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Lidar, optical distance and time of flight sensors

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Re: Lidar, optical distance and time of flight sensors

#21

Post by Animats »

tubs wrote: post #20
MezzoDelCammin wrote: post #5

Earlier quoted context omitted.

Also there's the simple physics view of the same problem. The advantage of scanning is that you can focus all the laser pulse energy into one narrow beam. Non scanning means covering the whole field of view at once with that same laser pulse. Then you have a choice. Either somehow deal with the exponentially weaker return pulse (since it's spread over the whole field of view), or try to increase the pulse energy (and…

Quadratically weaker, not exponentially.

Fourth power of the distance, actually. That's the radar equation. You have inverse-square losses going out, as the beam expands with distance. Then you have inverse-square losses coming back when the target is much smaller than the beam. That's the problem flash LIDARs face. It can be overcome with enough laser power out to 20-30 meters.

That's where the beam diameter at the target is much larger than the target, as for aircraft. With a small scanning dot from a LIDAR and a nice big target like a car, almost all the power hits the target, but you still have inverse square losses coming back.

Re: Lidar, optical distance and time of flight sensors

#22

Post by namibj »

jlokier wrote: post #11
MezzoDelCammin wrote: post #5

Earlier quoted context omitted.

Also there's the simple physics view of the same problem. The advantage of scanning is that you can focus all the laser pulse energy into one narrow beam. Non scanning means covering the whole field of view at once with that same laser pulse. Then you have a choice. Either somehow deal with the exponentially weaker return pulse (since it's spread over the whole field of view), or try to increase the pulse energy (and…

From a physics point of view, continuous transmission and correlation detection of a temporally- and spatially-diverse optical signal over the entire field of view addresses the problem of energy being spread out, and gets you better, more robust depth information for less emitted energy than scanning or full-field flashes. But from an optical and electronics point of view, it's much harder to process the return sign…

Actually the same rule as for RADAR also applies to LIDAR: single pulse has better energy efficiency, but requires untenable peak power at any vaguely-state-of-the-art signal quality/reception performance levels.

The reason is that you can time-gate the noise out that would otherwise be hitting your correlation accumulators if you have a vague idea of the supposed delay/ToF for the pulse.

However, once you add mechanical scanning, at least for systems with not that many orders of magnitude between range resolution and maximum detection range, you can use systems like mode-locked lasers that for example have around 0.1% native duty cycle, circumvent the issue of peak power through the aperture/scanning 's spatial focusing (each pixel only needs a managable amount of energy, and delivering that in a single pulse won't require unreasonable peak power levels), and still get all the energy-efficiency benefits of single-pulse ranging vs. spread-spectrum/correlation ranging.

The only but major downside is the requirement of mechanical scanning.

Re: Lidar, optical distance and time of flight sensors

#23

Post by namibj »

rowanG077 wrote: post #19
amelius wrote: post #16

Earlier quoted context omitted.

I'm not convinced by the market/economics argument. Cheap and small ToF sensors exist, even though the market initially was also small.

ToF sensors have huge applications. Initially ToF sensors also cost thousands of dollars. What huge applications do you see that should have driven the prices of over the counter integrable interferometers down? As I stated blu ray (and DVD, CD and laser disc before it) readers are tiny purpose build inferometers. And they are cheap.

Since when are CD reader optics interferometers? The linked video shows a CW homodyne LIDAR used for measuring vibration frequency and counting vibration amplitude in fringes.

Last I looked CD readers used a 4-detector sensor's differential low-pass signals for closed-loop track-following so the rotation need not be optically centered. I also see no reason why optical disc readouts would need homodyne let alone heterodyne readout.

Re: Lidar, optical distance and time of flight sensors

#24

Post by namibj »

amelius wrote: post #10

Slightly offtopic, why is it so difficult to find a cheap and compact laser interferometer that can do sub-micron measurements? This guy gets close: https://www.youtube.com/watch?v=MUdro-6u2Zg&t=770s But why isn't something cheap and small like this commercially available as an integrated system?

While not an integrated system itself, looking for convenient information on the matter I stumbled upon https://arxiv.org/abs/2011.05313 . As for integrated electro-optical hardware that does coherent optics, I'd like to point at SFP optics; the ones that are sufficiently non-fancy with their electronics (up to 10km single-mode optics; no high-end-multimode optics; no exotic coherent optics) should not have a retimer on the RX side. Once you go to 1-Gbit/s transceivers you'll find ones with a linear RX stage; those should be suitable for rigging simple homodyne interferometry by just feeding a little TX back to the RX taking care to let the RX AGC run the RX path at high gain.

Re: Lidar, optical distance and time of flight sensors

#25

Post by rowanG077 »

namibj wrote: post #23
rowanG077 wrote: post #19

Earlier quoted context omitted.

ToF sensors have huge applications. Initially ToF sensors also cost thousands of dollars. What huge applications do you see that should have driven the prices of over the counter integrable interferometers down? As I stated blu ray (and DVD, CD and laser disc before it) readers are tiny purpose build inferometers. And they are cheap.

Since when are CD reader optics interferometers? The linked video shows a CW homodyne LIDAR used for measuring vibration frequency and counting vibration amplitude in fringes. Last I looked CD readers used a 4-detector sensor's differential low-pass signals for closed-loop track-following so the rotation need not be optically centered. I also see no reason why optical disc readouts would need homodyne let alone heter…

The pits and valleys are spaced λ/4 apart. The reason there is such a stark difference in intensity is because this λ/4 spacing causes interference at pit->valley and valley->pit transition points. Of course this not a standard interferometer but rather a purpose build one. Wikipedia says:

> Interferometry is a technique which uses the interference of superimposed waves to extract information.

And a blu ray player directly uses interference of superimposed waves to extract information. It squarely fits in the homodyne category.

Re: Lidar, optical distance and time of flight sensors

#26

Post by MezzoDelCammin »

Animats wrote: post #21
tubs wrote: post #20

Earlier quoted context omitted.

Quadratically weaker, not exponentially.

Fourth power of the distance, actually. That's the radar equation. You have inverse-square losses going out, as the beam expands with distance. Then you have inverse-square losses coming back when the target is much smaller than the beam. That's the problem flash LIDARs face. It can be overcome with enough laser power out to 20-30 meters. That's where the beam diameter at the target is much larger than the target, as…

true. My original was just a quick jote on a phone sipping a coffee on Sunday. I admit I simply didn't want to go into the whole "square FOV for the sensor vs. one detector / diode and that combined with the time of flight loss over distance", so I just used "exponential" to mean "it loses power pretty quickly". Apologies for the sloppiness on my part.

Second part of the comment I omitted is was what You mentioned in the beginning. Those 20-30 meters of practical range is why we keep seeing small LIDAR sensors on things like iPhones / iPads (though there I believe the range is even a bit shorter due to the size / power constraints), but not really much beyond that.

For practical demo of what's currently available at the high end of solid state LIDAR (albeit at 40k+ USD), I'd suggest looking at Leica and their BLK2GO PULSE (solid state) vs the rest of the BLK line (rotating laser spot).

Re: Lidar, optical distance and time of flight sensors

#28

Post by viggity »

sega_sai wrote: post #9

I was always wondering if every car around you had a Lidar, would these systems be confused by light emitted by other cars ?

In general, no. And to be clear, this is my general understanding and I don't have the time to look up the specifics, but I'm pretty sure lidar uses a "carrier wave" in much the same way old telephone modems did, or your IR remote for your TV. The carrier wave is how these sensors don't get confused. If you've ever been in traffic, and you get a stray, one off "YOURE TOO CLOSE TO SOMETHING" when you in fact aren't, it is because a carrier wave from another car happens to line up and you have a collision (pun intended).

two seconds of googling: https://www.digikey.com/en/maker/tutorials/2021/understandin...

Re: Lidar, optical distance and time of flight sensors

#29

Post by mahirsaid »

NewsaHackO wrote: post #7

Very informative. I assume this is the type of industry work PhDs in physics do

There is still so much to be done with LIdar. I myself am so intrigued of the output these devices present. The data and capabilities that are possible with this tech is amazing to me. Terrain scanning to autonomous vehicles the list goes on. Lasers are amazing breakthrough for mankind and breakthrough for the physics that involved, light that can be manipulated to seek alternative outcomes of data or real-world applications.
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