Research On Ultra-stable And Ultra-low Phase Noise Cryogenic Sapphire Oscillator

Frequency is the most accurate physical quantity we can rely on for conducting pre-cision measurements.This has stimulated the development of modern fundamental mea-surement processes,high-speed communication,navigation and radar systems.Cryogenic sapphire oscillators possess ultra-high short-term stability and ultra-low phase noise,which make it the best candidate for applications with highly demanding requirements for frequen-cy sources,such as,deep space navigation and fundamental physics tests.State-of-the-art cryogenic sapphire oscillator can reach 2.4×10^-16at 32 s.For the phase noise,it achieves-100 dBc/Hz when the carrier frequency is 10 GHz and offset frequency is 1 Hz.Most sig-nificantly,the predicted achievable frequency stability is 3×10^-18,from 10 to 10⁴s.This ultimate performance would be comparable to optical clocks,but with more robust frame,easier technology and lower cost.To explore and break the current limitation of cryogenic sapphire oscillators,my work has contributed to the development of the system and in-vestigation of different limiting factors.This dissertation focuses on the development of oscillation loop system around the sapphire resonator?electronics package?.The first phase is to build a feedback controlled room-temperature sapphire oscilla-tor so that we can be familiar with all the due processes.The room-temperature sapphire oscillator frequency stability reaches 6×10^-11at 0.2 s in an extremely hot and humid am-bient environment,and is just worse than the experimental results,achieved by University of Western Australia in 1995,by about a factor 4.It is not unreasonable that we will reach better results with same design under optimum experimental conditions.Compared with room-temperature,the Q factor of sapphire resonator can be improved at least of three or-ders of magnitude,offering much more tunability to the sapphire resonator feedback control such as the stabilization on the sapphire temperature,incident microwave power and the implementation of Pound control system.However,being limited by the noise floor of our ultra-low phase noise frequency synthesizer?reference source?,the frequency stability that has been achieved so far is only about 1×10^-12?between 0.03 and 1 s?and the phase noise is just about-48 dBc/Hz@1 Hz offset from the 10 GHz carrier frequency.To push further the current performance of our cryogenic sapphire oscillator towards the theoretically achievable performance,new limiting factors need to be searched and studied.There are two types of cryogenic sapphire oscillators around the world,one is based on liquid Helium,whereas the other one is based on cryogen-free.It has been observed that the method employed for cooling a same resonator affects the performance of the cryogenic sapphire oscillator.Inspired by this phenomenon,we suspect that mechanical vibrations could be the cause.By applying precise finite element analysis to model the system under vibrations generated by a pulse-tube cryocooler,it has been shown that actually vibrations are one of the next new limiting factors of the current system performance.To reduce the noise floor due to mechanical vibration,one way is to lower down the vibration level from the source.Such as our cryogen-free system is exhibiting a performance one order of magnitude better in comparison with the cryocooler used for the state-of-the-art oscillator.Another way is to design a new configuration for sapphire resonator to be not so sensitive for the vibration.Now,combining new sapphire resonator design and the improved system with less mechanical vibrations,it is expected to improve by at least of one order of magnitude than the current performance of the state-of-the-art cryogenic sapphire oscillator.To prove it in experiment,it is required to reach a frequency stability level of about 10^-15@1 s integration time.