Diffractive optical elements for calibration of LIDAR systems: materials and fabrication

The promise of fully autonomous vehicles to replace the judgment of human drivers with real-time algorithmic decision-making based on optoelectronic systems relies fundamentally on the quality of the available data. Limitations imposed by sensor-resolution, available optical power, and achievable signal-to-noise ratios have been well studied in the light detection and ranging (LIDAR) application space. Additionally, the problem of integrating multiple sources of image data as well as the need to establish and maintain the system calibration over life are critically important to system reliability and safety. These latter concerns will receive even greater attention as self-driving vehicles begin to transition toward fully autonomous operation. Because of the importance of calibration to system performance and safety, the process of validating and recalibrating the system will ideally be integrated into the LIDAR system itself with calibration occurring automatically “anywhere and at any time,” without dedicated external infrastructure. Mass-market adoption is also being driven by the systems’ size and weight, as well as reliable manufacturability and resilience to environmental stresses. Due to their extreme stability, manufacturability, and small size, diffractive optical elements (DOEs) are well suited for use as optical calibration references. Current three-dimensional (3D) mapping systems based on structured light illumination already rely on DOEs as precision pattern generators to provide 3D depth sensing in a wide array mobile devices. We examine the potential use of DOEs as calibration elements in multicamera or LIDAR systems, including appropriate choices of materials, designs, and fabrication methods to ensure reliable long-term performance under automotive use conditions. We present simulations of the impact of DOE material properties on the accuracy of the generated dot patterns and consequently on the depth accuracy and lateral distortion of the 3D image. Additionally, we present requirements for DOE manufacture using conventional semiconductor fabrication technologies optimized for creating engineered surface nanostructures capable of transforming the output of a laser or other narrow-band source into a precise reference pattern.