Diode laser spectroscopy using a calibration free phasor decomposition approach to RAM nulling

Tunable diode laser spectroscopy (TDLS) detection schemes have been widely investigated [1,2] in the near-IR due to the presence of overtone absorption lines in the species of interest and the availability of lasers which operate in the region. However, overtone lines are much weaker than their fundamental absorption line counterparts which, for many species, lie in the Mid-IR. Traditionally the 3-5μm wavelength region has been addressed by lead-salt lasers, but this technology does not lend itself easily to the implementation of TDLS modulation schemes. This paper presents an investigation of TDLS measurements using a system based on difference frequency generation (DFG), in periodically poled lithium niobate (PPLN), which has been used to address fundamental absorption lines of methane (CH4) in the 3.4μm region