Mid-Infrared Tunable Laser-Based Broadband Fingerprint Absorption Spectroscopy for Trace Gas Sensing: A Review  
[PERSON]  
1 State Key Laboratory of Precision Measuring Technology and Instruments, Tianjin University, Tianjin 300072, China; [PERSON]L_ADDRESS] (S.Z. ); [EMAIL_ADDRESS] (K.T.) 2 Key Laboratory of Micro Opto-electro Mechanical System Technology, Tianjin University, Ministry of Education, Tianjin 300072, China; [EMAIL_ADDRESS]  
3 Key Laboratory of Advanced Electrical Engineering and Energy Technology, Tianjin Polytechnic University, Tianjin 300387, China; [EMAIL_ADDRESS]  
4 School of Mechanical Engineering, Hebei University of Technology, Tianjin 300130, China  
5 Correspondence: [EMAIL_ADDRESS]; Tel: +86-138-205-95185  
in this situation: (1) reduce the noise in the signal; or (2) increase the absorption. The former can be achieved by using modulation technique, e.g., wavelength modulation spectroscopy (WMS) and frequency modulation spectroscopy (FMS), with a typical sensitivity of absorbance \(\sim\)10\({}^{-5}\). The latter can be obtained by placing the gas inside a cavity in which the light passes through multiple times to increase the interaction length, e.g., multiple-pass or long path absorption cells, and cavity enhanced absorption spectroscopy (CEAS) [12; 13]. Both ways of reducing noise and increasing absorption can be applied to a same system, e.g., cavity enhanced wavelength modulation spectrometry [13] and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) [14; 15]. FMS is a method of optical heterodyne spectroscopy capable of rapid measurement of the absorption or dispersion associated with narrow spectral features. The absorption or dispersion is measured by detecting the heterodyne beat signal that occurs when the FMS optical spectrum of the probe wave is distorted by the spectral feature of interest. Recently, dispersion spectroscopy, namely chirped laser dispersion spectroscopy (CLaDS) [16] or heterodyne phase sensitive dispersion spectroscopy (HPSDS) [17], has attracted attention for its immunity to optical intensity changes and superb linearity in the measurement of concentration. CEAS and its new versions, e.g., cavity ring-down spectroscopy (CRDS) [18], broadband cavity ring-down spectroscopy [19], phase-shift cavity ring-down spectroscopy [20], integrated cavity output spectroscopy (ICOS) [21], and continuous wave cavity enhanced absorption spectrometry (cw-CEAS) [22], provide much larger pathlength enhancement by using a resonant cavity, and thus have highly sensitive absorbance \(\sim\)10\({}^{-7}\)-10\({}^{-9}\). Practically, the simplest and most promising method to enhance the signal of trace gas detection is to perform the detection at wavelengths where the transitions have larger line strengths, e.g., using fundamental rovibrational bands or electronic transitions. The fundamental rovibrational bands of a vast majority of gaseous chemical substances, located at the mid-infrared spectral region (MIR, 2.5-25 \(\upmu\)m), are due to the transitions of molecular rovibrational energy states. In general, these bands have stronger line strengths than the overtone and combination bands typically used in the visible and near-IR regions. The MIR spectrum depends on the physical properties of the molecule such as the number and type of atoms, the bond angles, and the bond strength. Thus, the MIR spectrum is uniquely characterized by highly specific spectroscopic features and is considered the molecular signature, which allows both the identification and quantification of the molecular species, especially suitable for larger molecules, e.g., volatile organic compounds (VOCs) [23]. VOCs are gaseous organic chemicals at the conditions of normal temperature and pressure (NTP, 293.15 K and 101.325 kPa). There are several hundred types of VOCs, some of which are dangerous to human health or cause harm to the environment. VOCs monitoring has attracted attention for long time.