Laser Interferometry Ranging Based On Quadrature Phase Detection With Wavelength Modulation

Laser interferometry ranging has been widely applied in precision measurement,positioning and manufacture thanks to its non-contact way,high precision and traceability.The homodyne interferometer is widely used because the optical path is relatively simple,however,the optical paths are usually spatially separated so that it is susceptible to environment interference and also not ready for application in small space.In this thesis,to meet the requirements for displacement sensing in the Casimir experiment and scanning probe microscope,we built a fiber interferometer with quadrature phase detection through wavelength modulation,which has a constant sensitivity over a wide range of displacement range.We analyze the main error sources of the displacement measurement,including wavelength stability,laser intensity modulation caused by wavelength modulation,measurement errors of experimental parameters,and assembly errors of the fiber probe.The nonlinear errors are mainly discussed,including the modulation of light intensity caused by wavelength modulation,the multiple reflections in the cavity,the measurement errors of the amplitude of the interference fringe and its first harmonics.A method is adopted to correct the nonlinear error in real-time according to an error table obtained in prior by sweeping the wavelength.The assembly of the fiber probe is described,as well as the measurement and control program developed with Labview.The performance of the fiber interferometer was finally tested.The modulation of the light intensity is reduced to 0.02% by the power stabilization module,which reduces effectively its contribution to the displacement error.The error table is obtained as the difference between the phase measured and expected phase when sweeping the wavelength.We check the repeatability of the phase error for the same phase value but different cavity length,and find the repeatability is mostly below 1 nm.This verifies the feasibility of realtime phase correction during the displacement measurement.With real-time phase correction and wavelength stabilization,the precision of the interferometer is estimated to be 0.1 nm(3?)with a sampling rate of 100 Hz.The current measurement accuracy and precision is mainly limited by the measurement error of the amplitude of the interference fringe and the error of the nonlinear real-time correction.Finally,the PID feedback control of the cavity length is demonstrated.