Diode laser linewidth and phase noise to intensity noise conversion in the gas-cell atomic clock

In order to clarify the mechanism of increased laser relative intensity noise (RIN) in the diode laser pumped gas-cell atomic clock, we have performed experiments examining the influence of laser linewidth, Δv_L, on RIN. Our measurements are consistent with the phase noise (PM) to intensity noise (AM) conversion theory: laser phase fluctuations induce fluctuations in the absorption cross section of the atomic vapor and these yield fluctuations in the transmitted intensity of the laser. In the experiments, transmitted RIN of a beam from a 100 kHz linewidth diode laser was compared with that from a 60 MHz linewidth diode laser after each was passed through a cell containing natural rubidium vapor. We present some results of computational theory showing that for laser linewidths less than the atomic dephasing rate of the atomic vapor, RIN is proportional to √(Δv_L); however, for laser linewidths greater than the atomic dephasing rate RIN is a decreasing function of Δv _L