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Improving long-path differential optical absorption spectroscopy with a quartz-fiber mode mixer

机译：石英纤维模式混合器改善长径差分吸收光谱

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Long-path differential optical absorption spectroscopy (DOAS) has become an increasingly important method for determination of the concentration of tropospheric trace gases (e.g., O-3, NO2, BrO, ClO). The use of photodiode array (PDA) detectors enhances long-path DOAS systems considerably owing to PDA's higher sensitivity resulting from the multiplex advantage. The detection limits of these systems are expected to be 1 order of magnitude lower than systems of similar optical setup with scanning detectors. When the scanning detector is simply replaced by a PDA, unwanted spectral structures of as much as 8 x 10(-3) appear. The size of these randomly changing structures exceeds the photon noise level by 2-3 orders of magnitude thus severely limiting the sensitivity. We show that an angular dependence of the response of the PDA causes this structure in combination with unavoidable changes in the illumination. A quartz-fiber mode mixer, which makes the illumination of the spectrograph-detector system nearly This independent of the angular intensity distribution of the measured light, was developed and tested. This new device reduces the unwanted structures in laboratory and held experiments by a factor of 10. The detection limits of long-path DOAS instruments with PDA detectors are improved by the same amount and are thus lower than those of currently used systems with scanning detectors. At the same time a much shorter measurement time (by similar to 1 order of magnitude)becomes possible. (C) 1997 Optical Society measurement techniques.
机译：长距离微分光吸收光谱法（DOAS）已成为确定对流层痕量气体（例如O-3，NO2，BrO，ClO）浓度的日益重要的方法。光电二极管阵列（PDA）检测器的使用大大增强了长距离DOAS系统，这是由于PDA因其多重优势而具有更高的灵敏度。预计这些系统的检测限比具有扫描检测器的类似光学装置的系统低1个数量级。当用PDA代替扫描检测器时，会出现多达8 x 10（-3）的不需要的光谱结构。这些随机变化的结构的尺寸超过光子噪声水平2-3个数量级，因此严重限制了灵敏度。我们表明，PDA响应的角度依赖性会导致这种结构与照明中不可避免的变化相结合。开发并测试了一种石英纤维模式混合器，该混合器使光谱仪-检测器系统的照明几乎与照明光的角强度分布无关。这种新设备将实验室和实验室实验中不需要的结构减少了10倍。带有PDA检测器的长距离DOAS仪器的检测限提高了相同数量，因此比当前使用的带有扫描检测器的系统要低。同时，可以实现更短的测量时间（近似1个数量级）。 （C）1997光学学会测量技术。

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