| The ultra-stable lasers with the characteristics of high spectral purity,excellent temporal spatial coherence and brilliant short-term frequency stability are of great significance in the field of optical clock development,gravitational wave detection,basic physical constant testing and so on.The research and development of ultra-narrow linewidth laser based on Pound-Drever-Hall?PDH?technology has been carried out,and the ultra-stable lasers of Hz and even sub-Hz linewidth have been realized.At present,the most ultra-stable lasers researched are working for laboratory and ultra-stable laser systems in orbit have not been developed yet.As one of the key components of the future optical clock for space application,the ultra-stable laser determines the short-term stability of the optical clock and affects the overall performance of the future high-precision time-frequency system for space application.As the core component of the space ultra-stable laser,the space ultra-stable optical reference cavity determines the frequency stability.And it is also relatively weak in the environmental adaptability of the whole space ultra-stable laser system.It is of great significance to study it thoroughly and systematically.Based on the developed ultra-stable laser with sub-Hz linewidth,the vibration sensitivity,temperature response and the dynamic response characteristics of the optical reference cavity are studied.A sub-Hertz linewidth laser system at 698 nm for strontium atomic optical clock has been built.Through the finite element simulation,the support structure of the existing 100 mm optical reference cavity is optimized and its vibration sensitivity is impreoved.The residual amplitude modulation produced by the electro-optic crystal is studied.It is found that the laser frequency is most insensitive to the temperature change of the electro-optic crystal when the residual amplitude modulation is maximized by controlling the temperature of the electro-optic crystal.Based on the acousto-optic modulator,the stability of laser power is improved by an order of magnitude,and the stability per second of the laser power is up to 6.4×10-5.Then the control system is optimized.By optimizing the parameters of the control system,the control bandwidth of the control system reaches 2.7MHz.By the frequency beating of the two separated and similar systems,the laser frequency stability of the single system is better than 2×10-15.After eliminating the relative frequency drift of the two systems,the laser linewidth of the single system is better than 0.9Hz.The micro-vibration level of The International Space Station is about three orders of magnitude higher than that of the super static optical laboratory on the ground,so it is necessary to optimize the vibration sensitivity of the optical reference cavity.By contrast,the ultra-rstable optical reference cavity based on the cubic chamber has great potential in both the realized vibration sensitivity and the resistance impact ability.Therefore,the cubic chamber is chosen as the cavity structure of the space ultra-stable optical reference cavity.In this paper,the change of the cavity length caused by the preload in a certain cutting depth is derived at first.According to the change of the cavity length before and after the acceleration change,the expression of the vibration sensitivity of the cubic cavity is obtained.According to the expression,the vibration sensitivity of the optical reference cavity in different cutting depth can be calculated.The restraint method of finite element analysis on vibration sensitivity of cubic cavity is improved.The change of reference cavity length under different preload and acceleration is studied quantitatively.By analyzing the change of reference cavity length under different amplitude acceleration load,the sensitivity of reference cavity length stability to the acceleration is obtained.The sensitivity of reference cavity length to acceleration and extrusion pressure is compared and analyzed.Finally,the optimized design size that the vibration sensitivity of the cube cavity is the smallest is obtained no matter the reference cavity is placed horizontally or vertically.And the ultra-stable optical reference cavity with vibration sensitivity as low as 2.2×10-11/g is obtained experimentallyThe inevitable change of ambient temperature is another main factor that affects the length of optical reference cavity,and it is also the reason that one constantly pursues high stability temperature control for the ground laboratory system.In this paper,the temperature response characteristics of the reference cavity in a vacuum chamber with three layers of thermal shields are studied by using the thermal network method.Through the differential equations of each node,the relationship between the temperature of the reference cavity and the temperature of the vacuum chamber is established,and the analytical function expression and response characteristics of the temperature of the reference cavity changing with the step temperature and periodic temperature are obtained.The influence of thermal radiation in the vacuum chamber on the thermal response time constant of the system is studied by using the finite element simulation method.It is found that the thermal radiation will greatly accelerate the rate of heat transfer from the outside to the reference cavity.At the same time,the influence of the surface emissivity of the shielding layers,the material and the thickness of the thermal shields on the time constant of the system is analyzed.The finite element simulation results and the theoretical analysis results confirmed each other.Then a passive thermal insulation system of reference cavity with low emissivity and large thermal capacity of thermal shields is designed.The simulation results are verified by the experiments.The thermal response time constant of finite element simulation is about 258 hours.The time constant measured by experiments is consistent with the simulation results.Finally,the passive thermal insulation system of the optical reference cavity is designed.Through theoretical calculation and finite element simulation analysis,the thermal response time constant of the optical reference cavity system is about 72.2 hours.Assuming that the thermal expansion coefficient of the reference cavity is 1×10-9/K,the temperature sensitivity of the reference cavity is 2.8×10-6 for the environmental temperature variation with a change period of 1.5 hours and an amplitude of 10mk The maximum value of the length stability per second of the reference cavity is 3.4×10-20.The temperature sensitivity of the reference cavity is 2.2×10-2,and the maximum value of the length stability per second of the reference cavity is 1.6×10-17 for the ambient temperature variation with a change period of 24 hours and an amplitude of 10mk.In addition,due to the presence of photodetectors and other heating devices in the cavity system,two active temperature control systems are designed to deal with the possible temperature rise problem.Through thermal simulation analysis,the double-layer temperature control system has the ability to control the temperature of the reference cavity near 30?.Because the space ultra-stable optical reference cavity is oriented to space applications,it is one of the key technologies to be able to withstand the huge mechanical vibration and impact of the rocket launch stage.At the same time,due to the limitation of space load resources,the structural optimization and weight reduction design of the optical reference cavity is also an important part of the realization of space ultra-stable optical reference cavity.Firstly,according to the theory of structural optimization,the structure of the space ultra-stable optical reference cavity system is designed,including the semi-spherical support structure of the reference cavity with self-adaptive characteristics,the light path structure before the cavity and after the cavity with weight reduction,and the buffer support system of the metal wire damping cushion.The simulation results show that the first-order resonance frequency of the space ultra-stable optical reference cavity system is 195Hz,which can meet the development of aerospace products.At the same time,taking the mechanical environment test condition of aerospace products as the input,the finite element simulation is carried out,including the sine-sweep with the maximum amplitude of 15g,the random vibration with the maximum power spectral density of 0.125g2/Hz,the shock spectral response with the maximum amplitude of 630g and the acceleration overload with the maximum amplitude of 7.5g.The simulation results show that the maximum stress of the reference cavity is less than 50MPa and the other main components are also within the allowable stress range of the material.That means the space ultra-stable optical reference cavity system has good mechanical environment adaptability.Finally,a large-scale sine-sweep test and a degraded random vibration test are carried out on the reference cavity simulator of the common quartz glass and its support frame.The acceleration sensor is attached to the surface of the reference cavity,and the maximum acceleration of the reference cavity in the mechanical test is measured to be about 40g,while the structure of the reference cavity support of the quartz reference cavity and the material of torlon4203 is not damaged.To some extent,it shows that the reference cavity system has good mechanical environment adaptability,and is a good choice for space application. |
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