Measurements of carbon monoxide mixing ratios in Houston using a compact high-power CW DFB-QCL-based QEPAS sensor

Sensitivity enhanced NIR photoacoustic CO detection with SF6 promoting vibrational to translational relaxation process

“…There are two basic approaches currently being adopted to improve sensitivity. One is increasing the excitation light power [13] , [14] , [15] , such as Yin et al boosted the NIR light to ~ 10 W, resulting in a ppb-level CO detection sensitivity [16] ; meanwhile, Yin et al detected multicomponent by-products of SF 6 decomposition by amplifying the light power to 1.724 W, achieving a minimum detection limits (MDL) of 435 ppb for CO [17] ; Mao et al implemented an all-optical photoacoustic spectrometer using 1 W light source, and the obtained detection limits for CO was 4.6 ppm [18] . However, increasing power light source will lead to high power consumption, high cost, degraded light beam quality and complex cooling system which significantly limits the commercialization of the CO sensor.…”

Monitoring of ambient carbon monoxide concentrations based on wavelength modulation direct absorption spectroscopy

“…傅立叶变换红外吸收光谱(FTIR)常利用 CO 分子 2150-2320 cm -1 特征吸收光谱 并结合多次反射池，检测限低至~0.2 ppb [4] 。 上述测量方法对预处理要求较高，系统复杂且响应速度慢。可调谐半导体 激光吸收光谱(TDLAS)具有波长选择性强、测量速度快、免标定等优点 [5,6] 。 其中，直接吸收光谱(DAS)原理简单，可测量气体吸收率函数，广泛应用于环 境监测和工业过程气体分析等领域 [7,8] 。 然而， 激光强度波动、 光电探测器暗噪声、 1/f 噪声等限制了 DAS 测量灵敏度进一步提高 [9,10] ，在大气痕量 CO 浓度测量中， 目前多选择中红外强吸收谱线以提升测量灵敏度，如采用 4.57 μm 的量子级联激 光器(QCL)并结合多次反射池，CO 检测限可至~1 ppb [11][12][13] ；或者采用高频调 制的波长调制光谱(WMS)方法提升测量信噪比，通过提取高信噪比的 2 次谐 波信号，并可结合光声光谱(PAS) [14] 或光热光谱 [15] ，实现低至~1 ppb 的检测限 [16][17][18][19] ；以及采用等效光程 km 级的积分腔输出光谱(ICOS) [20] 实现 ppb~ppt 级的 CO 检测。由于中红外 QCL 多为自由空间光输出且成本较高，因而也有科研工作 者采用易于光纤耦合的近红外激光器进行 CO 测量，如采用垂直腔面发射激光器 (VCSEL) [21][22][23] ，利用 2.3 μm 附近的 CO 特征吸收谱线，结合 WMS 方法，在较 短光程(40 cm)时检测限可达到~0.4 ppm；以及采用 2.3 μm 分布反馈(DFB) 半导体激光器结合石英增强光声光谱(QEPAS) ，最小可探测 CO 浓度达到 43.3 ppm [24] ；或采用腔增强吸收光谱方法，在常压和有效光程 1545.6 cm 条件下最小 可探测 CO 浓度~34 ppm [25] ；或采用近红外 1.57 μm 的 DFB 半导体激光器，利用 腔衰荡光谱(CRDS)方法实现大气 CO 监测 [2,26] 。上述 WMS 方法通常需标定， 而 PAS、ICOS、CRDS 操作及维护较复杂且成本较高。近年来，彭等 [27]…”

Mid-infrared surface transmitting and detecting quantum cascade device for gas-sensing

“…Quantum cascade structures operating in the mid-infrared region have proven to be a promising platform for a variety of applications, e.g. vibrational absorption spectroscopy 3 4 and quarz enhanced photoacoustic spectroscoppy 5 6 7 8 . QCLs show stable long term frequency stability appropriate for spectroscopy after an initial stabilization of the electric contacts 9 .…”