Absorption laser spectroscopy ALS is a powerful technique for qualitative and quantitative studies of atoms and molecules in trace concentrations. As such ALS has applications not only in physics and chemistry but also in biology, environmental monitoring, and medicine. In the later field the analysis of the human breath is very useful for health monitoring and a rapidly expanding field for medical instrumentation, In this connection we developed an optical multi-pass cell based on highly reflecting confocal mirrors, achieving both long optical paths and dense atom space (volume) coverage to obtain high-sensitivity and high-selectivity . The system uses six mirrors, and we demonstrate a path-length of 300 m in a cell of only of 0.5 m in lenght. Different volume fillings and path lengths were achieved by tilting the mirrors with angles ≤ 0.05 radians. Spectrally resolved absorption measurements in the near IR of the greenhouse gases CO2, CO, and CH4 were carried out using a broadband frequency comb Er+ fiber laser beam including Raman shifting in a highly nonlinear fiber to an optical range spanning from 1.5 μm to 1.7 μm. Initially we recorded the absorption spectra of the first overtone rovibrational band for CO2, CO, and CH4 , and in the future we plan to quantify minute concentrations of additional biomarker gases and measure isotope ratios. In the case of methane a signal to noise ratio S/N=120 was obtained, yielding the estimated theoretical sensitivity of 6 ppmv, which can be further improved by optimizing the number of passes. Our optical apparatus is portable and can be used for a wide range of applications, including environmental monitoring, combustion processes, other medical diagnostics, and fundamental atomic and molecular physics studies. This work is supported by the Robert A. Welch Foundation grant No. A1546. The statements made herein are solely the responsibility of the authors.
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