## 3 System Configuration
### New Types of Gas Cells  
Recently, new types of gas cells have been developed to achieve long optical path in a smaller volume, including modified MPCs [111; 112; 113; 114; 115; 116], circular multi-reflection (CMR) cells [117; 118; 119; 120; 121; 122; 123; 124], and HWG [69; 125; 126; 127; 128; 32]. [PERSON] and [PERSON] reviewed the progress in modified MPCs and CMR cells and compared them in terms of optical pathlength (OPL), volume, and path-to-volume ratio (PVR) [116]. HWG-based gas cells have been discussed in detail in books and reviews [129; 130; 131]. We compared the main features among the modified MPCs, CMR cells and HWG, as shown in Table 1.  
There are three categories of HWG employed in spectroscopic gas sensing, namely Ag/AgI-coated HWG (Ag/AgI-HWG), photonic bandgaps HWG (PBG-HWG), and iHWG [131], and the comparison  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{1}{c}{**Type**} & **Ref.** & **OPL/m** & **Volume/L** & **PVR/m/L** & **ADs** & **DADs** \\ \hline \multirow{8}{*}{Modified} & 2 mirrors & [111] & 22 & 0.55 & 40 & (c) (d) (e) & 2��⃝1 \\ \cline{2-7}  & 6 mirrors & [112] & 314 & 1.25 & 251.2 & (b) (c) (e) & 1⃝1 \\ \cline{2-7}  & 3 mirrors & [113] & 1.46 & 0.33 & 4.4 & (c) (d) & 1⃝1 \\ \cline{2-7}  & 2 mirrors & [114] & 57.6 & 0.225 & 256 & (a) (b) (d) (e) & 2⃝1 \\ \cline{2-7}  & 2 mirrors & [115] & 26.4 & 0.28 & 94.3 & (b) (c) (e) & 2⃝1 \\ \cline{2-7}  & 6 mirrors & [116] & 32.4 & 0.48 & 67.5 & (c) (d) (e) & 1⃝1 \\ \hline \multirow{8}{*}{CMR cells} & 1 mirrors & [117] & 1.05 & 0.078 & 13.5 & (c) (d) (e) & 1⃝1 \\ \cline{2-7}  & 6 mirrors & [118] & 3.1 & 0.024 & 129.2 & (a) (c) (d) (e) & 1⃝1 \\ \cline{2-7}  & 1 mirrors & [119] & 2.16 & 0.04 & 54 & (a) (c) (d) (e) & 1⃝1 \\ \cline{2-7}  & 1 mirrors & [120] & 4.08 & 0.04 & 102 & (a) (c) (d) (e) & 1⃝1 \\ \cline{2-7}  & 6 mirrors & [121] & 0.69 & 0.013 & 53.1 & (a) (c) (d) (e) & 1⃝1 \\ \cline{2-7}  & 1 mirrors & [122] & 12.24 & / & / & (a) (c) (d) (e) & 1⃝1 \\ \cline{2-7}  & 1 mirrors & [123] & 9.9 & 0.05 & 198 & (a) (c) (d) (e) & 1⃝1 \\ \cline{2-7}  & 65 mirrors & [124] & 10 & 0.14 & 71.4 & (a) (c) (d) (e) & 1⃝1 \\ \hline \multirow{8}{*}{HWGs} & Ag/AgI-HWG [32; 69] & 5 & 0.004 & 1250 & (a) (d) (e) & 1⃝1 \\ \cline{2-7}  & iHWG & [126] & 0.25 & 0.001 & 250 & (a) (d) (e) & 1⃝1 \\ \cline{2-7}  & iHWG & [63] & 0.075 & 0.0003 & 250 & (a) (d) (e) & 1⃝1 \\ \cline{2-7}  & PBF-HWG & [127] & 1 & \(5\times 10^{-6}\) & 200,000 & (a) (d) (e) & 1⃝1 \\ \cline{2-7}  & PBF-HWG & [128] & 0.08 & \(4.5\times 10^{-7}\) & 177,778 & (a) (d) (e) & 1⃝1 \\ \hline \multicolumn{7}{c}{Advantages index} & \multicolumn{7}{c}{Disadvantage index} \\ \hline \multicolumn{7}{c}{(a) easy configurations, fewer than three components} & \multicolumn{7}{c}{1⃝ multiple components (\(\geq\)3 optical elements) and requiring adjustment} \\ \multicolumn{7}{c}{(b) ultra-long pathlength (\(>\)50 m)} & \multicolumn{7}{c}{2⃝ large volume (\(>\)0.1 L)} \\ \multicolumn{7}{c}{(c) keeping focal properties, low-aberrations} & \multicolumn{7}{c}{3⃝ hard to align, sensitive to vibrations or input conditions} \\ \multicolumn{7}{c}{(d) compact structure} & \multicolumn{7}{c}{4⃝ spot diffusion introduced by aberrations} \\ \multicolumn{7}{c}{(e) suitable for MIR} & \multicolumn{7}{c}{5� slow gas exchanging} \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of different types of gas cells.  
of them is as shown in Table 1. HWGs work as both an optical waveguide and gas transmission cell that provide an extended OPL yielding high sensitivity measurements [69]. It is worth noting that filling PBG-HWG with analyte gas for sensing is difficult owing to the considerable back-pressure building up in the hollow structure. HWGs are ideal candidates for gas cells due to their high PVR and ability to transmit light at a fairly wide wavelength range, which means building a MIR sensor platform is feasible.