Theoretical And Experimental Research On Ultrastable Photonic Microwave Generation

Ultrastable oscillators demonstrate the highest frequency stability for short term,and the ultrastable microwave signals play very important roles in many applications.State-of-the-art primary frequency standards based on cold atoms exhibit higher frequency sta-bility of 10-14,which is limited by the quaGtum projection noise.However,the local oscillator based on quartz-oscillator limits the frequency stability to 10-13@1 s due to Dick effect.Therefore,an ultrastable frequency source is of prime importance for fre-quency standards to realize high short-term stability.Moreover,these low-noise frequency sources are also required by other applications,such as ultrahigh resolution radar,deep space navigation and ultrahigh resolution of communications.This paper demonstrates the ultrastable photonic microwave generation signal used on Cs atom fountain clock.The frequency stability of new ultrastable microwave signal can reach 10-16 level.And the photonic frequency source is composed of an ultrastable laser,an optical frequency comb,an optic-to-electronic converter and frequency synthesizer.The reference of the frequency source is a 10-cm ultrastable F-P cavity,whose stability can reach 10-16 level which is mainly limited by thermal noise of mirror coating.The result shows that the tunable frequency around 9.192 GHz with relative frequency instability of 7.4×10-15@1 s.This result can meet the demands of Cs atom fountain clock.The contexts mainly includes the following points.1.Ultrastable laserThis paper acheives an ultrastable laser with sub-Hz linewidth based on a high finesse F-P cavity through PDH(Pound-Drever-Hall)frequency stabilization technic.We employ a special designed frequency stabilization system to phaselock the phase between the com-mercial laser and the 10-cm F-P cavity.And the broadband noise and long-term frequency drift of the laser can be effectively suppressed as we employ a special control strategy which is composed by a fast control loop and a slow control loop.Finally,we have arrived the 1555 nm ultrastable laser with the frequency instability of about 7×10-16@1?20 s,and the frequency instability has been limited by thermal noise of the ultrastable cavity which made of ULE glass.We demonstrate a new proposal for making an ultrastable laser referenced to a multi-cavity for further reducing the thermal noise of F-P cavity,enabling a lower thermal noise limit due to the averaging effect.In comparison with a single-cavity system,relative fre-quency instability of the synthesized laser can be improved by a factor of the square root of the cavity number.We perform an experiment to simulate a two-cavity system with two independent ultrastable lasers.Experimental results show that the relative frequency instability(Allan deviation)of the synthesized laser is 5×10-16@1s,improved by a factor of (?) from a single-cavity-stabilized laser.2.Optical frequency combFemtosecond optical fequency comb is used as an optical frequency divider for trans-miting the optical freqency to microwave frequency.In this experiment,we use an Erbium fiber optical frequency comb.And we do more researches on the frequency stabilization systems for the fr and fceo.In this case,the fceo can be phaselocked by modulating the pump current or the special EOM in the optical cavity.And the frequency instability of fceo phaselocked by pump current is 9.6×10-17/?,with a phase noise range from-50 dB rad2/Hz to-60 dB rad2/Hz.The frequency instability of fceo is 1.6×10.17/? phase-locked by special designed EOM with a bandwidth of about 1.8 MHz.The frequency stabilization of fr is used the EOM and PZT,which are mounted in the optical cavity.And the frequency instability of fr is 7.4×10-15@1s,and the frequency instability of fbeat is 2×10-17@1 s,with the pahse noise is lower than-50 dB rad2/Hz.The comb build a bridge between the optical frequency and microwave frequency through stabilized on the ultrastable laser,and sucessfully shift the optical frequency to the 41st harmonic of the fr signal near 9.54 GHz microwave frequency with lower residual frequency stability.The optical comb can continuously work for more than 1 week3.Optic-to-electronic converter and microwave frequency synthesizerThe optic-to-electronic converter is used to detect the laser pulse train from optical frequency comb while fr is phaselocked to the ultrastable CW laser.Additional phase noise of the detection of the harmonic signal of fr is mainly contributed by the thermal noise,shot noise,and amplitude-to-phase conversion due to the saturation effect.Ac-tually,the saturation of the PD is due to charge accumulation in very short time,and is determined by the energy of a laser pulse.Thereby we can employ a repetition rate multiplier based on a series of cascaded Mach-Zehnder interferometer(MZI)to obtain a higher saturation optical current,which finally reduces the broad band noise floor of the generated microwave signal,we get a microwave near 9.54 GHz with a frequency stability of 3×10-15@1 s.After optic-to-electronic conversion,a low phase noise frequency synthesizer with the phase locked loop technic is used to shift the?9.54 GHz to tunable 9.192 GHz and 10 MHz.And finally,we get the 9.192 GHz microwave signal with the frequency instability about 7×10-15@1 s and a 10 MHz signal with the frequency instability of 1.8×10-14@1 s.4.Application of photonic microwave generationTo evaluate the performance of 9.192 GHz microwave signal from photonic mi-crowave generation,we build two independent and identical reference systems and per-form the measurement.The results show that the frequency instability of the 10 MHz signal is about 1.8×10-14@1 s,and the frequency instability of the synthesized 9.192 GHz signal is about 7.4×10-15@1 s and below 3×10-15@20 s.Benefiting from such a stable frequency source,the short-term frequency instability of the cesium fountain clock can be improved from a few 10-13?-1/2 to a few 10-14?-1/2 limited by quantum projection noise.