1. Introduction

Over the past few decades, environmental pollution problem has occurred to different degrees in the whole world, such as atmospheric pollution, marine pollution, and urban environmental problems. With the globalization of economy and trade, environmental pollution is becoming more and more internationalized [1]. In order to control environmental pollution, great demands and tremendous efforts for new technology to detect hazard gases such as CH4,

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

CO2, CO, HONO, H2S, and HCl have been performed. This would be beneficial for the implementation of global environmental protection policies for the reduction of gas pollution and for a general environmental management [2].

Several optical techniques have been developed to detect these hazard gases in the atmosphere [3–10]. Cavity-enhanced spectroscopy (CEAS) or cavity ring-down spectroscopy (CRDS) has been demonstrated to enable measurements of multiple gases with a low detection limit of sub-ppb [4–6]. However, these two technologies require critical optical alignment and regular cleaning of mirrors of the external cavity which affects continuous monitoring of atmospheric species in the field. Quartz-enhanced photoacoustic spectroscopy (QEPAS) technique was also developed for environmental and biomedical measurements [7, 8]. Nevertheless, the high modulation frequencies used in QEPAS may represent a problem for multicomponent gas mixtures containing varying amounts of water vapor such as ambient air, due to the strong influence of water vapor on the molecular vibrational-translational (V-T) relaxation times. Other spectroscopic methods such as open path Fourier transform infrared spectrometry (FTIR) and differential optical absorption spectroscopy (DOAS) have been reported for atmospheric molecule detection [9, 10]. But the minimum detection limits (MDLs) of FTIR usually exceed the requirements for high sensitivity measurements of the atmospheric species. The main disadvantage of the DOAS system is that its spatial resolution is rather poor with a path length generally greater than 1 km.

The technique based on tunable diode laser absorption spectroscopy (TDLAS) is an effective method to measure gas mixing ratios and multiple parameters with high selectivity, high sensitivity, high precision, and high response time [11–18]. Especially, with the development of multi-pass absorption cells, the effective optical path length can be extended from a few meters to several hundred meters; the sensitivity is significantly improved [19–21]. In order to further improve the signal-to-noise ratio (SNR), the wavelength modulation spectroscopy (WMS) technology with second harmonic (2f) signals is usually employed in the TDLAS system to measure the gas concentration.

The first commercial TDLAS gas sensor was introduced on the market in 1995 using the trademark laser gas by Norsk Elektro Optikk Company. Over the past decades, TDLAS has been extensively investigated potentially as an effective method to measure multiple gas parameters and is widely used in various areas such as gas mixing ratio detection, vehicle emissions, gas exhaust temperature monitoring, carbon isotope measurements, and so on [22–37]. Now NEO Monitors is one of the world leading suppliers of the TDLAS-based gas analyzers and dust monitors. Its products are widely used in the field of industrial process control and emission monitoring; nearly 6000 sets of laser gas analyzers were installed in more than 40 countries and regions in the world currently. We are also developing instruments based on TDLAS technology to satisfy the needs of environmental monitoring and industrial process control in China. Figure 1 shows several pictures of the gas sensors developed by our research team. In this chapter, we will briefly present several kinds of gas sensors developed by our research group for various field applications, which could expand from environment and public safety to medical science.

2. Basic principles of TDLAS

be simplified as

transmitted intensity I can be expressed as

Figure 1. Several pictures of the TDLAS system developed by our research team.

Based on the Beer-Lambert law, the relationship between the incident intensity I<sup>0</sup> and the

Environmental Application of High Sensitive Gas Sensors with Tunable Diode Laser Absorption Spectroscopy

http://dx.doi.org/10.5772/intechopen.72948

209

where k is the absorption coefficient and L denotes the path length (in cm). In the near-infrared region, the gas absorption coefficient is usually very small, i.e., kL ≤ 0.05 [38]. Eq. (1) can thus

I ¼ I<sup>0</sup> exp ð Þ �kL (1)

I ¼ I0ð Þ¼ 1 � kL I0½ � 1 � σ νð ÞCL (2)

Environmental Application of High Sensitive Gas Sensors with Tunable Diode Laser Absorption Spectroscopy http://dx.doi.org/10.5772/intechopen.72948 209

CO2, CO, HONO, H2S, and HCl have been performed. This would be beneficial for the implementation of global environmental protection policies for the reduction of gas pollution

Several optical techniques have been developed to detect these hazard gases in the atmosphere [3–10]. Cavity-enhanced spectroscopy (CEAS) or cavity ring-down spectroscopy (CRDS) has been demonstrated to enable measurements of multiple gases with a low detection limit of sub-ppb [4–6]. However, these two technologies require critical optical alignment and regular cleaning of mirrors of the external cavity which affects continuous monitoring of atmospheric species in the field. Quartz-enhanced photoacoustic spectroscopy (QEPAS) technique was also developed for environmental and biomedical measurements [7, 8]. Nevertheless, the high modulation frequencies used in QEPAS may represent a problem for multicomponent gas mixtures containing varying amounts of water vapor such as ambient air, due to the strong influence of water vapor on the molecular vibrational-translational (V-T) relaxation times. Other spectroscopic methods such as open path Fourier transform infrared spectrometry (FTIR) and differential optical absorption spectroscopy (DOAS) have been reported for atmospheric molecule detection [9, 10]. But the minimum detection limits (MDLs) of FTIR usually exceed the requirements for high sensitivity measurements of the atmospheric species. The main disadvantage of the DOAS system is that its spatial resolution is rather poor with a path

The technique based on tunable diode laser absorption spectroscopy (TDLAS) is an effective method to measure gas mixing ratios and multiple parameters with high selectivity, high sensitivity, high precision, and high response time [11–18]. Especially, with the development of multi-pass absorption cells, the effective optical path length can be extended from a few meters to several hundred meters; the sensitivity is significantly improved [19–21]. In order to further improve the signal-to-noise ratio (SNR), the wavelength modulation spectroscopy (WMS) technology with second harmonic (2f) signals is usually employed in the TDLAS

The first commercial TDLAS gas sensor was introduced on the market in 1995 using the trademark laser gas by Norsk Elektro Optikk Company. Over the past decades, TDLAS has been extensively investigated potentially as an effective method to measure multiple gas parameters and is widely used in various areas such as gas mixing ratio detection, vehicle emissions, gas exhaust temperature monitoring, carbon isotope measurements, and so on [22–37]. Now NEO Monitors is one of the world leading suppliers of the TDLAS-based gas analyzers and dust monitors. Its products are widely used in the field of industrial process control and emission monitoring; nearly 6000 sets of laser gas analyzers were installed in more than 40 countries and regions in the world currently. We are also developing instruments based on TDLAS technology to satisfy the needs of environmental monitoring and industrial process control in China. Figure 1 shows several pictures of the gas sensors developed by our research team. In this chapter, we will briefly present several kinds of gas sensors developed by our research group for various field applications, which could expand from environment and

and for a general environmental management [2].

208 Green Electronics

length generally greater than 1 km.

system to measure the gas concentration.

public safety to medical science.

Figure 1. Several pictures of the TDLAS system developed by our research team.

2. Basic principles of TDLAS
