Laser frequency standards

Self- and N2- broadening of CO in the low-pressure regime

“…5(B). The observed small shifts in low-pressure regime agree with previous studies [8] and coincide with the case where Γ < θ ku.…”

“…The Doppler-broadened spectra α 0 (ν) are described with line intensity from HITRAN2016 [3] and line broadening parameters from Predoi-Cross et al [21] and they are sharing the same line center with the measured Lamb dips.The fitted results of the line widths, as well as the power saturation parameter S for both self-and N 2 -broadening of CO R (9), are listed in Table 1 and Table 2. As been characterized in previous studies of Doppler-free spectroscopy [8,11,14,22], the elastic scattering of particles dominates the behavior of the line width. Therefore, we focus on the direct contribution of the elastic collision process to the collisional scattering of molecules at low pressures of around 1 mTorr.…”

“…A similar nonlinear character of the pressuredependent line shift would also be expected. The effect was observed in the early study of a nonlinear methane absorber around 3.39 µm using a He-Ne laser [8]. They reported an anomalous sharp decrease in the shift of the Lamb dip of methane to less than 10 Hz/mTorr, which was attributed to the decrease in the number of scattered molecules in the interaction and the increasing attraction forces.…”

“…The collisional shift in the low-pressure regime can decrease by more than one order of magnitude compared to that observed in Doppler-limited measurements. That is essential for gas laser frequency stabilization [8,9,20]. Shifts of the Lamb-dip centers were determined through the beating frequency referenced to the frequency comb in this work.…”

“…Early studies of nonlinear narrow transitions were mostly carried out with gas lasers in a saturated absorption cell inside the cavity. For instance, a CH 4 cell with a He-Ne laser around 3.39 µm [8,9], and the CO 2 absorption with a CO 2 laser at 9.6 and 10.6 µm [10,11], SF 6 and/or OsO 4 absorption with a CO 2 laser at 10.6µm [12,13]. One limitation of these measurements besides the stability of the laser frequency is the interference between the strong field and the nonlinear absorption since the gas cell is inside the laser cavity.…”