Phase Noise Cancellation Of Pulsed Clock Laser During Its Transfer To Interrogated Atoms

In recent years,optical clocks using narrow-line optical transitions of trapped neutral atoms or single ions have achieved rapid development,reaching frequency instability and uncertainty levels of the order of 10-18.Particularly,optical lattice clocks based on ytterbium(Yb)and strontium(Sr)atoms have realized fractional frequency instabilities on the order of 10-19.Optical clocks play a key role in time and frequency metrology,providing accurate frequency standards for the future redefinition of the SI second.They also find their applications in fundamental physics,such as test of fundamental physical laws,measurement of possible variations of fundamental physical constants,detection of gravitational waves and dark matters.In addition,optical clocks have unprecedent sensitivity to the geopotential of the earth,and thus enable the so-called relativistic geodesy.For our Yb lattice clock,the clock transition is interrogated by an ultra-stable clock laser with a wavelength of 578nm.The pulsed clock laser is delivered to the lattice-trapped atoms through optical elements(optical fiber,lenses,mirrors,etc.)and free-space propagation.Mechanical vibrations,temperature and air pressure fluctuations and some other factors will introduce additional phase noise,leading to residual Doppler shift and broadening of the clock spectrum.The effect of these phase noises on the measured fractional clock frequency may reach the order of 10-16,so it must be sufficiently suppressed.In order to detect and cancel the phase noises of the clock laser that occur in the optical path to trapped atoms,we have designed and built a phase noise cancellation system based on the principle of Michelson interferometer.Our main achievements are summarized as follows:1.We have built the electronic part of the phase noise cancelation system,including the low-noise,high-speed detector,the phase measurement circuit working at beating frequency,as well as the servo control circuit with a voltage-controlled crystal oscillator.The noise of the electronic system is very low,corresponding to a fractional frequency instability of 1.1×10-18/τ.2.Based on the principle of Michelson interferometer,we have designed and built a phase noise cancellation system for our 578nm clock laser.Phase noises are reduced by more 34dB in the frequency range below 1KHz.When applied to the clock laser,a fractional frequency instability of 1.8×10-17/τ is obtained for the residual phase noise,indicating an improvement by two orders of magnitude compared to the case without noise suppression.This frequency instability level is also much lower than that of the best ultra-stable clock lasers.3.We have realized coherent transfer of pulsed clock laser using our phase noise cancellation system.The settling time for phase lock is about 120μs.For a typical interrogation time of 200 ms,the chirp frequency shift due to the phase fluctuations in the settling time is estimated to be 1.8 mHz.