Optical Frequency Transfer From A 578nm Laser To Optical Fiber Communication Wavelength Based On A Ti:Sapphire Optical Frequency Comb

The state-of-the-art optical atomic clocks have frequency instability and accuracy at the 10^-18 level,outperforming the current definition of the second,the Cs fountain clocks,by two orders of magnitude.In the near future,the second will be redefined based on optical atomic clocks.Moreover,the advent of optical atomic clocks opens the door for precision measurements at the 10^-18 level,including geodesy at the centimeter level,tests of relativity,dark matter detection,etc.Those applications of optical atomic clocks require frequency comparisons between optical clocks separated in distant places.However,since most optical atomic clocks dominate in 250-1130 nm,far from the transmission window of optical fiber,the frequency accuracy of optical atomic clocks has to be divided accurately to 1550 nm for transmission in optical fiber.In this thesis,an optical frequency comb based on a Ti:sapphire mode-locked laser is employed to bridge the large frequency gap between an optical atomic clock and a 1550nm laser.The frequency of a 578 nm cavity-stabilized laser for ytterbium optical clock is divided to 1550 nm with an optical frequency divider based on the Ti:sapphire comb.Ti:sapphire combs have merits of low noise and high accuracy,but they can't run continuously.To overcome this obstacle,the cavity of the commercial Ti:sapphire mode-locked laser is specially designed to avoid the loss of mode-locking.With the help of a digital controller,turn-key operation is realized for the Ti:sapphire mode-locked laser.Under optimized design of the laser cavity,the laser can be mode-locked over a month,limited by the observation time.The combination of a fast and a slow piezo inside the Ti:sapphire mode-locked laser allows us to adjust the cavity length with moderate bandwidth and tuning range,enabling robust and stable locking of the repetition rate(f_r)to a hydrogen maser.By combining a fast analog feedback to pump current and a slow digital feedback to a wedge inside the Ti:sapphire mode-locked laser and the pump power as well,the carrier envelope offset frequency(f_ceo)of the comb is phase-locked robustly.We extend the continuous phase-locked time of the Ti:sapphire comb to 5 days.The residual jitter of f_r and f_ceo are 0.08 m Hz and 2.5 m Hz at 1 second averaging time,respectively.To realize optical frequency division from optical atomic clocks to the telecomm band,we firstly obtain the beat note between a 1550 nm laser with the Ti:sapphire comb with the help of cross modulation with a signal to noise ratio of 43 d B at 300 k Hz resolution bandwidth.With this beat note,as well as the beat note between the 578 nm laser and the comb,f_r and f_ceo,we realize optical frequency property transfer from the578 nm laser(corresponding to ytterbium atomic clock)to 1550 nm.An out-of-loop measurement shows that the additional noise in optical frequency division is 3.5×10^-18at 1 s averaging time.The division uncertainty is measured to be 1.3×10^-19,satisfying many applications of the most accurate optical atomic clocks for long-distance frequency dissemination.