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In a laser guided through a fiber, the beam is divergent upon exit. What normally is done is to put the fiber on the focus of a lens in order to achieve a collimated beam. This beam is then guided through focusing optics. The spot of the resulting beam after the focusing optics depends on the focal length of the collimating optics and the focusing optics. What I am trying to achieve is the possibility to have multiple spots without having to physically change the lenses. My idea is to put a beam expander between the collimating and focusing optics, which receives a collimated beam of a certain size and outputs another collimated beam of a larger size, simulating the use of a collimation optics of larger focal length. Such beam expanders are available for purchase

Is this idea feasible or am I forgetting something?

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    $\begingroup$ What do you mean by "multiple spots"? $\endgroup$ – Mark H Oct 23 '18 at 18:39
  • $\begingroup$ What I mean is that for one configuration my beam has for example 400 micrometers. Then I would change the beam expander and the beam would now have 500 micrometers. What I mean by multiple spots is the ability to change the spot size of my beam without changing the actual lenses. $\endgroup$ – Danilo Leite Ribeiro Oct 23 '18 at 18:45
  • $\begingroup$ a beam expander is made of 2 lenses. So you'd end up switching 2 lenses instead of one $\endgroup$ – Manu de Hanoi Oct 23 '18 at 18:51
  • $\begingroup$ However there are many variable beam expanders, which allow me continously increase my beam size from 2x to 10x. So I would only have to slightly adjust the beam expander instead of removing the full piece and putting another one. $\endgroup$ – Danilo Leite Ribeiro Oct 23 '18 at 19:09
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Yes, expanding your collimated beam in a variable way will allow you to control the focus spot size. The larger the collimated beam, the smaller the focus spot. Essentially, a larger beam allows you to take advantage of more of the numerical aperture of the focusing lens. Usually in a microscope or camera, this variability is achieved with an iris diaphragm in the collimated region to reduce the NA as needed. But since that method throws away power, perhaps you would prefer a variable telescope (i.e. beam expander).

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  • $\begingroup$ A system with a lens to collimate the light from a small source, then to refocus this to a small point is identical to a microscope. You might not be able to realize smaller spot size from the high NA due to the larger beam size - you're just imaging the source onto the output with magnification $M$. You'll also need to watch out for etendue gotcha. $\endgroup$ – D Duck Oct 23 '18 at 20:00
  • $\begingroup$ @DDuck indeed it is a microscope. Since the light originates from a fiber, I think it’s fair to treat it as a point source which will be focused with a point spread function of size inversely proportional to NA. $\endgroup$ – Gilbert Oct 23 '18 at 20:38
  • $\begingroup$ True, if a single mode fibre, but this often done with multimode fibre with disappointing results. $\endgroup$ – D Duck Oct 24 '18 at 8:13
  • $\begingroup$ @DDuck so with a multimode fiber it is feasible to achieve larger spot sizes than the fiber, but smaller ones may lead to undesirable results? Also, what is etendue gotcha? $\endgroup$ – Danilo Leite Ribeiro Oct 24 '18 at 14:20
  • $\begingroup$ Area $\times$ solid angle is constant. en.wikipedia.org/wiki/Etendue#Maximum_concentration $\endgroup$ – D Duck Oct 24 '18 at 19:25

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