A new way to enhance sensitivity in stimulated Raman spectroscopy: Hollow-core photonic crystal fibers.

Abstract

Stimulated Raman Scattering gain/loss (SRS) processes have a very low efficiency, so, for spectroscopic applications some combination of pulsed laser sources is normally used. Although continuous wave SRS spectroscopy was demonstrated in 1978 by Owyoung et al.1 getting the highest resolution up to now, shortly after that, he developed the quasi-continuous SRS technique which uses a high peak power pulsed pump laser, which is the best compromise achieved between resolution and sensitivity for these gas-phase spectroscopies. A new way to enhance sensitivity without sacrificing resolution, apart from using multipass refocusing cells and cavity enhanced techniques, is to use hollow-core photonic crystal fibers2 (HCPCF), which are very suitable due to the tight confinement of light and gas inside the core and the long interaction length. In our set-up for cw-SRS loss spectroscopy, a cw-single mode Ar+ probe laser and a cw-single mode tunable dye pump laser (both lasers are stabilized in frequency) interact with a gas sample. Whenever their frequency difference matches that of a Raman-allowed transition of the sample, the probe laser undergoes a loss of power. In our experiment, the gas is placed in a cell formed by a 1.2m length and 4.8µm core diameter HCPCF. We have demonstrated that this new technique enhances the sensitivity of SRS experiments around 6000 times over that of single focus cw-SRS. We will show several examples such as the Q-branch of the ¿1 and 2¿2 components of the Fermi dyad CO2 at 1388 and 1286 cm-1 respectively, or the Q-branch of O2 at 1555 cm-1.