Designation Of Single Sideband Modulator For Frequency Modulated Continuous Wave Source

With the continuous advancement of the study of laser technology,the related products of laser radar gradually replace other products because of their superior performance.Among them,the lidar ranging system has already become an important product in the fields of military industry,aircraft,satellite,automatic pilot and so on.Frequency modulated continuous wave laser ranging radar is a laser ranging technology based on coherent detection.Using linear frequency modulation radar in the process of work can reduce interference,and laser technology has high stereospecificity,high brightness,high energy density.These can detect a greater distance as using coherent detection methods compared with the direct detection has better detection ability,higher stability.They don't need cooperation target,and can direct detection of absolute distance.Because of the advantages of simple structure,high integration and high ranging resolution,FMCW laser ranging radar has gradually attracted the attention of researchers.The method of generating linear frequency modulation has always been a major research direction of FMCW lidar.In this paper,the frequency modulated continuous wave laser source is studied,and the internal modulation and external modulation methods are compared.The external modulation method is chosen for the experiment,and the working principle and advantages of lithium niobate crystal based on electro-optic effect are introduced.Finally,the carrier suppressed single side band(SSB)method is used to modulate the1550nm laser.Firstly,the principle of frequency modulated continuous wave laser ranging is deduced,and the principle of generating frequency modulated light based on external modulation is introduced.The system of frequency modulated continuous wave light source based on lithium niobate single side band modulator is designed.The DFB laser at 1550nm is selected as the input light source.The frequency noise of the DFB laser source is relatively small,which can reduce the nonlinear degree of LFM light.Single sideband modulator is a dual parallel Mach Zinder modulator,which requires more complex voltage input than MZI modulator.Its working modules include a three-way bias controller,an RF 90°phase-shifting bridge and an RF sweep source.The corresponding test receiving system is designed,which consists of APD avalanche photodiode,cross-resistance amplifier,two-stage amplifier filter and voltage module.The beam-splitting and beam-combining device is used to build an all-fiber testing environment to measure the beat frequency results.The results are analyzed and the better measures are showed.Experiments are carried out at 50m and 100m respectively.The beat frequency results of equal length optical path difference of 150m were tested,and the obtained beat frequency spectrum was obviously observed to have obvious linear spectrum shift at 0.5MHz,1MHz and 1.5MHz.Based on the analysis of the time domain waveform and frequency spectrum of the beat results,the causes and processing measures of stray signal and noise are put forward.The results based on the beat frequency carrier dispersion effect of silicon-based optical waveguide edge of single sideband modulator chip design,this paper proposes a three port silicon-based MZI modulator structure,structure of MZI by two lines of silicon PN junction phase modulator,compared with the conventional MZI structure,allow extra port as a monitor,this design can be used for limited silicon-based ridge waveguide imaginary part of refractive index change of quantitative research.Then,the formula of the carrier silicon-based ridge waveguide is derived,and the fitting results and the polynomial fitting equation are given.We provides a feasible test scheme for silicon based side loaded suppression single sideband modulator.Then the structure of single-sideband chip is designed and the bias voltage applied by the single sideband modulator is solved to achieve the purpose of single sideband modulation.Through the numerical search method,the solution that meets the requirements is obtained,which is put back into the equation.Most of the errors are in the order of 10^-8,which is in good agreement with the objective function.This result provides theoretical support for the future experiments of single side band modulation.