A dual-wavelength circular scanner with collinear transmit and receive axes has been developed for use in the SEAHAWK bathymetric lidar. The scanner optics consist of an achromatic prism pair located concentrically within a 11.3” diameter dual-zone holographic optical element (HOE). This scanner achieves coaligned green and infrared beams at a 20° off- nadir scan angle when using a 50W dual-wavelength laser (30W @ 532 nm and 20W @1064 nm) as the transmitter. The main engineering challenges in achieving the design were minimizing the optical pointing error between the four optical axes (two transmit and two receive) and developing a rugged prism pair design sufficient to withstand the high laser power. The design proved sensitive to fabrication and alignment errors so success depended on analyzing optical and mechanical tolerances, acknowledging fabrication limitations, measuring critical optical components, tailoring the design to the as- built components, and utilizing a custom alignment fixture featuring a digital autocollimator. Final measurements of the deployed scanner indicate its optical pointing error has a cone half angle error of less than 0.06° (1 mrad).
SEAHAWK is a high-performance, low-SWAP LIDAR for real-time topographic and bathymetric 3D mapping applications. Key attributes include real-time waveform and point cloud processing, real-time calculation of total propagated uncertainty (TPU), a novel co-located green and infra-red transceiver architecture based on a 12” circular scanner with holographic optical element (HOE), an ultra-compact Cassegrain telescope, custom detector architecture with dynamic load modulation (DLM), and analog-to-digital converters providing improved resolution, dynamic range, and sensitivity. SEAHAWK’s design yields higher sea-surface detection percentages than other circular scanning LIDARs and thereby enables more robust sea-surface correction strategies. The real time point clouds provide sensor operators with immediate, actionable intelligence about data quality while the aircraft remains on-station.
SEAHAWK is a new lidar for deep-water bathymetric surveying. Its performance and SWaP objectives generated requirements for the optical design affecting aperture, FOV, transmission efficiency, alignment accuracy, spectral filtering, and system size. Fabrication and other hardware limitations added constraints, particularly on the apertures of the detectors, filters, and custom scanner optics. An initial thin lens analysis produced a 3-channel receiver layout leading to the fabrication of an all-aluminum 300 mm diameter F/3.6 Cassegrain telescope having a total physical length less than 200 mm. An optimization of the relay optics maximized the narrowband filter performance by keeping the incidence angle constant across the system’s 38 mrad FOV. The resulting compact optical subsystem yields a smaller lidar head than other deep-water bathymetric lidars.
The Georgia Tech Research Institute (GTRI) is developing a transportable multi-lidar instrument known as the Integrated Atmospheric Characterization System (IACS). The system will be housed in two shipping containers that will be transported to remote sites on a low-boy trailer. IACS will comprise three lidars: a 355 nm imaging lidar for profiling refractive turbulence, a 355 nm Raman lidar for profiling water vapor, and an aerosol lidar operating at 355 nm as well as 1.064 and 1.627 µm. All of the lidar transmit/receive optics will be on a common mount, pointable at any elevation angle from 10 degrees below horizontal to vertical. The entire system will be computer controlled to facilitate pointing and automatic data acquisition. The purpose of IACS is to characterize optical propagation paths during outdoor tests of electro-optical systems. The tests are anticipated to include ground-to-ground, air-to-ground, and ground-to-air scenarios, so the system must accommodate arbitrary slant paths through the atmosphere, with maximum measurement ranges of 5-10 km. Elevation angle scans will be used to determine atmospheric extinction profiles at the infrared wavelengths, and data from the three wavelengths will be used to determine the aerosol Angstrom coefficient, enabling interpolation of results to other wavelengths in the 355 nm to 1.627 µm region.