# Laser spot size change through a beam expander

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?

• What do you mean by "multiple spots"? – Mark H Oct 23 '18 at 18:39
• 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. – Danilo Leite Ribeiro Oct 23 '18 at 18:45
• a beam expander is made of 2 lenses. So you'd end up switching 2 lenses instead of one – Manu de Hanoi Oct 23 '18 at 18:51
• 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. – Danilo Leite Ribeiro Oct 23 '18 at 19:09

• 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. – D Duck Oct 23 '18 at 20:00
• Area $\times$ solid angle is constant. en.wikipedia.org/wiki/Etendue#Maximum_concentration – D Duck Oct 24 '18 at 19:25