Solid State Automotive LiDAR: Physics Principles, Design ...
Transcript of Solid State Automotive LiDAR: Physics Principles, Design ...
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Solid State Automotive LiDAR: Physics Principles,
Design Challenges, and New Developments
Slawomir Piatek
New Jersey Institute of Technology &
Hamamatsu Photonics, Bridgewater, NJ
06.2. 2020
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β LiDAR Concepts: ToF & FMCW
β Light Sources & Beam Steering
β Solid State (Flash) LiDAR
Index
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LiDAR Concepts
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Basic Layout of Time of Flight LiDAR
start pulse
stop pulse
laser
target
photodetector
timer
beam steering
system
Distance d = Ξπ‘ β π/2
Measure the time of flight Ξπ‘
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FMCW LiDAR: Heterodyne Mixing
tunable
laser
BS
M
receiving optics
collimator
PD
returned light
M
M
M
M
to target
optical mixing
occurs on the
detector
frequency
shifter
electronics
Legend:
ππΏπ β Local oscillator frequency
πππππ ππ‘ β Offset frequency added by the frequency shifter, ~10 β 100 MHz
πππ β Power oscillator frequency of transmitted light
ππ β Frequency of returned light; Ξf due to distance and Dopplerβs effect
These are instantaneous values
ππΏπ
πππ = ππΏπ + πππππ ππ‘
ππ = ππΏπ + πππππ ππ‘ + Ξπ
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Example of Frequency Modulation: Double Linear Ramp
time
frequency
T ~ 10βs ΞΌs β 1 ms, B ~ 100βs MHz β 10βs GHz
0time
frequency
π΅π0 π0ππππ₯ ππππ₯
0 π 2π
π0
ππππ₯
π 2π
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Frequency Shift in FMCW LiDAR
time
local oscillator
received wave
π =π ππ΅1 + ππ΅2 π
8π΅π£π =
π ππ΅2 β ππ΅14π0
πΏπ =π
2π΅πΏπ£π =
π
π0π
Ξπ‘
ππ΅1
ππ·
ππ΅2
π
π΅
π0
π(π‘)
ππππ₯
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Heterodyne Optical Mixing
PDBS
fa
fLO
measure thisamplification!
πΈπ‘ππ‘2 = πΈπ + πΈπΏπ
2 = π΄π cos 2ππππ‘ + ππ + π΄πΏπ cos 2πππΏπ + ππΏπ2
πΈπ‘ππ‘2 = πΈπ
2 + πΈπΏπ2 + π΄ππ΄πΏπ cos 2π ππ β ππΏπ π‘ + ππ β ππΏπ
ππ ππ = ππ + ππΏπ + 2 ππππΏπ cos 2π ππ β ππΏπ π‘ + ππ β ππΏπ
ππ ππ =ππππ ππ
βπ= ππ + ππΏπ + 2 ππππΏπ cos 2π ππ β ππΏπ π‘ + ππ β ππΏπ
Ξπ = ππ β ππΏπ β πππππ ππ‘
Ξπ gives π and π£π
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Comparison Between ToF & FMCW Concepts
ToF FMCW
Pros
Easy optical layout
Easy distance calculation
Any wavelength of light can be used Pros
Optical amplification of the returned signal
Photon shot noise detection possible
Immunity to background and interference
Cons
Large detection bandwidth βincreased noise
Weak returned signal
Susceptible to background and
interference Cons
Complex optical layout
Expensive tunable laser with a long
coherence length needed
Complex distance and velocity calculation
Gives both distance and radial velocity
of the target
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Light Sources & Beam Steering
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Light Sources
ToF FMCW
High peak power: ~ 100 W
Short-duration pulses: ~ few ns
Repetition period: ~ ms - ΞΌs
Wavelength: NIR, e.g., 905 nm, 1550 nm
Tunable output frequency
Coherence length πΏ > 2ππππ₯
Stable phase
some light source offerings by Hamamatsu
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905 or 1550?
905 nm 1550 nm
Figure from Matson et al. 1983
Solar irradiance at sea level
ππ΅ @ 905 nm > ππ΅ @ 1550 nm S
pect
ral i
rrad
ianc
e ΞΌ
W c
m-2
nm-1
Wavelength [nm]
Solar background at 905 nm is higher than at 1550 nm
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905 or 1550?
H2O absorption @ 1550 nm > (100Γ) @ 905 nm
From Wojtanowski et al. 2014
Wavelength [nm]
Ab
so
rptio
n c
oe
ffic
ien
t o
f w
ate
r [c
m-1
]
Reference: βComparison of 905 nm and 1550 nm semiconductor laser rangefindersβ performance deterioration due to adverse
environmental conditions,β Wojtanowski et al. 2014
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905 or 1550?
1550 nm
- Requires IR (non-silicon) photodetectors
+ Best eye safety
905 nm
+ Lower background
+ Better transmission in atmosphere
+ Silicon-based photodetector
+ Coherence length πΏ βπ2
π΅
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Mechanical Beam Steering: Rotating Platform
π
Each laser is matched with its own detector Scan direction
Horizontal sweep
Vertical sweep
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Mechanical Beam Steering: Galvo Mirrors
Ο
Ο
Laser
π₯ β π¦ scan
Rotating Galvo mirrors are another example of a mechanical beam steering.
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Beam Steering: Optical Phase Array
Laser
Amplitude & phase
control unit
Beam in the far field
Amplitude and phase of the light emitted by
each pixel (emitter) can be controlled
electronically.
Optical phased array is an example of a solid state or βno moving partsβ beam steering.
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OPA Emitter and Receiver
Light feed for
emission
Local oscillator
light for optical
amplification
emitted beam detected beam
Control unit
Current output
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LiDAR Based on OPA Emitter and Receiver
First prototype of a FMCW LiDAR that uses
optical phased arrays for beam steering and
light detection.
The figure is from βCoherent solid-state LIDAR with silicon
photonic optical phased arraysβ by Poulton et al. 2017
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Beam Steering: Flash and Structured Light
Array of lasers such as
VCSELs
Laser and pulse
expander
Divergent pulse of light
β Wider the angle, smaller the surface brightness
β For a Gaussian beam, the surface brightness is
not uniform
β Lateral resolution limited by the 2D sensor
β Beams have the same intensity
β Lateral resolution limited by the angular
separation between the beams
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Solid State (Flash) LiDAR
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Basic Layout of ToF Flash LiDAR
projector
to the targetππ
ππ
focal plane array
control unit
π
The emission FOV, ππ, should be matched with the detection FOV, ππ.
2D detector
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Focal Plane Distance Measurement
Focal plane
image of the scene
detector array
A single βpixelβ in the 2D detector determines distance to a single element of the scene.
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Distance Resolution
πΏπ =π
2π΅
πΏπ
π΅~1
ππ β pulse duration
For π = 5 ns, πΏπ β 0.75 m β π΅ β 200 MHz
(distance resolution)
Better distance resolution requires pulses of even shorter duration. The limitation is in the laser
technology (parasitic inductance).
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Distance Uncertainty
ππ2~
πΏπ 2
π/π=
π2
4π΅2π/π
π
ππ
π
(distance uncertainty)
1. The larger the π
π, the smaller the uncertainty, all else being the same
2. Photodetector and electronic time jitters also contribute to the uncertainty.
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Photon Budget: Single Photodetector
TX
π π = π0ππ΄0ππ2
π0πβ2πΎπ
π π β Peak power received
π0 β Peak power transmitted
π β Target reflectivity
π΄0 β Aperture area of the receiver
π0 β Receiving optics transmission
πΎ β Atmospheric extinction coefficient
Lambertian reflection
Laser spot smaller than the target
Normal incidence
πΎ is constant
Assumptions:
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Photon Budget: Single Photodetector
π π = π0ππ΄0ππ2
π0πβ2πΎπ
Example: π0 = 100 W, π = 0.1, π΄0 = 3.14 Γ 10β4 m2, π0 = 0.5, πΎ = 0.5 km-1
for π = πππ m, π· = ππ nW
Reference: βComparison of 905 nm and 1550 nm semiconductor laser rangefindersβ performance deterioration due to adverse
environmental conditions,β Wojtanowski et al. 2014
1. Atmospheric extinction πΎ depends on weather conditions and wavelength. Its value can range
from about 4 km-1 to about 0.1 km-1 at 905 nm.
2. For a square 5-ns pulse (π = 905 nm), the number of emitted photons is ~2.3 Γ 1012 and the
number of received is ~1 Γ 103.
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Photon Budget: Flash
TX
RX
There is a tradeoff between spatial resolution and π
π.
π΄
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Photon Budget: Flash
π π = π0πππ2
Ξ2π΄0ππ2
π0πβ2ππΎ
(received peak power per pixel)
ππ β angular subtense (field of view) of a pixel
Ξ β angular field of view of the pulse projector (pulse divergence)
Reference: βComparison of flash lidar detector options,β McManamon et al. 2017
Note that ππ βͺ Ξ, so if the pulse peak power is the same, the amount of light received by a single
pixel is proportional to ππ2
Ξ2for a given distance π.
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Photodetection (Single Element)
TIA PD
Trigger
circuit
Pulse of light
Optical bandpass filter
Focal plane
To timerFOV
β Active area of the photodetector, focal length of the lens, and placement of the optical bandpass
filter determine the photodetectorβs field of view.
β Avalanche photodiode or silicon photomultiplier are commonly used photodetectors.
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Noise in Transimpedance Amplifier
β
+
π£π
π πΉ
πΆπ
ππ½
πΆππππ
ππ
ππππβ
π = π ππππ||π ππ
πΆπ‘ + πΆππ
π£π = ππ2 1 +
π πΉπ
2
+4π2
3Ξπ 2πΆπ
2π π2 + π π
2ππ2 + 4πππ π
1/2
Ξπ 1/2
πΆπ = πΆπ‘ + πΆπ + πΆππ + πΆπ
ππ = ππ2 + ππ½
2 + ππ2 + ππβ
2
All else being equal, noise increases with terminal capacitance of the photodetector.
(total capacitance)
noise output
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Bandwidth and Stability of Transimpedance Amplifier
β
+
π£ππ’π‘
π πΉ
πΆπ
Simplified equivalent circuit of a photodetector
connected to an uncompensated TIA
πΌ
π£ππ’π‘ = πΌβπ π
1 +1
π΄πππ½
π΄ππ β Open loop gain of TIA
π½ ππ =1
1 + πππ πΉπΆπ
where πΆπ = πΆπ + πΆππ
1
π½(ππ)= 1 + πππ πΉπΆπ
ππΉ ππ ππΊπ΅ππ
Gain peaking and
oscillations occur around
this frequency
ππΊπ΅ππ β Unity gain bandwidth of the op-amp
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Bandwidth and Stability of Transimpedance Amplifier
Simplified equivalent circuit of a compensated
photodiode connected to an uncompensated TIA
πΆπΉ
ππΉ ππ ππΊπ΅ππ
ππΉ =1
2ππ πΉ πΆπ + πΆπΉππ =
1
2ππ πΉπΆπΉ
πΆπΉ =1
4ππ πΉππΊπ΅ππ1 + 1 + 8ππ πΉπΆπππΊπ΅ππ
(Optimal value of the compensating capacitor)
β
+
π£ππ’π‘
π πΉ
πΆππΌ
π½ ππ =1 + πππ πΉπΆπΉ
1 + πππ πΉ(πΆπ+πΆπΉ)
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Importance of Noise and Bandwidth
time
High bandwidth β higher noise but high fidelity
trigger level
trigger time
time
Low bandwidth β lower noise and lower fidelitytrigger level
trigger time
flat top
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Importance of Excess Noise (F)
time
Sig
nal
Fixed trigger level
Constant fraction trigger
Waveform 1
Waveform 2
Fixed trigger level gives different round-trip-times, Ξπ‘1 β Ξπ‘2 β
Constant-fraction trigger gives the same round-trip-times, Ξπ‘1 = Ξπ‘2 β
Ξπ‘1 = Ξπ‘2
Ξπ‘1 Ξπ‘2
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Takeaway
Analog photodetection in ToF LiDAR, especially in flash LiDAR, is very challenging.
Is there anything else we can do?
What about a statistical measurement using SPAD?
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Single-Photon Avalanche Photodiode (SPAD)
steady state
transient
quench
recharge
π π must be large enough to ensure quenching
quasi-stable βreadyβ
state
(Quench resistor)
(Load resistor)
π π
ππ΅πΌπ΄π
π π
πππ΄π·
π π β« π π
πΌπππ΄π·
ππ΅πΌπ΄ππ π
ππ΅π· ππ΅πΌπ΄π ππππ΄π·
π£π
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Measuring Distance with a Single SPAD
t = 0
time
timer
targetSPAD
laser
Puls
e e
mis
sio
n
π‘ = π
Ξπ‘ =2π
π
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Measuring Distance with a Single SPAD
time
π‘ = 0 π‘ = πΞπ‘ =
2π
π
Multiple pulse illumination provides distance information to the target. The information comes
from a histogram of trigger times.
Histogram of trigger times
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SPAD Arrays
Micro-bumps
ASIC
(application-specific integrated circuit)
Photodetector array
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Detection Techniques with a SPAD Array
Time gating:
π‘ = 0 π‘ = π
Time window
Only the events in the pre-defined time window are counted. The choice of the time window
depends on the expected knowledge of the target distance.
Pixel 1
Pixel 2
Pixel 3
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Detection Techniques with a SPAD Array
Temporal and spatial correlation:
π‘ = 0 π‘ = ππ‘ = π‘1
π‘ = π‘1
π‘ = π‘1
Event not counted: temporal
correlation but no spatial
correlation.
Pixel 1
Pixel 2
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Detection Techniques with a SPAD Array
Temporal and spatial correlation:
π‘ = 0 π‘ = ππ‘ = π‘1
π‘ = π‘1
π‘ = π‘2
Event not counted: spatial correlation but
no temporal correlation.
π‘ = π‘2
Pixel 1
Pixel 2
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Detection Techniques with a SPAD Array
Temporal and spatial correlation:
π‘ = 0 π‘ = ππ‘ = π‘1
π‘ = π‘1
π‘ = π‘1
Event counted: spatial correlation but
no temporal correlation.
Pixel 1
Pixel 2
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SPAD Pixel for Correlated Detection
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SPAD Pixel for Correlated Detection
Pulse
coincidence
detection
Signal photon
Background photon
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Comments
1. This technique can be used both in a scanning and flash LiDAR.
2. In a scanning LiDAR, an OPA array is well-suited for beam steering.
3. The greatest advantage is a reduced sensitivity to the background light.
4. Additional advantages: less affected by gain variations, sensitivity in IR (using non-silicon
structures), compatible with CMOS-based ASICs.
5. Challenges: SPAD arrays can exhibit crosstalk and high dark count rates.
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Hamamatsu Assists LiDAR Companies
Photodetectors (Silicon or InGaAs, PIN, APD, SiPM, SPAD and more) for all LiDAR concepts
Light sources (PLDs or VCSELs) for selected LiDAR concepts
Custom integrated optical assemblies, from front-end electronics to complete ASICs
Support automotive grade qualifications (AEC, ISO and more)
Full customization of photodetectors, light sources, and optical assemblies
Because of our wide offering of optical components, Hamamatsu is unbiased
when recommending the correct detector and/or light source to each unique
LiDAR concept (customer) in the market.
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Photodetectors for LiDARS (850 nm β 940 nm)
Si PIN photodiode Si APD
High Photosensitivity;
Internal Gain = 1
High Photosensitivity;
Internal Gain ~ 100
SPPC (or SPAD)
Low Photosensitivity;
Internal Gain ~ 105 to 106
MPPC (or SiPM)
Low Photosensitivity;
Internal Gain ~ 105 to 106
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Photodetectors for LiDARS (1550 nm)
InGaAs APD
Wavelength [nm]
Photo
sensitiv
ity [A
/W]
High Photosensitivity;
Internal Gain ~10-20
Wavelength [nm]
Photo
sensitiv
ity [A
/W]
InGaAs PIN PD
High Photosensitivity;
Internal Gain β 1
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Closing Remarks
1. Two distinct LiDAR systems, ToF and FMCW, are actively researched
2. Each system presents a unique set of engineering challenges
3. Beam steering and photodetection are the two most outstanding challenges
4. Flash LiDAR together with a SPAD-based statistical detection is a new avenue of research
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Thank you
Thank you for listening
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