PNNL-13501 - Pacific Northwest National Laboratory

output signal processing. The initial modeling effort
focused on the mathematical modeling of the frequency
modulation system. Formulas and mathematical
techniques have been developed to predict the detected
frequency modulation harmonic amplitudes as a function
of wavelength, modulation frequency, modulation index,
and absorption line shape. A direct detection numerical
model for the detector and preamplifier has been
developed and implemented that incorporates the effects
of thermal background induced photo-current, dark
current, signal current, and Johnson noise. Initial
modeling calculations at medium ranges (5 km to 30 km)
indicated that very high detection sensitivity is required.
Therefore, a heterodyne detection model was
implemented to detect near the signal shot noise limit.
Conventional differential absorption lidar systems
developed using pulsed CO2 lasers are performance
limited by the effects of target-induced speckle. A
numerical model predicting the effects of speckle for
frequency modulation systems has been developed.
frequency modulation-based systems differ significantly
from conventional differential absorption lidar systems
because the laser is continuously modulated in frequency
rather than jumping between two discrete frequencies.
One-dimensional and three-dimensional numerical
simulations are used to develop formulas predicting the
signal-to-noise ratio performance of a frequency
modulation system due to target-induced speckle. These
formulas predict that speckle can be mitigated by using a
small illuminated spot (at close range) which maintains
the speckle pattern as common-mode, or by using a large
illuminated spot (at medium to long ranges) that allows
for significant speckle averaging. The frequency
modulation-based system will have additional averaging
due to the de-correlation of the speckle pattern as the laser
frequency is modulated. This may represent a significant
advantage over conventional lidar systems.
The atmospheric model includes the effects of
atmospheric turbulence and absorption. Turbulence
causes small index of refraction variations that induce a
finite transverse coherence length, additional beam
divergence, and intensity scintillation. Formulas
predicting these effects have been obtained through the
technical literature and have been incorporated into the
model. Extensive models of atmospheric absorption have
been developed by the U.S. Air Force. This software
(FASCODE/HITRAN) is available and has been
implemented at our Laboratory to predict the atmospheric
background transmission spectrum.
The models developed under this project were used to
predict detection limits for two unmanned airborne
398 FY 2000 Laboratory Directed Research and Development Annual Report
vehicle scenarios. A medium range Predator scenario was
assumed to operate at a range of 6.5 kilometers with
10 cm diameter optics, and a long-range Global Hawk
scenario was assumed to operate at 30 km with 20 cm
diameter optics. The modeling predicted that sensitive
chemical detection for both scenarios would require
heterodyne detection coupled with speckle mitigation.
Efficient heterodyne detection required that the returned
light have a relatively uniform phase-front. Speckle
mitigation through averaging required a highly speckled
(nonuniform) phase-front. Therefore, a speckled
wavefront would not be detected efficiently using a
heterodyne detection system. One solution may be to use
small two-dimensional coherent arrays for detection. The
phase-front can be made relatively uniform over each
array element if the element has a size comparable to the
diameter of a speckle lobe at the focal plane, and would
allow simultaneous speckle mitigation and sensitive
heterodyne detection.
Experiments were conducted using short-wave infrared
optics to determine possible heterodyne detection
architectures. A frequency offset homodyne architecture
was constructed for bench-scale experiments (Figure 1).
This setup used an acousto-optic modulator to frequency
shift a local-oscillator beam relative to the transmitted
beam to perform the heterodyne mixing. Experiments
were conducted with heterodyne beat detection limits
approaching theoretical levels.
Figure 1. Shown is a bench-scale system to study different
frequency modulation-formatting schemes. Also included is
a short-wave infrared lasers imaging camera and monitor
used to help align the heterodyne beams to high precision.
Heterodyne Chemical Sensing
Self-referenced homodyne offset detection was used to
enhance signal recovery of weak signals due to the
specular component of scattering. We postulated several