How do self-driving cars “see”? - Sajan Saini
It’s late, pitch dark, and a self-driving
car winds down a narrow country road.
Suddenly, three hazards appear
at the same time.
What happens next?
Before it can navigate this
onslaught of obstacles,
the car has to detect them—
gleaning enough information about
their size, shape, and position,
so that its control algorithms
can plot the safest course.
With no human at the wheel,
the car needs smart eyes, sensors
that’ll resolve these details—
no matter the environment,
weather, or how dark it is—
all in a split-second.
That’s a tall order, but there’s a
solution that partners two things:
a special kind of laser-based probe
called LIDAR,
and a miniature version of
the communications technology
that keeps the internet humming,
called integrated photonics.
To understand LIDAR, it helps to start
with a related technology— radar.
In aviation,
radar antennas launch pulses
of radio or microwaves at planes
to learn their locations by timing
how long the beams take to bounce back.
That’s a limited way of seeing, though,
because the large beam-size
can’t visualize fine details.
In contrast, a self-driving car’s
LIDAR system,
which stands for Light Detection
and Ranging,
uses a narrow invisible infrared laser.
It can image features as small as the
button on a pedestrian’s shirt
across the street.
But how do we determine the shape,
or depth, of these features?
LIDAR fires a train of super-short laser
pulses to give depth resolution.
Take the moose on the country road.
As the car drives by, one LIDAR pulse
scatters off the base of its antlers,
while the next may travel to the tip
of one antler before bouncing back.
Measuring how much longer
the second pulse takes to return
provides data about the antler’s shape.
With a lot of short pulses, a LIDAR system
quickly renders a detailed profile.
The most obvious way to create a pulse
of light is to switch a laser on and off.
But this makes a laser unstable and
affects the precise timing of its pulses,
which limits depth resolution.
Better to leave it on,
and use something else to periodically
block the light reliably and rapidly.
That’s where integrated photonics come in.
The digital data of the internet
is carried by precision-timed
pulses of light,
some as short as a hundred picoseconds.
One way to create these pulses is
with a Mach-Zehnder modulator.
This device takes advantage of a
particular wave property,
called interference.
Imagine dropping pebbles into a pond:
as the ripples spread and overlap,
a pattern forms.
In some places, wave peaks add
up to become very large;
in other places, they completely
cancel out.
The Mach-Zehnder modulator
does something similar.
It splits waves of light along two
parallel arms and eventually rejoins them.
If the light is slowed down and
delayed in one arm,
the waves recombine out of sync and
cancel, blocking the light.
By toggling this delay in one arm,
the modulator acts like an on/off switch,
emitting pulses of light.
A light pulse lasting a hundred
picoseconds
leads to a depth resolution of a
few centimeters,
but tomorrow’s cars will need
to see better than that.
By pairing the modulator with a super-
sensitive, fast-acting light detector,
the resolution can be refined
to a millimeter.
That’s more than a hundred times better
than what we can make out with
20/20 vision, from across a street.
The first generation of automobile LIDAR
has relied on complex spinning assemblies
that scan from rooftops or hoods.
With integrated photonics,
modulators and detectors are being shrunk
to less than a tenth of a millimeter,
and packed into tiny chips that’ll one
day fit inside a car’s lights.
These chips will also include a clever
variation on the modulator
to help do away with moving parts
and scan at rapid speeds.
By slowing the light in a modulator
arm only a tiny bit,
this additional device will act more
like a dimmer than an on/off switch.
If an array of many such arms, each with
a tiny controlled delay,
is stacked in parallel, something novel
can be designed:
a steerable laser beam.
From their new vantage,
these smart eyes will probe and
see more thoroughly
than anything nature could’ve imagined—
and help navigate any number
of obstacles.
All without anyone breaking a sweat—
except for maybe one disoriented moose.
