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Reflecting a laser with a mirror is there a limit to the power level?

  1. Sep 5, 2012 #1
    If I shine a laser at a mirror, it would be reflected back. If I got a super powered laser, could I still reflect it back? Would it start burning through the mirror?

    I'm thinking it's a case of imperfect reflection. If there were such a thing as a perfect reflector, you could reflect any laser, regardless of power level. However, if your mirror is only 99% perfect, 1% of the energy will be absorbed, and start to heat up the material of the mirror. Then it would just be a matter of time until your mirror gets too hot, depending on the material and the power of the laser.

    Does this sound correct? And does anyone know what sort of limitations there are for reflecting lasers?
     
  2. jcsd
  3. Sep 5, 2012 #2
    You're right that an imperfect mirror will absorb some energy and can be deformed or melted with enough energy, though, keep in mind, a mirror is basically just a thin sheet of metal, which requires an extremely high temperature to melt. I'm not sure what the exact limitations are, or if there are any lasers that could possibly achieve this. I wouldn't be too surprised, though.
     
  4. Sep 5, 2012 #3
    Does the glass of the mirror have any purpose other than providing a structure to deposit a metal film upon?
     
  5. Sep 5, 2012 #4
    The glass just supports the film, but that is not how high end optical mirrors are made. If someone did this for real, I'd expect them to use a highly polished beryllium plate with a redlectivity of about 98%. Able to survive much more heat transfer, lower absorption, and easier to provide active cooling.
     
  6. Sep 5, 2012 #5

    sophiecentaur

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    I understood that one reason that glass is used is that Perspex has a low coefficient of expansion (relative to affordable alternatives) so it distorts less as the temperature changes, and the optics are better, as a consequence.

    The ability to resist being vaporised by a laser beam would depend largely on the 'spot size' and the rate at which the local heat could be dissipated. A thick metallic coating would help because the thermal conductivity is much higher than that of glass (but that's not a quantitative answer, of course). Would they have to make shiny tanks and aircraft if laser weapons became viable?

    [Edit @ Pkruse - just missed your post as I was writing mine. When did high end telescopes stop being made with glass? For use in space, I guess there are much lighter alternatives but for terrestrial?]
     
  7. Sep 5, 2012 #6
    Some weapons already have countermeasures against LASERs. Many missles for example rotate in flight so the energy is absorbed over a much larger area.
     
    Last edited: Sep 5, 2012
  8. Sep 5, 2012 #7
    The military optical systems I've worked on all used beryllium. I've never worked on a telescope, but the optical design engineers who worked with me told me that the large telescope mirrors were also polished metal, but maybe not Be. We used it because it was the only material that met all our requirements, including weight, thermal expansion, young's modulus, and corrosion resistance. It also had to perform well in visible and infrared.
     
  9. Sep 5, 2012 #8

    mfb

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    I once read about a very high-intensity pulsed laser where they had to widen the beam in order to reflect it - otherwise the mirror would get too hot even from a single pulse.
    So heat can be an issue.
     
  10. Sep 5, 2012 #9

    Bobbywhy

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    Pa5tabear, In order to find information regarding your questions I thought about the project named “Medusa” which uses high power lasers and mirrors to concentrate optical energy onto a fuel pellet causing it to undergo fusion. During a Google search using these terms: “medusa free electron laser mirror” brought lots of pdf documents describing simulations of mirror performance in high power systems. I ran across this ad which I thought might be interesting:

    “Evaporated Coatings, Inc. manufactures highly reflective low loss dielectric laser mirror coatings with up to 99.95% reflection. Laser Mirror Coatings can be optimized for use from 248nm to 2500nm for laser line wavelengths, or multi band applications. Designs are available for deposition onto various types of optical materials including: glass substrates, fiber optic devices, and crystals and semiconductor materials.”
    http://www.evaporatedcoatings.com/mirror-coatings?WTX=gaw&gclid=COin0_3Dn7ICFQQGnQodJF8ACQ
     
  11. Sep 6, 2012 #10
    Perspex isn't "glass" is it? Wikipedia says it's plexiglas. Are you saying that plexiglas is better than standard silicon glass because it has a lower coefficient of expansion?
     
  12. Sep 6, 2012 #11
    If an infrared laser were shone into a person's eye, would they be blinded?

    I'm thinking yes, because you would still have absorption, which would cause heat damage. But you wouldn't see it... does laser blinding from visible light have other harmful effects in addition to heat damage?
     
  13. Sep 6, 2012 #12

    mfb

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    Powerful infrared lasers are extremely dangerous, because they produce heat similar to visible lasers - but you do not see them if they hit your eye.
     
  14. Sep 6, 2012 #13

    f95toli

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    Infrared lasers do not even have to be powerful, the fact that the beam is invisible makes them MORE dangerous than say a red laser since your blink reflex won't kick in to protect the eye, and it would take a while before you realized something was wrong (and by that time it would be too late).
    Hence, even a regular laser diode of few mW could damage your eyes very badly.
     
  15. Sep 6, 2012 #14
    So when you stare at the sun, what is it that actually causes the damage to your eyes?

    Is it too much radiation absorption, which heats up the "eye tissue" and kills cells?
     
  16. Sep 7, 2012 #15

    Redbelly98

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    The 1% (or whatever the number) doesn't have to be absorbed, it could be transmitted instead. That is pretty much what happens with a mirror used in a laser. What is transmitted becomes the emitted laser beam.

    At any rate, you check out some of these google hits to learn more about optical coatings and damage:
    https://www.google.com/#hl=en&biw=1...2,or.r_gc.r_pw.r_cp.r_qf.&fp=1ac6f9833d0f42d9

    To make it brief, any optical coating has a threshold for damage by laser beams. The threshold is typically measured in terms of W/cm^2 for continuously operating lasers, or J/cm^2 in the case of pulsed lasers.
     
  17. Sep 7, 2012 #16

    Integral

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    The 30W green beam laser system I worked with had 4 mirrors in the beam path. The mirrors were dielectric mirrors designed specifically for the frequency of the laser. They essentially were a stack of 1/4 wave plates. They had to be kept very clean as any spot on them would heat up causing a burn spot on the mirror. Cleaning and alignment were very critical parts of maintenance of this system.
     
  18. Sep 7, 2012 #17

    davenn

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    I suspect sophicentaur has made a mistake by saying perspex rather than pyrex
    perspex is a "plastic" and not used for commercial mirrors
    Pyrex on the other hand IS used for commercial telecscope and other mirrors and does as sophi stated ... have a low coefficient of expansion.
    ALL my telescopes have always used pyrex, all the big telescopes use pyrex, Hubble, Kec twins, Mt Palomar etc etc

    Dave
     
  19. Sep 8, 2012 #18

    sophiecentaur

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    Absolutely right. I think it must be down to my h t c and predictive txt!
    So even Hubble uses Perspex- despite how heavy it is? Interesting.
    [edit: I have to apologise; I mean Pyrex again.]
     
    Last edited: Sep 8, 2012
  20. Sep 8, 2012 #19

    Redbelly98

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    Uh, you said "Perspex" again :-)
     
  21. Sep 8, 2012 #20

    Bobbywhy

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    My Questar telescope has a Zerodur mirror. I paid a premium to buy it, but the results are fantastic!

    Zerodur, a registered trademark of Schott Glass Technologies,[1] is a lithium aluminosilicate glass-ceramic[2] produced by Schott AG since 1968.[3] It has been used for a number of very large telescope mirrors including Keck I, Keck II,[4] and SOFIA. With its very low coefficient of thermal expansion it can be used to produce mirrors which retain acceptable figures in extremely cold environments such as deep space.
    http://en.wikipedia.org/wiki/Zerodur
     
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