SPIE is working with SAE International to develop lidar measurement standards for active safety systems. This multi-year effort aims to develop standard tests to measure the performance of low-cost lidar sensors developed for autonomous vehicles or advanced driver assistance systems, commonly referred to as automotive lidars. SPIE is sponsoring three years of testing to support this goal. We discuss the second-year test results. In year two, we tested nine models of automotive grade lidars, using child-size targets at short ranges and larger targets at longer ranges. We also tested the effect of high reflectivity signs near the targets, laser safety, and atmospheric effects. We observed large point densities and noise dependencies for different types of automotive lidars based on their scanning patterns and fields of view. In addition to measuring point density at a given range, we have begun to evaluate the point density in the presence of measurement impediments, such as atmospheric absorption or scattering and highly reflective corner cubes. We saw dynamic range effects in which bright objects, such as road signs with corner cubes embedded in the paint, make it difficult to detect low-reflectivity targets that are close to the high-reflectivity target. Furthermore, preliminary testing showed that atmospheric extinction in a water-glycol fog chamber is comparable to natural fog conditions at ranges that are meaningful for automotive lidar, but additional characterization is required before determining general applicability. This testing also showed that laser propagation through water-glycol fog results in appreciable backscatter, which is often ignored in automotive lidar modeling. In year two, we have begun to measure the effect of impediments to measuring the 3D point cloud density; these measurements will be expanded in year three to include interference with other lidars.
Lidar tomography is a method that constructs high-resolution images of objects from multiple range projections along different projection axes. This approach is one way to overcome traditional limitations in remote sensing with focal imaging such as diffraction, optical aberrations, and air turbulence. We have shown previously through detailed modelling and simulation that lidar tomography can generate resolved imagery of objects from a moving platform if sufficient diversity of view angles and appropriate geolocation accuracy requirements can be met. Here we show that the geolocation accuracy requirements can be met through a data-driven approach that does not require accurate knowledge of the platform’s position relative to the object being imaged. This alleviates a significant technical burden of motion tracking and opens the way for a more practical implementation of the lidar tomography technique for remote sensing and imaging.
A new approach to a tunable Pancharatnam phase beam steering device is proposed that can steer NIR light to angles greater than 10 degrees with greater than 80% efficiency.
12530 This conference presentation was prepared for the Advanced Optics for Imaging Applications: UV through LWIR VIII conference at SPIE Defense + Commercial Sensing 2023
Most lidar approaches configured for long range applications rely on large aperture telescopes or else aperture synthesis to resolve objects. Here we describe a different technique that draws its inspiration from computed tomography (CT) scan technology. This method, called lidar tomography, relies on narrow laser pulses to generate a series of highly-range resolved measurements from different look angles as the lidar platform moves around the object. We compare the convolutional backprojection technique for image reconstruction to a model based iterative algorithm to reconstruct images of various objects in a lab. We show through modelling and simulation that the lidar tomography approach can generate <1-inch resolved imagery from an airborne platform if sufficient angular diversity and appropriate geolocation accuracy requirements can be met.
Risley prism beam steering uses pairs of wedge prisms that can be continuously rotated to steer light over a wide angular range. Due to the chromatic dispersion from refraction through wedge prisms, these devices typically only operate on narrowband light. Here we describe the design, development, and characterization of a broadband Risley Prism optical beam steering device capable of steering light with sub-mrad accuracy over a 40-degree field of regard. The design leverages compact rotation motors for a reduced volume profile and incorporates dual silicon and germanium prisms to achieve achromatic beam steering across an extended mid-wave infrared band from 2-5 μm. The existing device supports an optical aperture up to 110 mm, but is scalable to larger sizes depending on the application. The Risley prism was characterized in terms of its thermal response, speed, achromaticity, optical quality, and volume.
Two methods for designing a continuous optical beam steering device are to use a linear phase profile based on physical optical pathlength (OPL) , and based on Pancharatnam phase (PP) . There are challenges with both of these basic approaches when considering design goals of high steering angle range, resolution, speed, and efficiency. In this overview talk, a comparison will be made between these two approaches with regard to these design goals. The limiting effects on each of these design goals will be considered with detailed FDTD optical modeling, and compared with experimental measurements.
This Conference Presentation, “Towards a deep-learning aided point cloud labeling suite,” was recorded at SPIE Photonics West held in San Francisco, California, United States
While Spectralon panels are largely assumed to be ideal Lambertian surfaces, their actual polarized reflective responses deviate from the ideal by at least a small amount at illumination and viewing angles off surface normal. The Mueller matrix response of four different panels between 10% and 99% reflectance were measured and the radiometric response from two distinct monostatic or nearmonostatic polarimeter systems are compared, one at Montana State University and one at the Air Force Research Lab. The deviations from an assumed ideal Lambertian surface are reported.
A new design is developed for a non-mechanical, tunable Pancharatnam phase based optical beam steering device that drastically improves the steering resolution for large angles (up to 11°). A fringe field switching structure is used to construct a Pancharatnam phase, an in-plane spiral pattern with a given pitch length, in a liquid crystal cell. The pitch length of a Pancharatnam phase device is the length along the aperture corresponding to a phase increase of one wave. A design utilizing neighboring pitch lengths of different sizes achieves a steering angle corresponding to the average length of the neighboring pitches. The alternating pitch length design is advantageous to the resolution of the device because the length of a single pitch is restricted to an integer number of electrodes in the fringe field switching structure. By implementing the new alternating pitch design, the pitch length can change by a smaller non-integer number of electrodes resulting in finer steering.
Chalcogenide phase change materials (PCMs) are uniquely suited for spectral tuning applications due to their contrasting dielectric material properties. Recent headway has been made towards realizing tunable photonic devices using twodimensional, sub-wavelength resonators by carefully designing geometries that optimize optical, electrical, and thermal performances using multi-physics analyses and machine learning. In this paper, we tackle two other essential aspects for creating application-specific, tunable PCM devices: (1) scalability of the device size and (2) high-throughput fabrication techniques. We employ a deep ultraviolet (DUV) stepper projection lithography to manufacture over 100 densely packed GST metasurfaces, each with a sample size of 5×7 mm2, all on a 4-inch Al2O3 wafer. These metasurface structures were discovered using artificial neural network (ANN) techniques and confirmed by finite-difference-time domain calculations. The primary structures under investigation were nanobar configurations enabling amplitude modulation at short-wave infrared wavelengths to realize efficient optical switches for free space optical multiplexing. The DUV fabrication technique can easily be extended to other metasurface geometries to demonstrate multi-functional, non-volatile photonic devices.
With the emergence of advanced 2D and 3D sensors such as high-resolution visible cameras and less expensive lidar sensors, there is a need for a fusion of information extracted from senor modalities for accurate object detection, recognition, and tracking. To train a system with data captured by multiple sensors the regions of interest in the data must be accurately aligned. A necessary step in this process is a fine, pixel-level registration between multiple modalities. We propose a robust multimodal data registration strategy for automatically registering the visible and lidar data captured by sensors embedded in aerial vehicles. The coarse registration of the data is performed by utilizing the metadata, such as timestamps, GPS, and IMU information, provided by the data acquisition systems. The challenge is these modalities contain very different sets of information and are not able to be aligned using classical methods. Our proposed fine registration mechanism employs deep-learning methodologies for feature extraction of data in each modality. For our experiments, we use a 3D geopositioned aerial lidar dataset along with the visible data (coarsely registered) and extracted SIFT-like features from both of the data streams. These SIFT features are generated by appropriately trained deep-learning algorithms.
Point cloud completion aims to infer missing regions of a point cloud, given an incomplete point cloud. Like image inpainting, in the 2D domain, point cloud completion offers a way to recreate an entire point cloud, given only a subset of the information. However, current applications study only synthetic datasets with artificial point removal, such as the Completion3D dataset. Although these datasets are valuable, they are an artificial problem set that we can not apply to real-world data. This paper draws a parallel between point cloud completion and occlusion reduction in aerial lidar scenes. We propose a crucial change in the hierarchical sampling using selforganizing maps to propose new points representing the scene in a reduced resolution. These new points are a weighted combination of the original set using spatial and feature information. A new set of proposed points is more powerful than simply sampling existing points. We demonstrate this sampling technique by replacing the farthest point sampling in the Skip-attention Network with Hierarchical Folding (SA-Net) and show a significant increase in the overall results using the Chamfers distance as our metric. We also show that we can use this sampling method in the context of any technique which uses farthest point sampling.
Pancharatnam Phase Devices (PPDs) are an exciting new area for optical component development. Single layer active devices that provide optical beam steering over a range of several degrees will be discussed. The devices considered here use a comb electrode structure to provide an in-plane electric field to control the optical axis orientation of the liquid crystal director to have the desired spiral pattern of a PPD device. Two basic concepts will be discussed: one that used the in-plane fields to “pin” the only the ends of the spiral pattern; and another that uses sub elements to defines the desired director orientation at several locations in the spiral. The concepts, design details, and modeling results are shown.
Coherent nanosecond pulses with high peak powers in the 2μm region are in demand for applications such as LIDAR and atmospheric sensing. In this paper we present a PM pulsed laser based on a MOPA configuration providing up to 50W of peak power. The 2039nm seed laser is a pre-amplified DFB-FBG laser with <10kHz linewidth. Nanosecond pulses produced by an acousto-optic modulator are amplified by a single booster stage amplifier using a double clad PM thulium-doped fiber. We demonstrate >10W of output peak power for 50ns pulses over repetition rates from 50kHz to 2MHz. For 4-μs pulses and a repetition rate of 50kHz, our MOPA delivers 28μJ of pulse energy.
There has been an increasing demand for fast and efficient random access pointing within emerging applications such as LiDAR, space based optical communications, displays, and autonomous vehicles. Particularly, Electro-Optical beam steering approaches have been considered to replace current mechanical beam steerers which are dominant technology in these applications. However, mechanical approaches have some issues such as mechanical complexity, pointing stability, high cost, bulky and heavy. Therefore, there is a need to replace mechanical steering devices with less costly nonmechanically scanned ones. Liquid Crystal-based devices are among the top candidates with promising performance. Last year, we have introduced a novel concept to design a tunable liquid crystal beam steering device using dual fringefield switching (FFS) cell to create an in-plane electric field with local control ability on the director of the liquid crystal [1]. The architecture allows to form a Pancharatnam Phase shape with continuous phase across an aperture without any resets. In this article, we will review optimization process of such a device to provide maximum output efficiency. Step by step optimization of design factors as well as material factors are explained, and an efficiency table is represented for comparison. Finally, sample experimental data is shown to match the modeling expectations for high efficiency.
Electro-optic (EO) beam steering offers several important advantages over traditional mechanical steering approaches for applications such as laser communication, LiDAR and autonomous vehicles. These advantages include a lower cost, faster steering rates, and a random access beam pointing capability. While progress towards replacing mechanical steering approaches has been promising, challenges remain to achieve large aperture sizes with tunable steering directions. In this paper, we propose a unique liquid crystal based Pancharatnam Phase Device (PPD) for beam steering. The PPD has the advantageous property of allowing a continuous and unbounded optical phase delay across an aperture without requiring any phase resets, thereby maximizing steering efficiency. Our PPD architecture employs a linear array of phase control elements (PCEs) to locally orient the liquid crystal director into a cycloidal pattern for beam steering. The PCEs are comprised of a fringe field switching (FFS) electrode structure to create an in-plane electric field with low-operating voltages and a fast liquid crystal response time of around several milliseconds. Detailed modeling of the proposed design will be presented which demonstrates the design concept.
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