A Fast Measurement Method Based on Linearly Swept Wavelength Modulation and Frequency Tracking Demodulation for Photoacoustic Spectroscopy

Photoacoustic spectroscopy (PAS) based gas sensors with high sensitivity, high selectivity, zero background signal and real-time online monitoring are desirable in energy, environment and medicine. However, the stability and accuracy of PAS are decreased due to the resonant frequency shift of the photoacoustic cell, which is caused by finesse of photoacoustic cell and the temperature variety. In this paper, we propose a measurement method based on linearly swept wavelength modulation and frequency tracking demodulation to track and scan the frequency response of the photoacoustic cell, and obtain the resonant frequency of the photoacoustic cell directly and quickly. At the same time, it can be used as a gas concentration measurement method without measuring the resonant characteristics. The formula of the theoretical model is derived, and the feasibility and advantages are demonstrated in principle. Combined with the PAS system and the acetylene gas detection experiment, the measurement method was completed within 1 minute, and the error of resonant frequency was less than 0.1%. As a concentration measurement method, it has good linearity and stability. The research aims to eliminate the influence of resonant frequency deviation, improve the measurement speed, stability and accuracy of the PAS system, and promote the real-time online measurement application of PAS technology.