基于单模半导体激光器的模式跃变特性，用两个不同的驱动电流交替驱动激光器使其交替输出两个波长相近的激光，一个位于甲烷的一个窄带吸收峰处，被甲烷吸收；另一个错开稍许，不被甲烷吸收，用作参考光束.光束通过气体吸收室后，经光电转换输出方波电信号，两个波长光的强度分别对应方波信号的顶部和底部，随甲烷浓度变化的光，与其相对应方波的顶部也将随着甲烷气体浓度的变化而变化；参考光束的光不被甲烷吸收，与其对应的方波的底部将不随甲炕气体浓度的变化而变化，于是方波的幅值与甲烷浓度成一一对应关系.由于二者的波长相近，当它们通过媒介气体时，尘埃引起的散射或者衰减基本相同，可以用数学方法将其消去.实验结果表明，该方法在不采用昂贵的数字锁相放大器，且气体吸收光程只有10cm。的条件下，达到了0.05％的探测灵敏度.

Based on the mode-hopping of single mode diode laser, two different currents drive the laser alternately, the laser emits alternately two beam lights whose wavelength is differently, but closely. The first beam light＇ s wavelength is at the absorption peak of methane. The second beam light＇ s wavelength is a little shorter than the first one, which is not absorbed by methane, and it will serve as a reference light beam. The double beam will be converted into an electronic square wave after passing through the gas cell. The intensity of the double beam will correspond to the top and the bottom of the square wave respectively. The top of the square wave will vary if the methane concentration varies, but the bottom of the square wave will not vary because the intensity of the second light does not vary while methane concentration varying. Thus the amplitude of the square wave is proportion to the methane concentration. Because the two wavelengths are very close, the scatters or the losses caused by dust are almost the same after they passing through the methane, and they can be eliminated by mathematics. The result of experiment shows that the method can reach the sensitivity of 0. 05% even without using a phase-locked amplifier and the absorption light path is only 10 centimeters.