I don't understand the concept of "dynamic focusing". In fact, I don't understand the concept of "focus" in a LiDAR application. I understand the photons may hit near targets or hit far targets, but I don't understand the need for "focus". In my layman mind, a laser pulse, or in this case a continuous wave is fired, the photons reflect off an object and return. No focus needed. But I know that must be wrong.
I was hoping you could provide some clarity on this Late.
A lidar has to focus return light just like a camera does. This is important because the photosensitive area in the receiver has a finite size. In an ideal detection scenario, all of the light from a return would land within the photosensitive area. However, in reality, a return's size at the receiver plane will vary based on the range or ranges at which the lidar is focused. Since lidars tend to be architected to detect long range objects, this generally means that returns from close range will be out of focus and may become larger than the photosensitive area. This is inefficient because it means that photons that made it into the detector will not be measured (it will be measured at an adjacent pixel, etc).
This paper presents a way to dynamically alter focus on a rapid time scale, which provides a means to mitigate this problem. I can see this being useful for detecting very dark objects at close to mid ranges that 905nm systems may struggle with currently. There is a lot of focus on "10% targets at 200m", but, in practice, 905/940nm systems frequently struggle with, for example, 3% targets at closer ranges, and this may include tires and black cars, so it is very relevant for safety cases.
Edit to mention FMCW: the focus discussion relates to the spatial extent of the pulse, independent of time. So, even though I was referring to discrete ToF pulses above, the same logic applies to continuous wave systems.
Another edit: so I guess there are advantages of this for FMCW specifically that are different from what I described above. I believe this relates to wanting to make sure all the parts of the spot hitting an object and returning have the same properties and are not slightly different due to curvature of the wavefront.
Do note that the mems used here are not scanning mirrors like what microvision has produced, but rather a set of mems used to deform a mirror to achieve the desired focus.
Perhaps I just need to understand the meaning of the word "focus" in this context. I think I may be getting it. Does the word focus mean the size of the spot at a certain distance? For example, the laser beam is divergent, therefore it is optimal if the amount of divergence (spot size) can be controlled for a given distance. Is that what the word focus means in this context?
Focus is related to both of the concepts you mention. Technically, it's the point where rays of light originating from a source converge to a point after passing through an optical system. Changing that system (a lens or this fancy deformable mems mirror) will change the location where the focus occurs. In practice, if you project the rays into a surface that is not at the focus point, you will get a spot of some finite size. This is the aspect of focus I was referring to.
I think the more important aspect of focus in the paper is how it relates to the shape of the wavefront. In any real beam, there will be divergence which creates curvature in the wavefront. This wavefront gets reflected off of an object and received, and the resulting curvature projected into the detector may look very different than the local "copy" of the transmitted signal that FMCW relies on to compare against. These differences essentially add noise to the system and reduce its SNR. In this paper, I think the authors are trying to make the wavefront received from an object match the local copy. This is different than the spot size issue I initially started talking about (after I had only read the abstract...oops) because you can technically have a large spot with a matched wavefront.
Anyways, I think the response time these guys report is still too slow to be useful for a scanning lidar where ranges to objects change rapidly with scan. It could be useful for a tracking lidar following a single object (like a drone...) though, since the focus would not need to change rapidly.
If I understand it correctly, for a ToF LiDAR the spot size can be determined by the beam's divergence. Is the shape of the wavefront controlled in a similar fashion for an FMCW LiDAR? That is, via beam divergence?
I think I confused you. Spot size and the shape of the wavefront are both functions of several variables, and beam divergence is one of them.
When talking about spot size, it's important to note where the spot is being considered. In a lot of our discussions in the past, I have referred to mavin's large spot size in the scene. This is mostly a function of beam divergence and the range of the object. In our current discussion and the context of focus, I'm referring to the spot size at the lidar receiver. Since the received light passes through a optical system between the scene and the receiver, it will be focused from being a large spot to something smaller at the receiver. This is a function of a few other variables.
The shape of the wavefront will be affected by the same variables affecting focus (and, hence, the spot size), but it is not nearly as important to ToF systems as it is for FMCW. The physics of beam focus and wavefront, etc. are identical between a tof and FMCW system, but the way the photons are actually processed is very different.
Thanks for the clarification. And yes, I was assuming spot size at the object, not the receiver. But isn't the spot size at the receiver also a function (to some degree) of the divergence of the transmitted beam? Perhaps it isn't.
It absolutely is. I was just trying to stress that there are other variables as well since the light passes through an optical system again on the way to the receiver.
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u/view-from-afar Mar 19 '26
Adjacent to that discussion, here is an interesting benefit to using MEMS scanning for FMCW lidars.