A Wide-Range and Calibration-Free Spectrometer Which Combines Wavelength Modulation and Direct Absorption Spectroscopy with Cavity Ringdown Spectroscopy.

Zhen Wang,Zhimin Peng,Yanjun Ding,Yanjun Du

doi:10.3390/s20030585

Abstract

A wide-range, calibration-free tunable diode laser spectrometer is established by combining wavelength modulation and direct absorption spectroscopy (WM-DAS) with continuous wave cavity ringdown spectroscopy (CW-CRDS). This spectrometer combines the benefits of absolute concentration measurements, wide range, and high speed, using WM-DAS with enhanced noise reduction in CW-CRDS. The accurate baseline ringdown time, τ0, is calculated by the absorption peak (measured by WM-DAS) and the ringdown time containing gas absorption information (measured by CW-CRDS at the center wavelength of the spectral line). The gas concentration is obtained without measuring τ0 in real time, thus, greatly improving the measuring speed. A WM-DAS/CW-CRDS spectrometer at 1.57 μm for CO detection was assembled for experimental validation of the multiplexing scheme over a concentration ranging from 4 ppm to 1.09% (0.1 MPa, 298 K). The measured concentration of CO at 6374.406 cm−1 shows that the dynamic range of this tunable diode laser absorption spectrometer is extendable up to five orders of magnitude and the corresponding precision is improved. The measurement speed of this spectrometer can extend up to 10 ms, and the detection limit can reach 35 ppb within 25 s.

Highlights

Tunable laser diode absorption spectroscopy (TDLAS) [1,2,3] has the advantages of being noncontact, fast response, and wavelength selective
direct absorption spectroscopy (DAS) has been widely used in the case of high concentrations and strong absorption environments
The gas concentration can change from a trace to a high level in a matter of minutes and occasionally fluctuates violently

Summary

Introduction

Tunable laser diode absorption spectroscopy (TDLAS) [1,2,3] has the advantages of being noncontact, fast response, and wavelength selective. The direct absorption spectroscopy (DAS) [4,5,6] in TDLAS has a clear physical concept and is easy to operate, which is frequently used to quantify concentration, temperature, and other parameters of gases. DAS has been widely used in the case of high concentrations and strong absorption environments (absorbance 0.01 to ~1). For trace gas monitoring or weak absorption line measurements (absorbance

Methods

Results

Conclusion