Direct single-mode fibre-coupled miniature White cell for laser absorption spectroscopy.

We present the design, setup, and characterization of a new lens-free fibre-coupled miniature White cell for extractive gas analysis using direct tunable diode laser absorption spectroscopy (dTDLAS). The construction of this cell is based on a modified White cell design and allows for an easy variation of the absorption length in the range from 29 cm to 146 cm. The design avoids parasitic absorption paths outside the cell by using direct, lensless fibre coupling and allows small physical cell dimensions and cell volumes. To characterize the cell performance, different H2O and CH4 concentration levels were measured using dTDLAS. Detection limits of 2.5 ppm ⋅ m for CH4 (at 1.65 μm) and 1.3 ppm ⋅ m for H2O (at 1.37 μm) were achieved. In addition, the gas exchange time and its flow-rate dependence were determined for both species and found to be less than 15 s for CH4 and up to a factor of thirteen longer for H2O.