APPLICATION NOTE: AN027
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eGaN FETs for Lidar – Getting the Most Out of the EPC9126 Laser Driver
The pulse width of the transmitted optical
signal has a great influence on the distance
resolution of a lidar system [8, 9]. Figure 3 helps
show why this is the case. If we look at the top
case, we send narrow pulses of light out from
the lidar. Since the light pulse must travel to
the target, be reflected, and travel back, for a
target at distance d, the time t
d
between pulse
transmission and reception is:
t
d
= 2d/c
Where c is the speed of light in air,
approximately 30 cm/ns (about 1 foot/ns for
the imperialists among us). By measuring
the time t
d
, we can compute the distance.
Now suppose that we send longer duration
pulses, as shown in the bottom case.
We see that if the pulse length becomes
long enough, the reected pulses begin to
overlap, and it becomes harder to distinguish
features in the environment.
For an idea of what pulse lengths are
desirable in practice, consider an electrical
current pulse width of 1 ns driving the laser
diode, which corresponds to an optical pulse
length of 30 cm. As features of the target
approach 15 cm, the received pulses begin
to overlap and become harder to distinguish.
While various signal processing techniques
can improve the resolution for a given pulse
width, it is clear that a shorter pulse gives
better inherent precision, and that pulses on
the order of a few nanoseconds or less are
desirable for human-scale resolution.
Pulse energy determines the range of the
lidar. As demand for better resolution drives
designs towards narrower pulses, the diode
current must increase in order to maintain
sucient pulse energy. Typical pulse current
can range from a few amps to hundreds of
amps. A number of laser diodes are specied
with nominal pulse currents in the range of
several tens of amps. Under typical data sheet
test conditions, e.g. Pulse repetition frequency
(PRF) = 1 kHz, pulse width t
w
= 100 ns,
peak
current I
DLpk
= 30 A, operating temp T
OP
= 23-
25°C,
the peak electrical input power can
approach 300 W for a triple junction edge
emitting laser. The average test duty cycle is
often < 0.1% to prevent overheating of the
laser die. It is possible to operate these laser
diodes at higher currents with shorter pulse
widths and obtain greater peak optical power.
In summary, typical laser diode requirements
for commercial o-the-shelf laser diodes in
lidar systems suitable result in desired peak
pulse current ranges from a few amps to a few
hundred amps, with pulse widths from 1 ns to
10 ns. In the next section, we will see how to
obtain these extreme pulses.
t
p
t
p
Lidar
Transmission
Reection
Transmission
Reection
Lidar
Figure 3. Eect of lidar pulse width on resolution. Top: narrow pulses allows reections to be easily distinguished.
Bottom: wider pulses can overlap, making them harder to distinguish and reducing distance resolution.
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