Characterization of the phase noise in a cavity-enhanced frequency doubler

In optical atomic clocks, the low phase noise of the frequency conversion is essential to maintain the high frequency stability and phase coherence of clock lasers. Higher power of interrogation lasers is required for future optical clocks. In this work, we present a cavity-enhanced frequency doubler based on second harmonic generation with an intra-cavity PPLN crystal. The frequency doubler is highly efficient in obtaining the strontium atomic clock laser from the fundamental laser at 1397 nm. We measure the excess phase noise introduced by the enhancement cavity. The cavity-enhanced frequency doubler introduces excess phase noise with a power spectral density that is nearly an order of magnitude lower than that of state-of-the-art interrogation lasers for offset frequencies between 0.001 Hz and 1 Hz. The modified Allan deviation of fractional frequency instability is 5.3×10^-18 at a 1-second averaging time under a homemade housing. It indicates that the doubler can be used to improve the short-term stability of strontium optical clocks with the best up-to-date ultra-stable lasers. Meanwhile, its high efficiency can match increasing interrogation laser power for further applications.