基于可调谐半导体激光吸收光谱技术及波长调制技术, 采用波长为1 654 nm的分布反馈激光器, 结合开放式光学探头以及高灵敏度的铟镓砷光电探测器, 研制了近红外甲烷气体检测系统。 自主设计研发了分布反馈激光器驱动电路, 主要包括模拟PID温度控制电路与电流驱动电路。 其中, 温度控制电路具有较高的控制精度及稳定性, 长时间工作时激光器温度波动小于±0.02 ℃, 温度与激光器波长呈线性变化。 温度不变时, 改变驱动电流可以使激光器中心波长线性变化, 同时还提供了5 kHz正弦波和10 Hz锯齿波的调制信号, 用于谐波检测。 为了提取差分信号的一次谐波及二次谐波, 研制了正交锁相放大器, 一次谐波和二次谐波的提取误差分别为3.5%和5%。 系统中采用的开放式光电探头通过一次反射, 使有效吸收光程增加了一倍, 达到了40 cm。 通过对1%～5%的甲烷气体进行检测, 成功提取了一次及二次谐波, 得到了气体浓度与谐波信号幅值的拟合关系曲线。 在更换不同输出波长的激光器后, 该系统还具有检测其他气体的能力。

A methane (CH4) detection system based on tunable diode laser absorption spectroscopy (TDLAS) technique was experimentally demonstrated. A distributed feedback (DFB) laser around 1 654 nm, an open reflective sensing probe and two InGaAs photodiodes were adopted in the system. The electrical part of the system mainly includes the laser temperature control & modulation module and the orthogonal lock-in amplifier module. Temperature and spectrum tests on the DFB laser indicate that, the laser temperature fluctuation can be limited to the range of -0.02~0.02 ℃, the laser’s emitting wavelength varies linearly with the temperature and injection current, and also good operation stability of the laser was observed through experiments. Under a constant working temperature, the center wavelength of the laser is varied linearly by adjusting the driving current. Meanwhile, a 5 kHz sine wave signal and a 10 Hz saw wave signal were provided by the driving circuit for the harmonic extraction purpose. The developed orthogonal lock-in amplifier can extract the 1f and 2f harmonic signals with the extraction error of 3.5% and 5% respectively. By using the open optical probe, the effective optical pass length was doubled to 40 cm. Gas detection experiment was performed to derive the relation between the harmonic amplitude and the gas concentration. As the concentration increases from 1% to 5%, the amplitudes of the 1f harmonic and the 2f harmonic signal were obtained, and good linear ration between the concentration and the amplitude ratio was observed, which proves the normal function of the developed detection system. This system is capable to detect other trace gases by using relevant DFB lasers.