| As the equipment with the best stability and the highest measurement accuracy,optical atomic clock plays a vital role in the development of science and technology and the test of basic theory.In order to develop an optical atomic clock with high stability,a laser with excellent frequency stability is needed as the local oscillator of the optical clock.Pound-Drever-Hall（PDH）technology is a widely used and effective method to develop high stability laser,but the stable laser frequency of PDH technology is often limited by the thermal noise limit of the reference cavity.Using a high-precision long cavity is an important method to reduce the thermal noise limit of the reference cavity.Based on the 48 cm long cavity,the German Bureau of standards and Metrology（PTB）has developed the 698nm ultra stable laser with the best stability in the world,and the frequency instability has reached 8×10-17.Although the thermal noise limit can be reduced by using a long cavity,the stability of the cavity length is more sensitive to the change of the environment.Not only the requirements for noise suppression are improved,but even the same size of ambient noise will lead to greater changes in cavity length.Therefore,in order to develop a more stable laser with better frequency stability,higher requirements are put forward for temperature control,vibration isolation and photoelectric noise suppression in the frequency stabilization system.The main work of this paper is to systematically study the noise characteristics,suppression and measurement methods of ultra-stable laser in 698 nm band based on30 cm long reference cavity,and obtain a second stability of 3.6×10-16.The best level of frequency stabilization laser with 10-16 level is the same as that with 30cm reference cavity in the world.Firstly,this paper introduces the basic principle of PDH laser frequency stabilization technology,and summarizes the characterization and measurement methods of general laser stabili.Then the length stabilization method of the key component of the ultra-stable laser system,the ultra-stable cavity and the suppression method of optical noise are studied.Finally,a system is built for the 30cm super stable cavity,and the frequency stability is 3.6×10-16@1s.The research work mainly includes:The design of temperature insensitive reference cavity and the method of suppressing temperature fluctuation noise are studied.In order to reduce the thermal noise limit,the combination of ULE cavity and FS cavity mirror is generally used to make the reference cavity in recent years.This composite cavity changes the zero expansion temperature point of the whole cavity due to the mismatch between the expansion coefficient of the optical glue surface of the cavity mirror and the cavity.Although Thomas legero et.al proposed that the sandwich structure using the ULE outer ring can reduce the change of the zero expansion temperature point,the adjustment ability is limited when the zero expansion point of the ULE cavity is low.According to the theoretical basis of the offset of the zero expansion point,a double ring structure is proposed in this paper.Only a protrusion with the same size and thickness as the cavity mirror is cut out on the two end faces of the cavity,the adjustment range of the zero expansion temperature point can be increased by more than 7℃on the basis of the sandwich structure,without affecting the mechanical properties of the reference cavity.In order to reduce the ambient temperature of the reference cavity,the design method of the thermal shielding layer is also studied,and the heat transfer model of the shielding layer is established.According to the specific environmental noise,the design of shielding layer is analyzed in frequency domain for the first time,which provides further theoretical support for the passive temperature control theory.Due to the limitation of vacuum chamber and thermal shielding layer,the measurement of zero expansion temperature point is very time-consuming and prone to errors.When measuring the time constant of the thermal shielding system,it is found that the measured data of temperature change is very consistent with the theoretical analysis data,and because the heat transfer model in the vacuum system is relatively simple,the external noise interference is not strong.Therefore,the temperature value of any period in the descent curve can be inferred according to the starting temperature,ending temperature and descent time（thermal time coefficient）,and the zero expansion temperature point can be calculated by recording frequency.When the laser is locked in the reference cavity,the cavity power is directly proportional to the fineness of the reference cavity.When the power fluctuates,the cavity length of the high fineness cavity is more unstable.This paper theoretically analyzes this mechanism and the relationship between power jitter instability and frequency instability of frequency locked laser.Experimentally,a method of using in ring light instead of out of ring light is proposed to suppress the power stability of the reference cavity to less than 1ppm,and the corresponding laser frequency stability reaches the order of 10-19@1～100s.Based on the 30cm cavity,a system is built,and the temperature control,vibration isolation and noise suppression of various optical technologies are carried out according to the characteristics of the system.The beat frequency comparison analysis shows that the frequency instability of the system reaches 3.6×10-16@1s.At present,the international optimal result based on 30 cm cavity is 1.1×10-16@1s,the frequency stability of the system is very close to the result.The above research results are fully applicable to the needs of the national time service center to build an integrated sea,land and air time service system and develop a high stability optical atomic clock.Moreover,due to the poor vibration and noise environment in the laboratory,the experimental system will be moved to an air raid shelter with relatively good vibration environment in the future,and the laser frequency stability equivalent to the optimal index of similar systems in the world is expected to be obtained. |
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