Study On Ultra-precision Multi-degree-of-freedom Optical Measurement Base On Laser Interferometry

The TianQin mission is based on the long-arm transponding laser interferometer to measure the tiny displacement changes between two free-falling test masses caused by gravitational waves.One of the key technologies is high precision inertial sensing.The commonly used inertial sensors are based on capacitance displacement sensing,which measures the displacement by reading the changes of coupling capacitance between the surface of the test mass and the electrostatic plate.The capacitance sensing has advantages of high integration,good reliability,and technical maturity.However,the typical capacitance sensors have a limited measurement range,which makes it unsuitable to inertial sensing for free-falling test masses with a large measurement range.Besides,the capacitance sensing has a large feedback electrostatic disturbance,which influences the residual acceleration noise of the test mass as well.In recent years,the optical sensing has become a trend for developing inertial sensors.For example,in 2015,the LISA Pathfinder had been launched successfully,and achieved an acceleration noise level of 5×10^-15 m/s²/?Hz by using optical readouts of test masses.This thesis mainly discussed the multi-degree-of-freedom optical sensing of the test mass for TianQin mission.Through the analysis and comparison of various types of optical?translational and angular?displacement measurement methods,the method based on multi-beam laser interferometry is chosen to realise multi-degree-of-freedom optical sensing of test masses.Combining with high precision phase measurement,this multi-DOF laser interferometer system achieved a sensitivity of picometer for translation measurement and a sensitivity of nanoradian for tilt measurement,which meets the requirement of inertial sensing for TianQin mission.This thesis presents one two-degree-of-freedom high precisison displacement measurement system based on multi-beam laser interferometry.They could be used for spaceborne inertial sensing and other applications which required high precision translational and angular displacements measurement,such as gravitational experiments based on precision torsion banlance and multi-degree-of-freedom picometer positioning control,etc.The two-degree-of-freedom interferometer system achieved sensitivities of 2pm/?Hz and 0.4 nrad/?Hz at 1 Hz and 100 pm/?Hz and 10 nrad/?Hz at 2 mHz.The nonlinearity of translational displacement measurement is less than 0.1 nm.The dynamic range of the tilt measurement is 4 mrad with a nonlinearity of 0.25%.The angular measurement of sub-nanoradian sensitivity over a dynamic range of a few milliradian has been realized successfully.In addition,this thesis analyzed the theoretical model of multi-degree-of-freedom motion coupling of the test mass,and built a five-degree-of-freedom interference system for testing.The five-degree-of-freedom optical sensing system realized two translational DOFs and three rotational DOFs measurements,and achieved sensitivities of 10 pm/?Hz and 5 nrad/?Hz at 1 Hz.The nonlinearities of the translational and angular displacement measurements are less than 0.06%and 10%,respectively,and the range of tilt measurements is about±200?rad.