This paper presents theory of speckle noise for a
frequency-modulation differential-absorption LIDAR system along with
simulation results. These results show an unexpected relationship between the signal-to-noise ratio (SNR) of the speckle
and the distance to the retro-reflector or target. In simulation, the use of an annular aperture in the system results in a
higher SNR at midrange distances than at short or long distances. This peak in SNR occurs in the region where the
laser's Gaussian beam profile approximately fills the target. This was unexpected since it does not occur in the theory or
simulations of the same system with a circular aperture. By including the autocorrelation of this annular aperture and
expanding the complex correlation factor used in speckle models to include conditions not generally covered, a more
complete theoretical model is derived for this system. Obscuration of the center of the beam at near distances is also a
major factor in this relationship between SNR and distance. We conclude by comparing the resulting SNR as a function
of distance from this expanded theoretical model to the simulations of the system over a double-pass horizontal range of
10 meters to 10 km at a wavelength of 1.28 micrometers.
This paper discusses the ongoing development of a compact, unattended low-power radiation detection system designed
for autonomous operation in regions with limited or no supporting infrastructure. This application motivates our focus on
two of the more challenging system development problems: (1) the development of compact, low-power electronics for
gamma-ray spectrometers and neutron detectors, and (2) analysis algorithms capable of distinguishing special nuclear
material from benign sources in the opaque signatures of
mid-resolution spectrometers. We discuss our development
efforts on these fronts and present results based on implementation in a proof-of-principle system composed of two 5-cm
× 10-cm × 41-cm NaI(Tl) crystals and eight 40-cm <sup>3</sup>He tubes.
The Pacific Northwest National Laboratory has developed a remote-sensing LIDAR system designed to detect trace chemicals in the atmosphere. Atmospheric optical turbulence is the largest noise source for the system, causing both fluctuations in the returned signal strength and signal loss due to laser beam break-up and wander. Field experiments have been conducted over the past few years in an effort to better understand the impact of atmospheric turbulence and develop strategies for improving the system. Studies have focused on the propagation of infrared laser beams at 1.278 and 9.56 micrometers over double-pass, horizontal path lengths ranging from 2 to 10 kilometers roundtrip under a variety of turbulence conditions. In addition, numerical simulations of our experimental setup have been developed to complement the experimental work. A comparison of results from the simulations with those from the field experiments shows reasonable agreement. Therefore, similar simulations will be used to aid in the design of a next-generation system.