Detection of Trace Hydrogen by Coherent Anti-Stokes Raman Spectroscopy

Hydrogen is an emerging energy source requiring new technologies to reliably measure its emissions. This paper presents work on an optical sensing technique using coherent anti-Stokes Raman spectroscopy (CARS) to measure low concentrations of hydrogen in air as a step towards an emissions monitoring system. The setup uses a pulsed Nd:YAG laser and a Raman shifter cell to produce a pair of nanosecond beams (pump at 532 nm and Stokes at 683 nm) having a frequency difference matched to the vibrational energy shift of hydrogen. We employ a polarization scheme, where the polarization of the pump is rotated relative to the Stokes beam, to suppress the non-resonant CARS background (from the air) to enhance measurement signal-to-noise ratio. In addition to the measurement cell, a CARS reference cell is used to normalize against laser energy and beam profile fluctuations. Results of Raman cell pumping and polarization enhancement are presented. We also present a signal level study versus hydrogen concentration showing the ability to measure at levels below 10 parts-per-million (ppm). An outlook for future system improvements is also presented.