Fiber Bragg grating-based wavelength modulation spectroscopy technique for trace gas sensing

Optical gas sensing: a review

Abstract: The detection and measurement of gas concentrations using the characteristic optical absorption of the gas species is important for both understanding and monitoring a variety of phenomena from industrial processes to environmental change. This study reviews the field, covering several individual gas detection techniques including non-dispersive infrared, spectrophotometry, tunable diode laser spectroscopy and photoacoustic spectroscopy. We present the basis for each technique, recent developments in methods and performance limitations. The technology available to support this field, in terms of key components such as light sources and gas cells, has advanced rapidly in recent years and we discuss these new developments. Finally, we present a performance comparison of different techniques, taking data reported over the preceding decade, and draw conclusions from this benchmarking.

A Survey on Gas Sensing Technology

Abstract: Sensing technology has been widely investigated and utilized for gas detection. Due to the different applicability and inherent limitations of different gas sensing technologies, researchers have been working on different scenarios with enhanced gas sensor calibration. This paper reviews the descriptions, evaluation, comparison and recent developments in existing gas sensing technologies. A classification of sensing technologies is given, based on the variation of electrical and other properties. Detailed introduction to sensing methods based on electrical variation is discussed through further classification according to sensing materials, including metal oxide semiconductors, polymers, carbon nanotubes, and moisture absorbing materials. Methods based on other kinds of variations such as optical, calorimetric, acoustic and gas-chromatographic, are presented in a general way. Several suggestions related to future development are also discussed. Furthermore, this paper focuses on sensitivity and selectivity for performance indicators to compare different sensing technologies, analyzes the factors that influence these two indicators, and lists several corresponding improved approaches.

Tunable Diode-Laser Spectroscopy With Wavelength Modulation: A Calibration-Free Approach to the Recovery of Absolute Gas Absorption Line Shapes

Abstract: The principles and implementation of an alternative approach to tunable diode-laser spectroscopy with wavelength modulation are described. This new technique uses the inherent phase shift between diode-laser power modulation and frequency modulation to separate the residual amplitude modulation and the first derivative signals recovered at the fundamental modulation frequency. The technique, through analysis of the residual-amplitude-modulation signal, is absolute, yielding gas-absorption-line-shape functions, concentrations, and pressures without the need for calibration under certain defined operating conditions. It offers the simplicity of signal analysis of direct detection while providing all the advantages of phase-sensitive electronic detection. Measurements of the 1650.96-nm rotation/vibration-absorption-line-shape function for 1% and 10% methane in nitrogen at various pressures are compared to theoretical predictions derived from HITRAN data, and the excellent agreement confirms the validity of the new technique. Further measurements of concentration and pressure confirm the efficacy of the technique for determining concentration in industrial-process environments where the pressure may be unknown and changing. An analysis of signal strength demonstrates that sensitivity comparable to that of conventional approaches is achievable. The new approach is simpler and more robust in coping with unknown pressure variations and drift in instrumentation parameters (such as laser characteristics) than the conventional approach. As such, it is better suited to stand-alone instrumentation for online deployment in industrial processes and is particularly useful in high-temperature applications, where the background infrared is strong.

A novel fiber Fabry–Perot filter based mixed-gas sensing system

Abstract: A practical mixed-gas sensing system utilizing a fiber Fabry–Perot (FFP) filter and Fiber Bragg Gratings (FBG) was developed. By modulating the transmission wavelength of FFP within the width of one absorption line of the target gas, the system can detect different kinds of gaseous species selectively. A FBG with center wavelength being aligned to the absorption peak of the target gas was employed as wavelength reference to make the transmission wavelength of FFP modulate stably in a required region. The properties of the proposed system were demonstrated experimentally by simultaneous detection of acetylene (C 2 H 2 ) and carbon monoxide (CO). Approximate linear relationships between the system responses and the input gas concentrations were demonstrated by experiments. The minimum detectable C 2 H 2 and CO concentration of ∼15 ppm and ∼300 ppm were also achieved by experiments respectively.

Superluminescent Diode-Based Multiple-Gas Sensor for NH $_{\bf 3}$ and H $_{\bf 2}$ O Vapor Monitoring

Abstract: An approach of superluminescent light emitting diode (SLED)-based multiple-gas sensing (NH3 and H2O vapor) is proposed and demonstrated by using an absorption spectroscopy technique for emission monitoring (DeNOx process) applications. Such process normally occurs at temperature range of 250-400°C where significant interference effect due to H2O vapor is expected. Hence, measuring multiple gases and eliminating cross-interference effect is of great importance in sensing trace gases. In this study, an SLED-based sensor with center emission wavelength of 1530-nm region is chosen to measure the concentrations of pure NH3 and H2O vapor by probing its overtone and combination bands present in this region. Detection limit for the NH3 gas accounted in the case of aqua-ammonia (undiluted) is estimated to be 120 ppm ·m. A novel approach of introducing an open-space etalon in the path of the SLED beam is also proposed and demonstrated for adequate isolation from interfering species and improving the detection limit. By using an etalon with a free spectral range of 104 GHz and tuning it to appropriate wavelength, detection limit of about 22 ppm·m is attained for NH3 gas sensing in the presence of H2O vapor.