The fundamental, overtone, and difference bands are mostly concentrated in the infrared area, which features a lot of absorption lines. Figure 2 shows the linestrength of various infrared absorption transitions for a few common species in the range of 400–10,000 cm⁻¹ at 296 K. Therefore, it is important to select an appropriate absorption line for trace gas detection. Firstly, the absorption lines should be sufficiently independent to avoid interference from other gases that are abundant in the atmosphere, such as H₂O and CO₂ [12]. Furthermore, the line strength should be appropriate to improve the detection sensitivity of the TDLAS system [13, 14]. The line strength of the gas absorption spectroscopy represents the absorption capacity of a specific wavelength, which is related to the distribution of low-energy particles and the transition probability of the gas molecules. Therefore, the absorption line strength is most affected by temperature. Moreover, the lineshape function is highly significant when simulating the profile of the gas absorption spectroscopy, and different lineshape functions might reflect different broadening mechanisms. The Doppler effect of molecular motion dominates spectroscopy broadening in high-temperature and low-pressure conditions, and it is commonly characterized by the Doppler line profile. As for low-temperature and high-pressure conditions, the collision of molecular motion plays a dominant role and the Lorentzian line profile is generally used to describe the line broadening [15]. In most cases, the effects of Lorentz broadening and Gaussian broadening must be evaluated at the same time, and the Voigt profile is the convolution of the Gaussian and Lorentzian profiles [16].

Databases such as GEISA, HITEMP, and HITRAN provide detailed spectral parameters [17–19], covering more than 709,000 transitions for most atmospheric species. The individual spectral line data including line strength, wavenumber, and lower state energy, are suitable for analysis and selection of absorption lines. Furthermore, the relevant data are corrected and supplemented by related researchers, which helped to further improve the accuracy of gas detection [20].

As the light source, the linewidth of the laser should be narrow. It should also have the characteristics of stable performance, compact structure, and high output power. Therefore, the main applications for trace gas monitoring in the laboratory and industrial settings are distributed feedback laser diode (DFB-LD), vertical cavity surface emitting lasers (VCSEL), and the mid-infrared lasers including interband cascade lasers (ICL) and quantum cascade lasers (QCL), the characters are shown in Figure 3.

DFB-LD is a type of useful tunable diode laser that is operated on the principle of grating mode selection to get a specific wavelength depending on the Bragg gratings contained in the laser structure. The effective refractive index of the grating will be changed by temperature, leading to the change of output wavelength. Therefore, the tuning range of the DFB-LD is closely related to the effective refractive index of the grating material. In 1973, Nakamura et al. developed the first DFB laser, which achieved a narrow linewidth output near 830 nm [21]. However, the early GaAs DFB lasers have extremely high threshold conditions and need to operate at extremely low temperatures, making it difficult for large-scale applications [22]. In addition to GaAs, other III-VI group semiconductor materials, such as InP, can achieve longer output wavelength and are successfully achieved operation under low-temperature conditions (<0°C) [23]. Sakai et al. exploited the separate-confinement heterostructure to obtain a 1.5-μm continuous wavelength output that can operate continuously between −20°C and 58°C, with a threshold current as low as 50 mA at room temperature [24, 25]. Since then, the DFB laser’s application potential as a light source used for the optical sensors has been discovered. Morris et al. used graded-index separate-confinement heterostructure to achieve a 760 nm continuous single-mode tunable laser with a critical current of 25 mA, temperature tuning rate of 0.06 nm/°C, current tuning rate of 0.0075 nm/mA, and the tunable wavelength range is 4.2 nm, and successfully used for O₂ detection [26]. Doussiere et al. designed a novel structure that integrates a relatively short distributed-Bragg grating near the output facet and a high reflectivity facet coating on the rear facet [27]. And they achieved the single-mode power over 200 mW at case temperature up to 60°C is consistently obtained for current below 300 mA.

DFB-LD is usually modulated by the current and temperature to control the range of output wavelength. Its tuning efficiency is closely related to the frequency of the driving current and the material of the laser diode, and it will have a great impact on the signal-to-noise ratio (SNR) and residual amplitude modulation. Sun J et al. found that tuning efficiency is affected by the different materials inside the DFB-LD, which is a robust parameter guiding us to choose favorable DFB-LD [28]. Since the injection current has a great influence on the internal temperature of the DFB-LD, which leads to the fluctuation of output wavelength range, suppressing the change of the internal temperature of the DFB-LD is beneficial to improve the sensitivity of the gas detection system [29]. Compared with traditional LD sources, the biggest advantage of DFB-LD is that it can be used in complex industrial environments and on-site detection. Wei Y et al. employed the 1.62 μm fiber-coupled DFB diode laser as the light source to achieve a ppm-level high sensitivity detecting system for the multi-point ethylene concentration detection, meeting the needs of fire warning in coal mine goafs [30]. Wang Z et al. chose the absorption line of water vapor at 1,368.5597 nm for detection and achieved the lowest detection limit of 790 ppb [31]. Using a single DFB diode laser emitting at 2.33 μm, Shao L et al. presented a spectrometer for constantly measuring atmospheric CO and CH₄ [32]. According to the Allan variance, the respective minimum detection limits for CO and CH₄ are 0.73 and 36 ppb at 122 s and 137 s.

Although the DFB-LD may provide extremely narrow laser output, the tuning range is limited; nevertheless, VCSEL compensates for this shortage. The VCSELs have good wavelength tunability and can achieve a tuning range of 10 cm⁻¹ at a modulation frequency around 1 MHz. Soda H et al. proposed the concept of vertical cavity surface emission for the first time and obtained dynamic single longitudinal mode semiconductor lasers by shortening the cavity length [33]. And they successfully prepared the first VCSEL operated at 77 K, but its threshold current is as high as 0.9 A. Later, they further realized a pulsed VCSEL made of GaAs/GaAlAs. The threshold current under 77 K conditions was reduced to 350 μA, but the threshold current under room temperature conditions was 1.2 A [34]. K. Iga et al. developed a VCSEL that can operate continuously at room temperature in 1989. Since then, the advancement of VCSEL has ushered in a major turning point, with the technology progressing towards long wavelengths. The advent of the devices continuously operated at the wavelength of 1,310 nm and 1,550 nm under room temperature has greatly enriched the wavelength detection range of VCSELs [35, 36].

With the maturity of VCSELs, the excellent characteristics have become the focus of researchers in the field of gas sensing. Wang J et al. used the VCSEL with a wide current-tuning frequency range as the light source to detect oxygen in a high-pressure environment [37]. The wider tuning range enables VCSELs to obtain more spectral information and then realize the simultaneous detection of multiple gases with a single light source and the correction of gas concentration. Chen J et al. employed VCSEL to achieve simultaneous detection of CO and CH₄ at 2.3 μm at an effective absorption path length of 20 cm, with detection accuracy reaching the ppm-level [38]. Wang Y et al. used a VCSEL with output wavelengths over the range from 1,682.7 nm to 1,685.7 nm achieving a detectable concentration of 300 ppm with relative errors below 5% is sufficient for successful early warning of propane leaks in the petrochemical and oil-gas storage and transportation industries [39]. At the same time, VCSELs have well-developed uses in the near-infrared region for detecting the concentrations of CO, CO₂, CH₄, and other typical gas. Chen J et al. achieved the first 2.3 µm VCSEL-based CO sensor. They employed an absorption cell with a 10 cm optical path length that can detect CO in 1 s with a ppm resolution [40]. And with the development of miniaturization, the VCSELs with a micro-electro-mechanical system(MEMS) capable of spans of 2–110 nm and repetition rates of 10–1,000 kHz, which successfully applied to the absorption spectroscopy detection of H₂O and HF in the range of 1,321–1,354 nm [41].

Compared with the near-infrared band, the fundamental band of most gas molecules is mainly concentrated in the mid-infrared band. Therefore, many researchers are considering the use of tunable diode lasers in the mid-infrared band for trace gas detection to increase the SNR and sensitivity of the system. Theoretically, Lead salt diode lasers made from IV-VI semiconductor materials can operate in the 3–30 μm spectral region [42]. However, due to the structural of IV–VI materials, Lead salt diode lasers must work in cryogenic temperatures, and problems such as beam divergence, multi-mode competition, and low output power are not suitable for gas absorption spectroscopy detection [43]. The temperature properties and output light intensity of the telluride-based semiconductor laser are better than those of the lead salt laser, however, it is difficult to achieve a wavelength range of more than 5 μm [44, 45]. The appearance of Interband Cascade lasers (ICL) and Quantum Cascade lasers (QCL) ameliorates these problems. Different from the inter-band transition process of traditional semiconductor lasers, the laser generation process of QCLs only involves the conduction band and the electrons in it to achieve the inversion of the number of particles between different quantum well energy levels and radiate laser light. Based on this idea, Faist J et al. designed the first quantum cascade laser in 1994, with an output wavelength of about 4.2 μm, an output power of only 8.5 mW under 10 K conditions, and a threshold current density as high as 14 kA/cm² [46]. In the following 20 years, the rapid development of QCLs has been achieved in terms of operating temperature, output performance, and wavelength coverage. In 1997, a DFB-QCL capable of working at room temperature was successfully developed. The output wavelengths were 5.4 and 8 μm respectively, and the former can achieve a peak power of 60 mW at 300 K [47]. In 2001, Faist J et al. further realized the QCL with an output wavelength of 9.1 μm, which operated continuously at room temperature, with an output power of 17 mW at 292 K and a maximum continuous operating temperature of 321 K [48].

With continuous improvement, QCLs have gradually become the most competitive light source in the mid-to-far infrared band and have shown extremely high application value in trace gas detection. Although long-wavelength devices still require low-temperature cooling, the continuous output within 16 μm could be achieved at room temperature, through the design of quantum wells [49], which are suitable for monitoring the greenhouse gases like CH₄, NO, N₂O. Kasyutich et al. used a continuous wave DFB-QCL and a single-pass absorption cell with an optical path length of only 21 cm to detect CO, N₂O, and NO, with detection limits of 2.8, 0.6, and 2.7 ppm, respectively [50]. Using a novel compact MGC, Ren W et al. demonstrated the construction of a single-QCL based absorption sensor for the simultaneous detection of atmospheric CH₄ and N₂O at 7.8 μm. The detection limit of 5.9 ppb for CH₄ and 2.6 ppb for N₂O was achieved respectively, at the 1-s averaging time [51]. Maity et al. selected the absorption line of C₂H₂ at 1,311.76 cm⁻¹ and achieved the lowest detection limit of the ppb level within the integration time of 110 s [52].

The electrons of ICLs could transition between the conduction and valence band, enabling semiconductor materials with a narrow bandgap to be used. Therefore, the wavelength range of ICLs is narrower, which makes up for the lack between traditional semiconductor diode lasers and QCLs. Yang et al. produced DFB-ICL with a single-mode lasing wavelength of 3.3 μm by introducing distributed feedback Bragg gratings, and its working temperature reached 175 K [53]. In 2006, under the temperature of 78 K, the continuous output power of ICL was increased to 1.1 W [54]. In the same year, under the adjustment of the temperature controller, the 3.3 μm ICL achieved continuous operation at a temperature of 264 K [55]. In 2013, Nanoplus provided 3–6 μm commercial ICLs. This wavelength region includes fundamental C-H, O-H, and N-H stretch vibrations [56–58] and it covers an important gap in wavelength coverage between diode lasers and QCLs. Song F et al. proposed the sensor system with a 3,291 nm ICL and a multi-pass gas cell with a 16 m optical path length. A limit of detection of ∼13.07 ppb with an averaging time of 2 s was achieved using the DSP-based digital lock-in amplifier (DLIA) and a limit of detection of ∼5.84 ppb was obtained using the LabVIEW-based DLIA with the same averaging time [59].

In general, among the above tunable lasers, it can be found that DFB-LD is suited for large-scale production applications. DFB-LD could operate at room temperature without the use of a complicated cooling device, and the line width could exceed 1 MHz. However, the tuning range of DFB-LD is small, and the absorption line strength in the near-infrared region is lower than mid-infrared, so it is difficult to achieve extremely sensitive detection results. VCSELs are cheap to produce, have a wide tuning range and low thresholds. As mid-infrared laser sources, QCL and ICL could achieve higher detection sensitivity. But these types of lasers are expensive and need to operate at low temperatures. They are therefore suitable for extremely high demands on detection results or for experimental exploration.

In absorption spectroscopy gas sensor detection, in addition to the selection of an appropriate light source, the design of an effective absorption path is also very important to improve the detection sensitivity and SNR of the system. According to the Beer-Lambert law, the effective interaction length of the laser energy and the gas sample has a significant effect on the absorption signal. In the traditional absorption spectroscopy gas sensing technology, a multipath absorption cell is often used to increase the effective absorption optical path. In 1942, White proposed a long optical path absorption cell composed of three concave mirrors with the same radius of curvature and named by his name [60]. Although the White cell achieves a long absorption path through multiple reflections between the mirrors, the design of this three-sided mirror makes the entire device bulky and the mirrors need to be strictly aligned. Herriott simplified the structure of the white cell and designed a new type of multi-optical path absorption cell [61]. The Herriott cell only retains two concave mirrors. The incident light enters the absorption cell through a hole of one mirror, and after multiple reflections in it, it exits from the same hole. Compared with White cell, the structure of the Herriott cell has been greatly simplified, and by adjusting the structure of the two mirrors, an effective absorption path of tens of meters to hundreds of meters can be achieved [62]. Although Herriott cell improves the problem to a certain extent, the structure of this kind of absorption cell is often complex to achieve a long absorption path. Moreover, once the optical path is fixed, the number of reflections cannot be changed and the effective absorption path cannot be flexibly adjusted. This is not conducive to practical engineering applications, so researchers have been exploring the miniaturization of multi-path absorption cells. Through the precise design of the absorption cell, the light spot covers the entire mirror surface as much as possible, although the path of a single absorption is not long, it can finally achieve long optical path absorption through hundreds of reflections [63]. To further improve the detection limit, this kind of multi-reflection long optical path absorption cell is gradually applied to the mid-infrared gas detection system and realizes the detection of C₂H₆, N₂O, and CH₄ at the ppb or even ppt level [62, 64, 65].

Based on the idea of multiple reflections, another cavity enhancement technique that combines the resonant characteristics of the optical resonator with the laser gain characteristics is designed to obtain a long absorption optical path. In 1998, O’Keefe and Engeln almost simultaneously proposed the technique to detect the integration of the light intensity passing through the resonant cavity [66, 67]. The method uses a semiconductor laser as a coherent light source, and uses a cavity mirror with high reflectivity to form a resonant cavity to achieve an extremely long effective absorption optical path, and detects the target gas by detecting the time-integrated light intensity passing through the cavity. Paul et al. proposed an off-axis incident cavity enhancement method considering the sensitivity in different environments [68]. This method effectively suppresses cavity mode noise by exciting more high-order transverse modes in the resonant cavity and has a simple optical structure and stronger adaptability to different conditions, so it is widely used in the field of gas detection. Barry et al. applied the integrated cavity to the study of methane near 1.73 μm using a tunable diode laser and achieved a detection sensitivity of 1.8 × 10^–7 cm⁻¹ for mirror reflectivities of 99.84% [69]. With the maturity of mid-infrared semiconductor lasers, more and more researchers have begun to combine them with cavity enhancement technique. Rao et al. used a quantum cascade laser with a tuning range of 1,601–1,670 cm⁻¹ to detect the concentration of NO₂, and the lowest detection limit improved to 28 ppt [70]. In a recent study, Mhanna et al. used a 3.3 μm DFB-ICL as the light source and obtained an effective absorption path length of 1.39 km through the off-axis incidence. They finally realized the detection of the ppt level of benzene vapor [71].

The method for signal demodulation, as well as the suppression of background interference, plays a key role in enhancing the detection accuracy and sensitivity, in addition to selecting appropriate components for a gas detection system.