| In recent years,with the rapid development of ultra-precision manufacturing,the demand for micro-displacement measurement has also increased day by day.Optical fiber interferometers have received widespread attention due to their small size,high accuracy,and convenient measurement.Phase Generated Carrier(PGC)demodulation technology,as a common method for optical fiber sensor signal demodulation,has the advantages of high sensitivity,good linearity and high dynamic range.However,the traditional PGC demodulation method is more sensitive to changes in laser light intensity and modulation depth,and the laser modulation frequency and amplitude need to be improved under high-speed measurement requirements,which in turn will bring greater light intensity along with modulation and affect demodulation performance.In addition,the structure change under the requirement of dual-axis measurement leads to the non-linear error caused by the change of the modulation depth.Combining the above research background and requirements,this article has carried out the following research work:(1)In view of the problem that the measurement speed of the fiber interferometer is restricted by the modulation characteristics of the light source,thus it is difficult to further improve the accuracy.This paper establishes the relationship between the sinusoidal modulation signal characteristics and the phase modulation depth and the system measurement speed.It is pointed out that the premise of effectively improving the measurement speed of the interferometer is to increase the laser modulation frequency and to ensure that the phase modulation depth remains unchanged.Therefore,the corresponding modulation signal generation module is designed to achieve highspeed and large-scale laser modulation,thereby meeting the high-speed measurement requirements of fiber interferometer.(2)Aiming at the problem that due to the light intensity of the fiber interferometer is multiplied by the modulation amplitude during the high-speed measurement process,and the nano-level demodulation error is introduced,a set of light source intensity stability control system based on the semiconductor optical amplifier(SOA)is designed in this paper.The laser output light intensity fluctuation is measured in real time and the drive current of the optical amplifier is adjusted accordingly to achieve stable control of the light source power,thereby suppressing the change of the light source intensity under high-speed measurement conditions,and reducing the corresponding displacement demodulation error from nanometer to sub-nanometer.(3)Aiming at the problem that the optimal demodulation cannot be achieved at the same time due to the different working distances of the dual-axis optical fiber interferometer during synchronous measurement,this paper analyzes the relationship between the phase modulation depth and the interferometer measurement working distance,and finds that if the modulation depth cannot meet the working distance demand during the dual-axis displacement measurement,the demodulation error can range from a few nanometers to a hundred nanometers,and it is difficult to ensure that the biaxial working distances are completely equal in the actual measurement process.Therefore,a non-linear error elimination method based on normalized amplitude correction is proposed.The two channels of signals before demodulation are processed in equal amplitude to ensure that the residual error after demodulation is reduced from nanometer to sub-nanometer.(4)Based on the above analysis and design,the dual-axis optical fiber interferometer signal modulation and demodulation system can realize sub-nanometer displacement demodulation under the requirement of dual-axis high speed measurement,and it is verified in experiments that the system has dual-axis measurement capability and the measurement speed is better than 1m/s,measurement error is better than 0.15 nm,and the measurement resolution is better than 0.09 nm. |
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