This PDF file contains the front matter associated with SPIE Proceedings Volume 12537, including the Title Page, Copyright information, Table of Contents and Conference Committee lists.

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.

In laser detection and ranging (LIDAR) we observed that, in a scattering medium, the target signature is overlaid by scattered light. Here, we distinguish back scattered light resulting in a strong background signal which decreases with the distance (power law) and light that is scattered on the return path producing a halo of light around the target. Both effects can impede accurate ranging and lead to an error in estimating the size and shape of the target. In this paper, we analyze the halos around a target located in a fog tunnel with varying fog density. The measurements were performed using time-correlated single photon counting. Hence, the occurrence of the halos was investigated spatially and temporally with a resolution of a few picoseconds. We have simulated photon propagation in the forward and backward directions, allowing us to explore the optical properties of the fog a such as the mean scattering length and the phase function. Furthermore, we can reconstruct the shape and position of the target by back-projecting the data building a Huygens-Fresnel wavefront.

Remote cavity exploration is a matter of probing hollow structures such as caves and cavities and is applied in security and military scenarios, used for assessing cavities in disaster and damage scenarios or geological and archaeological excavations, and attracted interest for space applications. In contrast to detailed exploration and mapping approaches, we have developed a rapid and simple assessment of cavities that allows initial remote evaluation and classification of cavity sizes prior to further exploration by other means. Our method uses time correlated single photon counting (TCSPC) to measure the temporal response of cavities to excitation by short laser pulse illumination. We observe a temporal signature consisting of multiple peaks due to multiple-bounce photon path before the onset of intermixing of signatures lead to an integration and homogenization of intensity. The observed signatures represent photon paths with reflections from different surface areas within the cavity. Based on the temporal sequence and shape of the signatures, we can determine the size of the cavity investigated. In this publication we focus on the analysis of the recorded signatures using a multi-peak fitting algorithm and a propagation model assuming a spherical cavity shape

Enhancements to rapidly tunable photon orbital angular momentum (OAM) states have opened the door to enhancing free space optical applications across various domains. Photon OAM states have been proven to identify preferential optical transmission channels (eigenchannels) that reduce the impact of turbulent media on free space optical links. In this study, we initiate the use of photon OAM-induced preferential optical transmission channels to enhance the detection and sensing of concealed objects by investigating experimental thrusts into non-electronic feedback mechanisms such as diffuse scattering. This technology leverages the compact, rapidly tunable OAM generator technology developed by researchers at Clemson University to identify these preferential channels and take advantage of them to demonstrate improvements in detectability over traditional lasing techniques. In this work, we detail forthcoming experiments between NIWC Atlantic and Clemson that are focused on investigating this issue, which will enable future experimentation and research into the enhancement of detecting concealed objects among other remote sensing applications.

Kinematic laser scanning allows for efficient acquisition of large-scale and highly accurate 3D data. Georeferencing of the LiDAR data requires integration with auxiliary navigation systems. The standard processing pipeline consists of GNSS/IMU integration, georeferencing, and subsequent adjustment of the laser data. In contrast, we propose a holistic approach for GNSS, IMU and LiDAR integration where all measurements are incorporated in a single model, thereby enabling accurate estimation of both trajectory and system calibration parameters. This method is applied to the case of a 3D laser scanner mounted on a moving platform. We demonstrate precise georeferencing of the kinematically acquired data by comparison to statically acquired reference data.

The Army's High-Castle (HC) Airborne testbed 2021 flight campaign collected 3D point cloud data derived from a Geiger-mode LiDAR system. While the 3D data exhibited exceptional resolution of 3D ground targets, the horizontal resolution was 3-4 times worse than theoretical limits. The sources of resolution degradation remain a current research and calibration study. This paper presents a comprehensive virtual Kinematically Linked Model Framework that rigorously models the physical components of HC testbed hardware and the propagation of component errors on derived 3D point cloud products. In-depth examples of component errors and their impact on resolution degradation are presented.

In the present work, an experimental system is implemented, and a theoretical model is built that allows quantifying atmospheric depolarization in the city of Santiago de Cali, Colombia. The experimental setup uses a LiDAR coupled to a Polarotor, which allows the separation of the backscattered light into its parallel and perpendicular polarization components. This device allows the use of a single photomultiplier tube, thus facilitating calibration procedures. The theoretical model is based on the Mueller formalism and considers the contribution of each optical element of the LiDAR system on the polarization of the backscattered light. This is achieved by assigning to each element a Mueller matrix and subsequently calculating the matrix associated with the whole assembly. The contribution of the optical elements of the system on the depolarization parameter d is determined. The corrections to the signals obtained are established, so that the data is not altered by the particularities of the assembly used.

The use of LIDARs to characterize atmospheric properties has become commonplace since near the invention of the laser. The accuracy of these measurements is generally limited by the signal-to-noise ratio of the optical channels making the measurement – typically constrained by several environmental and systematic parameters, such as solar background, power-aperture product, etc. These measurements are usually modeled according to various forms of the atmospheric LIDAR equation along with a solar background model. However, one of the limitations of current approaches is failing to account for the effects of optical turbulence on the LIDAR signal. While researchers have already established a correction factor for the hard target LIDAR equation, this factor has not yet been applied to atmospheric LIDAR systems – some of which aim to measure optical turbulence as a data product. This paper investigates the application of a correction factor to the elastic LIDAR equation to account for the effects of turbulence. Using mathematical modeling and simulation, it was observed that the number of photons detected by the system is overestimated and it decreases as turbulence increases as expected and in agreement with previously published results. Additionally, a preliminary investigation of the effects of turbulence on the relationships between the LIDAR system signal-to-noise ratio (SNR), the field of view (FOV), and transmitter divergence is addressed and opened up for future discussion. For example, increasing the beam divergence and FOV mitigates the effects of turbulence but increases the number of background photons detected, effectively reducing the SNR. It is expected that these results can be used by system designers to understand how turbulence will affect the performance of atmospheric LIDAR systems and provide quantitative tradeoffs in design decisions.

Surface dust blown by a lunar lander is a threat to operations and assets. Multi-national lunar exploitation makes this a potential defense problem. To aid prediction and protection, we are developing a lander-mounted laser-based instrument to obtain empirical particle-size distributions in ejecta plumes. The method is based on analysis of laser propagation decay at multiple wavelengths. System design depends on expected laser propagation decay lengths in the cloud of lunar particles lofted by the lander rockets. We present laboratory experiments to confirm theoretical expectations for laser propagation decay constants for independently known particle size distributions. The method of extracting particle size distributions from measured decay constants at multiple wavelengths is demonstrated. Predictions are made for decay constants in lunar plumes with representative regolith size distributions and minerology.

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.

Global Shutter Flash Lidar (GSFL) is a powerful technology for a host of 3D ranging applications. GSFL generates real-time organized point clouds without motion distortion, which makes it an attractive technology for applications involving moving targets such as remote sensing, guidance navigation and control (GNC), and space situational awareness. In contrast to scanning lidar modalities, the field of view of GSFL is fixed, requiring an external gimbal in order to extend the field of regard without degrading the LIDAR range performance. To overcome this challenge, Advanced Scientific Concepts LLC has developed a GSFL that incorporates Liquid Crystal Polarization Gratings (LCPG) to steer the field of view of the GSFL, resulting in an increased field of regard. Passive LCPGs are thin birefringent solid films that steer light to one of two deflection angles, depending on the polarization handedness of the circularly polarized input light. Electrically variable liquid crystal wave plates included in the stack enable control of polarization handedness to select the desired steering angle at each LCPG. Not only does this result in a step and stare scanner that is simple to control and does not introduce motion blur, but it is also ideal for platforms that have low SWaP requirements. Here we present the operational concepts of the non-mechanical beam steering and quantify the effects of the LCPGs on the GSFL performance.

Semiconductor optical amplifier (SOA) has drawn much attention due to its critical need in coherent detection scheme such as FMCW (frequency-modulated continuous-wave) in automotive LiDAR (Light Detection and Ranging). Coherent detection provides more features than ToF (Time of Flight) such as speed and direction for autonomous vehicles. Instead of a bulky and expensive fiber laser, a coherent laser source with high gain SOA can achieve small form factor with Si PIC (Photonic Integrated Circuit). Here we present a proprietary SOA structure based on AlInGaAs material system with multiple quantum wells on InP substrate. The SOAs with curved and tilted straight waveguides were developed and tested. The saturated output power of such SOA at 1550nm and 1310nm can reach higher than 350mW and 450mW with high wallplug efficiency. The small signal gain exceeds 40dB for both 1310nm and 1550nm. The low anti-reflection (AR) coating can achieve 0.01% reflectivity, and the noise figure and near-field mode fields of various SOA configurations are presented and compared. An array of four SOA waveguides at 127m or 500m pitch can deliver total output power over 2 Watts with proper heat sinking. SOA arrays can also be processed as individually addressable with electrical and optical isolations. Such high-performance SOA array offers the design freedom to LiDAR systems with various scanning strategies such that long range detection can be realized. Gain chip, RSOA (Reflective SOA) based on the curved waveguide for external cavity laser configurations is tested and discussed. Self-alignment features can be built onto the SOA chipset to achieve integration of Si PIC for minimal footprint and low-cost mass production.

ICESat-2 is a space-based laser altimetry mission that provides accurate 3D representations of the earth’s surface. The Advanced Topographic Laser Altimeter System (ATLAS) onboard ICESat-2 can measure surface heights with great accuracy, providing critical measurements needed to better understand key surface structure variables. The Photon Research and Analysis Library (PhoREAL) was designed to provide a customizable analysis tool for NASA’s ICESat-2 Land and Vegetation (ATL08) data. With PhoREAL, users can resample, reproject, and recalculate terrain and canopy height statistics at any along-track resolution. PhoREAL is designed both as a command line tool and a Windows-based GUI and provides functionality such as reading ATL08/ATL03 data files and exporting the geolocated photon data to multiple output file formats; combining ATL08/ATL03 data products to label the individual photons as noise, ground, canopy, and top of canopy photons; comparing and aligning the measured ICESat-2 data relative to reference data; computing ICESat-2 height and radiometric statistics for ground and canopy photons at specified bin lengths; and plotting the ICESat-2, reference, and statistical data together. PhoREAL is available as free and open-source software on GitHub (https://github.com/icesat-2UT/PhoREAL). It can run as a Windows executable or in a Python environment for compatibility in both Windows and Linux environments. PhoREAL is a valuable tool for scientists who want to analyze ICESat-2 data. It provides a user-friendly interface for accessing and processing the data, and it offers a variety of features that can be used to extract valuable information from the data.

We present a Simple POint Cloud (SPOC) file format, suitable for efficiently storing and processing geospatial point cloud data. This format provides support for 64-bit floating-point precision coordinates, compressed storage, and data streaming. The code base is implemented as a header-only, modern C++ library with Python extensions under an open source license. The format can be applied in a wide variety of use case scenarios, and was motivated by a need for high-precision, transparent data storage and transmission for geospatial processing and machine learning applications. Existing file formats sometimes either do not support the precision and dynamic range necessary for certain applications, they do not support common interprocess communication protocols, or they are overly complex or rigid.