A New RAM Normalized 1$f$-WMS Technique for the Measurement of Gas Parameters in Harsh Environments and a Comparison With ${2f\!/\!1f}$

Abstract

A calibration-free first harmonic (1f) wavelength modulation spectroscopy (1f-WMS) technique for gas species parameter measurement is demonstrated. In this technique, the total magnitude of the 1f-WMS signal is normalized by a component of the 1f residual amplitude modulation signal. This method preserves the advantages of the traditional nf/1f-WMS (n ≥ 2) technique, such as the immunity to the non-absorbing systematic losses and the accurate recovery of gas parameters, without the requirement for non-absorbing regions for normalization at high pressure or high modulation index values (m-values). The proposed technique only requires the 1f signal, which has the largest magnitude of all the harmonics signals, and, therefore, fundamentally has a higher sensitivity to the nf/1f technique. Furthermore, since only the 1f-WMS signal is used, the technique is less complex in terms of signal processing and data acquisition. This paper also shows a comparison of the proposed technique and 2f /1f for measuring CO 2 in the exhaust of a combuster. The data highlight how nonlinearities in the optical detection system as a function of frequency have a considerable effect on the recovered 2f/1f spectra, causing variation in the recovered gas concentrations. This effect is not seen in the methodology proposed in this paper.

Laser Absorption Sensing Systems: Challenges, Modeling, and Design Optimization

“…Although the laser characterization process is not difficult by itself, the task of accurately tracking its variations in real time is nontrivial. More recently, Upadhyay et al have proposed a new calibration-free 2f -WMS technique to measure gas concentration and pressure without the need for laser pre-characterization [103][104][105][106]. Similar to the idea of DAS baseline fitting, the harmonic backgrounds, i.e., RAMs, are obtained by interpolating the non-absorbing wings of the X and Y components of the demodulated 1f, 2f, and 3f signals ( Figure 13).…”

Second Harmonic Phase Angle Method Based on WMS for Background-Free Gas Detection

“…In many cases, the RAM amplitude can be comparable to or even much larger than the absorption signal, which results in the absorption signal often drowning in the background and difficult to distinguish. The RAM also limits the [6], [7].…”

“…Also, Upadhyay et al demonstrated experimentally that the nonlinear gain of detectors varies with frequency and optical attenuation. During laser intensity fluctuations, the 2f /1f method will not be able to eliminate the nonlinear gain variation of the detector [7]. However, these problems can be well avoided if the same order harmonics are employed for normalization.…”

Research on Spectroscopy Modulation of a Distributed Feedback Laser Diode Based on the TDLAS Technique

“…Infrared absorption spectroscopy is widely researched due to numerous advantages including fast response, nonintrusive nature, and sensitive species-specific detection capabilities [1][2][3][4], compared with traditional gas-sensing techniques such as catalyst combustion [5][6][7], electrochemical [8][9][10], and semiconductor [11][12][13]. erefore, gas-sensing systems and sensors based on infrared absorption spectroscopy have been reported by many research groups in recent years, and many related techniques have been developed and improved to achieve high-sensitive sensing ability [14][15][16].…”