LiDAR system based on integrated optical phased array (OPA)

In a key step toward developing LiDAR systems suitable for widespread commercial applications, CEA-Leti has developed a genetic algorithm for calibrating high-channel-count optical phased arrays (OPAs), as well as an advanced measurement setup capable of wafer-level OPA characterization.

OPA is an emerging technology that consists of an array of closely spaced (about 108m) optical antennas that emit coherent light over a wide range of angles. The resulting interference pattern can then be changed by adjusting the relative phase of the light emitted by each antenna. For example, if the phase gradient between the antennas is linear, a directional beam will be formed. By changing the slope of the linear gradient, the direction of the beam can be controlled, allowing solid-state beam steering.

This can improve performance in terms of scanning speed, power efficiency, and resolution compared to the bulky, power-hungry, and expensive mechanical beam steering systems used in current LiDARs. Another feature of OPA-based LiDAR systems is that they have no moving parts, as solid-state beam steering is achieved simply by phasing the antenna, significantly reducing the size and cost of these systems.

CEA-Leti reported the calibration and characterization results at the Photonics West 2021 Digital Forum in a paper titled "Development, Calibration, and Characterization of Silicon Photonics-Based Optical Phased Arrays."

"The development of high-performance OPAs will pave the way for inexpensive LiDAR systems for autonomous vehicles, holographic displays, biomedical imaging, and many other applications," said Sylvain Guerber, lead author of the paper. "However, widespread adoption of LiDAR will depend on lower system cost and smaller form factor."

LiDAR, which stands for Light Detection and Ranging, has become a key enabling technology for future sensing and vision systems. In addition to automotive and medical uses, they can also enable autonomous mobility for drones and robots, as well as industrial automation. Commercial LiDAR systems must meet stringent requirements, especially in automotive applications. In particular, high power and low divergence beams are required to accurately resolve the scene. For example, to resolve a 10cm object at 100m requires an OPA operating in a circuit with a wavelength of 1-0.8m, which should contain at least 1,000 antennas, each spaced 1.00-8m apart. Therefore, for commercial OPA-based LiDAR systems, high-channel-count OPAs must be developed.

Guerber says that it is possible to take advantage of the mature silicon photonics platform to produce integrated chip-scale OPAs with solid-state beam steering capabilities. However, this is only the first step towards fully functional OPAs, as the beam scanning requires preliminary calibration. This calibration process can take a lot of time due to the large number of optical antennas required, which is incompatible with large-scale deployment of the technology. Therefore, the CEA-Leti team has developed what is probably the first wafer-scale OPA characterization setup, an important step towards the industrialization of OPA-based LiDARs. In addition, a genetic algorithm based on Darwinian evolution has been developed to quickly and reliably calibrate high-channel-count OPAs. They can speed up calibration by up to 1,000 times compared to previously used algorithms.

Guerber noted that widespread commercial adoption of LiDAR technology in the automotive industry and other markets is expected to take several years. OPA is a crucial step, and CEA-Leti will continue to work on it.

“There are still a lot of challenges, especially at the system level,” he explained. “LiDAR is made up of many elements: lasers, drive electronics, OPA steering systems, detectors, and data processing capabilities. They all have to work together; OPA is only one part of the system.”

This work was partly funded by the French ANR through Carnot, the ECSEL Vizta European project and the French national program “Investissement d’avenir program at IRD Nanoelec” (n°ANR-10-AIRT-05).