Frequency calibration method for improved precision and stability of direct laser absorption spectroscopy

Tunable diode laser absorption spectroscopy is widely used for detecting trace gases, as it combines good selectivity and sensitivity with noninvasive setups and fast response. Among the multiple implementation possibilities, direct absorption spectroscopy stands out, since it allows for straightforward determination of the number density. The frequency accuracy of the laser tuning is a fundamental limit to the precision of these setups, and in order to improve frequency calibration, a method utilizing variable frequency modulation is presented. It leverages the phase-sensitive behavior of mixing the light intensity signal and the modulation source to establish an absolute frequency reference of the absorption signal. The frequency modulation of laser diode sidebands in a range from 400 MHz to 1.1 GHz are used for frequency calibration without the use of external elements in the optical setup. These fixed frequency beacons are embedded in an algorithm that demonstrates improved performance of direct absorption spectroscopy using this method. The approach is implemented in a setup for molecular oxygen spectroscopy using a VCSEL laser near 394.3298 THz (760.7845 nm).