Professor David Guerra Phasers
Prototype Holographic Atmospheric Scanner
for Environmental Remote Sensing (PHASERS)
Located on the campus of Saint Anselm College in Manchester NH, PHASERS is a ground-based atmospheric lidar (laser radar) system that utilizes a Holographic Optical Telescope and Scanner to obtain atmospheric backscatter profiles.
Figure 1: Holographic optical element at the bottom of the cone-shaped baffles acts as the receiver telescope and scanning mirror. The laser beam is transmitted coaxially, and the atmospheric backscatter is diffracted to focus on the photomultiplier tube (PMT).
PHASERS is built around a volume phase reflection Holographic Optical Element. This single optical element both directs and collimates the outgoing laser beam as well as collects, focuses, and filters the atmospheric laser backscatter, while offering significant weight savings over existing telescope mirror technology. Conical scanning is accomplished as the HOE rotates on a turntable sweeping the 1.2 mrad field of view around a 42 degree cone. As part of the research atmospheric aerosol and cloud return signals have been received in both stationary and scanning modes. The success of this program has led to the further development of this technology for integration into airborne and eventually satellite earth observing scanning lidar telescopes.
Figure 2: Stationary mode. 100 one-minute averaged files with the laser running at 20Hz.
PHASERS has been continually operated and upgraded during its approximately six years of operation. In its stationary mode of operation, the HOE is oriented to point in one direction and data are taken over a given period of time. The 100 consecutive, one minute averaged, data files presented in the surface plot in the figure above were taken between 8:00 -9:40 PM on October 1996 with the system pointing north. The lines of maximum contour are at the backscattering ratio value of 1.203 and the additional contour lines are at 0.15 intervals. With this time sequence of data files it is evident that the system can detect the change in multiple aerosol layers as a function of time.
Figure 3: Relative backscatter signal strength from one scan as a function of range (m) and angle (degrees).
In its scan mode of operation the HOE is rotated at a
fixed rate and data are taken continuously and averaged in predetermined
intervals. This essentially divides the sky into equal segments
of a three dimensional hollow cone. A data set of a conical scans
using the 2 kHz laser was taken between 7:00 - 7:30 PM on November 3,
1997. During this data acquisition, the HOE was rotated at a constant
rate of one revolution every ten minutes and data files were stored in
thirty one-minute averages. Thus, each file represents one tenth
of the sky and each scan begins and ends in the north. It is important
to recognize that the cones of data produced in this process represent
the time evolution of the sky in the FOV swept out by the HOE. By
analyzing these data with this in mind, atmospheric structures, such as
clouds, can be seen advecting across the sky.
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