Dual-frequency laser at 1.5 μm for optical distribution and generation of high-purity microwave signals

We describe the stabilization of the beatnote of an Er,Yb:glass dual-frequency laser at 1.5 μm with and without an external microwave reference. In the first case, a classical optical phase-locked loop (OPLL) is used, and absolute phase noise levels as low as - 117 dBrad²/Hz at 10 kHz from the carrier are reported. In the second case one or two fiber-optic delay lines are used to lock the frequency of the beatnote. Absolute phase noise levels as low as - 107 dBrad²/Hz at 10 kHz from the carrier are measured, fairly independant of the beatnote frequency varying from 2 to 6 GHz. An analysis of the phase noise level limitation is presented in the linear servo-loop theory framework. The expected phase noise level calculated from the measurement of the different noise sources fits well with the predictions.