Linear calibration‐free wavelength modulation spectroscopy

Abstract

This article presents a new linear calibration-free wavelength modulation spectroscopy (LCF-WMS) technique for gas sensing. By combining the natural logarithms of the transmitted intensity and background signals, this technique eliminates the effects of the intensity, intensity modulation, and the modulation phase to obtain integrable harmonic signals along the line-of-sight and thus realize calibration-free measurements. To illustrate the validity of the LCF-WMS technique, it was applied to measurement of the water vapor concentration in air at ambient temperature. Because of its specific features of line-of-sight integrability and baseline independence, this method will be advantageous in absorption tomography and high-pressure applications.

In Situ Measurement of NO, NO2, and H2O in Combustion Gases Based on Near/Mid-Infrared Laser Absorption Spectroscopy

“…The theory of laser absorption spectroscopy is well understood and has been published previously [ 33 , 34 , 35 , 36 ]. The principles and techniques of the LCF-WMS method are briefly reviewed in Section 2.1 to define terms and guide the discussion.…”

“…The second harmonic was recommended due to its higher SNR compared to other order harmonics in the literature [ 33 ]. Accordingly, the second harmonic was used for NO and H 2 O detection in the present study.…”

“…This results in the harmonic signals being further distorted due to the nonlinear coupling of the characteristics of the non-uniform flow field and laser parameters, which increases the difficulty of resolving harmonic signals. Recently, linear calibration-free wavelength modulation spectroscopy (LCF-WMS) has been reported [ 33 , 34 ], which is suitable for trace gas detection in non-uniform fields. However, these works were primarily concerned with the performance of a single component, which cannot capture overall NO x emissions.…”

Weak absorption detection method under broadband interference based on linear wavelength modulation spectroscopy

“…The fundamentals of laser absorption spectroscopy have been published previously [10,11]. To define terms and guide discussions in subsequent sections, the theory of laser absorption and principles of LCF-WMS technique are briefly reviewed.…”

“…Furthermore, the traditional normalized wavelength modulation method (WMS-nf/1f) is inevitably influenced by the first harmonic, which limited the ability to address the broad absorption interference. In the previous work, we had shown the technique of linear calibration-free wavelength modulation spectroscopy (LCF-WMS) to be able to obtain the higher-order harmonics without introducing other harmonic components, which provides the possibility of eliminating broad absorption interference by using higher-order harmonics [10]. However, in order to determine the order of harmonics in practical measurements the evaluation for the ability of higher order harmonics requires further analysis.…”