Abstract
This paper presents a detailed experimental wavelength modulation spectroscopy approach and demonstrates its applicability to various types of semiconductor lasers in the near infrared and mid-infrared. A 5250 nm continuous-wave distributed feedback quantum cascade laser, a 2004 nm vertical cavity surface emitting laser, and a 1650 nm distributed feedback edge-emitting laser are used to extract the concentration and pressure values of nitric oxide, carbon dioxide, and methane, respectively, using the $2f$ wavelength modulation spectroscopy (WMS) technique under controlled conditions. The generality of the technique is demonstrated by extending it to $3f$ WMS for the three different kinds of lasers used in this study. The methodology required to provide in-situ real-time measurements of both gas parameters and operating characteristics of the laser are described in detail. Finally, the advantages and limitations of the technique are discussed in view of the fact that the characteristic behavior of the laser sources is significantly different. We specifically discuss the issue of targeting non-absorbing wavelength regions and the choice of modulation frequency and modulation amplitude of the laser, as well as the choice of the detection harmonic.
Highlights
Tunable diode laser absorption spectroscopy (TDLAS) based gas sensors have transitioned in the past few decades from a laboratory based gas sensor into a practical sensor for field applications, such as combustion monitoring [1]–[4], flow measurement [5], [6], fuel cell monitoring [7] and environmental monitoring [8]–[15]
It is known that QCLs such as the external cavity QCL (ECQCL) used by Chao et al [18] and the cw-distributed feedback (DFB)-QCL laser used in this study, have much higher nonlinearity in their intensity versus current characteristics as compared to DFB lasers
Higher order wavelength modulation spectroscopy (WMS) signals are weaker than the 2f WMS signal, they have a higher signal to the absorption independent background residual amplitude modulation (RAM) ratio
Summary
Tunable diode laser absorption spectroscopy (TDLAS) based gas sensors have transitioned in the past few decades from a laboratory based gas sensor into a practical sensor for field applications, such as combustion monitoring [1]–[4], flow measurement [5], [6], fuel cell monitoring [7] and environmental monitoring [8]–[15]. There are significant variations in the measured signals that are not due to variation in gas parameters but due to systematic issues such as vibrations, contamination of the optics and drift in laser characteristics due to temperature variation or aging. Periodic re-calibration in industrial systems is not a viable solution because post-installation access is limited in many cases For such applications, the 1f-normalised 2f technique (2f/1f ) [16]–[18] and its extension to nth harmonic, i.e. nf/1f technique where n ≥ 2 [19], [20] is widely used. Apart from being immune to absorption-independent systematic issues such as light scattering, variable coupling, and unintended beam deflection caused by vibrations, it has been shown that this method is applicable at high pressures when the adjacent absorption lines blend with each other
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