Rigorous comparison of the spectral SNR of FTIR and EC-QCL spectroscopy (Conference Presentation)
Rigorous comparison of the spectral SNR of FTIR and EC-QCL spectroscopy (Conference Presentation)
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FTIR 和 EC-QCL 光谱的光谱 SNR 的严格比较（会议演示）
DOI：
10.1117/12.2209063
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发表时间：
2016
期刊：
影响因子：
--
通讯作者：
Childs D
中科院分区：
文献类型：
--
作者：
Childs D
自20世纪50年代以来，使用热光源的FTIR光谱法一直是获得红外光谱的主要方法。不幸的是，有限的表面亮度和低空间相干性的黑体辐射限制了光谱信噪比的显微光谱学和stand-off检测。最近的两项创新正在解决这个问题a）由高空间相干宽带超连续光源照射的FTIR仪器和B）高空间相干窄带EC-QCL。 在这里，我们问这两种方法是否提供了相同的灵敏度。通过注意到与近红外光学相干层析成像的类比，我们严格地表明，EC-QCL的高时间相干性带来了额外的，非常大的SNR优势，在其他匹配条件下，由超连续谱光源照射的FTIR仪器。具体而言，如果两种仪器使用相同的照明强度和相同的检测器噪声水平记录包含N个点的光谱，则EC-QCL可以在比FTIR仪器短的时间xN内提供给定的光谱SNR。在实际应用中，该系数可以达到x100，甚至可能达到x1000。 我们利用类比与OCT进一步开发中红外“扫频激光器”，使用市售的组件，其中的调谐速率是远远高于商业EC-QCL设备。我们使用这种扫频激光器来证明SNR的优势实验，使用定制的EC-QCL光谱仪和PDMS聚合物样品。我们探索光谱采集速率的潜在上限，无论是从外部腔中的增益积累的基本动力学和可能的机械限制腔调谐速率。
FTIR spectroscopy using a thermal light source has been the dominant method for obtaining infrared spectra since the 1950’s. Unfortunately the limited surface brightness and low spatial coherence of black-body radiators limits the spectral SNR in microspectroscopy and stand-off detection. Two recent innovations are addressing this problem a) FTIR instruments illuminated by high-spatial coherence broad-band supercontinuum sources and b) high spatial coherence narrow-band EC-QCL’s. Here we ask whether these two approaches offer equivalent sensitivity. By noting an analogy with near-infrared optical coherence tomography we rigorously show that the high temporal coherence of the EC-QCL brings an additional, very large SNR advantage over an FTIR instrument illuminated by a supercontinuum source under otherwise matched conditions. Specifically if a spectrum containing N points is recorded by both instruments using the same illumination intensity and the same detector noise level, then the EC-QCL can deliver a given spectral SNR in a time xN shorter than the FTIR instrument. This factor can reach x100, potentially even x1000, in realistic applications. We exploit the analogy with OCT further by developing a mid-infrared “swept laser”, using commercially available components, in which the tuning rate is much higher than in commercial EC-QCL devices. We use this swept laser to demonstrate the SNR advantage experimentally, using a custom-made EC-QCL spectrometer and PDMS polymer samples. We explore the potential upper limits on spectral acquisition rates, both from the fundamental kinetics of gain build-up in the external cavity and from likely mechanical limits on cavity tuning rates.