Method for eliminating second harmonic distortion based on modulation phase difference in TDLAS technology

Abstract

Tunable diode laser absorption spectroscopy (TDLAS), as a branch of gas spectrum detection technology, has the merits of high resolution, high sensitivity, low detection limit, less environmental impact and suitable for various complex environments. It plays an important role in the field of air pollution monitoring, aeromechanics, mine development, industrial manufacturing, biomedicine and other fields. With technology accumulation over the past few years, TDLAS has become the most widely used and representative gas detection technology. The influence of the residual amplitude modulation brought by the phase difference on the harmonic signals is analyzed. The relation formula of gas concentration after the phase difference added is derived from the theoretical formula. Then the influence of the factors such as the modulation amplitude and the modulation frequency is further investigated. A set of BRD circuits is designed to make the reference beam and the detected beam normalization at the current level, which improves the signal to noise ratio and eliminates the asymmetry of the left and right peaks of the harmonic signals caused by the residual amplitude modulation, and improves the measurement accuracy. The experiment shows that the spectral line distortion is proposed by using a BRD circuit to eliminate spectral line distortion caused by the phase difference. After using the BRD circuit, the resolution of the direct concentration signal is increased by nearly 3 times.