Avalanche Photodiodes for
LIDAR Applications
To meet the
growing market of laser based distance measurement, laser scanning
and mapping, shape recognition and remote sensing, Avalanche
Photodiodes (APD) especially suitable for LIDAR (Light Detection
and Ranging) are needed. Preferentially, APDs are used for applications
with very low optical signal strength or with high modulation
frequencies, as it is the case with LIDAR.
LIDAR is an optical remote sensing technology. An emitter sends
out infrared laser pulses and the reflected light is detected
by a photodiode. The distance and relative speed of other objects
can be determined from the runtime of the signal (time of flight)
and the speed of light. This measurement method is similar to
radar. Both technologies offer similar ranges for similar applications
but LIDAR has the significant advantage of a much higher resolution.
In addition, LIDAR systems can usually be produced at lower costs
because of the less expensive components and a single system
can cover
short and long ranges.
The advantage of a much higher angular resolution gives APDs
an outstanding role, especially when it comes to more complex
automotive applications. Advanced driver assistance systems such
as adaptive cruise control (ACC) or collision avoidance systems
do not only provide a more comfortable driving experience but
are also able to reduce the severity of accidents or even prevent
crashes entirely. While some of these systems are already well
established in higher vehicle classes, even better
assistance systems are introduced to deal with more and more
complex traffic situations.
The angular resolution of LIDAR systems makes it possible to
identify the horizontal or vertical position of a car or pedestrian
more precisely, and to distinguish different objects that are
located in same distance and moving with the same speed. Unlike
radar, LIDAR sensors are also able to recognize the size and
thereby draw conclusions about the type of the object. LIDAR
technology sometimes still has the reputation of losing performance
at bad weather conditions, but new generations of LIDAR systems
insure a high functionality even with rain or fog by receiving
multiple echoes from a single laser beam.
For many years, conventional LIDAR range finding systems have
been in use for civil and defense applications. Similar technology
is being used to create a 3D image of fixed or moving objects,
the so-called laser radar (LADAR). This is interesting, especially
for surveillance and orientation tasks to complement conventional
image technologies in unmanned aerial vehicles (UAVs) and other
autonomous vehicle systems. In another application a Laser Radar
seeker can detect objects and identify specific features (such
as the shape of a flying object) with a resolution of 10-15 cm
from a distance of 350 m and can react with countermeasure activities.
The conventional approach was to use
a laser opto-mechanical scanner requiring sophisticated optics.
A matrix APD array enables a real-time LADAR imaging of moving
targets with a single pulse. Within a single laser pulse each
channel of the array receives the reflected signal from the object
in a timed sequence. In contrast to CMOS based solutions APD
arrays have a much higher sensitivity enabling detection and
identification of objects hundreds or thousands of meters away.
LIDAR systems also become more and more relevant for environmental
applications. Based on the differences in the backscattering
of various particles, LIDAR systems are used for atmospheric
measurements, e. g. for the analysis of clouds, layers of aerosols
and the status parameters of atmosphere or for the detection
of traces of gases relevant to the climate. Additionally, the
significance of the technology is steadily increasing for wind
energy industry where it´s used for the measurement of
wind speed and direction. Cartography or Mapping is another large
field for LIDAR applications.
Here, sensors are fixed on an airplane and scan the earth's surface
and objects on it. The data are used to create 3D maps which
are the basis for e. g. the analysis of coastal erosions or the
risk of landslides to save lives as well as for geographic information
systems or major construction projects all over the world.
First Sensor manufactures and improves silicon APDs for over
20 years for demanding sensor solutions and offers a wide selection
of APDs for range finding and distance measurement applications.
First Sensor’s series -9 APDs is optimized for 905 nm laser
radiation, has a high quantum efficiency, fast rise time, low
noise and slow slope gain curve and is therefore particularly
suited for LIDAR applications. The product line includes single
element APDs as well as arrays in 5 x 5 (25 APD-pixel) and 8
x 8 (64 APD-pixel) matrix geometries and in linear configuration
with up to 16 elements to serve specific needs. First Sensor
offers these arrays both as component level devices and as hybrid
modules simplifying the integration and evaluation into your
system. The available high reliability packages include standard
TOs and ceramic carriers as well as cost efficient organic carrier
SMD packages. All packages can be built with specific band pass
filters matching the laser wavelength to improve signalto-noise
performance.
July 2013
