Research On Orthogonal Phase Locking Technology Of Heterodyne Laser Interferometer

With the development of ultra-precision processing,microelectronics technology,biomedical engineering technology and other technologies,the requirements for ultra-precision nano-detection are increasing.Among many nanometer measuring technologies,the heterodyne laser interferometric technology has been widely used,because of its various advantages,such as high precision,wide range of measurement,no-contact,and so on.For the generated Doppler signal,two demodulation methods are commonly used:fringe counting and phase discrimination.The resolution of the fringe counting method is limited by wavelength,which isλ/2,and the accuracy can only reach a micrometer level.The demodulation accuracy of the phase discrimination method can break through the constraint of λ/2 and reach a level of nanometers.Therefore,this thesis uses a quadrature phase lock method to demodulate the Doppler signal generated by the heterodyne laser interferometric system.The specific research content mainly includes:1.The principle of laser Doppler measurement and the principle of optical heterodyne detection are introduced in detail,the theoretical analysis and formula derivation of the movement displacement of the measured object are carried out,and the principle of laser Doppler measurement and the principle of optical heterodyne detection are proposed to realize the displacement of moving objects.The specific measurement scheme is given,which includes the optical path,hardware circuit,and demodulation algoritlun.A heterodyne laser interferometric measurement system has been designed.2.A hardware circuit with a FPGA chip as the core is designed according to the signal characteristics of the optical path part of the heterodyne laser interferometric system.The optical power of the reference signal and the measured signal output by the system optical path is small,and the signal-to-noise ratio is low.First,the two optical signals are converted and amplified.The signal conditioning circuit is designed for this part to achieve the purpose,and an appropriate bias voltage is introduced.After reaching the working range of the subsequent A/D conversion circuit,the digital signal output after the analog-to-digital conversion is transmitted to the FPGA control circuit for demodulation,and the result is sent to the upper computer by the serial communication module,thereby realizing the measurement of the displacement of the moving object.3.Four kinds of phase demodulation algorithms are analyzed,including short-time Fourier transform method,Hilbert transform method,sine curve fitting method and quadrature phase lock method.Analyzed their principles,compared their advantages and disadvantages in Doppler signal demodulation,and proposes the use of quadrature phase lock method to demodulate the signal and solve the phase difference between the two signals,which can effectively suppress noise and improve the signal-to-noise ratio is directly calculated to obtain the phase difference at different times,which avoids the additional error that may be introduced by separately calculating the initial phase of the two signals.4.The all-fiber heterodyne laser interferometry system is simulated and verified by experiments.The motion of the piezoelectric ceramic oscillator is simulated by the quadrature phase-locking method.When the signal-to-noise ratio is 6dB,the phase and displacement solutions are The maximum adjustment error is 6.376° and 13.726nm,respectively.The signal-to-noise ratio of 6dB has been verified 20 times over.The minimum displacement measurement error range is 7.732nm to 19.058nm.After that,an all-fiber heterodyne laser interferometric system was built for experimental verification.Under different driving voltages,the movement displacement of the piezoelectric ceramic oscillator was measured.The displacement measurement range was between 4.152 μm and 14.009 μm.The experimentally measured maximum of the relative demodulation error is only 0.963%.