Coherent Multispectral LIDAR in Silicon Photonics using Optical Phased Arrays

该项目将研究在已建立的CMOS制造代工模式下，在低成本、紧凑的硅芯片上开发激光雷达传感器的不同方面。由此产生的光子集成电路将包括各种各样的光学元件，包括片上激光器、波导、金属加热器、放大器和光电探测器，但早期的重点将放在开发光学相控阵（OPA）上，以在远场中引导和塑造光束。为了使用opa获得具有可接受系统性能的可行激光雷达传感器，必须满足某些标准。完全集成的激光雷达设备在硅芯片上的潜在影响是巨大的；这些设备可以取代迄今为止在自动驾驶汽车和无人机上看到的笨重的激光雷达系统，而SWaP（尺寸、重量和功率要求）的一小部分。例如，光学相控阵需要通过多模干涉耦合器将来自总线波导的光分裂成一个波导阵列，每个通道都有光学移相器，以单独控制每个光束的相位，从而将光束在自由空间中引导到期望的远场方向。为了通过阵列中每个波导通道的不同光束的建设性干涉产生高质量的窄发散输出光束，必须解决几个问题。第一个问题是，移相器利用热光效应工作，将导致相邻波导之间的热串扰问题，并破坏相邻通道中光束的相位。这个问题可以通过在移位器的位置增加波导之间的间隙来缓解。然而，这导致了第二个问题，即波导在阵列输出端的间距太大，导致远场的光束宽度太大。然而，在小间距下，相邻波导之间的模式将相互作用并传输功率，就像定向波导耦合器一样。波导也将有不同的光路长度，由于发散和收敛的路径部分。为了实现一个大型光学相控阵，所有这些问题的解决方案将被研究、模拟和制造。这些因素将在Lumerical MODE溶液中进行模拟。除了各种其他因素外，由于制造误差也会引起相位不均匀。这些因素将在制造后进行监测和测试，以找到相移器的最佳电压值，以对抗相位不均匀性。项目的后期可能会涉及到观察不同波长的激光，对场景进行多光谱分析，这将提供额外的目标信息。考虑到眼睛的安全操作和目标检测效率等因素，我们还将研究连续波源的频率啁啾，并将其与使用脉冲源进行片上操作的可行性进行比较。

The project will investigate the different aspects of developing a LIDAR sensor on a low-cost, compact silicon chip under the established CMOS fabrication foundry model. The resulting photonic integrated circuit will comprise a large variety of optical components including an on-chip laser, waveguides, metal heaters, amplifiers and photodetectors, but the early focus will be on developing an optical phased array (OPA) to steer and shape the beam in the far-field. In order to have a viable LIDAR sensor with an acceptable system performance using OPAs, certain criteria must be met and satisfied. The potential impact of a fully-integrated LIDAR device on a silicon chip is huge; these devices could replace the bulky LIDAR systems seen on autonomous vehicles and drones up until now at a fraction of the SWaP (Size, weight and power requirements).An optical phased array requires splitting light from a bus waveguide via a multi-mode interference coupler, for example, into an array of waveguides with optical phase shifters attached to each channel to control the phase of each beam separately, and hence steer the beam in free-space in the desired far-field direction. Several issues have to be addressed in order to produce a high quality, narrow divergence output beam via constructive interference of the different beams from each waveguide channel in the array.The first problem is that the phase shifters, operating using the thermo-optic effect, will incur thermal crosstalk issues between adjacent waveguides and disrupt the phases of beams in adjacent channels. This problem can be alleviated by increasing the gap between the waveguides at the location of the shifters. However, this leads to a second problem in that the waveguides will be spaced too far apart at the output of the array, leading to a large beam width in the far-field. At small spacings however the modes between adjacent waveguides will interact and transfer power as in directional waveguide couplers. The waveguides will also have different optical path lengths due to the divergent and convergent path sections. Solutions to all of these problems in order to realise a large optical phased array will be investigated, simulated and then fabricated. The factors will be simulated in Lumerical MODE solutions. There will also be phase nonuniformities induced due to fabrication errors in addition to various other factors. These factors will be monitored and tested post-fabrication to find optimum voltage values for the phase shifters to counter phase nonuniformities. The later parts of the project could involve looking at different wavelengths of lasers for a multispectral analysis of a scene, which would deliver additional target information. Frequency chirping of a continuous wave source will also be studied and its feasibility of on-chip operation compared to using a pulsed source considering factors such as eye-safe operation and efficiency of target detection.