Evaluation of an ultra-stable laser system based on a linewidth transfer method for optical clocks

We have evaluated an ultra-stable laser system, which is based on the linewidth transfer method. In this system, a Nd:YAG laser at 1064 nm stabilised to a high finesse Fabry- Pérot etalon is employed as a master laser. Beat signals between the high-speed controllable optical frequency comb locked to the master laser and another ultra-stable laser operated at 1.5 μm are observed to investigate the frequency stability. The frequency stability measured by the fractional Allan deviation is about 2 × 10^-15 at 1 s averaging time, which almost coincides with the estimated thermal noise limit on the optical cavity. Furthermore, using the system, the ¹S_0-³P₀ transitions in ytterbium and strontium atoms are observed. We have demonstrated that linewidth transfer technique based on the optical frequency comb combining with the narrow linewidth laser serves as a useful ultra-stable local oscillator for optical clocks.