近日，來自安徽理工大學、安徽皖西學院、復旦大學大氣與海洋科學學院、上海期智研究院的聯合研究團隊發表了《通過實施光學條紋噪聲抑制方法的激光波長調制光譜技術實現氣體測量的高精度和高靈敏度檢測》論文。
 

　　Recently, the joint research team from Anhui University of Science and Technology, West Anhui University, Department of Atmospheric and Oceanic Sciences, Fudan University, Shanghai QiZhi Institute published an academic papers High precision and sensitivity detection of gas measurement by laser wavelength modulation spectroscopy implementing an optical fringe noise suppression method.
 

　　可調諧二極管激光吸收光譜(TDLAS)已被開發用于痕量氣體測量，因其高精度、高靈敏度和無需任何樣品準備的原位自校準的獨1特優勢。通常，長光程的多次通過腔體(MPC)被應用于增強基于TDLAS的傳感器的檢測精度和靈敏度。然而，MPC中出現的意外光學干涉紋嚴重影響了傳感器的檢測精度和靈敏度。基于MPC的TDLAS傳感器的檢測精度和靈敏度通常受到光學干涉紋的限制，這些干涉紋由衍射、鏡面表面瑕疵的散射、鏡面畸變、熱膨脹、冷收縮或應力變形引起。因此，MPC中觀察到的光學干涉紋由不同的光學干涉紋組成。這些光學干涉紋主要是由于少量的激光以與主激光束相差ΔL的光程到達探測器所致。這些問題對于TDLAS是普遍存在的，尤其是在使用密集重疊斑點模式的MPC時，提出了一些不同的方法來消除光學干涉紋的負面影響。
 

　　The Tunable Diode Laser Absorption Spectroscopy (TDLAS) has been developed for trace gas measurement, as its unique advantages of high precision, high sensitivity and self-calibration in situ qualification with-out any sample preparation. The multi-pass cell (MPC) with a long optical path is usually applied to enhance TDLAS-based sensor’s detection precision and sensitivity. However, the unexpected optical fringes occurring in the MPC often spoil the sensor’s detection precision and sensitivity seriously. The detection precision and sensitivity of the TDLAS-based sensors containing an MPC are often limited by the optical fringes that result from diffraction, scattering on the mirror surface imperfections, mirror aberration, thermal expansion, cold contraction, or stress deformation. Therefore, the complex optical fringe consisting of different optical fringe will be observed in the MPC. These optical fringes are due largely to a small amount of laser reaching the detector with an optical path length differing by ΔL from the main laser beam. Those problems are common for TDLAS, especially using dense overlapped spot pattern MPC and some di?erent methods are proposed to eliminate the negative influence of the optical fringes.
 

　　研究團隊提出了一種抑制可調二極管激光吸收光譜中光學條紋噪聲的新方法，并將其應用于由光學條紋擾動的CH4氣體傳感器，以提高檢測精度和靈敏度。
 

　　In this work, a novel method to suppress optical fringe noise in the tunable diode laser absorption spectroscopy is proposed and applied to the CH4 gas sensor perturbed by optical fringes for higher detection precision and sensitivity.
 

　　所開發的CH4檢測儀的示意圖如圖1所示。寧波海爾欣光電科技有限公司為此項目提供鎖相放大器(HPLIA 微型雙通道調制解調鎖相放大器)，從光電探測器輸出的信號發送到鎖相放大器，鎖相放大器相對于同步信號對2f模式進行解調，鎖相放大器的時間常數設為1ms。
 

　　The schematic diagram of the developed CH4 detection instrument is shown in Fig. 1 . HealthyPhoton Co.,Ltd provided a HPLIA Miniature dual-channel modulated demodulation lock-in amplifier for this project. The lock-in amplifier demodulates the signal in the 2f mode with respect to the sync signal. The time constant of the lock-in amplifier is set to 1 ms.
 

　　Fig.1. Schematic diagram of the developed CH 4 detection system
 

　　lock-in amplifier (Healthy Photon, HPLIA)
 

　　對于被光學條紋和隨機噪聲干擾的20 ppm CH4的二次諧波(2 f)信號，通過該新方法，2f信號的信噪比(SNR)從17提高到182，優化平均光譜范圍Δ???。與未經處理的原始信號相比，CH4測量精度改善了約1.5倍。相應的最小可檢測濃度可從3 ppb改善到0.78 ppb。系統的相應噪聲當量吸收靈敏度(NNEA)和噪聲當量濃度(NEC)分別為6.13 ×10-11 cm−1 W Hz−1/2 and 0.181 ppm。
 

　　For the 2nd harmonic(2f) signal of 20 ppm CH4 spoiled by optical fringes and random noise, by the novel method, the signal-to-noise ratio (SNR) of the 2f signal is improved about 6.5 times from 17 to 182 with an optimal averaging spectral range Δ???. A ∼1.5 times improvement in the measurement precision of CH4 is achieved compared to unprocessed raw signal. The corresponding minimum detectable concentration can be improved from 3 ppb down to 0.78 ppb. The corresponding noise equivalent absorption sensitivity (NNEA) and the noise equivalent concentration (NEC) of the system is 6.13 ×10-11cmW-1Hz-1/2 and 0.181 ppm, respectively.
 

　　Violet line from traditional averaging method and magenta line from the novel optical fringe noise suppression method.
 

　　Histogram plot of the 20 ppm CH 4 deviation.
 

　　20 ppm CH 4 Allan-deviation stability of developed overlapped spot pattern MPC.
 

　　參考文獻：
 

　　Reference:
 

　　Yanan Cao, Xin Cheng, Zong Xu, Xing Tian, Gang Cheng, Feiyan Peng, Jingjing Wang
 

　　High precision and sensitivity detection of gas measurement by laser
 

　　wavelength modulation spectroscopy implementing an optical fringe noise
 

　　suppression method, Optics and Lasers in Engineering 166 (2023) 107570
 

　　www.elsevier.com/locate/optlaseng