(1) Laser emitting sub-system: TPGAS2S09H diode
laser with wavelength of 905 nm is used for this
project. This laser emitting sub-system is to
generate high energy of laser. As a initial
research, a prototype with 5 x 5 pixels2 3D image
array with <5cm ranging accuracy, in real time at
rates up to 30 Hz, is designated.
(2) Receiver sub-system: The most common detector
used to detect the laser echo pulse is linear mode
avalanche photodiode (APD). This research
adopted the Geiger-mode APD for receiving
photon single. 3D imaging laser radar that uses
Geiger-mode APDs is being developed (Aull and
Marino, 2005; Daniel, 2003; Johnson, 2003; Aull
et al., 2002; Marion et al., 2003). Because of the
effect of background light noise, a new system
design and new data processing method need to be
developed. The details of this sub-system will be
reported in the near future.
(3) Micro-Control Sub-system: which is core of
system. The microprocessor controls the entire
system and enables real-time control laser
emitting, receiving, measure of time interval, data
sampling, storage, etc. The control device also
provides interactive communication with each
modules. The control device is, therefore, usually
designed as keyboard display unit.
(4) POS subsystem: which is used to provide the
attitude of each laser ray and central position of
laser emitter for calculation of 3D geodetic
coordinates in a given coordination system.
(5) LiDAR point cloud pre-processing: which is used
to generate XYZ coordinates on the basis of range,
attitude angles of each laser ray and central
coordinates of laser emitter.
2.2 Principle of Flash Laser Emitting
A detail of principle for flash laser is given by Hu (2005).
A brief description is given in this Section.
If the size of the emitting plan of a diode laser is
L and L, in length and width, and the divergence angle
of the bundle of lasers is g , and 9, in horizontal and
vertical plan, the focal length is f,, the laser source is
located at the focus (see Figure 2).
With geometry in Figure 2, we have
60, = 2arctan( L ) (D
t
Generally, L << 2 f;, so the divergence angle of plan is
a eis (2)
Ff,
As observed in Eq. 2, the divergence angle is negatively
proportional to the focal length.
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B3, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
To make the direction of emitting laser is parallel to the
direction of entry laser, the diameter of aperture must be
at least
D, zL, 4 2f, an) (3)
Usually, L << D,, , the least diameter of aperture is
expressed by
D, «2f, tan (4)
So, the least width of laser plan is
D -—D,-L £5)
Substitute Eq. 3 into Eq. 5, we have
D, =2f tan) (6)
As observed from Eq. 6, the width of laser plan is
positively proportional to the focus length.
Combined Eq. 2 and Eq. 6, it is hard to simultaneously
meet both conditions, which results in difficulty of
designing a reasonable and powerful laser emitter sub-
system.
With the same method, we have divergence angle is
2.5 (D
t
To make laser power in the emitting system in vertical
plan, the least diameter of aperture is
BD RAF tan) ( 8)
The least width of emitter laser in vertical plan is
D,, = 2f, tan) (9)
For example, if the emitter plan is 225x400um, the
divergence angle is 10° in planar direction and 25° in
vertical direction, and divergence angle is 30-33mrad
after alignment.
With the two cases above, this paper select the fast —axis
conical lens with a focal length of 7.7 mm, and 7 mm and
9 mm in height and length; select a slow-axis conic lens
with a focal length of 13.7mm, and 13 mm and 15 mm in
height and length.
The major purpose of the emitting system is to align the
emitting laser. Thus, when selecting an emitting laser
system, we have to simultaneously consider both the
alignment characteristic and the complexity of optical
system in structure, processing and manufacturing. With
the computational parameters, the 3D model is simulated
using ZEMAX software, as depicted in Figure 3. The
simulated 3D model, associated with its size is depicted
in Figure 4.