High Linearity FMCW Lidar Frequency Tuning Technique Based on Electro-optical Phase-locked Loop
In recent years,Light Detection and Ranging(Lidar)technology has gained significant attention due to digital advancements and its widespread applications in various domains such as active target detection,industrial manufacturing,robotics,and autonomous driving.The increasing demand for high-precision measurement technology has led to the Frequency-Modulated Continuous Wave(FMCW)approach emerging as a promising tool for achieving enhanced accuracy and resolution at lower received optical power,enabling direct detection target position and velocity.FMCW offers several advantages,including improved anti-interference ability,cost-effectiveness,wider measurement range,and faster measurement accuracy.However,it is essential to note that FMCW laser ranging relies on the ideal linear frequency modulation assumption,which can be distorted by strong nonlinearity and thermal effects of the tunable laser itself,resulting in substantial distortion of ranging results within a narrow frequency range.This issue becomes particularly critical for high-performance Lidar systems.In this manuscript,we propose a nonlinear correction system for tunable semiconductor laser frequency scanning,using a 1 550 nm DFB laser modulated with sawtooth waves to create beat signals.An iterative algorithm is employed initially to prevent lock loss due to large frequency differences,followed by an Electro-Optic Phase-Locked Loop(EO-PLL)which adjusts the pre-distortion current to effectively suppress nonlinearity and achieves linear light tuning.The impact of this linearization on FMCW Lidar resolution is confirmed through experiments measuring the thickness of target objects.Following the nonlinear correction of the semiconductor laser's frequency sweeping,the power spectra of the beat signal are compared in three scenarios:the initial state,pre-distortion,and EO-PLL.It can be observed that the initial beat signal has a wider frequency spectrum with many other frequency components.After pre-distortion correction,the spectrum slightly narrows,and the introduction of EO-PLL further compresses the spectrum,greatly suppressing the frequency sweeping nonlinearity.Additionally,the time-frequency diagram of the beat signal is obtained through the short-time Fourier transform.Before the nonlinear correction,the frequency of the beat signal fluctuates greatly within one period.However,after the introduction of EO-PLL,the frequency of the beat signal stabilizes at the pre-set reference signal frequency of 350 kHz.Moreover,we analyze the output optical frequency of the laser.It is evident that the output optical frequency of the laser before correction exhibits serious nonlinearity,with a Root Mean Square Error(RMSE)of 5.2 GHz.However,after being controlled by EO-PLL,the RMSE of the optical frequency difference is reduced to 23.7 MHz,shrinking to 0.033 9%of the original residual nonlinearity.Furthermore,we expand the frequency excursion to obtain better resolution and compare it with previous results.In the ranging experiment,the discrepancy between the measured target thickness and the actual thickness across various distances ranges from 0.05 to 0.15 mm,demonstrating that the nonlinear correction is effectively achieved.This paper presents a tunable semiconductor laser FMCW ranging system,which incorporates a high-order EO-PLL to achieve a linear optical frequency output by modulating the current slope of the semiconductor laser.The manuscript verifies the reliability and accuracy of the nonlinear correction algorithm by analyzing distance information from static targets.By comparing beat signals corrected under different frequency excursions,it is found that residual nonlinearities are greatly suppressed.Experimental results demonstrate how this technology can significantly improve the performance of FMCW laser ranging systems.This research holds both theoretical and practical importance,contributing to enhanced national competitiveness in relevant research fields.
Frequency Modulated Continuous Wave(FMCW)Semiconductor laserSawtooth modulationSweep nonlinearity correctionElectro-optical phase-locked loop
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