Abstract
Receiver design is integral to the development of a new remote sensor. An effective receiver delivers backscattered light to the detector while optimizing the signal-to-noise ratio at the desired wavelengths. Towards the goal of effective receiver design, a multi-channel optical receiver was developed to collect range-resolved, backscattered energy for simultaneous hyperspectral and differential absorption spectrometry (LAS) measurements. The receiver is part of a new, ground-based, multi-mode lidar instrument for remote characterization of soil properties. The instrument, referred to as the soil observation laser absorption spectrometer (SOLAS), was described previously in the literature. A detailed description of the multi-channel receiver of the SOLAS is presented herein. The hyperspectral channel receives light across the visible near-infrared (VNIR) to shortwave infrared (SWIR) spectrum (350–2500 nm), while the LAS channel was optimized for detection in a narrower portion of the near-infrared range (820–850 nm). The range-dependent field of view for each channel is presented and compared with the beam evolution of the SOLAS instrument transmitter. Laboratory-based testing of each of the receiver channels was performed to determine the effectiveness of the receiver. Based on reflectance spectra collected for four soil types, at distances of 20, 35, and 60 m from the receiver, reliable hyperspectral measurements were gathered, independent of the range to the target. Increased levels of noise were observed at the edges of the VNIR and SWIR detector ranges, which were attributed to the lack of sensitivity of the instrument in these regions. The suitability of the receiver design, for the collection of both hyperspectral and LAS measurements at close-ranges, is documented herein. Future development of the instrument will enable the combination of long-range, ground-based hyperspectral measurements with the LAS measurements to correct for absorption, due to atmospheric water vapor. The envisioned application for the instrument includes the rapid characterization of bare or vegetated soils and minerals, such as are present in mine faces and tailings, or unstable slopes.
Highlights
All remote sensors, including various types of lidar instruments, employ receivers to collect backscattered energy
The telescope has a diameter of 203 mm and an effective focal length of 2032 mm that focuses light into a multi-channel, polarization insensitive, optical relay mounted to the rear port of the telescope
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Summary
All remote sensors, including various types of lidar instruments, employ receivers to collect backscattered energy. Ground-based, atmosphere-focused laser absorption spectrometry (LAS) instruments, commonly identified as differential absorption lidars (DIAL), have often employed a telescope as the primary aperture of the receiver [1,2,3,4]. Repasky [6] and Moen [7] split light into near-field and far-field receiver channels to provide atmospheric measurements over short (1 km) and long (up to 12 km) ranges, respectively. In another iteration of the Moen [7] two-channel DIAL receiver, a shared telescope for transmission and receiving enabled stable alignment and eye-safe beam expansion [4]
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