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I've seen some videos of laser toys but they all seem to use either one laser or lens with some specific range of focus. What I was thinking is this:

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But it seems like it might violate the 2nd law of thermodynamics, since the energy on the left side is more spread out than on the right side. Can this be done? If it can, can this be done in a practical manner?

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  • $\begingroup$ Could you specify how that would violate the second principle? I'm not sure how that would apply here. $\endgroup$ – fffred Mar 10 '16 at 15:34
  • $\begingroup$ @fffred I said "it seems like" and I said it's because "the energy on the left side is more spread out than on the right side". Second law implies the energy is to spread out, rather than concentrate. $\endgroup$ – Tomáš Zato - Reinstate Monica Mar 10 '16 at 15:36
  • $\begingroup$ The entropy of an isolated system cannot decrease. That's about it. Here, there is no isolated system, and energy is not entropy. Or am I getting something wrong? $\endgroup$ – fffred Mar 10 '16 at 15:38
  • $\begingroup$ @fffred The way you discuss this me is making me very uncomfortable. I never said I'm confident about that statement and given I've got an answer showing it's both possible and easy I see it was wrong. However none of your comments helped me know what is specifically wrong with it, I rather feel you're just throwing your "nope!" at me as if I was deemed too stupid to understand any explanation. Speaking of isolated system - second type perpetuum mobile is not isolated system either still laws of thermodynamic apply quite fine. $\endgroup$ – Tomáš Zato - Reinstate Monica Mar 10 '16 at 16:28
  • $\begingroup$ I am sorry if my comments seemed rude. They were not intended to. As far as I can tell I don't see any reason why the second principle would have any impact here, and I was just asking for a more precise explanation on how that would have one. $\endgroup$ – fffred Mar 10 '16 at 19:39
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You can do this with a simple telescope or an infinite conjugate microscope objective. Because the input beams are not centered on the collection optics the output beam isn't perfect.

This can be improved with proprly designed aspheric lenses, or parbolic collection mirrors.

It's also possible to use fiber optics to combine multiple beams; a colleague did his PhD in this area.

The basic idea is that of a beam expander; if you run it backwards, it's a beam reducer: http://www.photonics.com/Article.aspx?AID=56477

Here's an article on the topic: http://www.laserfocusworld.com/articles/print/volume-48/issue-06/features/beam-combining-cranks-up-the-power.html

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  • $\begingroup$ At the National Ignition Facility, the lasers are not combined in a single beam. Not even on a single spot. They only hit the same target. $\endgroup$ – fffred Mar 10 '16 at 15:33
  • $\begingroup$ @fffred: I've removed the reference to NIF. $\endgroup$ – Peter Diehr Mar 10 '16 at 15:47

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