| LiDAR(Light Detection and Ranging,LiDAR)is an active remote sensing device that uses a laser to sense the surrounding information,which is analyzed by the cloud computing system to achieve real-time environmental monitoring.This device has become the most effective solution for environment sensing in smart cars because of its advantages of high range accuracy,strong directionality,fast response time,and no interference from external clutter.LIDAR is divided into mechanical and solid-state,and solid-state LIDAR has gradually become the preferred vehicle radar for major automobile brands because of its smaller size,more stable reliability,high measurement accuracy,simple structure,and easy to hide in the car body without affecting the aesthetics.The most promising one is the all-solid-state LIDAR which adopts the Optical Phased Array(OPA)principle to meet the requirements of vehicle regulations,and the beam deflection does not require any macroscopic and microscopic moving parts.All optoelectronic components are integrated into a silicon-based optical chip,which makes the deflected beam more flexible;the manufacturing technology is compatible with the traditional Complementary Metal Oxide Semiconductor(CMOS)technology and can realize mass production.At present,OPA technical difficulties are mainly focused on how to reduce array crosstalk,improve the radiation power,increase the scanning angle,improve the scanning accuracy,and other issues.In this paper,we hope to improve the radiation efficiency,far-field spot,scanning range,beam width,and grating intensity of the antenna during far-field diffraction through the structural design and parameter optimization of the grating antenna,to strengthen the far-field radiation capability of the OPA transmitter.The specific work is as follows:1.To improve the radiation efficiency of the grating antenna,we propose a Si3N4-based tapered-tip double-layer auxiliary grating antenna,which consists of two offset Si3N4 grating layers,with the upper layer in a conventional rectangular structure and the lower layer in an inverted tapered-tip structure.It was found that the free-space diffraction efficiency of the beam radiation of the grating antenna unit exceeds 72%in the 1450 nm-1600 nm band,with the highest value of 97.5%at 1550 nm,and the overall directionality is much better than that of the conventional rectangular double-layer grating.Subsequently,a grating antenna array of 10 grating units with antenna spacing greater than half a wavelength and crosstalk lower than-16dB over the entire wavelength range was demonstrated;the far-field divergence angle of the beam was 1.3°×3° in the studied wavelength range,and the field of view exceeded 28.12°×23.43°,with the maximum range not exceeding 108.45°×23.43°.If in the infinite plane,when the grating antenna reaches the limit length of 1.6 cm and the number of array expansion channels increases to 1000,the far-field divergence angle can be reduced to 0.0081°×0.03°.The analysis results show that the tapered tip structure and double offset design can effectively increase the reflection effect of the bottom grating,reduce the remaining optical power attenuation rate in the waveguide,and then improve the directionality of upward radiation and far-field radiation performance.This is because the difference between the upper and lower structures affects the far-field interference effect,with higher directionality indicating more focused far-field beam energy and stronger interference capability;while a specific offset will lead to a different effective refractive index for each segment,and the perturbation of the refractive index will directly affect the diffraction performance of the device.2.Based on the double-layer grating structure,we further investigate the subwavelength waveguide grating antenna(WGA)-a millimeter-scale WGA based on the CMOS process subwavelength crescent arc structure,in which the crescent arc-shaped subwavelengths are placed equally spaced on both sides of the conventional strip silicon waveguide wavelength silicon segments on both sides of the conventional strip silicon waveguide with equal spacing,where the crescent arc is designed using a Y=AX2 function with uniform curvature.It is found that the overall optical crosstalk is below-20 dB when the optimal array spacing of 1.35μm is optimized for this WGA,and the wavelength tuning accuracy of 0.134°/nm in the theta direction is achieved in the studied 1500 nm-1600 nm band with more sensitive longitudinal scanning.In addition,compared with the conventional rectangular subwavelength WGA,the residual optical power decays exponentially during optical transmission,and the power decays to 0 when propagating to 5 mm.The analysis of the results shows that the equally spaced placement of subwavelength segments in the fading field and the crescent arc structure can effectively improve the grating limit length and unidirectional radiative properties of the WGA.Since the crescent arc is designed with a uniform curvature Y=AX2 function,the degree of opening and closing of the quadratic function has a great impact on the "focusing" ability during diffraction.Optimization of the optimal crescent arc opening and closing value can further increase the limiting length of the grating antenna,and the far-field beam will be narrower.In addition,the antenna transmits light both vertically and horizontally,which results in severe optical energy loss,which is detrimental to the unidirectional emission of the device;it also deteriorates the spot quality in the far field,resulting in at least two spots in the far field.For this reason,we suppress the lateral outgoing emission by tuning the subwavelength band grating parameters,leading to a continuous bound state effect in the fading field to reduce the bottom light absorption capacity,and thus improve the unidirectional radiation and far-field spot quality of the WGA. |
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