A dual-tone modulation method to reduce the background fluctuation in tunable diode laser absorption spectroscopy

Optical interference fringe, known as etalon effect, is the main factor which causes background fluctuation in trace gas concentration detection based on tunable diode laser absorption spectroscopy(TDLAS). The fringes make the background signal present a sinusoidal oscillation, which reduces the system's detection ability on the gas of more low concentration. To reduce the background fluctuations, a dual-tone modulation (DTM) method is introduced to the model based on Beer-Lambert law and etalon effects. According to the results of simulation and actual experiments, the DTM method can significantly reduce the background fluctuations. More specifically, the standard deviation (STD) value of background fluctuations decreased from 2.6239 parts-per-million (ppm) to 0.19 ppm in our experiments, where the STD of 0.19 ppm is corresponding to absorption of 6.224*10−6 Hz−1/2 with effective optical path length of 2.8 m and integral time of 0.1 s. The results of long-term detection experiment further demonstrated the stability of DTM in reducing the background fluctuations and the reliability of trace gas concentration detection.