[Dprglist] New lidar technology

Tue Aug 22 13:27:54 PDT 2023

 Doug - no info beyond what's in the article. They leave several key techniques unspecified in the article, probably because it's their secret sauce. For example, does the heterodyning and detection of sum or difference frequencies happen in the optical domain or electronically.
The 4m sensor you linked is an interesting little unit, and cheap.
Paul

    On Tuesday, August 22, 2023 at 08:12:08 AM CDT, Doug Paradis <paradug at gmail.com> wrote:  

 Paul,     Interesting article. Claims large scale sampling in 2025. Do your sources have any info on its capabilities? 
     On the other end of the scale, DFRobot has a new product with 400cm range available,https://www.dfrobot.com/product-2727.html. One flaw is a single I2C address, making multiple sensor applications more difficult. Regards,Doug 
On Tue, Aug 22, 2023 at 4:53 AM Paul Bouchier via DPRGlist <dprglist at lists.dprg.org> wrote:

OK - maybe not really new technology so much as combining known techniques in a new way and applying them to lidar...
DPRG folks,
I saw an article in IEEE Spectrum here:https://spectrum.ieee.org/lidar-on-a-chipdescribing lidar-on-a-chip for automotive applications. It's being called 4D lidar, because in addition to providing range across a 2D field of view (thus providing a 3D point-cloud) it also provides velocity toward or away from the sensor for detected objects (hence the 4D). And it targets a $500 pricepoint.

The techniques that are new to lidar (AFIK), but have been used in radar for a long time, are:1. FMCW - Freqency Modulated Continuous Wave lidar. They change the emitted light frequency (called chirping in radar) and compare the emitted frequency with the reflected frequency to determine range and speed.
2. Phased array emitters and receivers produce an electronically steerable beam (called synthetic aperture in radar) to produce an electronically direction-controlled beam that can be steered to scan the scene.

FMCWThis involves "chirping" or changing the light frequency in a repeating fashion, via a triangular frequency modulation of the laser. When the reflected light is mixed with a local copy of the currently-emitted light, the frequency difference between the two is proportional to the range (because the local copy of the emitted light will have changed frequency between the time it was emitted and the corresponding reflection is received). Heterodyning provides the sum and difference frequencies which can be measured to provide range. Furthermore, if the target is moving toward or away from the sensor, the reflected frequency is modified from what a stationary object would have reflected due to the doppler effect. Comparing the reflection from a rising chirp with a falling chirp enables separating the doppler frequency shift from the range-related frequency difference as described (for radar) here: 
https://www.byte-lab.com/fmcw-and-the-doppler-effects/Looking at multiple heterodyne frequencies enables separating objects at different ranges that both fall within the detector beam width (imagine two distant cars on a road, one closer than the other, one traveling away from you and the other toward you).

Phased arrayA phased array antenna introduces a phase difference between adjacent closely-spaced antennas so as to control the direction of the beam, which can be done in 1D (linear) or 2D. The net of this is to produce an electronically steered laser beam that can be moved at megahertz-level rates. The same principles apply to the receiver. The antennas are about a wavelength apart, which means for infrared the lasers are about 1.5 micrometers apart and highly coplanar and there are thousands of them. Obviously this can only be done on a chip. There's a good graphic in the article illustrating how beam steering via multiple phased antennas works. Doing this with lidar involves introducing a variable delay in the light emitted from each laser. They appear to do this with an electro-optical element in front of the laser, where an applied voltage changes the speed of light propagation and thus the phase of the light emitted from a laser relative to the one next to it.
I would expect initial versions of this kind of chip to initially be limited to auto manufacturers (specs and devices), but maybe over time as they hone the techniques, they may become more widely used and available. These sensors are qualitatively different than previous lidar sensors, and seem to take lidar to the next level by far exceeding the capabilities of the old "spinning coffee can" Velodyne ($75k) sensors.
Whew! Sounds like a good RBNV discussion topic.
Paul
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