  We demonstrate the use of a fiber-based femtosecond laser locked onto an
ultra-stable optical cavity to generate a low-noise microwave reference signal.
Comparison with both a liquid Helium cryogenic sapphire oscillator (CSO) and a
Ti:Sapphire-based optical frequency comb system exhibit a stability about
$3\times10^{-15}$ between 1 s and 10 s. The microwave signal from the fiber
system is used to perform Ramsey spectroscopy in a state-of-the-art Cesium
fountain clock. The resulting clock system is compared to the CSO and exhibits
a stability of $3.5\times10^{-14}\tau^{-1/2}$. Our continuously operated
fiber-based system therefore demonstrates its potential to replace the CSO for
atomic clocks with high stability in both the optical and microwave domain,
most particularly for operational primary frequency standards.
