Straightforward Measurement of Saturation Intensity of Doppler-Broadened Transitions: Application to Rubidium Vapour

The commonly quoted saturation intensity of rubidium (Rb) for D2 transitions is 16.7
$\mathrm{W}.\mathrm{m}^{-2}$
. This value, however, is only accurate for the cycling transition, which renders the Rb atom to resemble a two-level system. In practice, the effective saturation intensity is influenced by various factors, such as the polarization of light, optical pumping, and atom collision which can redistribute the population. As a result, it is crucial to experimentally determine the effective saturation intensity. In this context, we demonstrate a novel yet simple experimental method to measure the effective saturation intensities of Doppler-broadened transitions in atomic vapours. Our method only requires on-resonance absorbance data for various input laser intensities, which can be obtained with a tunable CW laser without the need for precise frequency locking. We applied this method to measure the effective saturation intensities of all four Doppler-broadened D2 transitions in natural rubidium (Rb) vapour (27.8
^%87
Rb and 72.2 %
${}^{\text{8}} {}^{\text{5}}Rb$
) at
$r={35}^{\circ} \mathrm{C}$
. The results show that the effective saturation intensities for Rb range from 0.752 to 16.8
$\mathrm{W}.\mathrm{m}^{-2}$
. Notably, for certain transitions (specifically, repump transitions), the effective saturation intensities were smaller than 16.7 W.
$\mathrm{m}^{-2}$
(up to an order of magnitude), with potentially important implications for optical pumping in Doppler-broadened transitions.