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中文摘要：
腔衰荡光谱技术（cavity ring-down spectroscopy，CRDS）是一种高灵敏的激光吸收光谱技术，被广泛应用于镜片反射率测量、光谱分析以及痕量气体检测等研究领域。首先从理论上描述了 CRDS 技术的实验原理，然后基于555定时器自行设计了一种可应用于CRDS技术的阈值电路，并通过实验进行了验证。为了优化阈值电路的性能，研究了不同输入阻抗及容抗下阈值电路对衰荡事件测量的影响。通过直接接入不同值的电容与电阻，发现输入电容越大，输入阻抗越小，对衰荡事件的测量影响越大，尤其当阈值电路输入电阻小于50Ω时，衰荡事件的线型严重扭曲。对不同输入电阻与输入电容时C2 H2气体在6531．7805 cm-1处的吸收信号进行了采集与分析，发现输入电容与输入电阻的改变对测量结果有很大的影响，得到的吸收信号会失真，甚至无法进行拟合。最终给出了阈值电路的最佳设计方案，在阈值电路的设计中选取的输入阻抗越大越好，最小要在100Ω以上，选取的容抗越小越好，最好低于1 nF，同时要保证阈值电路的时间常数远小于衰荡时间。该研究对于CRDS技术应用过程中阈值电路的设计具有重要的参考价值。
英文摘要：
In the present paper,the authors simply describe the principle of cavity ring-down spectroscopy （CRDS）measurement technology and introduce a threshold circuit,based on 555 timer IC （integrated circuit）,applied to CRDS.And we verify the feasibility of the circuit.By means of changing the input resistor and input capacity,we analyzee the influence of threshold circuit characteristics on single ring-down event,and found that the greater the input capacitance,and the smaller the input impedance of circuit,the more serious the distortion of measurement of ring-down curve,especially when the resistance is less than 50 Ωthe ring-down time produces very big change.Then we collected and analyzed the C2 H2 absorption signal at 6 531. 780 5 cm-1 with different input resistance and input capacitance,and fitted the absorption line with Voigt profile.We found that the change of the input capacitance and resistance caused the distortion of fitting curve.Finally We give the optimal design of threshold cir-cuit：the input impedance of the threshold circuit should be great,minimum of 100Ω,the capacitive reactance should be as small as possible,preferably below 1 nf,at the same time it should be ensured that the threshold of the circuit time constant is much smaller than ring-down time.The experiment research on CRDS technique application has important reference value to threshold circuit design.
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