By Cnr Ifn
This approach involves observing the target transitions in a nonlinear fashion, by adoption of pump and Stokes laser fields whose frequency detuning matches the vibrational resonance. For the first time this approach is applied in combination with an optical frequency comb to achieve absolute calibration of the frequency axis. The experimental setup measures the Stimulated Raman Loss induced by an intense Stokes laser at 1064 nm on hydrogen molecules housed in 30-m-long multipass cell to enhance the signal. The detection is shot-noise limited and comb-referred, while experimental spectra are fitted with sophisticated line shape models relying on ab-initio quantum-scattering calculations of collisional parameters. In a recent achievement we showed this approach to be able to reach a combined uncertainty of 1.0·10-5 cm-1 (310 kHz) on the Q(1) line of the 1-0 band of H 2 at ≈ 4155 cm-1, improving by 20 times the experimental benchmark and by a factor of 2 the theoretical benchmark. Measurements are planned on other lines of the 1-0 band of H2 as well as on purely rotational S-type transitions.
