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This https://www.gentec-eo.com/blog/spot-size-of-laser-beam article states that laser beam width changes along the length of the ray (the function stated doesn’t look like it could be a constant 1)

Is it impossible to create optics that completely parallelize laser light? If not, why so?

matthias_buehlmann
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  • Every laser beam will suffer the effects of diffraction - therefore, it is impossible to have a beam with constant width. You can collimate it to decrease this effect, but it will always be present. – Woe Jun 01 '19 at 22:40
  • Is there some formula then that gives the minimal angle possible? – matthias_buehlmann Jun 01 '19 at 23:11

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You can’t have a perfectly parallel beam. There are two equivalent ways to see this.

The uncertainty principle: the photons making up the beam have to have some spread (uncertainty) in their transverse momentum because they’re confined in position to a certain beam width.

Diffraction: by creating a beam of a certain width, you must have a certain amount of diffraction that causes the beam to diverge.

Fundamentally, these are the same. They come from the wave nature of light.

Bob Jacobsen
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  • Simplified, a laser pointer creates a sharp light circle with some stray glow around it. Does diffraction mean the circle part is bound to grow, or is it possible to keep the circle constant with and it just gets more faint with distance and the glow around it gets more prominent? – matthias_buehlmann Jun 02 '19 at 07:42
  • The central part will grow, because the beam is diverging. – Bob Jacobsen Jun 02 '19 at 15:55
  • But if I can focus a beam into a coverging beam using a les, why can’t I focus a beam to infinity so that divergance is zero? – matthias_buehlmann Jun 06 '19 at 07:44
  • Because there is no such thing as perfectly parallel light. See answer for physical reasons why. – Bob Jacobsen Jun 06 '19 at 15:10
  • Experimental proof: get a long focal length lens, go outaide, and try to make a point image from sunlight. You can’t. The light from the left side of the sun and the right side of the sun follow non-parallel, diverging paths. The longer your lens, the bigger the spot (image of the sun) will be. – Bob Jacobsen Jun 06 '19 at 15:40
  • @bon jacobsen how is that experimental proof? – matthias_buehlmann Jun 06 '19 at 16:19
  • It shows your “converging lens” example won’t work. – Bob Jacobsen Jun 06 '19 at 17:28
  • jacobson but it doesn’t really say much since the sunrays are neither collimated nor does the sun aproximate a point-lightsource (like a laser diode does) – matthias_buehlmann Jun 06 '19 at 18:25
  • Examine both parts of what you just wrote. They’re both wrong. Once you see why, you’ll see why I’ve been correctly answering your question. – Bob Jacobsen Jun 06 '19 at 18:31