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.