Tech Briefs Magazine - March 2022 - PIT-19

whereas an FMCW system is captive for
their entire dwell time.
Compounding the captivity time is
the fact that FMCW often requires a
minimum of two laser frequency sweeps
(up and down) to form an unambiguous
detection, with the down sweep
providing information needed to overcome
ambiguity arising from the mixing
range + Doppler shift. This doubles
the dwell time required per shot above
and beyond that already described. The
amount of motion of a target in 10µs
can be typically only 0.5mm, making it
difficult to separate vibration versus real,
lineal motion.
Claim #4: FMCW Has Less Interference
Quite the opposite actually!
Spurious reflections arise in both ToF
and FMCW systems. These can include
retroreflector anomalies like " halos, "
" shells, " first surface reflections, off-axis
spatial sidelobes, as well as multipath,
and clutter. The key to any good LiDAR
is to suppress sidelobes in both the spatial
domain (with good optics) and the
temporal/waveform domain. ToF and
FMCW are comparable in spatial behavior,
but where FMCW truly suffers is
in the time domain/waveform domain
when high contrast targets are present.
Clutter: FMCW relies on window-based
sidelobe rejection to address
self-interference (clutter) that is far less
robust than ToF, which has no sidelobes.
To provide context, a 10µs FMCW
pulse spreads light radially across a 1.5
km range. Any objects within this range
extent will be caught in the FFT (time)
sidelobes. Even a shorter 1µs FMCW
pulse can be corrupted by high intensity
clutter 150m away. The 1st sidelobe of a
Rectangular Window FFT is well known
to be -13dB, far above the levels needed
for a consistently good point cloud. (Unless
no object in the shot differs in intensity
by any other range point in a shot by
more than about 13dB, something that is
unlikely in operational road conditions).
Of course, deeper sidelobe taper can
be applied, but at the sacrifice of pulse
broadening. Furthermore, nonlinearities
in the receiver front end (so-called
spurious-free dynamic range) will limit
the effective overall system sidelobe
levels achievable due to compression
and ADC spurs (third order intercepts);
phase noise6
; and atmospheric phase
modulation etc., which no amount of
window taper can mitigate. Aerospace
and defense systems can and do overPhotonics
& Imaging Technology, March 2022
come such limitations, but we are unaware
of any low-cost automotive grade
systems capable of the time-instantaneous
>100db dynamic range required
to sort out long-range small objects from
near-range retroreflectors, such as arise
in FMCW.
In contrast, a typical Gaussian ToF
system, at 2ns pulse duration, has no
time-based sidelobes whatsoever beyond
the few cm of the pulse duration
itself. No amount of dynamic range between
small and large offset returns has
any effect on the light incident on the
photodetector when the small target return
is captured.
First Surface: A potentially stronger
interference source is a reflection
caused by either a windshield or other
first surface that is applied to the LiDAR
system. Just as the transmit beam is on
near continuously, the reflections will
be continuous, and very strong, relative
to distant objects, representing a similar
kind of low frequency component that
creates undesirable FFT sidelobes in the
transformed data. The result can also
be a significant reduction of usable dynamic
range. Furthermore, windshields,
being multilayer glass under mechanical
stress, have complex inhomogeneous
polarization. This randomizes the electric
field of the signal return on the photodetector
surface, complicating (decohering)
optical mixing.
Lastly, due to the nature of the
time domain processing vs. frequency
domain processing, the handling of
multi-echoes-even with high dynamic
range-is a straightforward process in
ToF systems, whereas it requires significant
disambiguation in FMCW systems.
Multi-echo processing is especially important
in dealing with obscurants like
smoke, steam, and fog.
Claim #5: FMCW is Automotive Grade,
Reliable, and Readily Scalable
This is unproven at best.
FMCW's purported advantage comes
from the fact that it leverages photonics
and telecommunications technology
maturity, thereby facilitating scalability
to higher performance levels (in addition
to cost savings). True, FMCW allows
low-cost photodetectors, like PINs,
whereas ToF often use APDs and other
more costly detectors. However, the details
are far more nuanced.
The supply chain for LiDAR components
is relatively nascent, but components
like fiber lasers, PIN array receivers,
ADCs and FPGAs or ASICS have
been used in various industries for years.
These types of components are very low
risk from a supply base point-of-view.
By comparison, the critical component
for FMCW systems is the very low phase
noise laser, which has many tight requirements
and no other high-volume
user to help drive down volume manufacturing
costs.
The optical components used in ToF
LiDAR systems are derivatives of components
widely and routinely used in
commercial systems. The new developments
are the MEMS, which have been
previously used in virtually all automotive
pressure and air bag sensors, as
well as Gatlin guns, missile seekers, and
laser resonator q-switches in the military.
The components of FMCW systems
have been available in laboratory environments
for years, but no high-volume
production systems have deployed items
like the frequency agile long coherence
length diode laser needed to enable
such systems.
Furthermore, ToF LiDARs already
have multiple vendors selling automotive
qualified components across the
entire hardware stack: lasers, detectors,
ASICs, etc. Historically, a disruptive
technology (such as FMCW laser sources)
that is uniquely manufactured inhouse,
must have a 10x technical gain to
offset a product that enjoys a robust supply
chain with multiple vendors already
passing quality standards for a given customer
base.
Scalability ties directly to maturity.
One way of describing technology maturity
is a scheme developed by NASA in
the 1970s7
called the " Technology Readiness
Level " (TRL). This scheme assigns
numbers to a technology according to
how far along the path from technology
inspiration (TRL 1) to deployment in
multiple successful missions (TRL 9).
In the case of ToF LiDAR, we believe
the components and systems are at TRL
8, while the FMCW components and
systems are at TRL 4. This is a significant
gap in technology readiness that
will take many years to close. The major
scalability shortcomings of FMCW
systems include the low shot rate due to
the laser chirp pulse stretching, and the
high-speed ADC and FPGA required to
process returns. In the case where higher
shot rates at the system level are required,
parallel channels of the optical
path and electronics may be deployed.
These might use a single scanning
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Tech Briefs Magazine - March 2022

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