Earlier quoted context omitted.
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 manuf…
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.
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.