Characterization of coherent population-trapping resonances as atomic frequency references

A low-cost, potentially compact and robust microwave frequency reference ca n be constructed by use of vertical-cavity surface-emitting lasers and cohe rent population-trapping resonances in Cs vapor cells. Fractional frequency instabilities of 2 x 10(-11)/root tau /s have been achieved with a minimum of 7 x 10(-13) at tau = 1000s. The performance of this device as a functio n of external parameters such as light intensity, optical detuning, and cel l temperature is discussed. The dependence of the dark-line resonance signa l on these parameters can be understood largely by means of a simple, three -level model. The short-term stability depends critically on the optical de tuning, whereas the long-term stability is limited currently by line shifts due to drifts in cell temperature.