PNNL-13501 - Pacific Northwest National Laboratory

Figure 3. This portable system is being developed to recover
signals from rough surface back scattering. The use of a
commercially tunable cavity enhanced laser diode permits
coverage over many available absorption lines of gases.
There is a possibility of exploiting frequency modulation
spectroscopy at low frequencies if the laser beam has a
small target depth so that the return rays are embodied in
one or a few imaged speckles. In short-range
applications, the multi-path distortions are common-mode
when the pixels are matched to the telescope’s angular
resolution. This scheme is being static tested for possible
application on low-flying craft or where static platforms
can make use of returns from ~100 meters. The commonmode
rejection of multi-path distortions could allow more
sensitive signal recovery of narrow absorption lines. To
date, we have only tested results over ~50 meters indoors
with single pixel detectors, but the path-loss ratios bode
reasonable signal return for sensitive absorption
measurements in the short-wave infrared against low
reflective rough surfaces (R< 2%). Using a multiplexed
400 FY 2000 Laboratory Directed Research and Development Annual Report
detector array with optimal filtering for coherent signal
addition should produce substantial improvements. We
are also studying this setup for use with optical
amplification using commercial short-wave infrared
amplifiers for fiber communications. The noise
performance and gain of existing erbium doped fiber
amplifiers suggest that low-noise optical preamplifiers
could be used to help recover weak signals nearly as well
as a heterodyne receiver design. Optical alignment to the
fiber will require development of properly tapered multimode
fibers, but this alignment scheme may be easier to
effect in the field than heterodyne alignments which need
much better absolute angular match of two beams.
Summary and Conclusions
New and significant areas of coherent and frequency
modulation-differential absorption lidar research were
identified to optimize remote sensing strategies of our
lidar concepts. The development of the instrumentation
slowed due to our desire to understand the modeling
results. Nevertheless, three bench-scale systems have
been developed. The systems shown in Figures 2 and 3
can be applied against outdoor targets at ranges useful for
low-flying unmanned aerial vehicles and micro-airbornevehicles.
The latter could be used to position sensitive
short-wave infrared detectors for bistatic reception.
Multiple lidar architectures, using heterodyne or optical
amplification techniques will be studied next year in order
to provide a parametric basis for comparison. This field
work should provide valuable insights on atmospheric and
rough surface scattering properties.
Reference
Sheen DM. 2000. Frequency modulation spectroscopy
modeling for remote chemical detection. Pacific
Northwest National Laboratory, Richland, Washington.