Reducing PM-to-AM conversion and the light-shift in laser-pumped, vapor-cell atomic clocks

The alkali vapor-cell atomic clock, either conventional or coherent-population-trapping (CPT), offers one of the most viable approaches to an ultraminiature device. Unfortunately, this atomic clock suffers from two laser-induced noise processes, laser phase-noise (PM) to amplitude-noise (AM) conversion and ac-Stark shift fluctuations. Here, we demonstrate a method to circumvent these problems in a passive and miniaturizable manner based on pressure-broadening of the relevant optical transition. For this purpose, we have constructed a conventional atomic clock employing Rb⁸⁷ vapor confined with 100 torr of N₂. In this high-pressure environment, both PM-to-AM conversion efficiency and the ac-Stark shift are reduced. Though we employ a "phase-noisy," cleaved-facet diode laser, and lock the laser frequency to the pressure-broadened 1.6 GHz D₁ absorption line of Rb, we obtain excellent short-term frequency stability (i.e., σ_y(τ) = 1.8x10^-12/τ¹2/). Moreover, since a single resonance cell generates locking signals for both the laser wavelength and the crystal oscillator, the atomic clock has real potential for miniaturization.