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.
The infrared sensors group at the Pacific Northwest National Laboratory (PNNL) is focused on the science and technology of remote and in-situ chemical sensors for detecting proliferation and countering terrorism. To support these vital missions, PNNL is developing frequency-modulation techniques for remote probing over long optical paths by means of differential-absorption light detecting and ranging (LIDAR). This technique can easily monitor large areas, or volumes, that could only be accomplished with a large network of point sensors. Recently, PNNL began development of a rugged frequency-modulation differential-abosrption LIDAR (FM-DIAL) system to conduct field experiments. To provide environmentla protection for the system and facilitate field deployments and operations, a large, well insulated, temperature controlled trailer was specified and acquired. The trailer was outfitted with a shock-mounted optical bench, an electronics rack, a liquid nitrogen Dewar, and a power generator. A computer-controlled gimbal-mounted mirror was added to allow the telescope beam to be accurately pointed in both the vertical and horizontal plane. This turned out to be the most complicated addition, and is described in detail. This paper provides an overview of the FM-DIAL system and illustrates innovative solutions developed to overcome several alignment and stability issues encountered in the field.
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 ~1x10-4 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.
We consider the application of mid-infrared (MIR) wavelength quantum cascade lasers (QCL) as sources for free-space optical communications. QCL’s possess high modulation bandwidth and excellent optical performance in the atmospherically transparent MIR spectral range. In order to investigate this potential application area, we have performed a series of comparative evaluations on analog and digital free-space optical links operating in the near-infrared (NIR) (830nm, 1300nm and 1550nm) and mid-infrared (8μm). The measurements were made using well controlled atmospheric conditions in the 65ft long Pacific Northwest National Laboratory’s Aerosol Wind Tunnel Research Facility using water vapor, oil vapor and dust as the scattering media. We measured the transmitted intensity as a function of the density of scatterers in the tunnel. We also performed bit error rate analysis of signals transmitted at the DS-3 data rate. The QCL link consistently showed a higher performance level when compared to the NIR links for water fog, oil fog and dust scattering.
The small size, high power, promise of access to any wavelength between 3.5 and 16 microns, substantial tuning range about a chosen center wavelength, and general robustness of quantum cascade (QC) lasers provide opportunities for new approaches to ultra-sensitive chemical detection and other applications in the mid-wave infrared. PNNL is developing novel remote and sampling chemical sensing systems based on QC lasers, using QC lasers loaned by Lucent Technologies. In recent months laboratory cavity-enhanced sensing experiments have achieved absorption sensitivities of 8.5 x 10-11 cm-1 Hz-1/2, and the PNNL team has begun monostatic and bi-static frequency modulated, differential absorption lidar (FM DIAL) experiments at ranges of up to 2.5 kilometers. In related work, PNNL and UCLA are developing miniature QC laser transmitters with the multiplexed tunable wavelengths, frequency and amplitude stability, modulation characteristics, and power levels needed for chemical sensing and other applications. Current miniaturization concepts envision coupling QC oscillators, QC amplifiers, frequency references, and detectors with miniature waveguides and waveguide-based modulators, isolators, and other devices formed from chalcogenide or other types of glass. Significant progress has been made on QC laser stabilization and amplification, and on development and characterization of high-purity chalcogenide glasses, waveguide writing techniques, and waveguide metrology.