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Semi-OT: Any experts on solid state lasers?

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  • Semi-OT: Any experts on solid state lasers?

    I've been bouncing around a project idea that involves shooting a laser at a spinning mirror, ideally a short laser pulse that produces a visible dot on a target. The dot moves by changing the delay between when a sensor detects the location of the motor shaft and when the laser is fired.

    I've done some testing with a 5mW laser, and it works, but the laser is very faint when trying to switch it on and off at microsecond levels. My motor is running at 4-5k RPM, and using a microcontroller to trigger the laser for less than 10us does not produce a visible laser dot. Longer pulses produce a line instead of a dot, and it still remains quite faint. My driving circuit is only a MOSFET and current limiting resistor so it is not ideal, either.

    If I increase my laser power to say a 100mW capable laser, what kind of eye danger would this present if the laser was only fired in 1us pulses? The total laser energy delivered to the target might be low, but is it still potentially harmful to your eyes? My thinking is that it is about cumulative exposure, so an ultra short but powerful pulse will do the same damage as a continuous beam at a much lower energy.

    Interested in knowing what someone with more laser experience thinks of the problem.

  • #2
    Others have addressed this problem with a shutter that the low power laser shines through. Sync it with the mirror and you have a controllable strobe.
    At the end of the project, there is a profound difference between spare parts and extra parts.

    Location: SF East Bay.

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    • #3
      Originally posted by danlb View Post
      Others have addressed this problem with a shutter that the low power laser shines through. Sync it with the mirror and you have a controllable strobe.
      That would work but I am looking at speeds in the low microseconds, so a mechanical or electromechanical shutter won't be able to open and close in time. It seems to me the electrical method is necessary. Syncing two mechanical mechanisms would also prove tricky as I need firing time control.

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      • #4
        Drive electronics integrated to the laser head are probably too slow for short pulses.
        Low duty cycle also makes the laser dot very dim. If you have 10us ON and 10ms OFF timing that would give you 1:1000 duty cycle and 5mW laser pointer would become 0.005mW

        Eye damage is sort of related to average power, maybe more like RMS value or something like that. Single 100W 1 second pulse once per year is obviously pretty bad..
        See: " power density versus exposure time for various wavelengths"
        https://en.wikipedia.org/wiki/Laser_safety

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        • #5
          Originally posted by mattthegamer463 View Post

          That would work but I am looking at speeds in the low microseconds, so a mechanical or electromechanical shutter won't be able to open and close in time. It seems to me the electrical method is necessary. Syncing two mechanical mechanisms would also prove tricky as I need firing time control.
          You are thinking of a camera style shutter. I was thinking of a rotary shutter such as you find in movie projectors. A spinning disk with holes in it makes an effective shutter as long as you can tolerate a dot that goes through the phases of the moon.

          I read that a 5mw laser is deemed "safe(ish) because the instinct to blink and shut your eyes will block the light before damage is done. I'm not sure how that applies to more powerful lasers at lower exposures.

          Dan
          Last edited by danlb; 02-24-2020, 03:19 PM.
          At the end of the project, there is a profound difference between spare parts and extra parts.

          Location: SF East Bay.

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          • #6
            Don't know much at all about lasers but it does bring a question to mind. If its only a 10us duration dot, can the human eye and brain even "see" something that short of a duration?

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            • #7
              You have a spinning mirror so you could have a rotary shutter spinning at the same speed? You just need a means to change the speed of the shutter (very slightly) to 'move' the reflected spot. There must be a thousand and one ways of doing that by mechanical means. How fast do you need to shift the dot?

              Ooops! I see DanLB has already posted most of that!

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              • #8
                If your targets are fixed, you can use a mask to block physically the beam from the unwanted directions.
                Helder Ferreira
                Setubal, Portugal

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                • #9
                  I am not familiar with the laser power levels and durations that are dangerous. If I were doing this I would take precautions to prevent the beam from leaving the apparatus. That would be all around it because you never know when a stray beam may strike something else in the room/area and come back to you from an unexpected direction. That rotating mirror could send it in any direction.

                  And if you need to run any tests or alignments with the laser on for longer times, please do use a lower power. Accidents do happen.
                  Paul A.
                  SE Texas

                  Make it fit.
                  You can't win and there IS a penalty for trying!

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                  • #10
                    Originally posted by Sparky_NY View Post
                    Don't know much at all about lasers but it does bring a question to mind. If its only a 10us duration dot, can the human eye and brain even "see" something that short of a duration?
                    Can, as long as the pulse has enough brightness. Eye response to short pulses is averaging/ integrating.
                    1lumen 1ms pulse and 1000lumen 1us pulse appear roughly same intensity.

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                    • #11
                      Off topic a little, years ago Scientific American, in the amateur scientist section had an article on building a UV laser with a gigawatt output. The pulse duration was only a picosecond or so.

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                      • #12
                        What is the mechanism by which a laser can destroy eyesight? Is it power over time resulting in overheating, or is it the absolute light level, however short the pulse might be? It just seems that a gigawatt pulse for even a picosecond would fry some retinal cells- the same could be true of a 20 watt pulse or a 5 mw pulse. Still a lot of photons impacting a very small area of the retina, even if short duration.
                        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                        • #13
                          If you have very good control of the pulse length you can use a higher voltage. The average power would then be very important, as that would control the heat, which is what would destroy the laser.

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                          • #14
                            Originally posted by darryl View Post
                            What is the mechanism by which a laser can destroy eyesight? Is it power over time resulting in overheating, or is it the absolute light level, however short the pulse might be? It just seems that a gigawatt pulse for even a picosecond would fry some retinal cells- the same could be true of a 20 watt pulse or a 5 mw pulse. Still a lot of photons impacting a very small area of the retina, even if short duration.
                            The damage mechanism for short pulses is micro-cavitation (bubbles), but for longer pulses it's thermal denaturation (they blow up).

                            BTW: Despite being "solid", diode lasers are not normally considered to be solid state lasers -- they're semiconductor lasers.

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                            • #15
                              Thanks for the replies. I'll explain the mechanism and the problem more carefully;

                              The idea is to have a laser dot appear on a surface and have that laser indicate a distance from a subject. The laser, dot target and another location on the same plane as the laser dot form a triangle. If using a continuous laser mounted to a rotating base, the base would need angular accuracy and repeatability on a 10 arcsec level to be useful. My thought was that if I can spin a mirror with a consistent speed, it would be easier to fire a laser with precision at the mirror to produce a dot on the target rather than try to aim a continuous laser precisely. Then I can describe the system with trigonometric functions to calculate the time needed, and use a break beam optical sensor and 1 slit encoder wheel on the motor to locate the motor position every rotation. Detect the slit, wait the prescribed delay time, and fire the laser pulse. Repeat 60 times per second or so and you have a continuous dot on the target, to the user's eye.

                              Perhaps if I can tolerate a line instead of a dot and treat the tip of the line as the point of importance, that would suffice. This would mean that as long as the laser pulse is not so long the laser wraps back around again, there is still a useful "point" at the starting end of the line. Also if hte laser has a consistent "rise time" then it can be measured and subtracted from the calculated delay to correct for any positional error caused by the turn on speed or reaction speed of the laser driver.

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