We determine the absolute density of a sample of laser-cooled atoms in a two-level system by recording the absorption spectrum of the 85Rb 5S1/2 (F = 3, mf = 3) → 5P3/2 (F′ = 4, m′f = 4) transition. Trapped atoms were prepared in the (F = 3, mf = 3) ground state through optical-pumping techniques. We compare our results with an independent measure of the density that relies on a direct measurement of the number of atoms and size of the atomic sample. We also study the contributions of power broadening, laser line width, and Doppler broadening to the measured absorption spectrum. Our studies suggest that the natural line width (~6 MHz) can be measured to a precision of less than ~50 kHz if the laser line width is measured in real-time with a high-finesse Fabry–Perot cavity. PACS Nos.: 32.70.Cs, 32.70.Jz, 32.80.Pj, 42.62.Fi, 32.70.–n, 32.30.–r
Spatial-domain cavity ringdown from a high-finesse plane Fabry–Perot cavity
