PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
May 2016
309
In other words, the new Geiger mode and single photon
lidar offer a better use of the photons generated by the laser
source, resulting in a denser point cloud from the same or a
less efficient laser source.
To understand this concept better, think about shining a
flashlight on a wall. What you see on the wall is a circular
footprint of the light beam. Imagine now that you put
a filter made of 100 x 100 tiny meshed squares right in
front of the flashlight lens. What you see on the wall now
is a pattern of 100 x 100 individual squared footprints,
representing each of the filter mesh squares. In a way, we
created 10,000 cells of light using one pulse of light. That is
how the new lidar results in denser point clouds—by better
use of the light energy. Figures 1 & 2 provide a simplistic
illustration of the principal of operations of these new lidar
systems. Figure 1 illustrates the optical element used by
the single photon lidar to split the single laser pulse into
multiple sub-pulses, while Figure 2 shows how segmented
detectors receive the backscatter from an array of sub-
pulses.
Status of Geiger Mode and Single
Photon Lidar
The Lincoln Laboratory of the Massachusetts Institute
of Technology (MIT) championed the research and
development of the Single-photon Avalanche Photodiode
(SPAD) detectors used by Geiger mode lidar.
In the last decade or so, MIT developed a few airborne
lidar systems for military applications. The technology was
later commercialized through Princeton Lightwave, Inc.
(PLI) of Cranbury, N.J.
PLI is the leading supplier of single-photon counting
products based on indium phosphide SPADs. SPADs
provide orders of magnitude improvements in applications
such as target identification, 3D mapping, ranging and
navigation. These single-photon imagers enable extreme
long-distance, free-space optical communications and
SPAD-based receivers, which are critical components in
quantum communications and sensing systems. However,
PLI does not produce an integrated airborne lidar system
fully equipped with scanning mechanism, Inertial
Measurement Unit (IMU) and GPS. Harris Corporation was
the first company to produce an airborne lidar
system that is based on PLI’s SPAD technology
for civilian applications. As for the single
photon technology, Sigma Space Corporation
developed its first multi-beam Single Photon
Lidar (SPL), dubbed “Leafcutter.” This was
followed by their newer generation, the High
Resolution Quantum Lidar System (HRQLS).
Figure 2. Back-scattered laser sub-pulses directed towards segmented detectors.
Figure 1. A Diffractive Optical Element (DOE) in Single Photon Lidar.
“As with digital
cameras, lidar systems
based on these
detectors are scalable.
It gets larger, i.e.
more pixels, as the
manufacturing process
of the semiconductor
wafers advances with
time.”
