[OnlineGladiator]

For FMCW, there should be no interference at all for any number of sensors (there is more information in the link in my original comment). For pulsed lidar, it's an issue of how well you engineer it. I can confidently say a good sensor can regularly withstand dozens of sensors operating simultaneously in a small area because I've personally witnessed it. The devil is largely in the details of specifications and manufacturing quality (that latter one is a much bigger issue in reality), but there is no theoretical reason you couldn't make it work with hundreds of sensors simultaneously. And like I said, I know current off-the-shelf parts that will work with dozens of sensors simultaneously.

Maybe I can add a little color to my original comment this way: most lidar sensors today, including some very expensive ones from supposedly reputable vendors, are not very good. In my experience, it is more often a manufacturing problem than a design problem (this varies more by company than it does by technology).

[agoodthrowaway]

Imagine every car has a FMCW LIDAR. The chance of interference is very high. Additionally if those receivers are using SE PDs, then these sensors will also be “blinded” (TIAs and or PDs will saturate) when a nearby Lidar system shines its laser into the detector.

[kyzyl]

I suppose it will be a good while before we can approach an empirical proof for this sort of thing, since FMCW lidars are still very scarce, even more so than pulsed lidars right now. However, even if every car does have an FMCW lidar, the conditions required to get them to interfere with each other is are:

a) Have identical laser wavelength. Not just '905nm' or '1550nm', but _precisely_ the same wavelength. This is very hard to do even if you try.

b) Have a coincident beam path. Again, this needs to be very precisely aligned.

c) Have an overlapping coherence area. This is a bit technical, but it is a higher bar than just having spots spatially overlapping.

d) Have coherent+matching phase fronts at the detector. Again this is a fairly technical subject these properties vary along the beam path, and transversely. This also vary in time, temperature and many other things. The source lidar is able to 'interfere' with itself (in other words, get a signal), because it compensates for all of these effects with a local copy of the outgoing laser light. Other lidars' outgoing beams will in general, even for 100 cars, not be 'synced up' in this way.

Moreover, those conditions are just the intrinsic interference rejection properties of coherent lidars. Layered on top of that is that two lidars need to be using the same type of modulation, bullseye each other as they scan around the FOV, and provide enough photons to actually contribute to the signal. Then, if you satisfy all of those prerequisites, the interfering lidar also needs to overcome any heuristic/algorithmic rejection of spurious signals. Finally, if all of those conditions match up and you get a signal to punch through, and it's strong enough to over come the true signal, and you can't tell that it's an erroneous signal, then it will result on one bad/missing point in a frame of thousands of points, present for one frame.

You're correct, however, that there is a saturation issue. If you just DOS the photo diodes with photons you can potentially prevent any signals from getting through. But again, this isn't super easy to do. The detectors will almost certainly be balanced, not single ended, and AC coupled. So you really have to blast the photo diode, effectively bringing it up to it's damage threshold so it is just flooded with current and can't do anything, and/or just breaks. The raw laser light doesn't do much, both because the DC signal is rejected and because the balanced detectors will reject common mode signals (clearly you know this already). You also have the same issue with needing to shine into a very narrow field of view, at the right time, for long enough to matter.

[agoodthrowaway]

Unfortunately FMCW lidar is sweeping the lasers across the same band. You don’t need the exact frequency just a beat frequency that’s within your detection Bw.

Also balanced detectors have something called common mode rejection. This is not infinite. In high volume applications it’s difficult for this to be >25dB but you can buy some devices >35dB.

Given that Lidar dynamic range is ~100dB you will definitely see the DC. I’ve not thought about this too much but it seems like an issue for the AGC as your demodulator won’t be bothered by it.

[kyzyl]

It's true that the laser frequency is sweeping, but it very well may not be over the same band. The sweep bandwidth in a typical lidar is likely in the 1-10GHz range. The carrier frequency of the laser that this modulation is riding on is probably in the neighborhood of 200THz. Let's say you're using a telecom laser at 1550nm. The actual wavelength of that laser will centered on some channel in the 1530-1580nm band, with each channel spaced by say 100GHz. So already each laser might intentionally be in a different channel, depending on chance and how many cars are there. But even if they are in the same channel, the chirp bandwidth is small compared to the channel bandwidth, so there will likely be at most only partial overlap, depending on where the respective center frequencies actually are. Unless your lidar is using a very expensive, very fiddly laser system, this center frequency will be drifting around within the channel all the time. It varies with temperature, mechanical stress, output power and a bunch of other stuff, depending on the type of laser. However, even if the lasers are magically in the same channel, and perfectly locked to the same center frequency, you still need the light be coherent to produce an interfering RF signal. They will not be coherent.

Certainly the balanced detectors will have finite CMRR. In general you definitely have to make a good detector but it doesn't need to reject to 100dBc. A photodiode might have 100dB of dynamic range, but most likely your RF front end does not, and more importantly for most applications you will be dominated by photon shot noise, so you don't need to push common mode signals all the way to your electronic noise floor. 35dB of rejection works wonders.