对数变换-波长调制光谱在气体检测中的应用

为了提高痕量气体检测的稳定性并扩大动态范围，引入对数变换方法和差分检测电路，对常规波长调制光谱技术进行了改良。在使用锁相放大器提取与气体吸收相关的谐波信号之前，由自制的接收器完成对数变换和差分检测功能。通过对数变换，使激光强度调制与气体吸收引起的光功率衰减量实现了分离，再利用差分检测消除前者。受益于此双管齐下的策略，理论上可以捕捉吸收光谱的任意阶谐波分量，并且免受剩余幅度调制和谐波畸变的影响。为了验证理论，对NH3的P（6）吸收谱线的二次谐波进行了采集。在296 K的环境温度、1.01105 Pa的总压力和24.5 cm的有效光程下，假设信噪比降低为1时推导得到的理论检测下限为0.7 ppm（1 ppm=10-6）。以上结果表明该方案是痕量气体检测应用的一种理想选择。

Application of logarithmic transformed-wavelength modulation spectroscopy in gas detection

For the purpose of enhancing the stability and enlarging the dynamic range in trace gas sensing, the conventional wavelength modulation spectroscopy technology was improved by the introduction of logarithmic-transformed data processing method and differential detection circuit. Before the extraction of the gas absorption related harmonics using a lock-in amplifier, the logarithmic-transformation and the differential detection were fulfilled by a homemade receiver. Through the logarithmic-transformation, the optical intensity modulation of the laser emitting was separated from the absorption-induced power attenuation, and then the former was balanced during differential detection. Owing to this two-pronged strategy, each harmonic component of the absorption spectrum can be theoretically captured without the interferences of residual amplitude modulation and harmonic distortion. For the validation of theory, the second harmonic of P(6) absorption line for NH3 was acquired. The experimental temperature and total pressure were maintained at 296 K and 1.01105 Pa, respectively. Under the effective path length of 24.5 cm, a 0.7 ppm (part per million) detection limit was deduced on the assumption that the amplitude of signal was weakened to be equal with the noise. The above results indicate that this scheme is an ideal option for trace gas detection application.