High Power Distributed Bragg Reflector Laser And Its Application In Frequency-modulated Continuous Wave LiDAR

Light detection and ranging(Li DAR)technologies have been widely used in the area of autonomous driving,industrial manufacturing,mapping and other fields.Compared with the commercially available time of fight scheme,the frequency-modulated continuous wave(FMCW)scheme has advantages of high sensitivity,being hardly affected by the ambient light,and the ability simultaneously to obtain positon and velocity information in a single measurement.This thesis studies a distributed Bragg reflector(DBR)laser to meet the requirement of FMCW Li DAR light source for high-power,narrow linewidth and high linearity frequency sweeping.The main work of this thesis can be summarized as follows:(1)The high power DBR laser for FMCW Li DAR system is designed.In order to realize high output power,different from the sampled grating distributed Bragg reflector laser,the front mirror of the high power DBR laser uses a section of uniform grationg,which generates a low peak reflectivity of the reflection spectrum and high transmissivity,and the sampled grating of the back mirror generates the comb-like reflection spectrum with high reflection peak.In order to design the grating structure,the Transfer matrix method is used for the simulation of gratings in the front mirror and back mirror.The other sections of the laser,such as phase section and gain section,are designed so that the longitudinal mode spacing of the laser reaches 0.3 nm,which could meet the frequency sweeping range of 30 GHz.In order to increase the coupling effiency between the laser and the single mode fiber,the spot size converter is integrated at the output facet of the laser,so that the far field of the laser is approximately circular.And the spot size converter is simulated and optimized by the beam propagation method.(2)The high power DBR laser has been developed.In order to reduce the manufacturing difficulty of the laser,the high power DBR laser adopts the Offset quantum well structure.The grating is fabricated on the passive core waveguide.And the quantum wells are selectively removed before grating etching.The fabricated laser has been characterized.The test results show that the maximum output power of the DBR laser reaches 96 m W with linewidth of 313 k Hz and Side mode suppression ratio of 53 d B.The longitudinal mode spacing is about 0.29 nm,which agrees well with the design.The laser threshold current and series resistance are 23.5 m A and 6.5 Ω,respectively.By integrating the spot size converter at the output facet of the laser,the coupling efficiency between the laser and the lensed fibers,with beam waist diameters of 2.5 μm and 5 μm,reaches 64%and 44%,respectively.(3)The linear frequency sweeping is achieved by using the method of frequency predistortion,and measurements of speed and distance are successfully carried out.The nonlinear errors in frequency sweeping will reduce measurement accuracy,so calibration is performed before measurements.The method of frequency pre-distortion is used to calibrate the modulation signal loaded into the phase region.After several iterations,high linearity frequency sweeping is achieved.The calibrated modulation signal is loaded into the phase region of the high power DBR laser,and successful measurement of stationary and moving objects have been carried out,achieving the basic function of FMCW Li DAR.(4)The high power DBR laser is optimized and fabricated.The influence of laser phase noise on long distance measurement is analyzed,and the phase noise compensation method is used to realize the long-distance measurement in the fiber.In order to avoid the mechanical damage of the laser output waveguide during the cleavage process,the output port adopts the buried waveguide structure.In order to increase the heat dissipation capacity of the gain region and the semiconductor optical amplifier(SOA)region,and thus improve the saturation power of the laser,a thick gold electroplating process is used to fabricate the Pad electrode during the manufacturing process.As a result,the fabricated laser achieved an output power of 114 m W and a linewidth reduction to 140 k Hz.As the detection distance increases,the phase noise of the laser will cause the broadening of the frequency spectrum of the beat signal and reduce the measurement resolution.By using the phase noise compensation method,the effect of the phase noise on long-distance measurement is dramatically decreased,and finally the long-distance measurement of 100 m in the fiber has been realized,the error is less than 1%,and the spectral signal-to-noise ratio reaches 8 d B.