Laser-pumped atomic clock exploiting pressure-broadened optical transitions

The alkali-vapor-cell atomic clock, of either the conventional or the coherent population trapping type, offers one of the most viable approaches to making ultraminiature and chip-scale devices. Unfortunately, this atomic clock suffers from two laser-induced noise processes: conversion of laser phase noise (PM) to amplitude noise (AM) and ac-Stark-shift fluctuations. Here we demonstrate a method for circumventing these problems in a passive and miniaturizable manner based on pressure broadening of the relevant optical transitions. For this purpose we have constructed a conventional atomic clock employing Rb87 vapor confined with 100 Torr of N2. In this relatively high-pressure environment, both PM-to-AM conversion efficiency and the ac-Stark shift are reduced. Though we employ a phase-noisy, single-mode 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., σy(τ)=1.8x10-12/τ1/2]. Moreover, as a single resonance cell generates locking signals for both the laser wavelength and the crystal oscillator, the atomic clock has real potential for miniaturization