Research On Thermal Time Constants Of Vacuum Chamber Systems For Ultra-Stable Lasers

Compared with microwave frequency standards, optical frequency standards have much better frequency accuracy and stability, and they can provide better services to national defense and scientific researches. An optical frequency standard consists of three main parts: ultra-stable laser system, ion/atom trap system and femto-second frequency comb. As a local oscillator, the ultra-stable laser system is the core of an optical frequency standard, and a Fabry-Pérot?FP? cavity is the key element to the ultra-stable laser system as a frequency discriminator. The FP cavity is usually placed inside a vacuum chamber with a high vacuum level to reduce the effect of external environment. In order to achieve better frequency stability, the vacuum chamber should have a large vacuum thermal time constant to reduce the sensitivity to external temperature fluctuations. Currently, the determination of thermal time constants of vacuum chambers is based on either theoretical estimation or time-consuming experimental measurements. The first method can only apply to simple system, while the second method takes a lot of time to be carried out. To overcome the limitations, we propose thermal time constant simulation using finite element analysis?FEA? based on complete vacuum chamber models. In view of it, my main task is to study how to simulate and enlarge the thermal time constant of the vacuum chamber systems used in ultra-stable laser systems.We simulate the thermal time constants of two vacuum chambers existing in our laboratory using FEA, and the simulated results are 6.8×104 s and 8.7×104 s, respectively. We compare them with the results by performing experimental measurements. The simulation results and the experimental results agree very well, and the discrepancy between them are less than 6%. It proves the effectiveness and reliability of this FEA method. In addition, the time used to determine the thermal time constant is reduced from several months to several days by using the FEA method.Based on the FEA method of simulating the thermal time constant, we simulate several simplified design models to obtain design guidelines to increase the thermal time constants of vacuum chamber systems, while maintaining sufficient strength, enough stiffness and stability. We consider the selection of vacuum pressure, the effect of shielding layers' material, thickness, number and spacing, and the design of support structure's shape and material. We adopt the Taguchi method for shielding layer optimization, and demonstrate that layer material and layer number dominate the contributions to the thermal time constant, compared with layer thickness and layer spacing. In the design of support structure, we use three Viton balls as the support structure. At the same time, we found that the effect of the vacuum pressure on the thermal time constant is not obvious when the vacuum reaches a level of 10^-5 Pa.Besides, using similar FEA method, the thesis introduces the design of a cryogenic system used in a cryogenic sapphire oscillator, and determines the selection of material and thickness of the heat insulation shielding layers.