## 5 Conclusions  
This study successfully utilized TLS with mid-IR imaging, introducing a classical DAS modulation concept for the first time to enhance gas leak detection capabilities. A tunable ICL was modulated at 10 Hz across a methane absorption line at 3271 nm. The active DAS-OGI method requires backscattered laser light, so, in this study, a rough aluminum plate was used to simulate a nearly ideal non-cooperative target of 10 \(\times\) 10 cm\({}^{2}\). The detection was performed from a 2-m distance using a fast thermal camera in the mid-IR spectrum, capturing image stacks at 470 Hz for offline processing with application tailored DAS algorithms based on the Lambert-Beer law and HITRAN database.  
The innovative and application-tailored algorithms named DAS-F, DAS-f and DAS-2f are intended to efficiently analyze different features of a gas absorption line, such as the area (F), peak (f) or the second derivative of the absorbance (2f). These algorithms enableda calibration-free methane concentration determination in ppm*m and leak localization for rates as low as 0.25 mL/min, offering superior sensitivity and spatiotemporal resolution compared to conventional leak detection techniques.  
DAS-f and DAS-2f displayed the leak scenario in a single concentration image with a pixel-by-pixel sensitivity of approximately 1 ppm*m, making it possible to distinguish between leak gas and ambient methane. Furthermore, concentration image sequences show the spatiotemporal dynamics of the gas plume at a high contrast-to-noise ratio. This active OGI concept can be transferred to other gases with IR activity in the wavelength range between 3 and 6 \(\upmu\)m by changing the gas-specific laser without changing the camera. These findings could pave the path for future applications, including a real-time testing for leaks during production processes to ensure the functionality and/or legal requirements of products.  
Furthermore, the introduced baseline-free and calibration-free DAS-2f concept within this work demonstrates promising characteristics and potential, including accuracy, precision, 1/f noise rejection, simplicity and computational efficiency. In general, DAS-2f could expand the application scope of convention DAS.  
Conceptualization, T.S., M.B. and J.H.; methodology, T.S., M.B. and J.H.; software, T.S., E.M. and S.R.; validation, T.S., E.M. and S.R.; formal analysis, T.S., M.B. and J.H.; investigation, T.S., E.M. and J.H.; resources, J.H. and J.W.; writing--original draft preparation, T.S.; writing--review and editing, M.B., J.W. and K.S.; visualization, T.S.; supervision, J.H., J.W. and K.S.; project administration, J.W. and K.S.; funding acquisition, J.W. and K.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the DFG SBF 1537/1 Ecosense and Bundesministerium fur Bildung und Forschung (BMBF), 03 HY202G TransHyDE.  
The data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.  
Author [PERSON] was employed by the company Endress+ Hauser Process Solutions (DE) GmbH. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.