Signal processing method of ambient trace-gas monitoring with tunable diode laser absorption spectroscopy

Tunable diode laser absorption spectroscopy (TDLAS) is a new method to detect trace-gas qualitatively or quantificationally based on the scan characteristic of the diode laser to obtain the absorption spectroscopy in the characteristic absorption region. It needs to combine with a long absorption path in the ambient trace-gas measurements. TDLAS has a significant advantage not only in sensitivity but also in rapidity of response. It has been widely used in many atmospheric trace-gases detection, ground trace-gas detection and gas leakage detection. On-line signal processing is necessary to most trace gas monitor. In this paper, data processing method for retrieval of gas concentration, as well as the wavelength alignment algorithm based on correlation detecting technique is described, realizing the aim of wavelength alignment between the standard signal spectral line and the measurement signal spectrum. So the precision of the retrieving system is improved satisfactorily.

1. INTRODUCTION

Tunable diode laser absorption spectroscopy (TDLAS) has been widely employed in detecting atmospheric trace gases due to its high sensitivity, high selectivity, and fast time response^[1]The most important part in a standard TDLAS system is a tunable distributed feedback (DFB) diode laser. Over the past two decades due to high demand from the market significant progress in manufacturing tunable DFB lasers has been successfully made. The price for a DFB laser has been remarkably reduced. The key features of the DFB lasers are 1) their very narrow line-width, typically less than 50MHz, and thus much less than the line-width of a single rotational gas absorption line, and 2) the ability to modulate the wavelength of the laser output through the injection current. High sensitivity detection is obtained by wavelength or frequency modulation spectroscopy and by monitoring first, second, or higher order of the modulation frequency in the detected output signal^[2]. In addition, it could also greatly improve the detection sensitivity by adopting a long-path technique^[1]Compared with the multiple-reflection cell, the long open-path system can accurately represent a large spatial area particularly in poorly mixed atmosphere, and can directly provide accurate, simultaneous measurements of the average concentration of a number of trace gases over long open paths. Moreover, long open-path system contains the information of atmospheric flow during the measuring process of gas concentration, so it can also monitor the gas emission flux.