Research And Instrumentation Of Tunable Laser Absorption Spectroscopy With Quasi-continuum Technique

The Requirements of the monitoring of gas in envioroment protection and industry technique control have promoted the tenology of tunable laser absorption spectroscopy (TDLAS) greatly. The laser modulation spectroscopy which is the most effective method to improve the signal quality, to reduce the background noise and to improve the detection limit is significant for the development of gas detection technology and potential to trace gas analysis.Based on the TDLAS technology, the diode laser pulse modulation and quasi-continuous modulation have been studied in this paper. In the light of principle of the unbalanced Mach-Zehnder interferometer, combined with the location of the gas absorption signal, the tuning characteristics of the distributed feedback (DFB) diode laser in the pulse-driven mode were analyzed. Under the same experimental conditions, we detected the different concentrations of CO₂ with the same laser, and compared the technical characteristics, the signal features and the detection limit. It was shown that the detection limit of the continuous modulation spectroscopy and the quasi- continuous modulation spectroscopy was approaching. Through synthesizing the open long-path optical system, wide unable spectrum achieved by temperature tuning was used to detect the concentration of atmospheric CO₂. Furthermore, the variation of CO₂ in one day was obtained.A laser gas analyzer based on the tunable laser absorption spectroscopy was developed. Combined with multi-reflection open-path optical cell, the instrument which was board-level integrated with the FPGA-Single Board acquisition and control core of the United States National Instruments (NI) company, was small and portable. This instrument is suitable for mass productions and available in situ trace gas measurement.