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I have a pulsed laser (532 nm, 250 ps) operating with a period of 20 μs. I need to detect the location of the pulses in a 100 × 100 mm area with 5 mm resolution.

Being a complete novice to the field of optics, I am wondering if it is practical to try and focus the beam onto a high speed optical sensor array (capturing 50,000 fps) with some kind of lens arrangement and performing computer analysis on the resulting feed. If this is a reasonable approach, how does one go about selecting suitable optical elements for such a setup?

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  • $\begingroup$ How large is the beam you intend to locate? I am not sure why you consider focusing the laser which is pretty much collimated to start with. Like Transistor mentioned below, it would be really helpful to put some context into what you are trying to do. E.g. if you are trying to track the laser location and move it along a grid? $\endgroup$
    – NMech ♦
    Oct 3, 2022 at 8:33
  • $\begingroup$ Do you have to capture a single pulse? Or is it a continuous series of pulses where it's ok for your system to miss some? $\endgroup$
    – Drew
    Oct 4, 2022 at 5:03

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50,000 fps = 20 μs/frame. Your pulse is only one thousandth of that. A camera sensor will integrate the light input over the duration of the exposure but I suspect that you would have to run at high gain and that noise might be a problem.

I am wondering if it is practical to try and focus the beam onto a high speed optical sensor array (capturing 50,000 fps) with some kind of lens arrangement ...

enter image description here

Figure 1. Object, lens and image. Stemmer Imaging.

You won't be focusing the beam into a sensor or it will be destroyed (and the problem of placing the lens in the right place). Instead you will focus an image of the target and illuminated spot onto the sensor. This is how a film or digital camera works. Some of the light reflected by the subject is focused onto the film or sensor matrix.

... and performing computer analysis on the resulting feed.

That sounds reasonable - once you've got a decent image.

... how does one go about selecting suitable optical elements for such a setup?

It's basically a camera lens. There are good catalogues in paper and online form from companies such as Edmund Scientific and Stemmer Imaging. These often have good diagrams which are highly educational.

You don't say what the laser power is or what the target is but since it's visible green I'm wondering if you can run at reduced power for a long time to make it easy for the image capture.

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  • $\begingroup$ Thank you for the reply, Peak laser power is specified as 120w, so an average power of 1.5mw over the 20us period. At this point I've not decided on a target, I suppose it could be as simple as a printed grid pattern as I only need to detect the beam position with a rather coarse level of precision. $\endgroup$
    – Thomas Holle
    Oct 1, 2022 at 13:23
  • $\begingroup$ So what is the problem you're trying to solve? Are you trying to check that it's in the right place before you give it 100% power or what? My IR laser cutter has a prism where the beam exits the laser module and a visible laser is merged with the beam. This can be used for beam alignment and can be left on all the time. $\endgroup$
    – Transistor
    Oct 1, 2022 at 13:33
  • $\begingroup$ I think you will actually want very low gain. You want the camera to be exposing for as much of the frame time as possible. There is some unavoidable dead time when each row gets read, but ideally you will be exposing the rest of the time so you can capture the photons from the pulse. You may be able to use interleaving to your advantage, so that you never miss a pulse. If a row is getting read right as the pulse hits it, it will still register in the adjacent rows. $\endgroup$
    – Drew
    Oct 4, 2022 at 5:09

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