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.
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.
During the last several years, Pacific Northwest National Lab has
developed a remote sensing system designed to detect trace chemicals
present in the atmosphere. Using Frequency Modulated Differential
Absorption LIDAR (FM DIAL) techniques chemical signatures have been
observed over pathlengths ranging from several hundred meters to
several kilometers. Throughout the development process, we have
encountered many challenges. Some of these have been overcome but
others will require more novel solutions.
A trailer based sensor system has been developed for remote chemical sensing applications. The sensor uses quantum cascade lasers (QCL) that operate in the long wave infrared. The QCL is operated continuous wave, and its wavelength is both ramped over a molecular absorption feature and frequency modulated. Lock-in techniques are used to recover weak laser return signals. Field experiments have monitored ambient water vapor and small quantities of nitrous oxide, tetrafluoroethane (R134a), and hydrogen sulfide released as atmospheric plumes. Round trip path lengths up to 10 km were obtained using a retroreflector. Atmospheric turbulence was found to be the dominating noise source. It causes intensity fluctuations in the received power, which can significantly degrade the sensor performance. Unique properties associated with QCLs enabled single beam normalization techniques to be implemented thus reducing the impact that turbulence has on experimental signal to noise. Weighted data averaging was additionally used to increase the signal to noise of data traces. Absorbance sensitivities as low as ~1x10<sup>-4</sup> could be achieved with 5 seconds of data averaging, even under high turbulence conditions.
A trailer-based sensor system has been developed for remote chemical
sensing applications. The detection scheme utilizes quantum cascade
lasers operating in the long-wave infrared. It has been determined
that atmospheric turbulence is the dominating noise source for this system. For this application, horizontal path lengths vary from several hundred meters to several kilometers resulting in weak to moderate to strong turbulence conditions. Field experiments have simultaneously monitored meteorological and atmospheric quantities during remote sensing in order to better understand the impact of turbulence on horizontal beam propagation. A numerical model has been developed to simulate the performance of the system and comparisons between simulation and experiment have been encouraging.
The Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) will improve range measurements to the moon by at least an order-of-magnitude, with the goal of achieving millimeter precision. Lunar ranging provides the most stringent tests of Einstein's strong equivalence principle, as well as placing the tightest constraints on the time evolution of Newton's gravitational constant. At the heart of APOLLO is an integrated array of avalanche photodiodes (APDs) developed at MIT Lincoln Laboratories. These devices are capable of detecting the arrival of a single photon with high temporal precision (< 100 ps), with detection efficiencies as high as 50%. The thin APD arrays have breakdown voltages in the neighborhood of 25 volts, active areas 20, 30, or 40 microns in diameter, placed on 100 micron centers in a square pattern. APOLLO will initially work with a 4×4 array, but may eventually upgrade to a larger format. The potential use of APD array technology in other areas of astronomy is briefly discussed.