Fluctuations in the process of resonant atomic absorption: Laser phase-noise to amplitude-noise conversion

摘要

In the weak-field limit, resonant absorption is viewed as a passive process: an optical field impinges on an atom, and within some cross-sectional area the atom has a high probability for absorbing the radiant energy. Absorption, however, is a dynamic process. Consequently, though a singlemode laser is highly monochromatic, the field's phase noise (i.e., quantum noise) generates fluctuations in the atom's absorption cross section. Laser phase noise (PM) thereby gives rise to absorption cross-section noise, and hence fluctuations in the medium's transmitted light intensity (AM). Following a brief overview of the PM-to-AM conversion process, we consider the role of collisions on PM-to-AM conversion efficiency in the weak-field regime. Specifically, the relative-intensity-noise of a diode laser, tuned to the Rb D_1 transition, was measured after it passed through a Rb~(87)/N_2 vapor. Varying the N_2 pressure, we found that rapid collisional dephasing decreased the efficiency of PM-to-AM conversion. Examining the Rb~(87) hyperfine transition lineshape as a function of N_2 pressure, we then found that pressure-broadening increased the transition's signal-to-noise ratio when limited by the PM-to-AM conversion process.