Development Of Ultra-low Phase Noise Microwave Sources Based On Optical Frequency Downconversion For Cesium Fountain Clock

Ultra-low phase noise microwave sources can be used in cesium fountain clocks,radars and many other fields.With the emergence and development of ultra-stable lasers and femtosecond optical frequency combs,the generation of ultra-low phase noise microwave sources based on optical frequency synthesis has become a hot research topic.The microwave signal generated by the ultra-stable laser through femtosecond optical comb frequency down-conversion,referred to as the optical down conversion microwave source,has superior phase noise and short-term frequency stability performance,and has the lowest phase noise at small offset frequency.This thesis starts with the construction of the optical down-conversion microwave source and describes the author's main work during the postgraduate period,including the construction of the three components of the optical down-conversion microwave source:optical frequency comb locked to ultra-stable laser,pulse repetition frequency multiplication link and harmonic detection,and the integrated microwave chain.Construction of a 9 GHz ultra-stable microwave source is ultimately realized.Based on this,we generate an ultra-stable microwave output of 9.192 GHz,which meets the needs of the cesium fountain clock under development in our lab for an ultra-stable microwave local oscillator.The main contents of this thesis include1.By building the beat frequency optical path of the optical comb and the ultra-stable laser and PID feedback control,the comb frequency can be locked to the ultra-stable laser,and the frequency stability of the inner loop after locking is3.6×10^-16@1 s.The ultra-stable laser frequency stability is transferred to the optical frequency comb.2.The 9 GHz ultra-stable microwave signal is generated by low-noise photoelectric detection,and the link frequency stability of the 9 GHz ultra-stable microwave signal is3.3×10^-16@1 s.Based on commercial optical fiber devices,through optical fiber delay measurement,the optical fiber delay compensation based on fiber cutting and fusion splicing and optical fiber delayer was adopted,and two sets of two-stage pulse repetition frequency multiplication optical fiber links were fabricated to increase the optical comb pulse repetition frequency f_rep.The repetition frequency is increased to 4 times of the initial repetition frequency,which effectively increases the signal-to-noise ratio of the generated microwave signal.3.Based on microwave devices,direct digital synthesizer(DDS)and self-made power supply and PID feedback circuits,two sets of 9.192 GHz microwave integrated chains based on 9 GHz optical down conversion microwave signals are built and the microwave integrated chains are integrated into the chassis.The frequency locking of the microwave integrated chain is realized,and the servo bandwidth reaches 200 k Hz,and the frequency stability due to the additional noise of the microwave integrated chain is3.2×10^-15@1 s.4.Using the optical frequency comb locked to the ultra-stable laser as reference,the residual phase noise and frequency stability of the optical down conversion microwave source and microwave integrated chain were tested respectively,and the residual phase noise and frequency stability of the overall link were measured.The residual phase noise of the channel at 9.192 GHz can reach-108 d Bc/Hz@1 Hz,and the frequency stability can reach 3.8×10^-15@1 s.Integrating the phase noise of the ultra-stable laser,the electronic noise of the optical frequency comb locked to the ultra-stable laser,and the residual phase noise of the overall link,the calculated absolute phase noise of the optical down conversion microwave signal at 9.192 GHz reaches-90 d Bc/Hz@1 Hz.For the generated 9.192 GHz microwave source,the calculated frequency stability limit of the cesium fountain clock is 5.9×10^-15@1 s,which meets the preliminary requirements for the construction of cesium fountain clock.