Optical-pumping noise in laser-pumped, all-optical microwave frequency references

We demonstrate that optical pumping plays a significant role in determining the noise in certain types of laser-pumped vapor-cell microwave frequency standards by changing the way in which the laser's FM noise is converted to AM noise by the optical-absorption profile. When this FM-AM conversion is the dominant noise source, the noise spectrum of the transmitted intensity can be dramatically altered by the optical-pumping process. FM noise at Fourier frequencies larger than the optical-pumping time is converted to AM noise differently from noise at lower Fourier frequencies. This effect can modify the optimum design of vapor-cell frequency references and adds an additional FM-AM-related noise source that cannot be eliminated with laser tuning. (C) 2001 Optical Society of America.