Providing $10^{-16}$ Short-Term Stability of a 1.5- $\mu\hbox{m}$ Laser to Optical Clocks

We report on transferring $10^{-16}$ -level fractional frequency stability of a “master laser” operated at 1.5 $\mu\hbox{m}$ to a “slave laser” operated at 698 nm, using a femtosecond fiber comb as transfer oscillator. With the 698-nm laser, the $^{1}\hbox{S}_{0} - {}^{3}\hbox{P}_{0}$ clock transition of ${}^{87}\hbox{Sr}$ was resolved with a Fourier-limited linewidth of 1.5 Hz (before: 10 Hz). Potential noise sources contributed by the frequency comb are discussed in detail.