David W. Roberts

Initial measurements using a 1.54-μm eyesafe Raman shifted lidar

We have demonstrated an eyesafe lidar system for cloud and aerosol studies using 45-mJ/pulse 1.54-microm radiation generated by wavelength shifting the output from a pulsed Q-switched Nd:YAG laser using a CH(4)Raman cell.

Optimizing lidar dynamic range by engineering the crossover region

Accommodating the large dynamic range of lidar signals is always a challenge for optical engineers. Signals from low altitudes are much larger than signals from high altitudes because of their inverse-range-squared behavior, as well as atmospheric absorption and scattering. It is well known that the onset of received lidar signals with range can be controlled by adjusting the crossover of the laser beam into the receiver field of view. However, a careful analysis has shown that, in many lidar applications much of the systems dynamic range can be used up before the range where the crossover is complete. In addition, the analysis shows that defocus is the primary contributor to the geometrical overlap function in determining the range dependence of the signal, and that understanding defocus is necessary for the optical designer to optimize system performance. Examples are given to illustrate the improvements in dynamic range that can be achieved by optimizing the focus of a lidar receiver.

Covert camera for screening of vehicle interiors and HOV enforcement

This paper describes a covert means of photographing the interiors of moving vehicles at all times of the day or night through clear or tinted windows. The system is called the Georgia Vehicle Occupancy System (GVOS). It utilizes an infrared (IR) strobe light to illuminate passenger and cargo compartments through side windows or the windshield. A high-speed, digital, infrared camera records the images and is capable of providing clear, stop-motion images of the interiors of vehicles moving at highway speeds. A human screener can view these images, or pattern recognition algorithms can be used to count the number of passengers, identify particular individuals, or screen the types and placement of cargo. Examples of vehicle interior images recorded at highway speeds are shown. For homeland security, such a system can be used to screen vehicles entering military bases or other sensitive sites or it can be implemented on highways for identifying and tracking suspicious individuals.

Boundary layer height measurements with an eyesafe Lidar

We have developed and operated an eyesafe lidar in support of an intensive set of air chemistry measurements in Atlanta, Georgia, which were part of the Southern Oxidants Research Program (SORP) during the summer of 1992. The lidar was used to monitor the thickness of the mixed layer by measuring the vertical distribution of boundary layer aerosols. The lidar system is based on a Raman-shifted Nd:YAG laser source at 1.54 microns wavelength with a pulse energy of 40 mJ and a pulse repetition frequency of 4 Hz. The receiver aperture was 46 mm in diameter and an InGaAs PIN diode was used as the detector. The lidar data was typically averaged over 1000 laser pulses, which required about 4 minutes. The lidar returns were range corrected to yield profiles of signal versus altitude in which the signal is proportional to the atmospheric backscatter coefficient. The profiles showed the vertical extent of boundary layer aerosols, and this was interpreted to find the mixed layer thickness. Data was acquired on nine days in July and August 1992. Measurements were typically made at 15-minute intervals from early morning until midafternoon. Mixed layer thicknesses provided by the lidar have been shown to be consistent with balloon sonde results, and they have proved to be useful in interpreting atmospheric chemistry results.

Atmospheric laser radar as an undergraduate educational experience

The Georgia Tech Research Institute has teamed with a local undergraduate women’s institution, Agnes Scott College, to develop an eye safe atmospheric laser radar (lidar) system as a research experience for undergraduates. The students constructed the lidar under the supervision of Georgia Tech researchers after attending lectures and doing laboratory work on the technologies required to design and build the system. The course succeeded in making lidar technology accessible and appropriate for undergraduates and as a model for other schools. The associated research projects include studies of the planetary boundary layer, tropospheric aerosols and clouds, and the stratospheric aerosol layer.

Experimental validation of the differential image motion lidar concept

We have experimentally validated the concept of a differential image motion (DIM) lidar for measuring vertical profiles of the refractive-index structure characteristic C(n)(2) by building a hard-target analog of the DIM lidar and testing it against a conventional scintillometer on a 300-m horizontal path throughout a range of turbulent conditions. The test results supported the concept and confirmed that structure characteristic C(n)(2) can be accurately measured with this method.

Photoactivated methods for enabling cartilage-to-cartilage tissue fixation

The present study investigates whether photoactivated attachment of cartilage can provide a viable method for more effective repair of damaged articular surfaces by providing an alternative to sutures, barbs, or fibrin glues for initial fixation. Unlike artificial materials, biological constructs do not possess the initial strength for press-fitting and are instead sutured or pinned in place, typically inducing even more tissue trauma. A possible alternative involves the application of a photosensitive material, which is then photoactivated with a laser source to attach the implant and host tissues together in either a photothermal or photochemical process. The photothermal version of this method shows potential, but has been almost entirely applied to vascularized tissues. Cartilage, however, exhibits several characteristics that produce appreciable differences between applying and refining these techniques when compared to previous efforts involving vascularized tissues. Preliminary investigations involving photochemical photosensitizers based on singlet oxygen and electron transfer mechanisms are discussed, and characterization of the photodynamic effects on bulk collagen gels as a simplified model system using FTIR is performed. Previous efforts using photothermal welding applied to cartilaginous tissues are reviewed.

Eye-safe lidar as an undergraduate research experience

Agnes Scott College and the Georgia Institute of Technology are jointly developing an eye safe atmospheric lidar as a unique hands-on research experience for undergraduates, primarily undergraduate women. Students from both institutions will construct the lidar under the supervision of Agnes Scott and Georgia Tech faculty members. The engineering challenges of making lidar accessible and appropriate for undergraduates are described. The project is intended to serve as a model for other schools.

A new type of lidar for atmospheric optical turbulence

We are developing a new type of lidar for measuring range profiles of atmospheric optical turbulence. The lidar is based on a measurement concept that is immune to artifacts caused by effects such as vibration and defocus. Four different types of analysis and experiment have all shown that a turbulence lidar built from commercially-available components will attain a demanding set of performance goals. The lidar is currently being built for testing scheduled in 2007.

Design and performance of a 100 inch lidar facility

A lidar system based on the 100 in. optical collimator at Wright- Patterson Air Force Base has been developed for middle atmosphere studies. The system has been demonstrated by recording Rayleigh backscatter returns from mesospheric air molecules at altitudes up to 90 km. These returns were then used to develop atmospheric density profiles. The design nof the system provided several unique engineering challenges due to the long focal length and size of the collimator used as the receiver telescope. Careful optical engineering in the receiver and an innovative, modular approach led to a design that eliminates potential problems due to defocus, detector nonuniformity, and detector saturation.