| Autonomous vehicle technology is an automated technology with the ability of sensing its surroundings,planning its own route and operating without any human intervention.With the accelerating urbanization process,traffic safety and traffic congestion problems are becoming increasingly serious in China.Since autonomous vehicle technology can avoid 80% of accidents caused by human drivers,it has become the most effective solution to the increasingly traffic congestion and safety problems.Li DAR as an active detection system can achieve full-time,long-range,superior accuracy and angular resolution owing to the short wavelength of light.Because of its excellent performance,Li DAR is widely accepted as an indispensable sensor for autonomous vehicle.There are two fundamental challenges in a Li DAR system: optical beam steering technique and ranging method.Among the dominant ranging methods,To F is limited by the low precision,lack of velocity measurements and anti-interference capability.AMCW,another ranging method,is limited by the complex structures,high costs and lack of long-range ranging capability.The last dominant ranging method is FMCW,which is plagued by low stability,high complexity,and high insertion loss of the frequency modulation source.Thus,Li DAR requires an easily achievable,costeffective,high-performance ranging method.As for optical beam steering technique,Optical Phased Array(OPA)is considered as one of the most promising beam steering schemes due to its solid state,compact size,and high reliability.However,as the ranging methods commonly utilized in OPA Li DAR,To F and FMCW are impractical to commercial applications due to either requiring excessive optical power or the poor stability,high complexity,and high insertion loss of the FMCW source.Therefore,we demonstrated a ranging method referred to as phase-modulated continuous-wave.And a proof-of-concept OPA-Ph MCW Lidar is developed.The main content of the article includes:(1)The phase-modulated continuous-wave(Ph MCW)ranging method is proposed.Firstly,theoretical study on the Ph MCW ranging mechanism is carried out.Additionally,we built a Ph MCW system based on a mono-static configuration.Excellent accuracy of0.1 cm was obtained in close-range 1 m-8 m ranging experiments using a transmitting power of 10 m W.In addition,we varied the distance of target placed at the furthest 50-m,and the ranging error was as low as 2.2 cm,demonstrating the accurate ranging capability of Ph MCW.Given the SNR is proportional to the reciprocal of distance for heterodyne detection,the maximum ranging distance for the current setup is calculated to be 100.8 m.(2)The precision model of the Ph MCW method is explored.We conducted two sets of experiments with different distance and reflectivity,respectively.The minimum and maximum standard deviations are 2.8 cm and 3.3 cm,demonstrating exceptional precision of the Ph MCW system.Based on the ranging results,a modified precision model is proposed as well.According to such model,a vehicle with 10% reflectivity locating at 100 m would result in a precision of 6.62 cm,which is comparable to commercial Lidars,suggesting promising application prospect of the Ph MCW method.(3)The anti-interference capability of Ph MCW is explored.We conducted two sets of anti-interference capability experiments against To F and coherent Li DARs.Through an analysis of the difference in the measured precision with and without interference using the Ljung-Box method,we arrived at a quantitative conclusion on the antiinterference capability of Li DARs.The p values of the Ph MCW system were 0.0589 and 0.6327 with To F and coherent Lidar interference,respectively,demonstrating the excellent capability of the Ph MCW against interference.(4)The simultaneous velocity measurement mechanism of Ph MCW is proposed.Theoretically,we semi-quantitively explored the capability of velocity measurement of the Ph MCW method and explained the relationship between the frequency peaks and the target velocity.Experimentally,by measuring the radial velocity at different positions of the rotating disk,the capability of velocity measurement of Ph MCW was verified.And the velocity error ranges from-0.17 cm/s to 0.15 cm/s,suggesting exceptional velocity measurement accuracy.(5)The mathematical models of OPA are reviewed and the performance of the OPA chip is measured.Experimentally,the damage threshold of Si and Si N waveguides is measured to be 2.15 W and 35 W,respectively.Moreover,characteristics of the phase shifters were tested using a Mach-Zehnder interferometer.The rise and fall time were23.1 μs and 19.9 μs,respectively,and the power of π-phase shift is 20 m W.Finally,we tested the scanning characteristics of two dimensional OPA chip.The field of view(FOV)is 94°×14.4° with beam divergence of 1.41°×1.49°.(6)A proof-of-concept OPA-Ph MCW Lidar is developed.In the system,laser beam is emitted and steered by the OPA chip and recieved signal is collected by a lens.The target is composed of two letters “C” and “N” in respective planes separated by22.5 cm.After ranging all resolvable points,three dimensional point cloud is formed.The differential distance between the letters is 21.32 cm,which is in accordance with the experimental setup.The excellent 3D image fidelity demonstrated the feasibility of OPA-Ph MCW Lidar.And our work paves a novel way approaching to autonomous vehicle Li DAR. |
