Heterodyne optical phase-locking of extended-cavity diode lasers at 9 GHz

In order to stimulate atomic velocity=selective Raman transitions on the 852 nm caesium D2 line in an atomic fountain clock, two extended-cavity diode lasers have been optically phase-locked at a frequency offset of 9.192 GHz. The measured linewidth (fwhm) of the free-running lasers is 50 kHz. The phase-locked loop bandwidth, evaluated by observing the frequency noise spectrum, is 3.7 MHz and the phase error variance is found to be no more than 4X l0 -3 rad 2.