球面镜相隔50mm摆放。透镜L2在外腔的旁侧,用于收集喇曼散射光,焦距为40mm。长通滤光片F(BLP01-405R-25,Sem-rock)用于滤除漫反射光和瑞利散射光。使用的光谱仪是实验室自制的光谱仪[14]。
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使用共焦法布里-珀罗(Fabry-Perot,F-P)腔作为外腔。当激光注入F-P腔时,若想让激光在F-P腔内积累,并透过F-P腔,需要进行模式匹配,包括横模匹配(注入激光束光强分布与共振外腔横模的匹配)和纵模匹配(注入激光束频率与共振外腔)。
匹配的方法如下:把激光聚焦,使得束腰半径w1等于共焦腔本征模式的束腰半径w0,使光束的波前和腔镜相重合。试验中把激光器出射光束的慢轴聚焦为50μm,共焦腔本征模式的束腰半径为46μm。由于使用的是对称共焦腔,对称共焦腔具有自再现传输的功能,所以对模式匹配的要求不高,仅需要w1大致等于w0。
按如下的方式考虑纵模匹配。外腔形成驻波腔,自由光谱范围为:
$ \nu_{\mathrm{FSR}}=\frac{c}{2 L} $
(2) 式中,c是光速,L为外腔腔长,由于L=50mm,νFSR=3GHz。
精细度公式为:
$ F=\frac{\pi\left(R_{1} R_{2}\right)^{1 / 4}}{1-\left(R_{1} R_{2}\right)^{1 / 2}} $
(3) 左腔镜的反射率R1=96.5%, 右腔镜的反射率R2=99.5%, 精细度大概为160。带宽Δν易得:
$ \Delta \nu \approx \frac{\nu_{\mathrm{FSR}}}{F} $
(4) 求得带宽Δν=18.75MHz。
所使用的激光器的线宽为1716.5GHz左右,纵模间隔大概为48.9GHz。从自由光谱范围上来看,由于外腔和激光器的自由光谱范围相差很大,光线中的大部分无法透过外腔,但是由于半导体激光器的光反馈效应,半导体激光器的出射波长会受外界光影响,半导体激光器和外腔相同波长的光线会得到反馈和增强,内外腔形成共振,光线能够透过外腔。
为了使腔镜3的直接反射光不直接返回激光器,干扰光反馈,共焦腔与光轴倾斜摆放[15-17],具体光路如图 2所示。
光束Ⅱ倾斜射出,外腔出射的光束Ⅰ返回激光器,形成光反馈。
外输入光功率P1=500mW,输出功率P2=38mW,反射率R =98%,腔内总功率P3=P2/(1-R), 求得腔内单向功率为7.5W,总功率为15W, 腔内功率增强倍数为30。
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直接在实验室的空气环境中进行实验。经过对准和聚焦之后,积分50s,在CCD中获得了如图 3所示的图像。
图 3中,左边白线是泄漏的激发光和瑞利散射信号,从左往右两个白点依次是O2、N2信号。
积分时间1s,获得光谱,扣除基线后得到图 4。
在图 4中,能清晰看到O2, N2, H2O信号。去除基线后,1s积分时间,获得N2信号900个计数,说明在1s这样短的积分时间内,能获得比较强的喇曼散射信号,这说明了共振腔有效地增强了信号。另外,基线比较平坦,说明荧光背景比较弱,这有利于提高检测灵敏度,用于痕量气体的分析。
积分50s获得光谱,扣除基线获得图 5。
积分50s的情况下,以3000cm-1~3300cm-1范围内的平均强度为基线,获得N2信号强度为31792。以其标准偏差为噪声,得到信噪比为1989。信号的分辨率为40cm-1。
积分50s的光谱细节图如图 6所示。
如图 6所示,1处信号为激发光信号;2处的阶梯形状是长通滤光片的作用造成,在这之前荧光背景较弱,在这之后荧光背景较为明显;3处信号为SiO2; 4处信号为O2;5处信号为N2;6处信号经过前后对比,为激发光的信号;7处信号为H2O;8处信号在4600cm-1附近,它的来源还有待分析。
蓝紫光的腔增强空气喇曼光谱
Cavity-enhanced Raman spectroscopy of blue-violet light
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摘要: 喇曼光谱是一种无损、快速检测物质成分的方法。为了提高监测灵敏度, 对408nm波段半导体激光器的腔增强自发喇曼散射进行了研究。利用输出功率500mW、线宽0.9nm的408nm半导体激光器作激发光, 把激光耦合入共焦球面镜腔, 两面共焦球面镜的反射率分别为96.5%和99.5%, 部分激光返回半导体激光器形成光反馈, 半导体激光器与共焦腔形成共振。对装置的光反馈过程进行了探讨, 并对外腔的模式匹配和频率匹配分别进行了分析。结果表明, 共焦腔内功率达到15W, 功率增强了30倍; 用90°探测构型收集喇曼信号, 完成了空气喇曼信号检测; 1s积分时间, 获得N2信号900个计数。此共振增强腔大大增强了喇曼散射信号, 有潜力应用于多种气体的在线检测或高灵敏度检测。Abstract: Raman spectroscopy was nondestructive and rapid method for the determination of substance composition. In order to improve the sensitivity of monitoring, the cavity-enhanced spontaneous Raman scattering of 408nm band semiconductor laser was studied. 408nm semiconductor laser with output power of 500mW and linewidth of 0.9nm was used as excitation light. The laser was coupled into confocal spherical mirror cavity. The reflectivity of two-sided confocal spherical mirrors was 96.5% and 99.5%, respectively. Some lasers returned to the semiconductor laser to form optical feedback. Semiconductor laser resonated with confocal cavity. The optical feedback process of the device was discussed. The mode matching and frequency matching of the external cavity were analyzed respectively. The results show that, the power in the confocal cavity reaches 15W. The power is increased by 30 times. Raman signals are collected with 90° probe configuration. Air Raman signal detection has been completed. 900 counts of N2 signal are obtained in 1s integration time. The results show that the resonance enhanced cavity greatly enhances the Raman scattering signal and has the potential to be used in the on-line detection or high-sensitivity detection of a variety of gases.
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Key words:
- spectroscopy /
- laser diode /
- optical feedback /
- cavity enhanced /
- spontaneous Raman scattering
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