Heterodyne Raman dispersion spectroscopy with harmonic demodulation for hydrogen detection

A heterodyne scheme for stimulated Raman dispersion gas spectroscopy with phase-sensitive harmonic measurement is demonstrated. It is accomplished by optically manipulating the Raman transition through pump modulation and harmonically extracting the associated change on the beatnote generated by the sidebands of a Stokes beam. Theoretical formulation describing the spectral signal is developed and verified experimentally. Experiments on a 5-cm-long, 700-nm-diameter optical nanofiber have demonstrated a noise equivalent concentration of 24 ppm with 99 s integration time and a dynamic range of four orders of magnitude for hydrogen detection. This new approach provides a sensitive spectroscopic tool with the potential for widespread gas sensing applications.