Research On Phase Measurement Of Optical Beat Frequency Based On I/Q Demodulation

In recent years,laser ranging technology represented by laser absolute distance measurement technology has become increasingly mature.With the emergence of femtosecond optical frequency combs and the continuous advancement of related technologies,laser absolute distance measurement technology has reached a new level.The femtosecond optical frequency comb benefits from its superior frequency stability,which can well realize the frequency correction of traditional ranging light sources,and can also be directly used as a ranging light source to participate in measurement.However,due to the characteristics of femtosecond optical frequency combs that are different from traditional light sources,the solution of distance measurement results has a corresponding change.For example,in the Comb-calibrated FMCW absolute distance measurement,it is necessary to calculate the phase change curve of the beat signal between the femtosecond optical frequency comb and the FMCW and use this curve in combination with the ranging signal to achieve distance measurement Correction of results.At this time,because the femtosecond optical frequency is equivalent to the superposition of a large number of sine waves of different frequencies,a large number of frequency components will appear in the curve,and an appropriate frequency interval needs to be extracted to reduce the complexity of the calculation.At the same time,the frequency tuning characteristics of FMCW will also cause the phase information to continuously change over time;some traditional phase calculation methods may not be able to correctly solve the required results,that is,frequency correction cannot be achieved.Therefore,how to select the appropriate signal phase processing technology and use the corresponding phase calculation module for the signal characteristics has become the focus of research.This topic focuses on the phase calculation of the femtosecond optical frequency comb for FMCW frequency correction in FMCW absolute distance measurement,and aims to solve the problem of inaccurate range measurement caused by laser frequency tuning nonlinearity in FMCW absolute distance measurement technology.The principle of the basic FMCW absolute distance measurement technology and the characteristics of the beat signal for frequency correction of the femtosecond optical frequency comb are studied.Analyzed five basic phase calculation methods and chose to apply I/Q demodulation method in this subject,and then considered the problem of calculating the beat signal between femtosecond optical frequency comb and FMCW in practical applications.Finally,an I/Q demodulation algorithm based on an optical mixer is used to achieve the correct calculation of the beat signal.The main research contents of this paper are as follows:(1)Analyze the basic principles of FMCW absolute distance measurement technology,and study the influence of frequency tuning nonlinearity on the distance measurement results.Analyze the characteristics of the basic phase calculation method,research the corresponding phase calculation method according to the characteristics of the signal obtained when the femtosecond optical frequency comb is used to perform frequency correction on the FMCW,and analyze the feasibility of the I/Q demodulation algorithm calculation;(2)Focus on the signal processing problems that will be brought about when applying the I/Q demodulation method,especially the characteristics of the femtosecond optical frequency comb in signal processing.According to the problem that the femtosecond optical frequency comb is not suitable for digital signal processing when performing frequency correction on FMCW,an I/Q demodulation algorithm based on an optical mixer is adopted.The feasibility of using this algorithm is analyzed,and the effect of using an optical mixer on the final ranging result is studied.And use simulation calculations to verify the results,and build an I/Q phase calculation module based on an optical mixer.In the system phase calculation accuracy experiment,a phase measurement resolution of 0.5° is obtained;(3)Participated in the design and construction of an FMCW ranging system based on femtosecond optical frequency comb frequency correction.In the distance measurement experiment,the actual distance measurement experiment was carried out in two scenarios: an optical fiber with a length of 1.5m and a long rail space with a length of 0-30 m.High-precision and high-repeatability measurement results are obtained,and the feasibility of the I/Q demodulation algorithm based on the optical mixer in the ranging system is verified.