## 5 Discussion and Conclusions  
In this paper, we reported the design and performance of a \({}^{13}\)CO\({}_{2}\)/\({}^{12}\)CO\({}_{2}\) isotope ratio sensor based on the TDLAS technique. The sensing system consisted of a mid-infrared ICL, a mid-infrared MCT detector, and an MPGC with a physical size of \(20\times 7.6\times 10.5\) cm\({}^{3}\). The ICL with a center wavelength of 4.3 \(\upmu\)m covered the carbon isotope absorption line pairs located at 2314.36 cm\({}^{-1}\) and 2315.10 cm\({}^{-1}\). In order to achieve high-precision measurements of carbon isotope ratio, a high-precision temperature control system for MPGC was fabricated. In the indoor experiment, the response linearity of the proposed system reached 0.9996%, and the MDL was as low as 0.0139% when the integration time was 92 s. Finally, the sensor was combined with a gas-liquid separator to measure the \({}^{13}\)CO\({}_{2}\)/\({}^{12}\)CO\({}_{2}\) isotope ratio of CO\({}_{2}\) gas extracted from water. In conclusion, although the performance of the proposed sensor can meet the requirements of NGH exploration, this was only achieved in laboratory conditions, which suggests the proposed prototype of the \({}^{13}\)CO\({}_{2}\)/\({}^{12}\)CO\({}_{2}\) isotope sensor has potential application in deep-sea natural gas hydrate exploration. Further research is needed before the sensors can really be applied in NGH exploration. In terms of measurement technique, as 2f/1f WMS technique had a negligible absorption-independent background and was immune to the absorption-independent systematic losses. Thus, the 2f/1f WMS technique is a possible approach to reduce noise from mechanical vibration and to make the sensor more robust when applied in NGH exploration. In terms of sensors, cooling devices, antistatic devices, and shock absorption/isolation devices should be investigated. In terms of loading equipment, a cylindrical deep-sea hull that can withstand tens of mega-pascals on the sea floor is needed, as well as a mechanical fastening device and electrical interface cable. Compared with the high-precision carbon isotope measured methods of mass spectrometry, although the mass spectrometer can achieve MDL of ppt level for single chemical elements and ppm level for isotopes, the requirements of a high vacuum and high stability make it more suitable for ultra-high-performance detection in a laboratory environment, whereas it is not suitable for application in complex environments. For chromatographic analysis, its advantages lie in the detection of a variety of chemical components, and the MDL can reach to ppm level; however, it seems inadequate to meet the MDL requirement of ppb level. As far as flame ionization is concerned, it is responsive to almost all organic materials, and the response is proportional to the number of carbon atoms; however, it is insensitive to inorganic substances such as CO\({}_{2}\), H\({}_{2}\)O, CS\({}_{2}\), etc. Based on the above discussion, the mid-infrared TDLAS technique can realize the MDL of ppb level for single gas components, as well as tens of ppm for isotopes, and it can be applied in complex environments. In many practical applications, the ICL is subjected to low-frequency environmental perturbations such as thermal fluctuations and mechanical vibrations, resulting in the laser wavelength fluctuating slightly. A laser wavelength locking (LWL) technique is required to keep the operating point of the ICLconstant, thereby reducing the fluctuation. Thus, the precision of the proposed sensing system can be further improved by using the LWL technique, which requires a redesign of the system structure. The stability of the system can be further improved by adding a damping and waterproof device to strengthen the antijamming capability. [PERSON] performed the research. [PERSON], [PERSON], [PERSON], and [PERSON] conceptualized the research and wrote the paper. This research was supported by the National Key R&D Program of China (No. 2016 YFC0303902, No. 2018 YFC1503802), the National Natural Science Foundation of China (No.