Measuring Transmittance in Rubidium Vapor Cells Using an Extremely Weak Coherent Field of Light

The absorptive properties of the atomic transition in the Rb⁸⁵D2 (A) line (5S_1/2-5P_3/2) at 780.0 nm are measured by using a continuous tunable diode laser and a CCD detector in Rb vapor cell under the condition of extremely weak incident intensity. When the intensity of incident light decreases from 420 pW/cm² to 0.03 pW/cm², the transmittance is found to be independent of the incident intensity within our experimental accuracy.