Over the past year the automotive lidar manufacturers have been providing more details on their proprietary technology.
Timothy B. Lee provides an in-depth look at 10 of the leading companies for Ars Technica.
Automotive Lidar
Today, there are three big ways that automotive lidar products differ from one another. And after laying these approaches out, it’s easier to grasp the technology of nine leading automotive lidar companies.
To keep this survey of the automotive lidar landscape manageable, I’m sticking to independent companies that focus primarily on the lidar business. That means I won’t cover Waymo’s homebrew lidar technology, the lidar startups GM and Ford acquired in 2017, or the lidar efforts of bigger companies like Valeo (maker of the lidar in Audi’s 2018 and 2019 versions of the A7 and A8), Pioneer, or Continental. It’s hard to get these larger companies to give us details about their lidar technology—and there’s plenty of ground to cover without them.
The three big factors that distinguish lidar sensors
The basic idea of lidar is simple: a sensor sends out laser beams in various directions and waits for them to bounce back. Because light travels at a known speed, the round-trip time gives a precise estimate of the distance.
While the basic idea is simple, the details get complicated fast. Every lidar maker has to make three basic decisions: how to point the laser in different directions, how to measure the round-trip time, and what frequency of light to use. We’ll look at each of these in turn.
Beam-steering technology: Most leading lidar sensors use one of four methods to direct laser beams in different directions (two companies I cover here—Baraja and Cepton—use other techniques that they haven’t fully explained):
Spinning lidar. Velodyne created the modern lidar industry around 2007 when it introduced a lidar unit that stacked 64 lasers in a vertical column and spun the whole thing around many times per second. Velodyne’s high-end sensors still use this basic approach, and at least one competitor, Ouster, has followed suit. This approach has the advantage of 360-degree coverage, but critics question whether spinning lidar can be made cheap and reliable enough for mass-market use.
Mechanical scanning lidar uses a mirror to redirect a single laser in different directions. Some lidar companies in this category use a technology called a micro-electro-mechanical system (MEMS) to drive the mirror.
Optical phased array lidar uses a row of emitters that can change the direction of a laser beam by adjusting the relative phase of the signal from one transmitter to the next. We’ll describe this technique in detail in the section on Quanergy.
Flash lidar illuminates the entire field with a single flash. Current flash lidar technologies use a single wide-angle laser. This can make it difficult to reach long ranges since any given point gets only a small fraction of the source laser’s light. At least one company (Ouster) is planning to eventually build multi-laser flash systems that have an array of thousands or millions of lasers—each pointed in a different direction.
Distance measurement
Lidar measures how long light takes to travel to an object and bounce back. There are three basic ways to do this:
Time-of-flight lidar send out a short pulse and measures how long it takes to detect the return flash.
Frequency-modulated continuous-wave (FMCW) lidar sends out a continuous beam whose frequency changes steadily over time. The beam is split into two, with one half of the beam getting sent out in the world, then being reunited with the other half after it bounces back. Because the source beam has a steadily changing frequency, the difference in travel distance between the beams translates to slightly different beam frequencies. This produces an interference pattern with a beat frequency that is a function of the round-trip time (and therefore of the round-trip distance). This might seem like a needlessly complicated way to measure how far a laser beam travels, but it has a couple of big advantages. FMCW lidar is resistant to interference from other lidar units or from the Sun. FMCW lidar can also use Doppler shifts to measure the velocity of objects as well as their distance.
Amplitude-modulated continuous wave lidar can be seen as a compromise between the other two options. Like a basic time-of-flight system, AMCW lidars send out a signal and then measure how long it takes for that signal to bounce back. But whereas time-of-flight systems send out a single pulse, AMCW systems send out a more complex pattern (like a pseudo-random stream of digitally encoded one and zeros, for example). Supporters say this makes AMCW lidar more resistant to interference than simple time-of-flight systems.
Laser wavelength
The lidars featured in this article use one of three wavelengths: 850 nanometers, 905 nanometers, or 1550 nanometers.
This choice matters for two main reasons. One is eye safety. The fluid in the human eye is transparent to light at 850 and 905nm, allowing the light to reach the retina at the back of the eye. If the laser is too powerful, it can cause permanent eye damage.
On the other hand, the eye is opaque to 1550nm light, allowing 1550nm lidar to operate at much higher power levels without causing retina damage. Higher power levels can translate to longer a range.
So why doesn’t everyone use 1550nm lasers for lidar? Detectors for 850 and 905nm light can be built using cheap, ubiquitous silicon technologies. Building a lidar based on 1550nm lasers, in contrast, requires the use of exotic, expensive materials like indium gallium arsenide.
And while 1550nm lasers can operate at higher power levels without a risk to human eyes, those higher-power levels can still cause other problems. At the CES show in Las Vegas this year, a man reported that a powerful 1550nm laser from an AEye lidar permanently damaged his camera. And, of course, higher-power lasers consume more energy, lowering a vehicle’s range and energy efficiency.
With this background out of the way, let’s look at 10 of the leading lidar companies.
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