Having built a LiDAR system for an autonomy company in the past, this is a great write-up, but it omits what I found to be one of the more interesting challenges. For our system (bistatic, discrete edge-emitting laser diodes and APDs; much like a Velodyne system at high level), we had about an inch of separation between our laser diodes and our photodiodes. With 70 A peak currents through the laser diodes. And nanoamp sensitivity in the photodiodes. EMI is... interesting. Many similar lidars ignore the problem by blanking out responses very close to firing time, giving a minimum range sensitivity, and by waiting for maximum delay to elapse before firing the next salvo -- but this gives a maximum fire rate that can be an issue. For example, a 32 channel system running at 20 kHz/channel would be limited to ~200 m range (468 m round trip delay, some blanking time needed)... so to get both high rate (horizontal resolution) and high channel count (vertical resolution), you need to be able to ignore your own cross-talk and be able to fire when beams are in flight.

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jandrese 2 hours ago | parent | next [-]

200m range seems adequate for passenger vehicle use. Even at 100kph that's over 7 seconds to cover the distance even if you aren't trying to slow down. I think there is diminishing returns with chasing even longer ranges. Even fully loaded trucks are expected to stop in about 160m or so.

	▲	addaon 2 hours ago \| parent [-]
		Yep, 200 m is pretty close to standard. Which is why 32 channel and 20 kHz is a pretty common design point. But customers would love 64 channel and 40 kHz, for example. Also, it's worth noting that if your design range is 200 m -- your beam doesn't just magically stop beyond that. While the inverse square law is on your side in preventing a 250 m target from interfering with the next pulse, a retro-reflector at 250 m can absolutely provide a signal that aliases with a ~16 m signal (assuming 234 m time between pulses) on the next channel under the right conditions. This is an edge case -- but it's one that's observable under steady-state conditions, it's not just a single pulse that gets misinterpreted.

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newpavlov 2 hours ago | parent | prev [-]

>we had about an inch of separation between our laser diodes and our photodiodes

Why can't you place them further away from each other using an additional optical system (i.e. a mirror) and adjusting for the additional distance in software?

	▲	addaon 2 hours ago \| parent [-]
		You can, but customers like compact self-contained units. All trade offs. Edit: There's basically three approaches to this problem that I'm aware of. Number one is to push the cross-talk below the noise floor -- your suggestion helps with this. Number two is to do noise cancellation by measuring your cross-talk and deleting it from the signal. Number three is to make the cross-talk signal distinct from a real reflection (e.g. by modulating the pulses so that there's low correlation between an in-flight pulse and a being-fired pulse). In practice, all three work nicely together; getting the cross-talk noise below saturation allows cancellation to leave the signal in place, and reduced correlation means that the imperfections of the cancellation still get cleaned up later in the pipeline.